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Lenses such as the Canon EF 17-40mm f/4 L USM offer rear gelatin filter holders, but also accept front filters via a 77mm screw on type. If I was using a circular polarizer, I would need to put it on the front element to manipulate it. But if I was adding a solid ND filter or a color filter, what are the reasons to use or not to use the rear gelatin filter over the front screw on type?

This is the Canon EF 17-40mm f/4 L USM lens rear element. You can see the outline for the gelatin filter holder if you look closely. It is noted by the white square around the glass: enter image description here

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  • \$\begingroup\$ I do want other people to answer this since I am not an expert. I just answered it myself since I had a few ideas. \$\endgroup\$
    – dpollitt
    Commented May 20, 2013 at 1:40
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    \$\begingroup\$ Holy crap, I didn't even know that my 17-40 accepted rear filters. How did I miss that... \$\endgroup\$
    – AJ Henderson ♦
    Commented May 20, 2013 at 3:58
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    \$\begingroup\$ @AJHenderson, I would recommend reading all of your equipment manuals. The can tell you quite a bit. \$\endgroup\$
    – Evan Pak
    Commented May 20, 2013 at 12:38
  • \$\begingroup\$ @EvanPak - that's the scary part... I do! (Typically before the device is even in my possession.) I still missed it somehow. Or perhaps forgot it because I didn't care at the time. \$\endgroup\$
    – AJ Henderson ♦
    Commented May 20, 2013 at 12:49
  • \$\begingroup\$ @AJHenderson, I'm sure that I've done the same thing with my cameras. \$\endgroup\$
    – Evan Pak
    Commented May 20, 2013 at 16:17

2 Answers 2

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Gels aren't generally available as screw-on type filters. To place a gel on the front of the lens requires a holder of some kind and a much larger quantity of the filter media. Many "sample" sets of various color gels come in sizes that allow for trimming to fit a rear filter holder but not large enough to fill a front sized filter holder. Due to the less durable materials a gel filter is made of, it is protected from potential damage when inserted in a rear holder.

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    \$\begingroup\$ The sample swatches are almost always of lighting filters ("cine gels"), which are not the sort of thing one would ordinarily use in the optical path when creating an image. Imaging gels are usually small, delicate (traditionally, actual gelatin), optically flat and relatively expensive. \$\endgroup\$
    – user2719
    Commented May 20, 2013 at 3:12
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    \$\begingroup\$ They're not particularly common anymore, since their main uses (colour correction and B&W contrast) have largely become the province of post-processing. The main supplier was, at one time, Kodak (the brand to search for is Wratten). B&H still sells them. \$\endgroup\$
    – user2719
    Commented May 20, 2013 at 10:28
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    \$\begingroup\$ Thanks! So for use in a lens like the EF 17-40 f/4L you would still need to purchase a larger size of the material and trim to fit the holder at the rear of the lens? Do you know if more than one thickness of the ND gelatin would fit in the internal holder? I could also see using the ND material to create an internal graduated filter by trimming it smaller than the holder size in the vertical dimension. \$\endgroup\$
    – Michael C
    Commented May 20, 2013 at 12:34
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    \$\begingroup\$ The gels (real gelatin gels, at least) are really, really thin, so yes, you could fit multiples. The problem is that they'll stick together when it's humid and warm, and Newton's rings aren't an unknown side effect of stacking if the filters are warped/rippled. They're also really easy to tear, snap and permanently fingerprint (hey, they are gelatin after all). They're optically much better than glass, but such a royal pain — there's no getting around the fact that they're expensive disposables, so if you're not making money with 'em... \$\endgroup\$
    – user2719
    Commented May 20, 2013 at 19:27
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This isn't meant to be an exhaustive list or answer as I have very little experience with this but wanted to point out of the obvious reasons:

Cons of rear filters

  • Having to remove the lens each time to add or remove a filter
  • Does not allow filters requiring manipulation(vari-ND or circular polarizers for example)

Pros of rear filters

  • Allows filters to be used when screw on isn't an option(fisheye lenses)
  • No chance of vignetting from rear filter
  • Ability to stack even more filters with less potential for vignetting
  • Smaller size might cost less
  • Less chance of scratching filter
  • Smaller size to carry and travel with

Unknown as pro or con off rear filters

  • Cost
  • Optical quality
  • Availability
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  • \$\begingroup\$ Note that many lenses need not be removed to change a gel filter. You only slip out the holder change filters, slide it back in and lock. \$\endgroup\$
    – Itai
    Commented May 20, 2013 at 1:52
  • \$\begingroup\$ Yeah good point itai. I've never owned one of those so I always forget about them. \$\endgroup\$
    – dpollitt
    Commented May 20, 2013 at 2:03
  • \$\begingroup\$ Some of the holders supplied with super telephoto lenses do allow for manipulation (rotation) of polarizers in the holders from outside the lens. Most, if not all of them, have been introduced since 2013. \$\endgroup\$
    – Michael C
    Commented Mar 23, 2019 at 6:02

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