Originally posted by cali92rs You cant compare a Q lens to one designed for much larger format...the larger the format, the larger the f number it will begin to show the effects of diffraction.
You could compare the Q 8.5/1.9 and the Samyang 10/2.8 adapted to the Q.
Diffraction effects ultimately depend on the size of the aperture blades' opening.
What you are describing is more a function of different "equivalent" focal lengths,
and different enlargement ratios for a given print size.
Originally posted by cali92rs No lens designed for APS -C is going to show diffraction at f4 beyond a very negligible amount
Look at the Photozone test of the Sigma 8-16 at 8mm:
Sigma AF 8-16mm f/4.5-5.6 DC HSM (DX) - Review / Lab Test Report - Analysis
Diffraction is already degrading the central image noticeably between f/4.5 and f/5.6,
from 2850 to 2675 lw/ph on APS-C (16MP).