[GUB]

This is only true when the pixel pitch is the same. So k5ii vs K1 for instance. Otherwise IF (BIG IF) the lens can resolve at a higher resolution than the FF sensor's pixel pitch in the central part of the frame then the apsc portion will have closer lines on apsc sensor than shown.

If from memory a good lens achieves 100 lines per mm (1/100mm) then that is about 2 pixels in the K-1 or K01. And considering a 24mp apsc pixel is only marginally smaller (remember mp is a square of pixel diameter.) I think the basic real estate argument is more relevant. And the OP is talking lens and sensors not pixels.
I have experimented with FF lenses on the tiny sensor (and extremely tiny pixels) of the Q I can assure you it is the real estate that matters.

[MrB1]

How does magnification affect the result - e.g. the same field of view on the two sensors must be magnified more from the APS-C sensor than from the FF sensor to make a 12-inch print?

Philip

[ZombieArmy]

Yah but I'd argue for most cheap vintage lenses it'll actually be the opposite. A lot of cheaper lens designs just don't hold up to the demanding requirements of even APSC.

Good designs like the old tak 50/55s will easily keep up and hide bad corners while having exceptional sharpness in the center.

[UncleVanya]

I have experimented with FF lenses on the tiny sensor (and extremely tiny pixels) of the Q I can assure you it is the real estate that matters.

That is exactly my point about medium format lenses. In that case a ff lens isn't going to resolve well enough for the high density of the q sensor. And sure the pixels aren't equal. A large pixel is more tolerant of lower light and total light does matter. But a small sensor needs a higher resolution lens.

[Bob 256]

The same principle applies upwards. I have a 645 lens and K mount adapter that I've used on my K-1 and the image is fantastic, sharp to the edges with very low chromatic distortion, again, because the camera is using the central portion of the image and discarding that portion where the IQ falls off.

[GUB]

Yah but I'd argue for most cheap vintage lenses it'll actually be the opposite. A lot of cheaper lens designs just don't hold up to the demanding requirements of even APSC.

Maybe true with old zooms . If you are using old el cheapo primes it is because you are looking for other qualities rather than sharpness and these (vignetting - soft corners) are going to present better in FF. Things like flare and ca are probably aggravated by apsc.

A lot of cheaper lens designs just don't hold up to the demanding requirements of even APSC.

I believe FF is less demanding. It is why I have never regretted going FF - I use almost exclusively old glass.

[DWS1]

Depends more on the specific lens than the sensor size. Some will, some won't.

[pschlute]

How does magnification affect the result - e.g. the same field of view on the two sensors must be magnified more from the APS-C sensor than from the FF sensor to make a 12-inch print?

Philip

I think the size of the sensor is not the only point when you talk about "magnification" and prints, but the MP count too.

The Pentax K1 FF camera can produce a maximum print of roughly 24.5" x 16.3" at 300 ppi
The Canon EOS 90D aps-c (32.5 MP) can produce a maximum of 23.3" x 15.5" at 300ppi
The Pentax KP (24MP) aps-c can produce can produce a maximum of 20" x 13.3" at 300 ppi
My old *ist DS (6MP) aps-c can only produce a 10" x 6.7" print at the same ppi

[UncleVanya]

I think the size of the sensor is not the only point when you talk about "magnification" and prints, but the MP count too.

The Pentax K1 FF camera can produce a maximum print of roughly 24.5" x 16.3" at 300 ppi
The Canon EOS 90D aps-c (32.5 MP) can produce a maximum of 23.3" x 15.5" at 300ppi
The Pentax KP (24MP) aps-c can produce can produce a maximum of 20" x 13.3" at 300 ppi
My old *ist DS (6MP) aps-c can only produce a 10" x 6.7" print at the same ppi

That's true enough but it doesn't address the question of how lenses perform across platforms.

[GUB]

that's what I was talking about in Post # 7

thanks for the image

Great - it made sense to someone then!!

[Michail_P]

Took me a while to find this image I had created to visualise the difference. Please excuse the rough graphical approach but it does the job.

