First 28mm f1.8G review up

Started Jun 6, 2012 | Discussions thread
Joseph S Wisniewski
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Flawed assumption about AF...
In reply to harold1968, Jun 7, 2012

harold1968 wrote:

You might want to rethink this part...

  • This poses quite a challenge for people relying on normal AF-operation because AF grabs focus at fully opened aperture (f1.8) and does not compensate for any focus shift.

The AF "grabs focus" at either f2.8 (central AF zones, when an f2.8 or faster lens is used) or f5.6 (outer AF zones, or any lens slower than f2.8). It uses two pretty tightly focused portions of the exit pupil at +/- 5 degrees from perpendicular (f5.6) or +/- 10 degrees (f2.8).

It then does "compensate" for focus shift by applying the lens's default "focus tuning" to get a focus offset for wider apertures, and any correction the user has dialed in, so, depending on how Nikon set the default and how the user adjusted focus, you could be correct at f1.4 and off at 5.6, or off at 1.4 and correct at 5.6. The former is more likely, as Nikon calibrates fast lenses to be correct wide open.

In any case, there will be a focus shift between wide open and stopped down on pretty much every lens. Which brings us to...

  • I've never seen a similar behavior before.

That sort of blows your credibility as a reviewer, because it's a well-documented characteristic of many lenses, seen pretty strongly in all double Guass variations, such as the Nikon 50mm f1.4 AF-D.

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Rahon Klavanian 1912-2008.

Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed.

Ciao! Joseph

http://www.swissarmyfork.com

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