[brofkand]

SLR mirror boxes are quite well flocked, I doubt the extra light from the larger image circle is a measurable problem.

[pschlute]

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

I am not sure what you mean by "narrower focus". Each pixel on the sensor is getting the same amount of light whether you use a FF lens or aps-c lens, assuming both are the same FL and f-stop (ignoring potential vignetting at the edges/corners).

while the ff lens is going to disperse the same light in a more diffuse pattern, resulting in a loss of analog resolution.

Nope. Each pixel is getting the same light as above. Assuming the two lenses (FF and aps-c) have identical resolving abilities, the resulting images will be identical.

On my K-1 I use FF lenses. If I put my K-1 into aps-c mode I get a narrower FOV image from the camera. I am using a smaller "bite" out of the image circle than the lens produces. How can switching the camera to aps-c mode affect the amount of light each central pixel receives or the resolving ability of the lens ?

[simon_199]

On my K-1 I use FF lenses. If I put my K-1 into aps-c mode I get a narrower FOV image from the camera. I am using a smaller "bite" out of the image circle than the lens produces. How can switching the camera to aps-c mode affect the amount of light each central pixel receives or the resolving ability of the lens ?

In this particular example it won't make a difference. What is being put forward is that using a lens made to project the larger image circle on an APS-C camera could (in principle) introduce some stray light that might bounce around the mirror box area, in places where it is not necessary expected to happen when using lenses designed for APS-C, and result in flare or contrast loss. I'm no optical designer but my gut feeling is that this effect is negligible, but theoretically it could exist.

[Unregistered User 8]

Projecting a round light source onto a rectangular sensor has 'wasted' light that has been controlled since the very early days of photography methinks.

My very crude little unproportioned creation made in Paint gives a clue as to how much light isn't used.....

Attached Images

 

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Last edited by Unregistered User 8; 08-13-2022 at 12:17 AM. Reason: typo

[GUB]

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

Interesting comparison. On the basis that these tests are accurate and comparable then it would seem that apsc was an opportunity to make cheaper lenses and little else.
Here is the two test placed beside each other.

[slartibartfast01]

Interesting comparison. On the basis that these tests are accurate and comparable then it would seem that apsc was an opportunity to make cheaper lenses and little else.

Here is the two test placed beside each other.

Interesting comparison. On the basis that these tests are accurate and comparable then it would seem that apsc was an opportunity to make cheaper lenses and little else.

I am pretty sure that was the reasoning put forward when the APSC lenses were first introduced. Canon EF-S in 2003 IIRC.

[photogem]

Interesting comparison. On the basis that these tests are accurate and comparable then it would seem that apsc was an opportunity to make cheaper lenses and little else.

I am pretty sure that was the reasoning put forward when the APSC lenses were first introduced. Canon EF-S in 2003 IIRC.

The SMC Pentax DA 16-45/f4,0 was introduced as well in 2003, the DA14 in 2004.
None of them seem to an attempt to make cheaper lenses.
Knowing a little bit of the story behind the DA16-45 it was quite a special lens.

[MrB1]

It was due to the more consistent MTF performance across the whole APS-C frame that I obtained the FA 35 mm f/2 as a standard prime for my KP and I have not been disappointed.

Philip

[reh321]

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.

I have sometime been surprised by using lenses on my “Q”; some lenses give better or worse results than I expected. The only thing I can think of is that they have better or worse resolution in the middle of the barrel, resolution that doesn’t show up on standard tests. For example, my eight-element Takumar 50mm and my Pentax-M 50mm are not as sharp on my “Q”’as my Canon FD-mount 50mm is; my Sigma 70-300mm {which is apparently “FF”} does marginally better than my Pentax-DA 55-300mm does, at least at 300mm. I have to try each lens on a “Q”, rather than looking at standard tests.

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.

See, that's the assumption I'm questioning - the lens does not crop the image, it's the sensor that's doing that. I am also assuming, by the way, that not only is the focal length identical, but also the aperture and diameter of the objective lens. My argument is that exactly the same quantity of photons are entering both lenses, and that the APSC design actually concentrates the light more than an FF design would. But this raises a question of physics of the optical design of lenses as to which I am not qualified to express any opinion.

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

But "f-stops" are relative to the lens and body combination they happen to be on, and do not represent either the amount of light or the scope of the viewing angle, right?

...
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.

I assume that to be true, and is part of the reason I never stocked up on APSC lenses. The whole question may be a matter of how many disembodied spiritual entities can dance on the head of a pin. I don't notice any significant differences in what my FF lenses produce on either system. No one looks at a picture on the wall and wonder whether an APSC lens might have made a crisper image.

[UncleVanya]

See, that's the assumption I'm questioning - the lens does not crop the image, it's the sensor that's doing that. I am also assuming, by the way, that not only is the focal length identical, but also the aperture and diameter of the objective lens. My argument is that exactly the same quantity of photons are entering both lenses, and that the APSC design actually concentrates the light more than an FF design would. But this raises a question of physics of the optical design of lenses as to which I am not qualified to express any opinion.

Many many articles have been written about this. Essentially the total amount of light differs but the light per square millimeter (or any measurement of area) is the same (within reason, lenses can have minor variation at the same f stop, t stops are more accurate). The lenses for apsc don’t concentrate the light - if you want that, use a focal reducer on a ff lens mounted on apsc. You’ll gain light, and in some cases you will gain resolution.

[StiffLegged]

…I've come to the conclusion 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.

Your conclusion is faulty. The difference between a FF lens and an APSC lens of the same focal length is coverage: an FF lens covers the 24x36mm sensor while an APSC lens is only able to cover an APSC size sensor. The same thing happens with large format film lenses where a 200mm lens might be able to cover a 5”x4” film size with ease but can’t fully cover 8”x10” film, leaving corners or even a circular edged area dark on the much larger film.

The analog resolution is a property of the lens design, not the sensor or film size it’s used for.

[Wheatfield]

It was due to the more consistent MTF performance across the whole APS-C frame that I obtained the FA 35 mm f/2 as a standard prime for my KP and I have not been disappointed.

Philip

This is the beauty of using larger format lenses on smaller formats. Getting away from the edges of the image circle puts your images in the lens's sweet spot.
When I shot 4x5, my best lens was a Nikor 210mm that covered the 5x7 format quite generously. There was no possibility of getting soft corners with that lens.

---------- Post added Aug 13th, 2022 at 08:29 AM ----------

See, that's the assumption I'm questioning - the lens does not crop the image, it's the sensor that's doing that. I am also assuming, by the way, that not only is the focal length identical, but also the aperture and diameter of the objective lens. My argument is that exactly the same quantity of photons are entering both lenses, and that the APSC design actually concentrates the light more than an FF design would. But this raises a question of physics of the optical design of lenses as to which I am not qualified to express any opinion.

This is incorrect. The smaller format lens does not "concentrate" the light, it just projects a smaller circle.
Your argument is based on an incorrect premise.