Think of this as a focused image circle of a series of concentric lines that are spaced at the maximum resolution capability of the lens. Obviously they get further apart on the edges as the lens performance drops off.
Superimpose the two sensor sizes on it and you can see what I mean that the apsc presents the lens at a lower resolution. It is simple to see that the apsc image has less resolution lines on it.

Your graphical approach is very explanatory indeed. Thanks for sharing. It is really a matter of cropping. The analytical limits of any lens are standard. The construction parameters are the reason. The real consideration is exactly the portion of the coverage circle that is used by a sensor. Of course when it comes to image quality in , let’s say, an enlargement, the parameters of the sensor (pixel density etc) are more significant , assuming that the shot is as sharp (—>steady) as it can be. If we targeted to the exact same sharpness, for the same image, measured on two different prints , then we would have two different print sizes (aps-c / ff).

[Alex645]

Can a lens perform better on a small sensor than it does on a full frame sensor? For example, I have seen a review that reported that the FA 31mm performed better on a K-70 than it did on a K-1. Is the central portion of the lens used on the small sensor compared to all of the lens on the larger sensor?

The lens itself is not performing better, but as other have pointed out, the main reason it appears better on an APS-C sensor for a full frame image circle is that you are only getting the sweet center spot and none of the potential issues of resolution, vignetting, distortion, or chromatic aberration in the corners and edges.

Three other potential factors:
a) The FA lens, unlike the DFA lenses, were designed for film which allow light to strike the film from a greater angle than a digital sensor that has glass and layers of RGB pixels. These issues are exacerbated in the edges and corners on a film lens vs. a DFA or DA.

b) The 36Mp K1 is going to reveal optical flaws that a 24Mp K70 won't. I've seen this with the Nikon Df (FF 16Mp) users getting amazing results with older lenses that higher Mp FF Nikons don't.

c) Sample variation: With the lens tested and what and how it was tested by the reviewer may have skewed the results in favor of the K-70.

Took me a while to find this image I had created to visualise the difference. Please excuse the rough graphical approach but it does the job.

Think of this as a focused image circle of a series of concentric lines that are spaced at the maximum resolution capability of the lens. Obviously they get further apart on the edges as the lens performance drops off.
Superimpose the two sensor sizes on it and you can see what I mean that the apsc presents the lens at a lower resolution. It is simple to see that the apsc image has less resolution lines on it.

Perhaps this picture shows just circles with a blue and red square and has nothing to with resolution at all. To me these circles seem to reflect the lens and in the middle it is very sharp and sharpness drops off near the sides of the glass. It could be used to explain why an FF lens used on APS-C might give the experience that the lens is sharper than when used on an FF camera.
I was led to believe that resolution is measured with the use of straight lines, not circular lines. And about what kind of resolution where we discussing here? The word resolution in (digital) photography has to be defined.

[Kiddo70]

Thank you all for the discussion. But honestly, it makes me yearn for the days of simple film photography. Maybe I should look for a good used K1000.

[GUB]

Perhaps this picture shows just circles with a blue and red square and has nothing to with resolution at all. To me these circles seem to reflect the lens and in the middle it is very sharp and sharpness drops off near the sides of the glass. It could be used to explain why an FF lens used on APS-C might give the experience that the lens is sharper than when used on an FF camera.
I was led to believe that resolution is measured with the use of straight lines, not circular lines. And about what kind of resolution where we discussing here? The word resolution in (digital) photography has to be defined.

I like to think of resolution in practical terms of how well fine details can be resolved.
Hence the description - " Think of this as a focused image circle of a series of concentric lines that are spaced at the maximum resolution capability of the lens."
The red and blue squares sit on the same image circle so it is a no-brainer that the FF image contains more detail (resolution).
Yes the apsc sits on the area of the image with the most detail but the FF also sits over that area as well plus some.

Thank you all for the discussion. But honestly, it makes me yearn for the days of simple film photography. Maybe I should look for a good used K1000.

It is easy to fall into the complexity traps but really if you just think about two sensors sitting on the same image circle it is an accurate and simple comparison.