teddan
04-08-2009, 06:22 PM
When I look at the ISO 12233 Chart of the Canon EF 35 F2 taken with 1Ds Mark II , it seems sharper on f11 on the very edge, than f8. is this correct?
Daniel Browning
04-08-2009, 07:30 PM
When I look at the ISO 12233 Chart of the Canon EF 35 F2 taken with 1Ds Mark II , it seems sharper on f11 on the very edge, than f8. is this correct?
For the benefit of the reader: 35mm f/2 1Ds2 comparison ("http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=122&Camera=9&Sample=0&FLI=0&API=5& LensComp=122&CameraComp=9&SampleComp=0&FLIComp=0&A PIComp=6).
I, too, see that f/11 is sharper on the edge.
Can a lens be sharper at f11 than f8?
Yes, it is possible. Stopping down always increases diffraction (by the same amount for every lens). It *usually* decreases aberrations. If the decrease in aberrations going from f/8 to f/11 is greater than the increase in diffraction, then the results will be as we see in this comparison. The difference in the effect of diffraction between f/8 and f/11 on the 1Ds2 is so small that it's below the threshold of detection, because the pixels are too large to notice the difference. (The diffraction effect is still there, it's just at a higher spatial frequency than the camera is capable of sampling.) One that camera, one must stop down to f/16 before the diffraction comes down to the level where even the low-resolution 1Ds2 pixels can start to pick it up. Other cameras, such as the 1Ds3, sample higher spatial frequencies, and are therefore capable of recording the effect of diffraction at wider f-numbers.
There are also several other possible explanations for the difference in the above comparison,.