Using a Neutral Density Filter on a Rear Lens Element

Over time, there have been a fair number of threads that discuss using filters, especially glass rather than gelatin filters, on the back of a lens rather than the front. I have a specific question that doesn't appear to have come up before.

I've just acquired a Schneider Super-Symmar XL f/5.6 150mm lens. On 8x10, this is quite wide (in 35mm terms, about 21mm). I'll be using Schneider's IVa centre filter, which has a front-facing filter thread of 112mm. There's also a 62mm filter thread on the lens's rear element.

For cost reasons, I'm interested in using a glass, screw-in neutral density filter on the rear element rather than in front of the centre filter. In order to avoid focus shift, I would focus after screwing the ND filter onto the rear element. I'd have to try it, but I'm hoping that in good light I can focus with 2 stops of ND. I'd add the centre filter after focusing, which would reduce the light by another 1.5 stops. I'm thinking that this might give me as much ND control as I need.

Two questions...

What negative impact, if any, is using an ND filter on the rear element likely to have on overall image quality?

Does the answer to the question change if I use an 82mm ND filter on the rear element, via a step-up ring, rather than a 62mm ND filter?

If this whole idea is asking for trouble, and I should just arrange to mount ND in front of the centre filter, don't hesitate to say so :)

Thanks

If there are any imperfections, smudges, dust, etc on the filter they will all effect the performance of your lens. And that assumes that you are using a very high quality nd filter.

Just about any question on this -- or any other forum -- is asking for trouble.

My advise? Just run a couple of simple tests -- with X vs without X, with X vs Y, etc.

I'd also advise that you are free to experiment with CND filters. That is, what the manufacturer recommends may not be what fits your needs, lens(es), situation, etc. best. Just use them as a starting point -- much like ISO.

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Originally Posted by Bob Salomon

If there are any imperfections, smudges, dust, etc on the filter they will all effect the performance of your lens. And that assumes that you are using a very high quality nd filter.

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Thanks Bob. I use ND constantly for video, so I take your point about smudges, dust, etc, which I gather is a particular issue when mounting a filter to a rear element. I'm OK on ND filter quality. If I use a 62mm to 82mm step-up ring, I have Heliopan and B+W XS-Pro MRC Nano.

I actually use a 6x ND filter screwed on the rear element of 150mm ssxl lens a few times and quite pleased with the results. I used small f/32 or 45 for exposure and didn't notice any focus shift.

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Originally Posted by Hugo Zhang

I actually use a 6x ND filter screwed on the rear element of 150mm ssxl lens a few times and quite pleased with the results. I used small f/32 or 45 for exposure and didn't notice any focus shift.

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Thanks Hugo, this is interesting. Good to hear that you were happy with the results. By 6x, I assume that you mean 0.6 or 2-stop. When did you add the filter to the lens? If before focusing, my understanding is that there shouldn't be a focus shift issue.

I meant 6 stops like this one:

https://www.bhphotovideo.com/c/produ...l?sts=pi&pim=Y

I took the lens off the front standard after focusing, screw the filter on the rear element and then put it back on the camera and click the shutter. I could not focus with 6 stop ND filter. Too dark.

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Originally Posted by Hugo Zhang

I meant 6 stops like this one:

https://www.bhphotovideo.com/c/produ...l?sts=pi&pim=Y

I took the lens off the front standard after focusing, screw the filter on the rear element and then put it back on the camera and click the shutter. I could not focus with 6 stop ND filter. Too dark.

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Thanks, very helpful. So no noticeable focus shift, although as you say you were shooting at f/32 or f/45. Good to know that it worked for you, and that you were happy with the images.

Two things happen with a glass filter behind the lens.

1. There is a focus shift of about one third of the thickness of the filter. The exact value depends on the refractive index of the filter glass but if you focus after fitting the filter the on-axis focus point can be found accurately.

2. The filter imposes a bit of spherical aberration on the native image of the lens. This is because increasingly oblique rays effectively traverse an increasing thickness of filter glass. This results in an increasing focus shift for each image ray as it becomes more oblique toward the edge of the format. The effect is most severe for wide angle lenses. However stopping well down might create enough depth of focus to mask the problem. Testing will show.

This post, from earlier today, explains why I started this thread, and talks about the cost of conventional solutions. There are basic financial reasons to consider using certain filters on the rear element of this particular lens, given the 112mm outside thread of both of the centre filters (Heliopan and Schneider) that will work with the lens:

I say earlier in this thread that I'm standardised on 82mm screw-in filters and Lee100 100mm square filters. This means that I'm not set up for the 112mm outside thread on Schneider's 4a centre filter. Filter choice in that size is limited and prices are high.

