Originally posted by Wheatfield What were your testing methods and test parameters (TOC)?
Originally posted by Gooshin I am eagerly awaiting what Falconeye will say
I feel urged now to respond
Thanks for the interest.
Wheatfield asks a very good question (how measure a lens' resolution beyond the sensor's resolution?) and here is my method:
Assume, you want to measure a lens A: Reverse-mount lens A (with a filter-to-filter-screw adapter) to another lens B with significantly longer focal (say the difference in focal length is a factor M like M=6x) and mount the long lens B to the body as usual. Focus both lenses at infinity. Place a subject with tiny details in front of lens A, at a distance of the focal length of A. Note that, because all lenses are focussed at infinity, both lenses are operated at their sweet spot for resolution. This is not challenging either lens' macro capabilities!
Because everything captured by lens A is magnified by M, the resolution figures for lens B can be worse by a factor M and the quality of lens B doesn't matter as long as if it fully exploits the sensor's capabilities.
Assume A=Zeiss 50/1.4, B=DA*300/4, M=6, K20D (5µm pixel pitch). Then, theoretically, a detail just 0.8µm large should fill one pixel. *IF* lens A has a resolution of 0.8µm. If not, you'll see the real resolution capabilities of lens A in its focal plane (and near the center). This is so because by some fundamental law of optics, light rays are always reversible in direction.
Now, what is the outcome? Well, see at the attached image yourself. It is a 100% crop (so, each pixel corresponds to 0.8µm in the real world). The image was taken at f/4.0. The green lines are 1.5µm wide and 3µm apart and are created by interference of (green) light. Red light interference leads to a somewhat coarser patterns, as expected. I see no blue light pattern, so it either may not have been resolved or it may not have existed in the subject. A 3µm wide line pair corresponds to 333 lp/mm and the contrast still is
very high! This is more or less the diffraction limit at f/4 (which is at 1.29 µm)! Zeiss quotes 300-400 lp/mm for their 35mm primes and I confirm it. In comparison, the K20D sensor requires 100 lp/mm.
BTW, what you see is a part of a bee's eye and you see its many segments reflecting some sun light (the segments are invisible to the naked eye...). The photo is mine.
BTW, 1.5µm are 2 * SQRT(2) * lambda_green and I think the interference is coming from double reflection of light at the gap where two segments meet. But I am not sure. It could be reflection from something
within the eye segments as well...
BTW, if you would like to repeat my measurement: Please take into account that the combo of lens A and B behaves as a lens like 1800mm f/24. So, you basically have NO light and no chance to use long exposure! 1800mm shakes like hell even on a tripod. I had to use a tripod, sunlight, high ISO AND staking many frames together to come up with a result. And did I talk about focussing?
Originally posted by jbinpg Check out this post from Yvon Bourque. I think your statement about all DA lenses not working on FF sensors is incorrect:[/url]
Good link. May I refer to our own information here, too?
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/31629-da-lens-...ts-thread.html
Last edited by falconeye; 06-15-2011 at 05:28 AM.