Digital SLRs almost universally have two filters that are stacked immediately on top of their sensors. One is the anti-aliasing (AA) filter and the other is the UV-IR cut filter, or ICF. While each of these filters performs critical functions, each also reduces image quality significantly. For underwater photography, an interesting question is whether we might be better off with one or both of these filters removed.
The Anti-Aliasing filter:
The AA filter is necessary to prevent image detail too fine for the sensor from causing distortion in the form of aliasing artifacts. In bayer pattern sensors (all digital cameras other than Foveon), aliasing is particularly noticable in the form of color artifacts, or moire. While moire can often be detected and suppressed in software, not all aliasing errors show up as moire.
AA filters vary in strength depending on the tradeoffs the manufacturer decides to accept. Camera makers desire to deliver the best image quality possible but "best" is very much dependent on the intended use of the camera. It's possible for the manufacturer to use an AA filter sufficient to suppress all aliasing but doing so is not always the best choice. The Nikon D100 was notorious for the strength of its AA filter and the resulting softness of its images. The Kodak SLR/n and 14n had no AA filter at all and were quite sharp. Most recently, there has been discussion regarding the Canon 1D3 and the Nikon D3. Which camera offers better image quality is a matter of debate since the D3, in spite of its extra pixel count, includes a stronger AA filter. Which is preferable depends on who you ask and how they work.
But do we need an AA filter underwater at all? For large pixel cameras like the 5D, 1Ds (Mark 1), and the D3/D700 maybe, but for most current cameras we may well not. First off, it's well known that today's cameras often are limited by available lenses and very few lenses are of interest underwater. If the lens can't deliver fine enough detail we don't need an AA filter to remove any! There are a number of underwater-specific issues to consider as well. Underwater systems have ports that damage sharpness and have several inches to several feet of water to shoot to through that seriously limit resolution. For wide angle, image quality is seriously hampered by the performance of our dome ports and some shooters are being driven shoot at very small apertures to maintain image quality. For macro, small apertures are used as a matter of course. While there is a separate debate on just what the term "diffraction limited" really means, current cameras become noticably affected by diffraction anywhere from f/8 to f/16 depending on camera. This doesn't mean small apertures aren't useful, but it does mean that by f/11 or f/16, diffraction renders the AA filter moot. If all your underwater shooting is above f/11 chances are you never need an AA filter. Basically, the AA filter is robbing us of image quality in return for nothing. Even with wide angle at f/8 or larger, how much ultrafine detail are we going to get through feet of water column?
The UV-IR cut filter (ICF):
The ICF is necessary because sensors respond to ultraviolet and infrared light and image quality will be degraded if they are allowed through. There are two kinds of ICFs, absorptive and reflective. DSLRs use absorptive filters which are made using dyes and have a cyan appearance. Like AA filters, not all camera use ICFs of the same strength, so some cameras are vulnerable to image degradation due to a weak ICF. Another way to look at it is some cameras are better for infrared photography than others.
So how does the ICF contribute to image degradation? The ICF causes an imbalance in sensitivity to certain colors and, as a result, DSLRs are typically much more sensitive to green light than to blue and red. How bad? Look at these tables to see how far off some gain coefficients can be. You will notice that you will never see a gain of 1,1 which is what we always want!
Readers of the dpreview and luminous landscape forums may be familiar with UniWB and the technique of using magenta filters while shooting. There are a number of experienced pro shooters who advocate using a CC40M filter or stronger at all times while shooting a DSLR. This is a direct consequence of the absorptive ICF in the camera. Most of us are familiar with the concept of "expose to the right" (ETTR). The net effect of a typical ICF is to force all our underwater shots to underexpose our red channel by more than a stop! In other words, the ICF destroys more than a stop of our potential dynamic range. Being underwater, the red channel is already our weakness so the news is even worse.
Of course, the ICF performs a critical function as well, but neither infrared nor ultraviolet travels far underwater. Ambient light won't have any. Strobes will produce some but it will quickly dissipate. Surfaces shots will obviously need an ICF.
What is interesting is the other kind of ICF, the reflective kind. Reflective ICFs are far more effective and have no effect on color balance. They aren't used in DSLRs because they're more expensive and have limited angle of view. Reflective ICFs can be added externally to macro lenses but cannot be used with wide angle. Odds are good that they could be used inside the SLR as a replacement but they have to be custom manufactured for that purpose because of the delicate nature of their coatings. A reflective ICF inside an SLR might cause lens interactions which may explain why manufacturers don't use them. I would use one that way if I could.
What can we do?
Certainly, removing the AA filter will likely result in a universal benefit to underwater photography just as it does for many other kinds. When was the last time you saw color moire in an underwater shot? Every shot will benefit from improved appearance of detail with this modification.
Removing the ICF is more problematic. Assuming you can't replace the ICF with a reflective one inside the camera, you will be forced to compensate for each type of shot. Macro is easy enough as you simply add an ICF or replace your haze filter with a B+W 468. You can also install an effective ICF on your strobes. For wide angle, strobe filtering is a better approach. Lastly, you will probably have to accept the inability to shoot splits.
But what do you get in return? First off, you will NOT get an effective increase in ISO because that is governed by green light sensitivity. You will get better dynamic range and lower color noise. You will also get the elimination of the cyan posterization that haunts digital shooters of sunballs! You will get better sensitivity, lower strobe powers, and better image quality and color fidelity over using the CC40M approach advocated on the net today.
There is a small industry that caters to the needs of astronomy and infrared photographers. One such company is MaxMax. They will sell you modified cameras or modify your existing ones and they offer both AA and AA+ICF removal. One thing they lack is support for higher end models. The D300 and 5D are there but pro bodies are not. Perhaps if underwater photographers become more aware of the benefits of modifying their cameras for better performance, a company like MaxMax may be willing to support the models of most interest to us.