Enter Nikon's fairly new Z-mount lens, the ultra-wide Nikkor Z 14-24mm f/2.8 S. This is a US$2400 lens with a 112mm filter thread. In response, several manufacturers are suddenly making 112mm filters, marketing them specifically to owners of this lens. In addition to the usual UV filters, they are offering polarisers and neutral density filters. Prices are high, but still lower than some brands. These are niche filters, and brief, attractive offers are quite liable to come up. Indeed, that's how I obtained the filter mentioned in post #76, NiSi's NiSi Pro Natural Circular Polariser (112mm), at 50% off (B&H Flash Deal). In the next while, these filters may also start coming up on the second-hand market.

There are a couple of other options for ND, but they aren't cheap. The first is Lee's SW150 System for ultra-wide lenses if it can be set up to work with this lens. Lee's/Panavision's excellent ProGlass IRND filters are available for the SW150 system, but pricing is stiff. The other is to use my Arca-Swiss compendium shade and rig a filter holder for appropriately sized rectangular ND filters. This leads me in the direction of what is essentially a matte box system. As someone who shoots video, a matte box and filter tray was precisely what I wanted to avoid when I standardised around 82mm screw-in filters and Lee100. C'est la vie :) As mentioned in post #78, I'll also take a shot at trying limited ND - up to, say, two stops - on the rear element of the Schneider XL 150mm lens. If it doesn't interfere too much with focusing, and taking into account the light loss from the centre filter (added after focusing), this just might give me enough ND control. I'll have to do some reading on what impact, if any, an ND filter on the rear element may have on overall image quality.

The original post is at https://www.largeformatphotography.i...=1#post1626910

EDIT: See post #27 re Lee's SW150 Holder.

If it's a gel, no problem... Others might work, but you have to test in advance...

Changing ND filter while inside camera will slow operations down, as lens or back will have to be removed to change filter for focusing etc...

Steve K

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Originally Posted by LabRat

If it's a gel, no problem... Others might work, but you have to test in advance...

Changing ND filter while inside camera will slow operations down, as lens or back will have to be removed to change filter for focusing etc...

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The Lee and Rosco ND gels are used for lighting, e.g. on windows. It's unclear how suitable they are for camera lenses. Also, a roll isn't cheap, although suppliers may sell it in small quantities.

See posts #5-#7, if you haven't already, on adding an ND filter after focusing.

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Originally Posted by r.e.

The Lee and Rosco ND gels are used for lighting, e.g. on windows. It's unclear how suitable they are for cameras. Also, a roll isn't cheap, although suppliers may sell it in small quantities.

See posts #5-#7, if you haven't already, on adding an ND filter after focusing.

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There are Kodak Wratten/Calumet ND camera gels still floating around unused still in good shape... I can still buy them for $1- $3 in camera store clearance bins, but might take some time to find all densities you desire... Start looking in pro dealers, older camera stores, rental house surplus etc...

Steve K

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Originally Posted by LabRat

There are Kodak Wratten/Calumet ND camera gels still floating around unused still in good shape... I can still buy them for $1- $3 in camera store clearance bins, but might take some time to find all densities you desire... Start looking in pro dealers, older camera stores, rental house surplus etc...

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Thanks. It turns out that Kodak ND gels are still sold. The is the result of a B&H search: https://www.bhphotovideo.com/c/produ...d_name%3Akodak

B&H has the Kodak gels in 3"x3", 4"x4" and 4"x12". It appears that 6"x6" may also be available, but not via B&H. The 3"x3" should work with the 62mm rear element of the Schneider XL 150mm. However, at US$97.50 each the Kodak gels range from about the same price as B+W/Heliopan 62mm glass to more expensive. As you say, Kodak gels might be a lot cheaper from clearance bins.

Some may be interested in this 2016 thread (you wrote two of the posts): Wratten Gel, Acrylic or Glass Filters?

By the by...

While looking at options, I've learned that B+W is replacing its F-Pro line with "Basic" and its XS-Pro line with "Master. The announcement was made in October (see below). There are changes to the rim design of the filters and to polarisers. B&H has started selling the "Master" filters. Looks like this involves a price hike.

B+W press release and .PDFs: B+W Filter launches two new filter lines: BASIC and MASTER

Gels collect grit, fingerprints, and kink marks awfully easily, and being fragile, are hard to clean. Any such anomalies behind the lens are going to degrade the image more than if the filter were in front. But don't use polyester pseudo-gels like the Lee ones for any kind of precision imaging application; they're basically just cut-down lighting gel material. And if you're shooting in color, most ND filters actually have a hue bias and are not completely neutral, so that's another issue. Yet one more problem : never assume that the density given on the label is exactly the actual density. You need to check each one with a transmission densitometer yourself.

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Originally Posted by Drew Wiley

... never assume that the density given on the label is exactly the actual density. You need to check each one with a transmission densitometer yourself.

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I always TRY to remind myself -- "Trust, but TEST".

