I've been saying for years that a full-frame lens will work just fine on an apsc body, since all that's really happening is that you're selecting a narrower circle of illumination to fit over the sensor than the lens is capable of.

I've come to the conclusion that, while the statement is fundamentally correct, and I'm still mostly using FF lenses on my KP, it occurred to me this morning that the difference is that, given two lenses of identical focal length, the apsc lens is going to concentrate the same light in a narrower focus, while the ff lens is going to disperse the same light in a more diffuse pattern, resulting in a loss of analog resolution.

I've read where people have remarked on the difference in the theoretical result, I've never seen an explanation before, plausible or otherwise.

Perhaps some of the physicists among us can enlighten us more fully.

[simon_199]

Maybe I don't understand all the assumptions under which you are making the comparison. If the two lenses have identical focal length and identical aperture...there should be no theoretical difference. Maybe one could perform better than the other but it would not be due to the size of the image circle, but to other intrinsic characteristics of the two lenses that might differ.

I'm thinking about the following counterexample: if you take an FF lens and then add a flange that physically blocks light so that it covers only the APS-C image circle, will anything change ? I don't think so...

[Jonathan Mac]

The lens gathers exactly the same light regardless of sensor size. In the case of an APS-C sensor, half the amount of light falls on the sensor compared to a FF lens, the rest of the light falls on whatever surrounds the sensor (plastic, metal, whatever) and serves no photographic purpose.

The reasons for using APS-C lenses on APS-C cameras are to save size, weight and cost by not making the lens any larger than it needs to be for the intended sensor size.

[biz-engineer]

it occurred to me this morning that the difference is that, given two lenses of identical focal length, the apsc lens is going to concentrate the same light in a narrower focus, while the ff lens is going to disperse the same light in a more diffuse pattern, resulting in a loss of analog resolution.

Theoretically, there is no difference between a FF and APSC lens of the same focal length. It's just that the image circle projected by the APSC lens is cropped by the smaller sign of lens elements. With the same optical formula, if the lens elements weren't cropped, your APSC lenses would project an image circle large enough to cover an 8x10" film plane. The point of an APSC only lens is just to be small, lighter and cheaper, by having all lens elements of the optical path cut smaller than they would if they would be cut to cover a full frame image circle.

[kypfer]

Lots of valid advice above
I'd add that "apsc-specific" lenses are likely to be of a later design, not only in coatings but possibly optically as well, so may simply be capable of better performance than an "older" full-frame lens … not always true, obviously, but a point for consideration

[brofkand]

It's really just that APS-C lenses are often lighter and smaller than full frame lenses in my opinion.

[pentasonic49]

The advantage of using full frame lenses on an APSC camera is that the outer parts of the image are not used and most lenses have some drop off of resolution towards the edges. APSC lenses are no exception, but the drop off is still on the sensor area. Modern lenses should have improved antireflection coatings which should give them an advantage in certain lighting conditions. Full frame lenses tend to be larger, but my 16-50 Pentax is no compact lens, and that is an example of zero full frame compatibility.

[cdw2000]

The only difference I can envision is that because the FF lens has a larger image circle, there is extraneous light hitting features in the shutter box that normally do not get hit with light with an APS-C lens. That light might bounce around and reflect back onto the sensor (or the back element of the lens), causing a reduction in IQ or contrast.

Just to be clear, this is something I could envision happening, I don't know if it actually happens and has a visible effect.

On the other side of the coin, the advantage of the FF lens is that you are only using the center portion of the lens for the image which should mean better edge-to-edge sharpness and there should be virtually no vignetting, even at fully open aperture.

[mlag]

......, given two lenses of identical focal length, the apsc lens is going to concentrate the same light in a narrower focus, while the ff lens is going to disperse the same light in a more diffuse pattern, resulting in a loss of analog resolution.

I disagree on the wording here, focus is same (focal length! ) and difuse would mean unsharp...
I think you can look at it like this: an apsc lens can be created from a FF lens by cutting away an outer rim of the lenses, the outer 1/2 (just guessing) cm of the diameter is not needed. The sensor sits on the same place as the FF one, but is smaller and hence diameter can be smaller.
A good FF lens will be good on both sensors.
But we know sharpness into the corners is sometimes a problem on cheaper lenses and complex zoom lenses...
Hence an apsc usage of a FF lens might suffer less from corner unsharpness or vignetting, as it doesn't use the FF corners.


