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11-21-2010, 10:15 PM   #1
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AF Fine Tune inconsistent on the Pentax AF 50mm f/1.4 for you too?

Coming from the K10D I just got the K-5 and am loving the AF Fine tune (am loving a lot actually ). I just ran through all my AF lenses and a few MF lenses. I found a very peculiar thing when tuning my AF 50mm f/1.4. As I stopped down from f/1.4 to f/2.8 to f/4 the focus point would move forward. So I couldn't dial in one number to offset focus and have it be reliable across the lower f stops.

So I ended up tuning to f/2.8. The center of the focus region at f/1.4 is ever so slightly behind the focus point (while still reaching to the focus point), and at f/4 the wider critical focus region averages slightly in-front of the focus point (where the actual focus point still falls within the critical focus region.)

Now this may not be news at all and may have been widely discussed and known to many when the K-7 came out. If so I never read it; I'm sorry for my ignorance if so. But maybe not. In any case, is this something others see as well?

11-21-2010, 10:43 PM   #2
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By coincidence I was tuning the AF on a FA50/1.7 tonight and noticed the same problem going from f2 to f4. Like you, I picked f2.8 and will try to make that work. It seems to be one of the never discussed issues as i don't recall threads covering this. Anyway, you are not alone. How in the world did the real photogs handle poorly aligned AF in the "olden" days other than memorizing the characteristics of each lens?
11-22-2010, 01:02 AM   #3
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QuoteOriginally posted by imtheguy Quote
By coincidence I was tuning the AF on a FA50/1.7 tonight and noticed the same problem going from f2 to f4. Like you, I picked f2.8 and will try to make that work. It seems to be one of the never discussed issues as i don't recall threads covering this. Anyway, you are not alone. How in the world did the real photogs handle poorly aligned AF in the "olden" days other than memorizing the characteristics of each lens?
Another coincidence: I was playing with my FA50 1.7 this morning, pointing it out of the window at the view, and noticed that it shifted focus quite noticeably between aperture settings pretty much in the range you mention. (Wide open it tends to back-focus ever-so-slightly on my K-x. Unfortunately on the K-x you can't adjust for individual lenses.)

On my old K10D the DA21 used to shift between f/4 and f/5.6. It seems quite a common thing.
11-22-2010, 02:14 AM   #4
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The answer might be here.
diglloyd: A Brief Introduction to Focus Shift

What I have found is that the Pentax AF actually compensates this if you choose the desire f-stop before AF. I didn't notice this until I got the DA14 which would AF closer at f8 than wide open. I did some tests which also indicated images were sharper if AF & shot at f8 than AF wide open but shot at f8.


Last edited by wlachan; 11-22-2010 at 02:24 AM.
11-22-2010, 08:28 AM   #5
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Fine article Alan. Thanks for jumping in.
11-22-2010, 08:30 AM   #6
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Well written article. If I understood correctly, the writer is saying to use the aperture ring and physically stop down? ... I guess that makes sense. But some 'modrun' lenses don't have one, and the subject's lighting may not lend itself to the camera auto focusing on it when the lens is physically stopped down to f/5.6 or smaller. But true, it's an option.
11-22-2010, 09:25 AM   #7
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Focus shift means that the point of focus depends on the aperture. This effect is especially noticeable with fast lenses, esp. the 50/1.2.
The problem now is that the AF measurement itself uses light only from about f5.6. So even if the lens is fully open, it does not use all the light. Thus it is setting the focus to a point that is correct for f5.6. Your shots at f1.2 will be all soft.
The same is true for the split image screen. The prisms also are designed to use light from about f5.6. (There are some special versions though for faster lenses.)

The only way to overcome this problem is to use Liveview.

check this important article:
http://dougkerr.net/pumpkin/articles/Split_Prism.pdf
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