If you got a UV filter from the same manufacturer as your ND filter, I'd assume they would have the same base glass, ergo the same focus shift. (You might want to ask the manufacturer about this.) You could focus with the UV without light loss, then switch to the ND.

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Originally Posted by Mark Sawyer

If you got a UV filter from the same manufacturer as your ND filter, I'd assume they would have the same base glass, ergo the same focus shift. (You might want to ask the manufacturer about this.) You could focus with the UV without light loss, then switch to the ND.

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What a clever idea. If you're correct, that fixes the focus shift problem and the only remaining issue, apart from making sure that the filter is spotless, is Maris Rusis's second point in post #9.

I've been seeing focusing as a limitation on the amount of ND that can be used. Your suggestion, if it works, would remove the limitation.

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Originally Posted by Maris Rusis

Two things happen with a glass filter behind the lens.

1. There is a focus shift of about one third of the thickness of the filter. The exact value depends on the refractive index of the filter glass but if you focus after fitting the filter the on-axis focus point can be found accurately.

2. The filter imposes a bit of spherical aberration on the native image of the lens. This is because increasingly oblique rays effectively traverse an increasing thickness of filter glass. This results in an increasing focus shift for each image ray as it becomes more oblique toward the edge of the format. The effect is most severe for wide angle lenses. However stopping well down might create enough depth of focus to mask the problem. Testing will show.

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+1

Try it and see. If the introduced spherical abberation is noticeable, it will be at the edges of the image circle first.

As suggested, a gel filter will minimize the focus shift. You can get holders for them and you get to focus without the ND in place. I think that would be my preference. Just be aware that gel filters are fragile and need careful handling.

Doremus

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Originally Posted by r.e.

What a clever idea. If you're correct, that fixes the focus shift problem and the only remaining issue, apart from making sure that the filter is spotless, is Maris Rusis's second point in post #9.

I've been seeing focusing as a limitation on the amount of ND that can be used. Your suggestion removes the limitation. Following up on your idea... My screw-in ND filters include a 1-stop. As a stand-in, it's from the same series of filters and would probably work almost as well, from a focusing standpoint, as a UV or clear filter.

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While not true for all filters Heliopan makes filters of different densities by grinding the glass to reach the required density. So Mark’s solution would not work.

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Originally Posted by Bob Salomon

While not true for all filters Heliopan makes filters of different densities by grinding the glass to reach the required density. So Mark’s solution would not work.

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Just one more reason to run a simple test.

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Originally Posted by xkaes

Just one more reason to run a simple test.

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In post #3, you wrote:

Just about any question on this -- or any other forum -- is asking for trouble.

My advise? Just run a couple of simple tests -- with X vs without X, with X vs Y, etc.

I'd also advise that you are free to experiment with CND [center neutral density] filters. That is, what the manufacturer recommends may not be what fits your needs, lens(es), situation, etc. best. Just use them as a starting point -- much like ISO.

This is now the second post in which you've recommended "simple tests".

Re the centre filter... If you're just talking about whether or not to use it, I imagine that anybody who uses a centre filter knows that that's a judgment call. That said, based on discussions about this particular lens when used for 8x10, I expect to use the centre filter most of the time.

I'm now considering options for solid neutral density. I've asked for, and received, some very useful comments on one option, which is ND on the rear element. Given the cost of 8x10 sheets of film, this discussion is likely to save me quite a lot of money, not to mention time, compared to testing the various ways of doing this, one of which would require the purchase of gelatin filters that I don't have. If I do test, this discussion will narrow what's tested and how it is tested. Posts #18 and #21 have also prompted me to send a message to B+W asking about the thickness of the glass on its XS-Pro filters. Due to the design objective of the XS-Pro line, it's possible that the glass thickness is uniform, or close to it. The discussion is also helping me to evaluate this option against two options for placing ND in front of the lens. Those options are in post #10 and are quite expensive. To give you an idea, a single high quality 112mm screw-in ND filter costs US$200 and up, and a glass rectangular ND filter made by Schneider, Tokina or Lee/Panavision, large enough to cover 112mm, costs about $500.

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Originally Posted by Maris Rusis

Two things happen with a glass filter behind the lens.

1. There is a focus shift of about one third of the thickness of the filter. The exact value depends on the refractive index of the filter glass but if you focus after fitting the filter the on-axis focus point can be found accurately.

2. The filter imposes a bit of spherical aberration on the native image of the lens. This is because increasingly oblique rays effectively traverse an increasing thickness of filter glass. This results in an increasing focus shift for each image ray as it becomes more oblique toward the edge of the format. The effect is most severe for wide angle lenses. However stopping well down might create enough depth of focus to mask the problem. Testing will show.

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So problem #1 could be addressed by focusing with a UV filter in place, so long as the glass is the same type and thickness.

