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Brandon Dube wrote:
Thank you for your input. The trouble of using a lower spatial frequency is that it basically 'fattens' the plot and when the lens gets wide enough the DoF will fill the entire range (e.g 11-24 at 11mm).
Yes, but I think you don't always need f/8. F/5.6 will nicely clean up spherical aberration and coma with many lenses.
Anyway, 30 lp/mm is not a bad choice.
I would say strictly speaking this is a measure of MTF vs Field vs Focus (hence the titles on the plots) - but field curvature is the most valid name for it still. Astigmatism is purely the difference between the location of the sagittal and tangential planes, which we aren't measuring (but could compute). Of course we are not measuring petzval.
Let's meet halfway. You measure the curvatures of the sagittal and tangential fields.
The measurement time grows exponentially as one stops down because of the loss of light. A lens with good transmission at f/2 uses about 1/40s shutter time, at f/2.8 it is about 1/20s, at f/4 about 1/10s, f/5.6 1/5s, and so on. A measurement of MTFvsFieldvsFocus requires 441 exposures, not to mention the time for the collimator to swing and so on. f/11 is approximately the border before we seriously approach the noise floor in the sensor, but it hasn't proved too big an issue yet for the stop down data.
A-ha. I did not know that SNR is a serious issue with MTF benches.
Thank you for measuring all these interesting data.
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