MTF Curves: Theoretical Versus Actual
All camera companies (with the exception of Sigma and Leica) publish MTF curves for their lenses that are “theoretical” and not actually measured. Should you care?
Personally, I believe in the old President Reagan saying “trust but verify”. What follows is a dose of reality, compared to theory. I have chosen what most people would agree are among Nikon’s best pro lenses for this study, lest I get accused of measuring lenses that were manufactured using lesser standards.
The MTF curves I’m referring to are the traditional mix of MTF10 (contrast) and MTF30 (sharpness). I used the “mtfmapper” software version 0.5.8 to create the following charts. Personally, I place much more stock in the 2-dimensional MTF50 plots that measure the whole camera sensor. Unfortunately, getting 2-D MTF50 plots is hard to come by outside of this site.
The MTF charts are traditionally generated for a wide-open aperture, so that’s how mine are measured. It’s unknown what focus distance is used by Nikon; mine will be measured at the distance needed to photograph an “A0” resolution chart filling the frame.
I took the measurements in shade on a clear sunny day. Light wavelengths can affect measurements; I like to test using the same lighting conditions that I normally shoot.
105mm f/2.8G ED‑IF AF‑S VR Micro Nikkor
This lens is supposed to be optimized for “close” distances, but I’m measuring it at a more conventional distance.
Nikon Theoretical Chart (from Nikon site) for 105mm
Measured MTF10 and MTF30 for 105mm at f/2.8
I don’t want to appear cynical, but I was 99% sure that my measurements would show less sweetness and light than the Nikon claims. This is pretty much borne out by the measurements.
Take a look at the edge of the lens, though. It actually performs better than theoretical!
Measured MTF10 and MTF30 for 85mm at f/1.4
Again, not quite as good as theoretical. The edges have a few pleasant surprises, however.
85mm at f/4.0
Just for fun, I tried an f/4.0 test. It really cranks up the quality, doesn’t it?
24-70mm f/2.8E ED VR AF-S Nikkor
The wide end of this lens looks dramatically different than theoretical.
Again, this lens at 70mm looks quite a bit different than the claims.
It appears that Reagan had some good advice. Bear in mind that these lenses don’t represent the whole population; your mileage may vary.
My biggest surprise is that the FX frame edges fared better than expected.
Trust but verify.