As for problem #2, spherical aberration from the rays of light travelling through thicker glass the more oblique the angle, the problem is the same whether the glass is in front of or behind the lens. And if the lens is designed to take that into account in front of the lens, he is already using a center filter and I very much doubt the lens is designed to compensate for 2 stacked filters.

Mark Sawyer, in post #18, suggested focusing with a UV filter as a stand-in and replacing it after focusing with an ND filter. Two forum posts from 2006 also suggest this. They are in a thread called filter for Schneider 210mm SS XL. On that lens, the rear element thread is 72mm and the front thread is 135mm:

Ron Marshall:

"I use rear mounted filters on one of my lenses, because some of my filters would contact the front element. I focus with a UV filter in place, then replace it with a colored filter."

Michael S. Briggs:

"Assuming an index of refraction of 1.5 (which is reasonable), a glass filter on the rear shifts the focus by 1/3 the thickness of the filter. Obviously this is much less than a centimeter. In some cases the shift won't be important compared to the depth of focus. It's best to refocus with the filter in place. If the filter is too dense, you can substitute a lighter filter of the same material and thickness, as suggested by Ron.

"The rear mounted filter (in the converging beam) will also introduce some spherical aberration (as mentioned by Eric [Leppanen]), but I suspect that the effect will be unimportant at typical taking apertures.

"If you use a gel or thin polyester filter, both effects will be negligible."

In post #21, Bob Salomon notes that the glass for Heliopan ND filters varies in thickness. I have a set of B+W XS-Pro ND filters that I could use. I've asked B+W about glass thickness for the XS-Pro series, and will provide the response, if I receive one, here.

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Originally Posted by Jody_S

... As for problem #2, spherical aberration from the rays of light travelling through thicker glass the more oblique the angle, the problem is the same whether the glass is in front of or behind the lens. And if the lens is designed to take that into account in front of the lens, he is already using a center filter and I very much doubt the lens is designed to compensate for 2 stacked filters.

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As far as I know, placing the filter in front of the lens, where the light rays are random and not yet collimated by the lens does not introduce a focus shift nor any aberrations as long as the filter glass is plano-parallel and of good quality. It's the proportional refraction of the collimated (directional) rays from the rear of the lens passing through glass at different angles of incidence, which causes it to pass through different thicknesses, that causes the distortion.

Two stacked filters of good quality will also not add distortions. The problem with stacking filters is that there are more air-to-glass surfaces for reflections (from interfaces of different refractive indexes) and that can degrade the image somewhat through flare (although I stack filters all the time and find this effect negligible to non-existent).

Best,

Doremus

Picking up on Michael Briggs's comment, quoted in post #25, about gelatin...

Steve K (LabRat) suggested Kodak gelatin filters earlier in this thread, and Doremus Scudder, in post #20, says:

"As suggested, a gel filter will minimize the focus shift. You can get holders for them and you get to focus without the ND in place. I think that would be my preference. Just be aware that gel filters are fragile and need careful handling."

A search of old threads reveals that some very experienced photographers share Doremus's view.

In Kodak's Wratten system, gelatin ND filters, regardless of density, are #96 Wratten filters. As I note in post #14, B&H prices for new Kodak filters are fairly high. However, new old stock, in a variety of sizes, appears to be readily available on eBay. Assuming that the vendors are legitimate, and that the filters haven't deteriorated due to age, the prices strike me as quite good.

Kodak gelatin could be used either behind or in front of the lens. Behind, I think that 3"x3" would work well with the rear element of the Schneider XL 150mm. In front, 6"x6" would not only cover a diameter of 112mm, but could be used handheld or taped to the lens, or in a holder that fits to the lens. The author of one older post on the forum says that S.K. Grimes made a holder for this purpose.

In my case, one option is to use my Arca-Swiss compendium lens shade and rig a filter holder (the Arca-Swiss holder takes 4"x4" filters). I could use either gelatin or resin square/rectangular filters. This would also make it possible to use resin graduated ND filters. I exclude glass for cost reasons. Large, rectangular, high quality glass ND filters run US$500+ apiece. I'll have to check my Schneider 150mm lens+compendium pairing for vignetting.

Note: I've determined that Lee's SW150 holder can't be fitted to this lens's centre filter. The largest lens adapter is 105mm. Assuming that it didn't cause a vignetting problem, I suppose that S.K. Grimes could make a 112mm adapter. In any event, it's possible to use Lee's SW150 filters without its holder.

I'd like to thank everyone who contributed comments.

I've made a decision, at least for the time being, about filters for Schneider's Super-Symmar XL f/5.6 150mm when used with its IVa centre filter.

Note that the centre filter requires the lens to be used at f/11 or smaller, and comes with a loss of about 1.5 stops of light.

