How to fully test a 24-70mm f2.8?

Started Jan 18, 2013 | Discussions thread
Leonard Shepherd
Leonard Shepherd Forum Pro • Posts: 13,253
Re: How to fully test a 24-70mm f2.8?

I appreciate some on this forum recommend FoCal. On the other hand I have seen several threads by those who were abandoned using it because of variable results between tests. Why might this be?

The site explanation on how autofocus works is a little outdated in referring to a fixed aperture. Nikon use just three "apertures" for 51 autofocus points. Nikon say the size of each of the three lenses is about the size of a grain of rice. The "aperture concept" that applied to some early autofocus systems would result in apertures too small to transmit enough light for autofocus to work if one aperture smaller than a grain of rice illuminators 15 or more autofocus points, as it does with 51 point Nikon autofocus.

The explanation is silent on the all important detail that a segment of the image is projected on to twoautofocus detection lines, and that if the autofocus system wrongly identifies which part of the image is meant to be at the centre of the viewfinder mark the system can apply a wrong focus correction

This issue is why the autofocus system sometimes decides something in the background away from the centre of the viewfinder mark is the intended subject and focuses at a distance the photographer did not intend.

The FoCal targets seem to fall into categories where Nikon say autofocus may not perform accurately - if testing a D800 see page 100 of the instruction book for more detail as to why.

For those getting variable indications between AF points using FoCal my advice is retest with a better quality target to see if you get a different result. If you get a different (including tests at different times with FoCal) it think it would be unwise to make an adjustment based only on using FoCal.

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Leonard Shepherd
Many problems turn out to be a lack of intimate knowledge of complex modern camera equipment.

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