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.

Quote:

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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.

Quote:

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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.

Quote:

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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.