FF vs APSC - IQ differences after considering equivalence

Gear

There seem to be two groups of people in this subreddit:

those who don’t know about aperture equivalence and say f/2.8 = f/2.8 across different formats (and therefore believe there is no significant difference between APS-C and FF given the same sensor technology, same pixel size, same exposure, etc.), and
those who believe that the IQ advantages in FF can be matched when using APSC lenses with equivalent aperture and focal length(i.e., no IQ difference between something like f/1.8 on APS-C and f/2.7 on FF).

For the first group, there are already a number of articles explaining equivalence, but it still seems like a hard thing to convince them of.

This post is mainly addressing the second group. Kasson wrote a blog post on this that points out the limitations of equivalence. Basically:

Slower FF lenses are generally easier to design than their faster APSC equivalents, which can bring advantages in off-axis bokeh, off-axis aberrations, vignetting, and in-focus-to-out-of-focus transition rendering.
FF sensors tend to have better dynamic range than APSC(even at the same pixel pitch), assuming the same sensor generation and base ISO. If you are shooting at base ISO and ETTR, shadow noise on FF will be about one stop better.
Larger sensors generally have higher sensor resolution. Even if a smaller sensor has similar or higher megapixels than FF (e.g., 40MP Fuji vs 33MP Sony FF), the smaller pixel pitch increases the likelihood of CFA crosstalk, which can "pollute" color information.

and he concluded that “For IQ, size matters, and bigger is better.” That is assuming that an equivalent lens exists for comparison, which often is not the case when looking for equivalence to something like an f/1.4 FF prime or a lens like the Sigma 14mm f/1.8.

I believe there are also sharpness advantages to a larger-format setup. Some people argue that using an FF lens on APSC produces a sharper image because the smaller sensor “throws away” the weaker corners. I don’t think this is true in most cases:

A smaller sensor requires a larger enlargement factor for the same output size, so lens imperfections and diffraction blur are magnified more.
A way to compare sharpness across formats is by measuring line pairs per picture height (lp/ph) at MTF50.
If we use the same lens on both APSC and FF (for example, the lens resolves 100 lp/mm at the center), the FF resolution would be 100 lp/mm * 24mm vs the APSC resolution at 100 lp/mm * 16mm sensor height.
Thus, the center sharpness (in lp/ph) is always better on the larger format when using the same lens. An APS-C lens would need about 1.5× higher lp/mm to match an FF lens
Away from the center, the enlargement factor still favors the larger format unless the FF lens has very poor edges. The edge lp/ph of the FF lens would need to be 1.5× lower than its APS-C edge (or "part way") lp/ph for the opposite to be true, which is usually not the case. (note: 1 lp/ph = 2 lw/ph.)
- 35mm GM (@f/1.4): 2842 lw/ph (part way), 2396 lw/ph (edge) → 1.18× lower
- 50mm f/1.4 GM (@f/1.4): 3177 lw/ph (part way), 2620 lw/ph (edge) → 1.21× lower
- 135mm f/1.8 GM (@f/1.8): 3450 lw/ph(part way), 2948 lw/ph (edge) → 1.17× lower

Another counterexample to equivalence is by looking at smartphone IQ. For instance, the iPhone 17 Pro has a 1/1.28" main sensor, which is roughly equivalent to f/6.2 on FF and f/4.1 on APSC. I think it would be absurd — and experimentally disprovable — to claim that an iPhone image has the same IQ as an APS-C image taken at f/4.1, even if the phone has a lower base ISO and potentially newer sensor technology.

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I am not trying to claim that FF is better for most people. But I do think FF has image-quality advantages that APS-C cannot fully match, even after invoking equivalence to normalize SNR.

Whether that IQ difference is noticeable at a given output size—and whether APS-C is already “good enough” for most people—is a separate matter.

Posted Feb 22, 2026, 4:58 PM UTC

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