150XL and Rear Filters - Who is doing this?

[turtle]

Can anyone tell me whether one needs to use wide angle filters on the rear of the 150XL in order to avoid vignetting/a reduced image circle?

Have any of you had problems with focus shift?

I am considering the use of polyester filters on the rear, as they are thinner so should not result in any real focus shift, but will have to see if the holder will fit OK. Has anyone used gels/polyester filters on the rear?

[Bob Salomon]

Real? Put anything back there and there is a focus shift. And image degradation since the lens did its job on the image when the rays came out the back. Now any marks, smudges, imperfections, distortions, lack of flatness of the filter will effect the image before it reaches the image plane.

Filters belong in front of the lens unless they are designed by the lens manufacurer to be part of the optic.

[Helen Bach]

You don't need a holder to use filters behind the lens. You can attach them with tape. One way is to tear off a strip of camera tape about 10 mm wide, then fold it over lengthwise, sticky side out, before wrapping it round the outside of the back of the lens, fold to the rear of the lens and slightly proud - thus forming a sticky circle onto which you place the filter. Easier to demonstrate than to describe. You can also use little rolls of 'snot tape' - Scotch ATG tape.

I don't usually use behind-the-lens filters with LF, but I have used gelatin and sometimes thin polyester filters behind the lens for motion picture. A friend and colleague of mine, who has way more status as a commercial LF photographer than I do, uses Cokin resin filters behind his LF lenses and you can't tell the difference between his shots and mine in terms of resolution and sharpness until you print larger than 16x20 - but I use 'better' lenses than he does, so who knows. There's no point in me trying to persuade him that one shouldn't use Cokin filters behind the lens when he has had so much success when using them - he often tells me how many millions each of his ancient lenses have made for him, and he always uses an 81A. I think that experience should be your guide. How much does it cost to find out if using a filter behind the lens will make a difference with your lenses, your method, your print size, your standards?

[turtle]

Yes Bob, I am considering it! I know the theory, but focus shift is not a game changer as one can easily focus through the filter. Image degradation would be a concern, but there is theory and practice. In short, I am looking at the options for filters with the 150XL. The large front diameter means more expenditure and bulk, whereas I already have almost all of what I need were I to mount filters behind the lens. Polyester filters are so thin that I very much doubt that they will cause focus shift of note.

Worst case scenario I could mount filters in front, but this will entail compromises of its own. At this stage I am looking for info.

[Noah A]

Of course it's ideal to use the filters in front, but the 150XL uses huge filters!

There will be focus shift, but as long as you focus with the filter in place it shouldn't matter.

It really comes down to what you're doing. If you're contact-printing 8x10 then I'd be willing to bet you'll see no difference in sharpness. If you're drum scanning and printing huge, then, well, you may see a tiny bit. But in that case it's worth your trouble to do a test shot, scan and some test print sections just to be sure.

I think it'll be fine.

In terms of needing a slim or wideangle filter, I doubt it, but all you need to do is slap a normal filter on the back and check for vignetting on the camera.

[Helen Bach]

I know that we have covered this a few times before, but remember that the focus shift is not constant: it depends on the angle of incidence, so it causes spherical aberration as well as field curvature. This may or may not matter, and the thinner the filter the less the difference.

[turtle]

I cannot imagine that Schneider would have put a rear thread on there if it was likely to be a disaster and it is interesting to note that they did not put a rear filter on the 72 XL, which shares the same front diameter. I guess I will have to find out. I only use Lee polyester filters on LF at the moment, because they are very small and light. Considering that they are something less than 100mm (they are in a plastic mount) vignetting might be an issue with full movement. Certainly, carting about a selection of large filters that would cover the even bigger CF is not exactly appealing!

[Helen Bach]

Here's the tape method, in this case with a 165 mm Super Angulon and an old resin filter for illustrative purposes.

[E. von Hoegh]

Yes Bob, I am considering it! I know the theory, but focus shift is not a game changer as one can easily focus through the filter. Image degradation would be a concern, but there is theory and practice. In short, I am looking at the options for filters with the 150XL. The large front diameter means more expenditure and bulk, whereas I already have almost all of what I need were I to mount filters behind the lens. Polyester filters are so thin that I very much doubt that they will cause focus shift of note.

Worst case scenario I could mount filters in front, but this will entail compromises of its own. At this stage I am looking for info.

Assuming a spotlessly clean filter, the image degradation will depend on the thickness of the filter and the refractive index of the filter material.

[Eric Leppanen]

It is possible to do front filtration with the SS150XL without mechanical vignetting issues. There are a variety of threads covering this subject, here is one:

http://www.largeformatphotography.in...s150xl+filters