Using a Neutral Density Filter on a Rear Lens Element

[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...

Steve K

[r.e.]

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...

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.

[LabRat]

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.

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

[r.e.]

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...

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?

[r.e.]

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

[Drew Wiley]

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.

[xkaes]

... 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.

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

[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.

[r.e.]

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.

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.

[Doremus Scudder]

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.

+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