Polariser, 112mm screw-in

I made a decision about a polariser before starting this thread. I would much rather add a 112mm polariser to the centre filter than spend time trying to get the rotation right on a 62mm polariser screwed onto the lens's rear element. Indeed, I see a 112mm diameter as a benefit. I can use my hand to hold up a filter that large in front of my eyes and easily see how rotation is affecting the image. Normally, I use B+W and Heliopan screw-in filters, but neither makes a 112mm polariser. As noted in post #10, I chose NiSi's Natural Polariser, which appears to be well-regarded.

Neutral Density, 112mm screw-in

Very few companies currently make 112mm ND filters. With one exception, those that do offer very limited selection in terms of strength. Japan's Tokina makes a line of prime and zoom cinema lenses that have 112mm filter threads. It makes its Cinema Pro IRND filters in 112mm, eight strengths, for these lenses*. IRND means that the filters cut infrared as well as visible light, which has become more or less standard in digital filmmaking. Yesterday, I acquired two of the Tokina filters, which will serve my needs for the time being.

Rationale

I'm satisfied that adding filters to the front of the lens is preferable, from both ease of use and optical perspectives, to adding them to the rear element. My decision was made easier by the fact that I've been able to acquire the three filters at prices that make them affordable, which means that I should be able to sell them eventually without losing a meaningful amount of money. I was just lucky in coming across the two Tokinas yesterday.

I have a Nikkor W f/6.5 360mm lens that also made the decision easier. This lens has a 95mm filter thread. I think that a step-up ring to the 112mm filters may be more convenient in some cases than using my Lee100 system.

Follow-up

I plan to speak with S.K. Grimes after the holidays about how I might be able to use Lee's SW150 System graduated neutral density filters. I also plan to ask Lee whether it sees a problem if I have S.K. Grimes make a 112mm adapter for its SW150 holder. Lee's own largest adapter is 105mm.

If B+W responds to my query about the thickness of the glass on its XS-Pro line of filters (see post #25), I'll post what it says.

I think that the ideas in this thread are of on-going interest because they offer ways to address vignetting from stacking filters.


* Tokina also makes its Cinema Pro IRND filters in five other other diameters, as well as in 4"x5.65" and 6.6"x6.6" for matte boxes. Tokina owns Formatt-Hitech, but my understanding is that the Cinema Pro filters are made in Japan rather than in the U.K.

Mat Marrash, a member of this forum who has a large format YouTube channel, frequently uses Schneider's Super-Symmar XL f/5.6 150mm for 8x10. In this video, which I came across yesterday, he uses the lens with a screw-in R72 filter and Efke infrared film. His filter is 77mm, not large enough for the lens's 95mm front thread. At 18:10, he uses a 62mm to 77mm step-up ring to screw the filter onto the lens's rear element. This is one of the options that I was considering (post #1), except in my case with a 62mm to 82mm step-up ring*:


Shooting Infrared 8x10" B&W Film - Large Format Friday

https://www.youtube.com/watch?v=nDOT-kX_nOQ


* Marrash doesn't use a centre filter with the Schneider lens, at least in his videos. I don't know whether he addresses fall-off in processing, although he appears to be content with a certain amount of fall-off in his images. Marrash also uses a 100mm Benro Filter Holder for square/rectangular filters, including graduated neutral density filters. That holder won't fit Schneider's IVa centre filter, and in any event doesn't accept filters that are large enough to cover 112mm.

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Originally Posted by r.e.

I plan to speak with S.K. Grimes after the holidays about how I might be able to use Lee's SW150 System graduated neutral density filters. I also plan to ask Lee whether it sees a problem if I have S.K. Grimes make a 112mm adapter for its SW150 holder. Lee's own largest adapter is 105mm.

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I sent an e-mail to Lee support this morning and received a reply two hours later. Lee doesn't make adapter rings to order, but the support person thinks that a custom 112mm adapter ring should work. He's asked me for some info, now sent, about the Schneider lens and centre filter. This info will be looked at by one of their engineers in the U.K. after the holiday break.

I've had three other occasions to contact Lee support since purchasing one of its Lee100 holders about two years ago. Every time, I've had a quick, helpful response. Panavision owns Lee, and in North America I suspect that the support comes from its office in Burbank. In any event, I have nothing but good things to say about my dealings with Lee support staff.

In addition to S.K. Grimes, @Kiwi7475 has pointed out to me that 3D printing might be an option for a custom adapter ring.

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Originally Posted by Doremus Scudder

placing the filter in front of the lens, where the light rays are random and not yet collimated by the lens does not introduce a focus shift nor any aberrations as long as the filter glass is plano-parallel and of good quality

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No. No stray light is used in making an image. Only the rays that come into the lens directly from the subject are. And no the light that goes out of the lens to make the image is never collimated. Collimated light consists of parallel rays that never focus.

