[Marc Sabatella]

Which always makes me cringe inwardly, as the usual conclusion is often "APS-C is better for telephoto", quite stupid for the simple reason that FF pics can always be cropped to the same amount as APS-C (often with still superior IQ), negating the perceived telephoto advantage...

I think the "often with superior IQ" part is debatable - where do you think the advantage would come from if you're dealing with the same lens on the same sensor area with the same pixel density? - but in any case, with this approach you're paying for pixels you aren't using, and carrying around a larger and heavier camera just for the privilege of having to work that much harder to get the results you would have gotten more easily and cheaply with APS-C. Of course, it's not like the extra money and weight is wasted - FF obviously has advantages in *other* respects. But really, to extent it makes sense to generalize and compare across systems at all, one *can* conclude that APS-C has *some* advantages for telephoto, if you're sensitive to price, camera size, or PP effort. Whether those advantages outweigh the disadvantages for a given person is another matter, of course.

Still, you are right of course that's it's no free lunch for APS-C here.

[simico]

FF camera often have nearly the same pixel density than APS-C sensors
...
And given that FF has better high-iso,

In that case (same pixel density) there's no high ISO advantage. High ISO advantage can come from lower pixel density/larger photosites and advances in electronics (and to some degree, advances in firmware noise reduction).

[dlacouture]

In that case (same pixel density) there's no high ISO advantage. High ISO advantage can come from lower pixel density/larger photosites and advances in electronics (and to some degree, advances in firmware noise reduction).

Well, actually, in that case (same pixel density), there IS an advantage to FF, as it will have a lot more pixels (thus a less visible noise) for the same output size.

Say you have a 14.5MP aps-c, with a 200/2.8 lens, vs. a 21MP FF with a 300/4 lens : equivalent DOF and FOV, you have to use an one and a half higher iso on the FF, but you also have some 6.5MP extra resolution.

The pixel-level noise (seen when pixel-peeping both pics at 100%) is indeed more present in the FF pic.
(but in real life, FF photosites being usually bigger than those on APS-C, noise handling is better, and the noise difference between the two system comes well below the 1-1/2 stops difference of my example)

But, for the same final output size, the extra noise will be offset by the greater resolution, where the extra 6MP will "hide" the additional noise. And that's not even with the same pixel density, where the difference would have been even more to the advantage of the FF system.

So, for a given DOF/FOV, equivalent lenses are not actually much that cheaper/smaller in APS-C, and sometimes do not exist altogether (there is no equivalent to the good old 50/1.4).
What would cost a 50-135/1.7? I bought my Tamron 70-200/2.8 for 650€, and it does wonders on my film camera, but there is no equivalent lens for my K7, and there will never be one...

Only real price/weight difference is in the body...

[Marc Sabatella]

Well, actually, in that case (same pixel density), there IS an advantage to FF, as it will have a lot more pixels (thus a less visible noise) for the same output size.

Yes. I think the best way of summarizing things is this:

The single biggest factor influencing noise is sensor size. The bigger the sensor, the less noise, all else equal.

Pixel density has *some* effect on noise, but nowhere near as much as people commonly think. Part of that impression comes, no doubt, from trying to do comparisons at "100%" rather than using the same output size. It also comes from certain reviewers incorrectly implying/suggesting/claiming that the difference in noise levels between a typical P&S and a typical DSLR was due to the difference in pixel densities as opposed to the difference in sensor size.

So, for a given DOF/FOV, equivalent lenses are not actually much that cheaper/smaller in APS-C, and sometimes do not exist altogether (there is no equivalent to the good old 50/1.4).

Very true, although relevant to the point being discussed above, the one area where the equivalence does sometmes favor APS-C in terms of size and to some extent price is in the telephoto department. Not for all possible combinations of focal length and maximum aperture, but a 135/2.8 is indeed usually smaller and lighter than a 200/4.

[Blue]

The way a lot of people struggle with this is unfortunate and does stem from poorly chosen terminology and perpetuated by the marketing departments from the manufacturers including Pentax.

I have compared the fov or AOV of a focal length on 110, 6x7, 6x6, and 135 lenses before I ever had an aps-c sensor. That's the way I would visualize what focal length was a wide angle, normal and tele lens on the different formats.

Edit: Conversion factor for different formats may have been a better name.