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PS: The 17-55 has the same DOF of the 24-70 but the f2.8 defeats the ISO advantage of a FF f4.0.
Full-frame has a 1 and 1/3 stop advantage at same f-number (at equivalent focal length, i.e. 50mm f1.8 on APS-C compared to 80mm f1.8 on full frame). But this is simply because the entrance pupil, i.e. the actual aperture diameter is equal to the focal length divided by the f-number. So even though the f-number is the same the actual aperture is larger. For equal aperture equivalent lenses (in this example a 50mm f1.8 on APS-C and a 80mm f2.9 on full-frame, both 28mm aperture) the amount of light gathered is equal for equal angle of view (i.e. equivalent focal length). Assuming both sensors can collect the same number of photons before overflowing the image quality will be exactly the same for the same exposure time.
So it's the most correct way to compare lenses across sensor sizes to find the equivalent both focal length as well as f-number. This will result in lenses with the same aperture diameter and the same angle of view that collect the same amount of light and have the approximately same amount of glass (unless one needs to be much more retrofocal than the other because of the flange distance, as is the case for 30/35mm on crop vs 50mm on FF).
Canon makes many good APS-C lenses. The 10-18mm STM, 18-135mm STM, 55-250mm stm and 24mm stm are all really great lenses. Other than the pancake one, they don't have wide apertures, but they are great lenses.
While the lenses have good image quality their aperture is horrible! The 10-18mm is a 16-29mm f7.2-9 FF-equivalent! The 18-135mm is a 29-216mm f5.6-9 and the 55-250mm is a 88-400mm f6.4-9. Meanwhile the full frame Sigma 100-400mm f5-6.3 is being disparaged for having too small an aperture! Finally the 24mm is a full frame equivalent of 38mm f4.5. I wouldn't call that a wide aperture.
To be fair I like some of these lenses. I have the 55-250mm and I love it, and I'm planning to get the 24mm. But the problem is that Canon
could make wider aperture EF-S lenses that could compare to full frame lenses on full frame bodies. Sigma proves this. They would cost similarly to full-frame lenses of course, but there's a lot of people that want to buy one or two big expensive lenses like this, while still having the option of using their cheap small aperture lenses and not having to buy an extremely expensive full frame body. Canon
chooses not to offer this option for the single purpose of 'forcing' people to buy full frame. This is what p*sses so many people off.
Who told you full frame lenses are suboptimal on APS-C? They are just as good as APS-C lenses are for the most part.
Full frame lenses are made for a larger image circle than APS-C lenses. Look at the Sigma 30mm f1.4 ART. It costs 500 dollars. Canon have two similar focus length full frame lenses that are the same size and cost the same, a 28mm and a 35mm. But their aperture are f1.8 and f2. The point I'm making is that either you are paying for more glass than you use, or you're stuck with a larger aperture than it could have been if it were a crop lens. Lenses like the 18-35mm and 50-100mm would either have to be a smaller aperture, or much larger and more expensive if they were full-frame.
For an average enthusiast with 3 or 4 lenses a difference of 900 dollars is a lot.
Is it really though, when most fast glass costs $400-600 on either format?
It's the difference between having a certain set of lenses and having a similar set of lenses + one more. I would say that is a significant difference.
