Using Behind the Lens Filters

[Drew Wiley]

You'll have some image degredation regardless. Behind-the-lens should always be a last-resort option. It always has a penalty. That might not be a nuisance
in a contact print or deliberately soft image, but it is there and will compromise any image intended to be truly crisp.

[Pere Casals]

You'll have some image degredation regardless. Behind-the-lens should always be a last-resort option. It always has a penalty. That might not be a nuisance
in a contact print or deliberately soft image, but it is there and will compromise any image intended to be truly crisp.

It would be interesting to make a side by side comparison...

[Bruce Barlow]

"It would be interesting to make a side by side comparison..."

Why? Instead we can just endlessly speculate from our armchairs about things we don't know or understand.

Besides, if we DON'T do a side-by-side comparison, we'll avoid learning anything useful. Not to mention all the other things I seem to learn when I actually do real work.

[Drew Wiley]

Side by side real comparisons have been done for in labs for decades. I've done em myself, lab and field. This is ancient history for optical designers, and should be for photographers too. The era of any speculation about it was over long before the web was invented, and how the web created a tsunami of uninformed opinion about everything, which seems to assume that nothing was either known or in print before the web. But since various factors can cumulatively contribute to problems on the focal plane, you need to isolate them one by one even to attain an objective test. The one I keep harping on is the lack of film retention flatness in conventional sheet film holders. Your comparison test itself could be potentially bunk because you're seeing the consequence of more than one problem at a time. In the meantime, why not just believe the people who make lenses to begin with? Their own literature will point out those rare exceptions where a lens was in fact designed for an intermediate or rear filter application.

[Greg]

With a thin gel you won't notice a focus shift, because the shift it is said to be 1/3 of the filter thickness. But if you place a thicker filter (like a Pol) behind the lens then you should be able notice the shift, like bob stated.

I remember reading one time in a in a Kodak publication that their gel filters caused a shift of 1/2 of the thickness of the gel... Heck, if their gels caused a shift of the full thickness of their gel filters, I seriously any of us would notice it.

[Mark Sampson]

The question was far more important back in the days of thick glass-mounted filters; see somewhere in Ansel Adams' writing.
I've not found any degradation of image quality from using gel filters behind the lens, but then I'm an ignorant picture-maker not really concerned with theoretical perfection. I'm a long way from that in many ways, despite my best efforts toward it.
A good photograph (I have made a few of those in my life) is my goal; if that means putting a gel behind the lens, so be it.
None of my corporate supervisors, professional clients, buyers from galleries, or viewers of my prints have ever suggested that they weren't sharp enough, or wondered how I could possibly destroy the picture's resolution by putting a filter behind the lens.
I will admit that I started using gels BTL because I didn't want to spend out on a whole set of 82mm filters for the front of the 121/8 SA, but that answer has worked for me.
YMMV.

[Drew Wiley]

Going around saying you've never seen the far side of the moon doesn't mean it doesn't exist!

[consummate_fritterer]

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?

[Bob Salomon]

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.

[Christopher Barrett]

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