With a filter, the focus shift is present in any case. But its amount influences the distance between the lens and the subject when the filter is in front of the lens, and the distance from the lens to the image when the filter is behind the lens. Compare the shift of about 1/3 of the filter thickness to the hundreds of feet distance in a landscape or even to a couple of feet in a portrait to get the idea why we don't care about that shift in general photography with a filter in front of the lens. In 1:1 macro, the influence of the focus shift caused by a filter is equal in the amount with either filter positions. With bigger than life-size images, front-mounted filters make larger focus shifts than the rear-mounted ones.

Focusing through an anti-VU or another visually bright filter before replacing it with a dark one is a good method. Though different filters differ not only in their thickness but also in their glass refraction indices which are to be taken into account, too. So just moving the camera's back for the correction (a common 19th century practice to compensate for the 'chemical focus') may actually be a better option.

Filter imperfections influence the image differently with filters in different places.

1. Dust, dents and so on reduce the image quality virtually the same way regardless of the filter's place, especially when a good lens hood is in front of the optical system so any additional sidelong light on a front-mounted filter is negligible.

2. Any optical power in a filter (non-plane surfaces) has less influence when the filter is in front of the lens as any aberrations introduced by the front optical elements of a system are diminished by the next positive lens elements, along with the image itself.

3. Fluorescing filters (think all the Yellow - Orange - Read - IR glasses, the vast majority of UV-blocking glasses, and a number of different gels) have far less influence on the image contrast when put behind the lens as in that case, far less UV strikes the filter.


And regardless of the filter quality, there is one more problem: the additional light fall-off at the edges due to the fact that oblique rays have to travel through the filter a longer way. That does not depend on the filter thickness, so gels perform exactly the same as the glass. The darker the filter, the more prominent the light fall-off. (That does not happen with sharp-cut filters though (mostly the Yellow - Orange - Red ones) as their absorption does not depend on thickness.) Naturally, that is noticeable with wide-angle lenses only and is also independent of the front vs. rear filter position if the lens is more or less symmetrical, and not a retrofocus one. The only way to avoid the additional light fall-off is to put the filter inside the lens close to the aperture, where the oblique rays' paths are far less different from the image-center-forming rays' paths. This is the major benefit of gels - they are OK to be put inside the lens without any trouble.

Dichroic filters (i.e. the above-mentioned anty-IR 'hot mirrors') have more problems with wide-angle lenses as their mirror surfaces are calibrated for certain ray angles only, so such filters have quite different properties for the center parts of wide-angle images than for the corners. The effect is also the same for the front- and for the rear-mounted filters if the lens' type is close to a symmetrical one.

Besides, dichroic filters produce way more stray light if put behind the lens while with the dichroic surface in front of the whole optical system, the light reflected by the dichroic mirror just goes away from the system.

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Originally Posted by ridax

Focusing through an anti-VU or another visually bright filter before replacing it with a dark one is a good method. Though different filters differ not only in their thickness but also in their glass refraction indices which are to be taken into account, too. So just moving the camera's back for the correction (a common 19th century practice to compensate for the 'chemical focus') may actually be a better option.

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It's an interesting option if the lens has a threaded rear element. Of the ten modern lenses that I have by Fujinon, Nikon, Rodenstock and Schneider Kreuznach, ranging from 55mm to 600mm, the Super-Symmar XL 150mm is the only lens that does. For the others, the rear element option is Kodak gels or, I suppose, a glass filter attached with an adhesive. I don't know whether this is accurate, but there's a post on the forum that says that early copies of the XL 150 don't have a rear thread and that it was added during production, presumably at the request of customers.

Enjoyed your post. Interesting observations.

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Originally Posted by r.e.

Mark Sawyer, in post #18, suggested focusing with a UV filter as a stand-in and replacing it after focusing with an ND filter. Two forum posts from 2006 also suggest this. They are in a thread called filter for Schneider 210mm SS XL. On that lens, the rear element thread is 72mm and the front thread is 135mm:

Ron Marshall:

"I use rear mounted filters on one of my lenses, because some of my filters would contact the front element. I focus with a UV filter in place, then replace it with a colored filter."

Michael S. Briggs:

"Assuming an index of refraction of 1.5 (which is reasonable), a glass filter on the rear shifts the focus by 1/3 the thickness of the filter. Obviously this is much less than a centimeter. In some cases the shift won't be important compared to the depth of focus. It's best to refocus with the filter in place. If the filter is too dense, you can substitute a lighter filter of the same material and thickness, as suggested by Ron.

"The rear mounted filter (in the converging beam) will also introduce some spherical aberration (as mentioned by Eric [Leppanen]), but I suspect that the effect will be unimportant at typical taking apertures.

"If you use a gel or thin polyester filter, both effects will be negligible."

In post #21, Bob Salomon notes that the glass for Heliopan ND filters varies in thickness. I have a set of B+W XS-Pro ND filters that I could use. I've asked B+W about glass thickness for the XS-Pro series, and will provide the response, if I receive one, here.

