Hi Jack,
I don't agree on this. An MTF measurement involves the whole surface of the lens and almost all lenses improve in MTF when stopped down. On a perfect system maximum MTF is gained at maximum aperture but most systems reach optimum MTF stopped down a bit.
A lens having high MTF will also have high MTF stopped down, well corrected is well corrected, after all. But a lens that is unusable at full aperture can be very good stopped down. Therefore, I would say the stopped down figure is the most important, unless shooting at full aperture.
It can be argued that if one buys an f/1.4 lens he/she pays for that lens to perform at f/1.4.
For me it is a bit interesting. The reason I bought the 90/2.8 G was that I wanted a lens around 85 mm that:
- That was very sharp at f/5.6
- That also was very sharp at maximum aperture
- Had very little axial chromatic aberration
Two lenses that obviously filled the bill were the Zeiss Otus 85/1.4 and the Zeiss 135/2 APO Sonnar, but the Otus was both to large and to expensive and the APO Sonnar is 135 mm, and I have found that 135 mm is something I seldom use. Other options were the Sony 90/2.8G and the Batis 85/1.8. The Sony 90/2.8G produced very good figures in Imatest based testing at Lensrentals and also very good results at DxO-mark. I am not a tester having access to both lenses, needed to buy one.
Now, I am quite happy with my 90/2.8G. It outresolves the A7rII sensor across the field at full aperture and it shows no or little green/magenta fringing on out of focus areas in the images I have shot this far. If this is due to software correction Sony writes very good image.
I include MTF plots for the Hasselblad Distagon 60/3.5 CF, the Hasselblad HC 50/3.5 II and the new Zeiss Loxia 21/2.8. All these are very fine lenses. Distagon 60/3.5 CF is an old design and improves significantly when stopped down. The HC 50/3.5 II is a very good performer even fully open while the new Loxia shows very little improvement when stopping down.
Best regards
Erik
This has been repeated a few times in this thread but I don't believe it applies.
In typical photography and tests, scene-reflected light is assumed to arrive at the camera as a large wavefront perpendicular to the lens, which is then focused by the entire lens surface onto the sensing medium. Larger apertures in this case mean less diffraction but more contributions from more glass, which may mean more aberrations - so at low f-numbers, as aperture is increased performance tends to decrease as a result of the typically larger aberrations.
I have never seen or used an optical bench. However from the images of OLAF I have gleaned it looks to me that it shoots a laser-like beam of light to a single spot on the lens (as opposed to a wavefront) in order to determine something similar to the spot's related Point Spread Function, from which MTF and aberrations can be seen/derived. In other words it measure the performance of the lens one spot at a time, only at that one spot. If this is correct, aperture makes no difference whatsoever to aberrations (they are what they are at that one spot), although diffraction would still affect the results - so performing these tests at maximum aperture would provide the highest figures of merit for the lenses.
So I think Roger's optical bench tests are as objective and accurate as any especially because they are shot with the lens at maximum aperture. On the other hand he himself cautions about the fact that the lens is tested at infinity focus and performance may vary if the subject is closer than that.
Jack