[ElJamoquio]

I specifically don't get into the equivalency with f values and depth of field because it creates way too much confusion.

I do use equivalencey. I don't feel like it adds very much confusion, and it allows us to compare two sets of equipment (cost, size, etc) taking the same picture.

[Ikarus]

When enlarged to the same size, however, there would be an apparent decrease in DoF - but this is just a magnification effect. As such, any benefit from FF must be with proportionally increased resolution from that of the smaller format sensor.

The conception of the DoF changing with the format comes from a different experimental setup, in which it is not the position of the camera that is kept unchanged, but the FoV in the subject plane. This implies that with a crop format, the camera needs to be moved further away, which consequently results in a growth of the DoF.

[Ash]

The conception of the DoF changing with the format comes from a different experimental setup, in which it is not the position of the camera that is kept unchanged, but the FoV in the subject plane. This implies that with a crop format, the camera needs to be moved further away, which consequently results in a growth of the DoF.

Understood, and agreed.

[Fogel70]

The reverse teleconverter would make the lens faster by virtue of increased light intensity rather than larger aperture. But that is essentially the same as just having the lens itself on the larger format camera, only with more resolution.
I don't know of the practical utility of such a reverse teleconverter. The degradation in IQ would negate any apparent benefit. If there were great advantages for such a gizmo, they would surely have been made available by now.

The problem is no one can make a reversed TC as long as Kodak own the patent, and they will not let anyone get a licence to use it either. But as this patent will soon expire I think we will see this type of adapters within a few years. And on a short register distance mirrorless systems adapters like this make more sense.

[Fogel70]

Is there any more to the optical qualities of the lens than this, or does the different sized formats actually somehow change the lens's characteristics?

As long as FOV changes with different sensor size, the characteristics of the lens change. Or would you use a K-mount lens the same way on Q than on a FF K-mount camera?

[Ash]

As long as FOV changes with different sensor size, the characteristics of the lens change

By virtue of cropping only AFAIK. Or is it by any other mechanism?

[timo]

As long as FOV changes with different sensor size, the characteristics of the lens change.

Fundamentally they don't. What changes is the amount of the projected image you are using, and hence the degree by which you are magnifying it to screen or print size. This is why a given lens with different-size sensors gives different FOVs (FsOV?), which have to be considered not in terms of different focal lengths (because that never changes), but FOVs equivalent to different focal lengths in 35mm, which is different.

Or would you use a K-mount lens the same way on Q than on a FF K-mount camera?

No.

And as someone said earlier, different depths of field related to different sensor sizes are the consequence of greater or lesser magnification of the image, rendering degrees of blur less or more acceptable, respectively.

[lytrytyr]

With smaller formats and subsequently more cropping of the image circle from the lens, there will surely be a more consistent optimal IQ coming from the lens.
With good lenses such as the FA Limiteds, there isn't that concern.

Unfortunately, this doesn't seem to happen,
because of the increased resolution demands of small sensors.

Look at the Photozone reviews of the Nikkor AF-S 28mm f/1.8 G
on the 24x36mm D3x, APS-C D7000, and the 1" (CX) V1.

4 stars on FF, 3-3.5 stars on APS-C, and 2.5 stars on CX.

At best "good" border or extreme MTF50 on CX,
despite "excellent" optimal extreme MTF50 on FF.

[Fogel70]

Fundamentally they don't. What changes is the amount of the projected image you are using, and hence the degree by which you are magnifying it to screen or print size. This is why a given lens with different-size sensors gives different FOVs (FsOV?), which have to be considered not in terms of different focal lengths (because that never changes), but FOVs equivalent to different focal lengths in 35mm, which is different.

For me FOV is one of the most important parameter of a lens. So on FF and APS-C I use different lenses for the same purpose. FA 31/1.9 might work well as a "normal" lens on APS-C, but on FF I need FA 43/1.9 or FA 50/1.4 the same thing. But I would need a f/0.9 -1.3 on FA31 to get the same DOF as with FA50 or FA43 on FF.

Sure the lens don't physically change on different sensor size, but they change in how the can be used. This is why equivalence is important when using different formats. It's was so even back in the days of film, if using both 135 and medium format.

[Anvh]

Don't forget that the Circle of Confusion, that's the maximum size of a lightpoint in the photo, for typical print and viewing distance.
The CoC for ASP-C is lower then for FF, so that means that something will look less sharper earlier.

[Ash]

Unfortunately, this doesn't seem to happen,
because of the increased resolution demands of small sensors.

Look at the Photozone reviews of the Nikkor AF-S 28mm f/1.8 G
on the 24x36mm D3x, APS-C D7000, and the 1" (CX) V1.

4 stars on FF, 3-3.5 stars on APS-C, and 2.5 stars on CX.

At best "good" border or extreme MTF50 on CX,
despite "excellent" optimal extreme MTF50 on FF.

How would this best be explained? CoC? This is a good case for FF then, as apparent IQ would be better for all FF lenses on an FF camera vs the existing APS-C cameras, which mean better quality images from the same lenses.

[ihasa]

The conception of the DoF changing with the format comes from a different experimental setup, in which it is not the position of the camera that is kept unchanged, but the FoV in the subject plane. This implies that with a crop format, the camera needs to be moved further away, which consequently results in a growth of the DoF.

Ouside of the testing lab, it's the FoV and the composition of the subject that matters. The camera is moved to attain the desired composition, and therefore the DOF changes. Talk of the DOF not changing between the different formats is fine, but ignores the way we shoot in the real world, which is where technically questionable perhaps, but nevertheless handy, rules of thumb like 'equivalency' come in.

[lytrytyr]

How would this best be explained?

Briefly, in terms of the resolution required for,
let's say, a 12MP image,
on smaller and smaller sensors.

This is a good case for FF then, as apparent IQ would be better for all FF lenses on an FF camera vs the existing APS-C cameras, which mean better quality images from the same lenses.

On the whole, yes.
Bigger is better.

'Twas ever thus.

Back in film days, even a simple Tessar lens on 6 x 6 cm
could give a better image than a more sophisticated lens on 35mm.
(Assuming the film was flat in the camera.)

[Blue]

This is wrong, as the APS-C sensor is smaller, less than half the light is captured by this sensor compared to 135 film or FF sensor. Light per area unit will be the same for both, but the sensor/film with larger area will collect more light.

But if using a converter to make a FF lens to APS-C lens, so the APS-C sensor can collect all the light from the lens. Then light per area unit will be higher on the APS-C sensor. Shutter speed will then be faster on the APS-C camera if using the same aperture and ISO as on a FF camera.

The FA 77/1.8 lens will still let in the same amount of light at ISO 100 and 1/250. This size of the sensor/film does not change that fact.

For me FOV is one of the most important parameter of a lens. So on FF and APS-C I use different lenses for the same purpose. FA 31/1.9 might work well as a "normal" lens on APS-C, but on FF I need FA 43/1.9 or FA 50/1.4 the same thing. But I would need a f/0.9 -1.3 on FA31 to get the same DOF as with FA50 or FA43 on FF.

Sure the lens don't physically change on different sensor size, but they change in how the can be used. This is why equivalence is important when using different formats. It's was so even back in the days of film, if using both 135 and medium format.

[ElJamoquio]

The FA 77/1.8 lens will still let in the same amount of light at ISO 100 and 1/250. This size of the sensor/film does not change that fact.

I don't care about the amount of light that comes in the lens.

I care about the amount of light that is collected by the sensor... and of course, the FOV, DOF, Bokeh, SNR, Resolution...