Using Behind the Lens Filters

[Pere Casals]

Focus shift of about ½ the thickness.
Loss in resolution from less then flat gel or filter
Loss in resolution from any defects like dust, grease, fingerprints, scratches.

Bob, perhaps we can also add that gels lack anti-reflective coatings, and that has an impact in certain situations.

[Tin Can]

I wrote I removed a DIY rear filter mount. I did that years ago under the influence of some on this forum.
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CB I respect highly for his actual images and will now reconsider rear filters in certain situations.

The Chicago way is still an influence on me, despite my escape to rural life.

Thanks, Chris!

For the first dozen years of my career I shot architecture on 4x5 film at the studio of Hedrich Blessing. All of our photographers, up to a dozen shooting in the heyday, had custom filter holders 'permanently' mounted behind our lenses. We used 1 or 2 filters on every shot. Behind the lens mounting removed the possibility of filter flares. The studio produced tens of thousands of photos this way and were renown for the quality of our images. What does focus shift matter if you're focusing through the filter?

As with anything, you should do your own testing and weigh the benefits of convenience versus any loss in quality.

CB

[Pere Casals]

What does focus shift matter if you're focusing through the filter?

It does not matter.

The problem may happen if we want to focus without the (behind the lens) filter because we have a dim image in the GG, and with a deep red we have an even harder image to focus.

A front filter does not have focus shift, if we are not doing macro.

Also if filter is a gel focus shift shuld be irrelevant. But if stacking 2 glass filters behind the lens... this is another thing !

[bvy]

All very interesting, but more than the potential effects to image quality (or lack thereof) I was just concerned if I had the damn thing mounted acceptably!

[brucetaylor]

If you focus your ground glass through a filter mounted behind the lens focus shift is obviously accounted for. On the subject of focus shift using (single) gel filters behind the lens it is inconsequential. Many motion picture cameras (including Panavision and the workhorse Bolex) had behind the lens filter slots for gel filters both for ease of use and getting the filter out of the reflex viewfinder path. This on formats (35mm and 16mm) with depth of focus tolerances are small fraction of any of the formats discussed here. Additionally there were MoPic lenses made that required the use of a rear glass filter to maintain depth of focus.
Not that this in any way helps answer your question, bvy (sorry!), but I wanted to add to the behind the lens filter discussion.

[Pere Casals]

Many motion picture cameras (including Panavision and the workhorse Bolex) had behind the lens filter slots for gel filters both for ease of use and getting the filter out of the reflex viewfinder path.

Very interesting, an smart solution...

[consummate_fritterer]

Assuming one gel filter.

Precisely how much does a BTL gel filter in new condition affect image quality? Any measurements? Any direct comparisons of front-mounted gel vs. BTL gel?

Focus shift of about ½ the thickness.
Loss in resolution from less then flat gel or filter
Loss in resolution from any defects like dust, grease, fingerprints, scratches.

Thanks, Bob, but filters in new condition don't have dust, grease, fingerprints or scratches. I think gels in new condition should be flat enough. A very gradual wave affects nothing measurable. A new filter won't have sharp waves or wrinkles. What's the average half-thickness of a gel filter? I think it's far less than most focus errors.

Bob, perhaps we can also add that gels lack anti-reflective coatings, and that has an impact in certain situations.

This 'might' have some tiny effect but I suspect very little considering it's only one uncoated surface that matters.

[Pere Casals]

This 'might' have some tiny effect but I suspect very little considering it's only one uncoated surface that matters.

Two uncoated surfaces, a gel has two sides... I guess it would have an effect if another uncoated filter is stacked, of it the lens is single coated, reflecting back to the film the reflections in the gel.

[consummate_fritterer]

Two uncoated surfaces, a gel has two sides... I guess it would have an effect if another uncoated filter is stacked, of it the lens is single coated, reflecting back to the film the reflections in the gel.

Right, if two or more filters were stacked then yes, rear-mounted filters toward the front would have two reflective surfaces that matter. The rear uncoated surface of one filter is so far away from the film that it has no effect. I'd not use stacked rear filters though. I can't imagine ever needing more than two filters in 99 percent of any images. I might use a color (8, 11, 25, 29, 58, etc.) filter plus a ND (or CF) plus a polarizer but that would be a very rare event. A polarizer and/or CF I'd front-mount. If I ever did stack rear-mounted filters (I won't), the one with the most filter factor would go in front. I'd also never rear-mount an IR filter or very deep ND filter due to light bouncing from the front surface of the filter.

[cdavis324]

For the first dozen years of my career I shot architecture on 4x5 film at the studio of Hedrich Blessing. All of our photographers, up to a dozen shooting in the heyday, had custom filter holders 'permanently' mounted behind our lenses. We used 1 or 2 filters on every shot. Behind the lens mounting removed the possibility of filter flares. The studio produced tens of thousands of photos this way and were renown for the quality of our images. What does focus shift matter if you're focusing through the filter?

As with anything, you should do your own testing and weigh the benefits of convenience versus any loss in quality.

CB

It was standard practice in ny as well... We'd also put a ring of tape around the lens and stick the gel to the tape either in front of or behind the lens. Fingerprints, dings or other issues with the filters had no effect on the image unless the filter was in really, really bad shape.

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