Making larger diameter lens glass is more difficult than smaller diameters. They will be also thicker in the center or edge of the glas (think about the 2 typical shapes of a lens https://www.kindpng.com/imgv/TRomixm_convex-and-concave-lenses-concave-lens-convex-lens/ ) and thicker/larger lenses are more difficult to produce...
Here the smaller size difference might benefit apsc designs: they are easier to produce because using smaller and thinner lenses. Hence an purposely designed apsc lens design is more compact...as we all see on our apsc lenses (DA series)

Given a certain skillset of designer and poducer of lenses, there is bit more margin, degrees of freedom he can exploit and invest in image quality, hence an apsc lens might be perceived better as FF if build with same production skills or tools. Or he can just produce cheaper for same quality exploiting the size production-advantage.... Hence I would not always assume apsc is always a better lens design, it might be just a cheaper design...

Unfortunately a smaller sensor is more difficult to produce for the same image quality... Otherwise we would all use smartphones by now? So the various sensor sizes have justification, but advances in sensor technology allow more and more the smaller sensors... this market shift to smaller sensors and smaller, cheaper lenses is ongoing for some time and existed also in film. The desired quality level is the bottom. Oh, and digital postprocessing, AI or not, is a new factor pushing the limits, it's not only sensor hardware.

[CarlG]

… On the other side of the coin, the advantage of the FF lens is that you are only using the center portion of the lens for the image which should mean better edge-to-edge sharpness and there should be virtually no vignetting, even at fully open aperture.

Not really. The point of an ‘’apsc-lens’’ is that that outer lens rim can be cut away for a lighter lens. Or actually never be a part of the production process at all.

[UncleVanya]

Apsc lenses need slightly higher resolution than ff ones due to the pixel density (for a given focal length). This effect is mostly a non-factor with the small differences between these formats.

[AfterPentax Mark II]

Usually I read that an APS-C camera benefits from an FF lens. It seems that the APS-C sensor gets the image from the sharpest part of the lens: the Center. So less chance of vignetting as well as no unsharp corners.

[MrB1]

You might find interesting the Optical Limits comparisons of the FA 35 and the DA 35 Ltd, both tested on the APS-C K10D :-

Pentax SMC-FA 35mm f/2 AL - Review / Test Report - Analysis
Pentax SMC DA 35mm f/2.8 Limited macro - Review / Test Report - Analysis

[Wheatfield]

I've been saying for years that a full-frame lens will work just fine on an apsc body, since all that's really happening is that you're selecting a narrower circle of illumination to fit over the sensor than the lens is capable of.

I've come to the conclusion that, while the statement is fundamentally correct, and I'm still mostly using FF lenses on my KP, it occurred to me this morning that the difference is that, given two lenses of identical focal length, the apsc lens is going to concentrate the same light in a narrower focus, while the ff lens is going to disperse the same light in a more diffuse pattern, resulting in a loss of analog resolution.

I've read where people have remarked on the difference in the theoretical result, I've never seen an explanation before, plausible or otherwise.

Perhaps some of the physicists among us can enlighten us more fully.

Ummm no, all that is happening is the sensor is seeing a smaller section of the image circle. However, as the full frame lens projects a bigger image circle, it is possible for light to bounce off of stuff in the mirror box and cause flare.
It's not a likely possibility, in fact it is an extremely remote possibility, but it is the only conceivable downside.

[Lowell Goudge]

snip…..given two lenses of identical focal length, the apsc lens is going to concentrate the same light in a narrower focus, while the ff lens is going to disperse the same light in a more diffuse pattern, resulting in a loss of analog resolution.

Actually they both concentrate the same amount of light in the same area, that is the definition of f stop. The only thing the full frame does, is for the larger image format, it puts more light into the mirror box of the crop sensor body. That might or might not lead to a little more loss of contrast than diffusing that light in the baffles of the lens itself