LensRentals Nikon AF test just posted

Started Sep 14, 2012 | Discussions thread
RCicala
Regular MemberPosts: 109
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Re: LensRentals Nikon AF test just posted
In reply to Leonard Shepherd, Sep 16, 2012

Leonard Shepherd wrote:

Unfortunately there seems no link to the test target - which might be good - or less than good - for either AF system.

All Nikon fine tune body instructions confirm phase detect and contrast detect can be fooled by different types of subject.

A Nikon link about phase detect AF, and quote from it, is

https://nikoneurope-en.custhelp.com/app/answers/detail/a_id/48334/kw/Auto%20focus

Quote “Cameras AF systems require a difference in contrast to achieve focus. Plain subjects or subjects without much detail or a pattern may mean that the camera is unable to focus accurately".

Phase detect works best in single point AF - as the detection lines are shorter with less chance of failing to detect where the intended AF point is.
Contrast detect uses a smaller segment of the image but is less flexible.

The comment some Canon lenses of different focal lengths have different accuracy raises
similar queries.

Whether this is because a constant image magnification was not maintained, the target was less good for maybe Canon wide angles (Nikon explain about this in 14-24 and 16-35 lens instructions) or something else is unclear.

The conclusion may, or may not, be fair. More information would be useful.

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Leonard Shepherd

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

Leonard, that's a very pertinent point and something I should have included in the article. We use an Imatest Focusing Star - basically an 80% / 20% gray target with horizontal, nearly diagonal, and nearly vertical edges about 1.5 inches in diameter.

When we first started testing we ruled out standard stars, checkerboards, hatches, etc. for just the reasons you mention. I can't say for certain these are equal on all cameras, but they certainly seem better than anything else we've tried.

It's kind of a crappy picture, but you can see the focusing chart here: http://store.imatest.com/catalog/product/view/id/219/s/sfrplus-chart-inkjet-on-paper/category/12/ if you pul the slider all the way to large. There's one inside each of the gray boxes.

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