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The main advantage to using a color sensor for B&W photos is that you can do color contrast filtration (adjusting the relative brightness of different colors) as part of the post-processing workflow instead of at the time the image is captured. This is a big deal. It means you're not locked in to the filtration decisions you made when you captured the photo and have access to a far more nuanced range of filtration options than you would with optical filters.
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It also means you don't have to buy and carry around dozens of optical filters in different colors. In my color kit I just have a polarizer (which you can't do in postprocessing) and some ND filters (for long exposures). This represents a big savings in expense, weight, and general cumbersomeness.
So why use an achromatic sensor at all?
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Unfortunately, the way color sensors work can, under some conditions, slightly degrade image sharpness for some subjects. This is because (most) color sensors don't actually capture color information at each photosite. The underlying sensor is an array of monochromatic pixels. But it's covered with a filter, usually a "Bayer filter", that masks adjacent pixels with red, green or blue. An algorithm derives (guesses) the correct color of each pixel in the final image based on adjacent pixels.
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How much this reduces the sharpness of the final image depends on the color and contrast of the subject. In practice, the Bayer algorithm is quite good on almost all real-world subjects, but every now and then, a true monochrome sensor can render finer detail than a filtered color sensor would be able to. Again, the effect is fairly small in practice, and may be subsumed by the resolving capability of the lens (which is often already very close to that of the sensor).
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On my medium-format digital system, I'm capturing so much resolution to start with that I don't generally worry about the possibility that I might lose a bit of detail in a rare subject that the Bayer filter performs poorly on.
But there's another advantage to achromatic sensors, which is the real reason I have one.
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Achromatic sensors generally lack not only a color filter, but also the "hot mirror" ("high pass") filter that blocks infrared light from hitting the sensor. Normally, you want to filter out IR light; it focuses at a slightly different plane, and so can make daylight photos seem burry if you don't filter it out. But having an IR filter permanently attached to the sensor means you can't capture past the visible part of the spectrum.
And this forecloses making some interesting photos.
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IR photography is a bit of a niche, and is easier to do badly than well, but when done well, it creates some very interesting creative options. With the optical spectrum filtered out, a clear sky at midday contains very little IR and renders as almost completely black. Most plants, on the other hand, reflect IR extremely well, and render as almost completely white. And IR light reflects off other objects differently than does the visible spectrum.
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@mattblaze@federate.social I don't know if you do people/portraits but IR is interesting there too because skin is quite translucent around the 800-900nm band. Strong lighting can glow through skin, which looks quite alien and beautiful in some contexts.
@phenidone@mstdn.social Yeah, it's not the sort of work I do, but I've seen some really stunning IR portrait photos.
So you can usually easily make IR photos with an achromatic sensor. It means, however, that not only do you have to have the various color contrast filters to make regular B&W photos, but you also need both an IR-cut filter (for making photos with visible light) and an IR-low-pass filter (for making IR photos). So you're carrying around a lot of extra stuff.
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Are there other advantages to the achromatic sensor?
Not really in my experience. You can capture just about the same tonality and dynamic range with an equivalent color sensor, and you get a lot more flexibility in post processing. Plus you can make color photos if you want.
Again, I'm glad I have access to an achromatic sensor, but I use it only occasionally, for maybe 20% of my photos.
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By the way, I've spoken to photographers who insist that using an achromatic sensor is essential for artistic purity and integrity, but I think those are the same people who a decade earlier were claiming that digital photography isn't sufficient pure.
If I wanted to make it gratuitously harder to make photos I'd just go out without my eyeglasses.
For me the bottom line with achromatic sensors is to focus on what they actually do and don't contribute to your particular needs, and avoid indulging in sweeping romanticism about "purity" and whatnot.
Also, another practical disadvantage of achromatic sensors: there are hardly any cameras on the market that have them, and those that do are generally at the high end of the price scale. Phase One and Lecia have achromatic versions, and maybe a few others. And you typically pay *more* for the privilege of not getting the Bayer and IR filters (because the achromatic versions are much more a specialty product).
As always, use whatever tools work for you.
@mattblaze@federate.social Thanks for this explanation. Your conclusions map pretty well onto the choice of monaural or stereo cartridges for playing 78s: the “proper” solution (e.g. Denon DL-102) is expensive and obtains only a very marginal benefit.
@mattblaze@federate.social And when they want color photos they take multiple exposures with different colored filters and then combine in post? :D
@gregersn@snabelen.no Technicolor forever!
@mattblaze@federate.social I had a really interesting answer from an exhibition curator last year who insisted that non-chemical photography simply isn't photography. While I don't agree he was at least coherent about it which made for an interesting perspective.
@mattblaze@federate.social What is purity in photography anyway if not a delusion some people had since the beginning of the media.
@mattblaze@federate.social My favorite picture that anyone ever took of me was with a Leica Monochrom but that guy is a terrific portraitist and probably could have got the same result with a conventional color sensor. So I probably agree with you.
Also, Silver Efex.
@mattblaze@federate.social Only very very peripherally related to what you are describing, but this was the thing that occurred to me as a place I'd seen where using a true monochromatic sensor is better/necessary: