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Author Topic: Focus shift with aperture  (Read 942 times)

Rhossydd

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Focus shift with aperture
« on: June 17, 2013, 06:39:19 AM »

An interesting post about this at:-
http://www.reikan.co.uk/focalweb/index.php/2013/06/lens-focus-shift-reikan-focal/

It will be interesting to see the data produced when this feature is added to Focal and people start wider testing.
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BartvanderWolf

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Re: Focus shift with aperture
« Reply #1 on: June 17, 2013, 08:02:42 AM »

An interesting post about this at:-
http://www.reikan.co.uk/focalweb/index.php/2013/06/lens-focus-shift-reikan-focal/

It will be interesting to see the data produced when this feature is added to Focal and people start wider testing.

Hi,

Yes, interesting, but not always a big issue for many lenses and for subjects that are more 3D in nature. That doesn't mean that one shouldn't consider the effect when dialing in a micro-adjustment for a given lens/body combination. It's part of understanding the behavior of one's tools. It would be a useful addition to the Reikan software.

Another one of the variables in the test procedure is field-curvature, but I assume they tested in the image center only. Also, the color of the subject itself may cause a shift of  the focus plane, due to (Apo-)chromatic lens correction errors.

Another issue may be the color temperature and spectral composition of the light under which the AF test is conducted. In my experience, AF micro-adjustment with tungsten light, can give different results compared to daylight scenarios. That may be caused by a combination of, the lack of color correction of the phase-detect AF sensors, lack of apochromatic correction of the lenses, and field curvature.

In order to accurately test how well the resulting focus at a given aperture corresponds to the real optimal focus position, one can use a focus rail after autofocusing, and a method that gives a readout in physically defined units such as I did here with my optimal Capture sharpening evaluation after closing down the aperture to f/5.6:


When the measured amount of blur is minimized at the AF focus position, then all is fine. It also gives a sense of how far off the focus is, if it's meaningful in real life scenarios, and what is needed (to compensate) in Capture sharpening.

Cheers,
Bart
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