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APS-C/Super35 cameras are less staple for film than they were before, but are still widely used. Besides, there are quite a few semi-professional APS-C DSLRs on the market.

As far as I understand, a frequently useful 24-70 f2.8 on Full Frame would be analogous to 17-55 f2 or f1.8 on APS-C. But we rarely see that. Even with the 17-55, the closest there is is 17-55 f2.8, which is closer to 24-70 f4 on Full Frame. And the only f1.8 zooms in the DSLR market that I know are the Sigma 18-30 and 50-100.

We frequently see this in the MFT market, but not in APS-C. Why not?

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  • This question is likely to be answered with opinions rather than facts and citations. It should be updated so it will lead to fact-based answers. Aug 2, 2021 at 8:23
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    a frequently useful 24-70 f2.8 on Full Frame would be analogous to 17-55 f2 or f1.8 on APS-C. Your assumption re: aperture and crop factor is incorrect, or at least needs to be contextualized. The assumption seems to motivate the question. Please see the following questions to get more context on crop factor with respect to aperture: Do I use the crop factor in calculating aperture size and area?, and Why does crop factor apply with APS-C lenses, and why aren't these brighter than FF ones at same aperture?
    – scottbb
    Aug 2, 2021 at 16:30
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    In terms of depth of field (DoF) the f-number should be "translated" when discussing different formats. But in terms of exposure, f/2.8 is still f/2.8. Most users of constant aperture f/2.8 lenses use them for the speed they provide in low light (i.e. for the faster exposure times they allow) rather than for the shallower DoF. If one wants really shallow DoF, one is likely going to use a prime with a much wider aperture than f/2.8. There's no such thing as "true equivalence" between different formats. One can only preserve 2 of the 3 factors (ISO, Tv, DoF).
    – Michael C
    Aug 2, 2021 at 18:15
  • @scottbb Though not entirely clear, it seems to me the OP is saying if one wants to get the same shot in terms of angle of view and depth of field with an APS-C camera that one could get with a FF camera + 24-70/2.8, then one needs a 17-55/2 lens. The question does not seem to be saying that using a FF 24-70/2.8 on an APS-C camera gives the equivalent of a 17-55/2.
    – Michael C
    Aug 2, 2021 at 18:21
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    @MichaelC Understood, and I agree. It took me a couple reads, but that was the conclusion I came to as well. Or mostly, at least. My guess was that the equivalence assumption was about exposure, not DoF. I just wanted to make sure, or at least get more context, because I'm hoping to get clarification to make the question clearer in order to improve possible answers.
    – scottbb
    Aug 3, 2021 at 0:44

2 Answers 2

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Mostly because the market has not demanded them, or at least the marketing departments at the camera and lens makers have not perceived much of a demand for them.

Most photographers who choose to use cameras with sensors smaller than full frame do so due to cost considerations. This includes not only the cost of camera bodies but also the cost of lenses to use on them. In 2021 the cost difference between full frame and APS-C sensors is nowhere near as wide a gap as it was just a few years ago.

But the camera body is only half the equation.

A lens like the Canon EF-S 55-250mm f/4-5.6 STM can be made much more cheaply and yet perform just as well within the smaller image circle area needed by an APS-C camera as a lens like the EF 70-300mm f/4-5.6 IS. Due to the smaller image circle needed, the lens can also be lighter.

For those who choose smaller formats for reasons other than cost, the main one is compactness and portability. Wide aperture, constant aperture zoom lenses are typically not the lightest and most compact lenses available within the range of possibilities in any format. Putting a relatively large and heavy lens like the Sigma 18-35mm f/1.8 DC or Sigma 50-100mm f/1.8 DC, compared to other lenses available for the APS-C format, on a smaller, lighter body kind of negates the advantage of a smaller, lighter body.

While it may be true that a 50-100/1.8 on an APS-C body is smaller and lighter than a 70-200/2.8 on a FF body, there doesn't seem to be enough demand for such lenses to motivate manufacturers to offer them. Buyers willing to pay what such lenses cost are usually going to buy a FF camera at some point anyway, particularly since most newbies seem to think the camera is more important than the lens.

Remember, there's no such thing as "true equivalence" when comparing different formats. It's always an approximation. The only way one could have even theoretical true equivalence is if we could scale the wavelengths of various colors of light (and the energy and mass of photons) by the same factor as we scale everything else.

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You are badly underestimating the difficulty of making fast zoom lenses. If one could reasonably make an f/2 zoom, it would be out there for full frame cameras. Even constant aperture f/2.8 seems to be a stretch. When I searched bhphoto I found only one MFT zoom lens faster than f/2.8, the Panasonic 10-25 f/1.7. It is $1800.

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