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Further to the last sentence in the above quote, B+W tells me that the XS-Pro ND filters are of uniform thickness but have thicker glass than the XS-Pro UV and Clear filters. I guess the question is whether the ND/UV difference in focus shift, for a given photograph, is meaningful. The XS-Po filters are thin to begin with because they were designed to avoid vignetting with wide angle lenses. For example, the difference in thickness between an XS-Pro ND and a UV may not be significant for a landscape photo made with a wide angle lens, such as Schneider Kreuznach's Super-Symmar XL 150mm on an 8x10 camera. That's the lens that I'm particularly interested in. B+W also tells me that the new MASTER filters, which replace the XS-Pro filters, have the same thickness as the XS-Pro filters. This makes sense because the MASTER series appears to involve a change to the rims, but not the glass.

B+W via email:

[Re the ND filters:] "same thickness, only the inside folio is darker" [I assume that "inside folio" means the glass]

"The ND‘s are thicker than the the UV’s and the Clears."

"The Glass Thickness of MASTER to XS PRO Series is exactly the same, doesn’t matter which filter you use. There was no change."

Out of curiosity, I used my ssxl 150mm lens on my 5x7 camera yesterday for some close-up still life yesterday. One the ground glass with a 5x loupe, I could not see any focus shift with a 6x ND filter screwed into the rear element. I exposed two sheets of film, one with ND filter and one without at F/32. Developed the film this afternoon and could see no difference in sharpness between the two. Only difference is density and contrast of the negatives due to exposure time. Again, this is only judged by my naked eyes.

There is an interesting page here stating that a 62mm filter can be used with rear element of this lens.

https://www.bhphotovideo.com/c/produ....html/overview

Quote:

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Originally Posted by Hugo Zhang

Out of curiosity, I used my ssxl 150mm lens on my 5x7 camera yesterday for some close-up still life yesterday. One the ground glass with a 5x loupe, I could not see any focus shift with a 6x ND filter screwed into the rear element. I exposed two sheets of film, one with ND filter and one without at F/32. Developed the film this afternoon and could see no difference in sharpness between the two. Only difference is density and contrast of the negatives due to exposure time. Again, this is only judged by my naked eyes.

There is an interesting page here stating that a 62mm filter can be used with rear element of this lens.

https://www.bhphotovideo.com/c/produ....html/overview

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I don't doubt that focus shift is real, but it's interesting that you didn't see any on a 5x7 image made with Schneider's XL 150mm (on a 5x7 camera, roughly 31mm equivalent) close to the subject and at f/32.

Quote:

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Originally Posted by r.e.

I don't doubt that focus shift is real, but it's interesting that you didn't see any on a 5x7 image made with Schneider's XL 150mm (on a 5x7 camera, roughly 31mm equivalent) close to the subject and at f/32.

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Maybe because you went beyond the optimism aperture and beyond its optimized range.

Quote:

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Originally Posted by Bob Salomon

Maybe because you went beyond the optimism aperture and beyond its optimized range.

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Hi Bob,

I was commenting on Hugo Zhang's post #34. He could comment on the points that you raise. I do know that Schneider Kreuznach doesn't recommend the XL 150mm beyond 1:3.

I remember Edward Weston shot his peppers at f/256? Hours of exposure time? I actually misremembered my f stop of my shots yesterday. I used F/16 and see no difference. I used f/32 for another shot at much closer distance.

Again, I only contact print so I won't see if there is any focus shift.

Quote:

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Originally Posted by r.e.

Follow-up

I plan to speak with S.K. Grimes after the holidays about how I might be able to use Lee's SW150 System graduated neutral density filters. I also plan to ask Lee whether it sees a problem if I have S.K. Grimes make a 112mm adapter for its SW150 holder. Lee's own largest adapter is 105mm.

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This subject is a different issue, but for anyone who's interested Lee has replied to my question. See this post in a related thread: https://www.largeformatphotography.i...=1#post1627886

Quote:

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Originally Posted by ridax

No. No stray light is used in making an image. Only the rays that come into the lens directly from the subject are. And no the light that goes out of the lens to make the image is never collimated. Collimated light consists of parallel rays that never focus.

With a filter, the focus shift is present in any case. But its amount influences the distance between the lens and the subject when the filter is in front of the lens, and the distance from the lens to the image when the filter is behind the lens. Compare the shift of about 1/3 of the filter thickness to the hundreds of feet distance in a landscape or even to a couple of feet in a portrait to get the idea why we don't care about that shift in general photography with a filter in front of the lens. In 1:1 macro, the influence of the focus shift caused by a filter is equal in the amount with either filter positions. With bigger than life-size images, front-mounted filters make larger focus shifts than the rear-mounted ones.

Focusing through an anti-VU or another visually bright filter before replacing it with a dark one is a good method. Though different filters differ not only in their thickness but also in their glass refraction indices which are to be taken into account, too. So just moving the camera's back for the correction (a common 19th century practice to compensate for the 'chemical focus') may actually be a better option.

Filter imperfections influence the image differently with filters in different places.

1. Dust, dents and so on reduce the image quality virtually the same way regardless of the filter's place, especially when a good lens hood is in front of the optical system so any additional sidelong light on a front-mounted filter is negligible.

2. Any optical power in a filter (non-plane surfaces) has less influence when the filter is in front of the lens as any aberrations introduced by the front optical elements of a system are diminished by the next positive lens elements, along with the image itself.

3. Fluorescing filters (think all the Yellow - Orange - Read - IR glasses, the vast majority of UV-blocking glasses, and a number of different gels) have far less influence on the image contrast when put behind the lens as in that case, far less UV strikes the filter.


And regardless of the filter quality, there is one more problem: the additional light fall-off at the edges due to the fact that oblique rays have to travel through the filter a longer way. That does not depend on the filter thickness, so gels perform exactly the same as the glass. The darker the filter, the more prominent the light fall-off. (That does not happen with sharp-cut filters though (mostly the Yellow - Orange - Red ones) as their absorption does not depend on thickness.) Naturally, that is noticeable with wide-angle lenses only and is also independent of the front vs. rear filter position if the lens is more or less symmetrical, and not a retrofocus one. The only way to avoid the additional light fall-off is to put the filter inside the lens close to the aperture, where the oblique rays' paths are far less different from the image-center-forming rays' paths. This is the major benefit of gels - they are OK to be put inside the lens without any trouble.

Dichroic filters (i.e. the above-mentioned anty-IR 'hot mirrors') have more problems with wide-angle lenses as their mirror surfaces are calibrated for certain ray angles only, so such filters have quite different properties for the center parts of wide-angle images than for the corners. The effect is also the same for the front- and for the rear-mounted filters if the lens' type is close to a symmetrical one.

Besides, dichroic filters produce way more stray light if put behind the lens while with the dichroic surface in front of the whole optical system, the light reflected by the dichroic mirror just goes away from the system.

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ridax,

Thanks for the education! My comments were a bit simplistic, to say the least.

The point about the focus shift of the filter being between lens and subject when the filter is front mounted and the relative effect of that with different lens-to-subject distances is especially important for close-up work. I'll watch out for that in the future.

Still, can we safely say that, for anything but close-up work, mounting a glass filter on the front of the lens has no significant/appreciable effect on the focus (since the error introduced is so small)?

As for "collimated" light being projected by the lens: obviously, I used the wrong term. However a lens does something; it redirects all those rays and projects them in a cone of light; the light passes through a focal point somewhere and then onward from there to the film. All those ray tracings I did in my physics of optics classes led me to believe that the light was "ordered" or "directed" or whatever after passing through the lens. What would be the correct term for that?

And, after the rays of light have taken on whatever order the lens imparts to them, if I understand what you are saying, there is no more focus shift introduced than if the filter were mounted on the front of the lens, it's just the fact that the lens-to-film distance is much smaller than the lens-to-subject distance and that the introduced shift is a larger proportion of that distance. Is that a correct way to think about that?

I also have a bit to learn about how different filters work, apparently :) I always assumed that the common dyed glass or gel filters used in black-and-white photography (yellow, orange, red, green, etc.) worked by absorbing certain parts of the spectrum and changing it to non-image-forming energy as the light passed through the filter, and that the transmitted light was affected by the filter only by the refractive properties of the glass/gelatin (including the reflections due to impedance differences between media at the air-to-glass/gel surface). Your comment that darker filters have more light fall-off regardless of thickness and that that depends somehow on the transmission qualities of the filter confuses me. If it's not too much trouble, could I ask you to elaborate? (In a PM if you think it wouldn't be relevant to this thread). I'd love to know more about "fluorescing filters" too.

TIA,

Doremus

A new filter device to be used behind the lens:

https://petapixel.com/2022/01/12/ben...-lens-adapter/

What on earth does that Benro gadget have to do with LF camera technique? If it hypothetically could be adapted to fit, there is still the question of filters behind lenses not being a very good idea to begin with, optically. And trying to keep true Wratten gels in good shape, especially outdoors? ... I gave up on that long ago. They're just way tooooo susceptible to fingerprints, grime accumulation, creases, and scratches. Not cheap either.

Also a new way to add dust!

The bigger irony is how that Benro gadget is way bulkier and more cumbersome than simple front-mounted adapters. Plus it screws up normal optical distance logistics. Kinda like selling square auto tires. I recall one of those old Far Side cartoons labeled, "Early Business Failures", with some cave man and his fast food stand, selling grilled Porcupine on a Stick, quills n all.

It's actually an adapter for putting foreign lenses on various 35's

I have a couple, not that crap

I have Leica R to Nikon Z-fc as example

These are are great adapters but not with a dust hole!