1.6 crop IQ tests don't look as sharp compared to full frame, why?

mordor_74 wrote:

I am a newcomer here, but based on my reading:

1) generally speaking not all FF lenses permorm well on a crop sensor. the crop sensor uses only the central part of the lens, so the FF lens must have a very good center resolution to perform well on a crop. On the other side corner to corner the performance tends to be consistent. A good example is my 40mm STM (i know, not in the focal range we are speaking), not extremely sharp but very consistent across the frame already at f2.8

Only part I disagree with in 1 is the sharpness of the 40mm, it's pretty sharp, unless you meant just not sharp in the company of lenses like the 300 f/2.8 and 85 Zeiss Otus, etc... It beats most other lenses at its price point by miles.

2) after we pass 250mm and a certain distance, we do not have only the sensor noise, but also light noise become evident (there is a dpreview article that talks about that kind of noise)

I think you're mixed up here.  That isn't because of the focal length at all.  You always have photon shot noise.  A smaller sensor for the same exposure receives less total light and so has more photon shot noise.  This shouldn't hurt the sharpness per se though, but I guess it does as higher ISO noise reduction turns things to mush.  You may be thinking of degradations from the atmosphere when shooting at very long distances, which can be made even worse by anything else in the air, smog, pollen, high humidity, haze, rising heat causing shimmer.  Even though these long focal lengths may be used under these conditions in practice, the testing is all conducted at very modest distances where these don't really come into play, so they aren't paying any additional penalty in testing for that.

So i think that the performance of a lens on different sensor size should not be compared at the same focal lenght but at the same equivalent focal lenght.

In terms of framing?  In that sense I believe they already are.

We should compare the performance of 300mm in FF with 187mm on crop (same distance etc) or compare the long end 300mm of the lens on a crop with a 480mm on a FF (again, a more similar effort) and there evaluate if the difference of performance (i'd expect that the crop to be less good) and price (i expect the 480mm FF lens to be a lot more pricy) and make your mind if can live with the cropped result or you need the better solution.

.... to what end?  We already have compensated for focal length by changing our shooting distance to frame the shot the same way.

This is how understand this matter so far...