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03-02-2012, 09:55 AM   #1
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Another question on FF or BF

Hi
Hopefully this is a new question (quite theoric).
I learned how to use a focus test chart to adjust ff or bf. I print this piece of paper, flat on a table, camera at 45 degree above it, flash, tripod etc. OK.
But the issue is that I must use as much aperture as possible right? so the DoF is limited and only few lines before and after the central focus point are in focus. If I use f8 all the paper is in focus. Also I need to use a certain focal lenght because say I have a 17-50mm, only with 50mm I see only the piece of paper and not all the table.
So I identify the correction in these conditions, say 50mm and f2.8 and I adjust my camera settings until is perfect.
But then I go out and shot all other apertures and focal lenghts and distances from the subject.
The question is if my correction is still valid having completely different conditions? I am afraid that I introduce a problem rather that fixing it with the correction. How do I know that is the best correction also in all the other conditions? (of course I can shot to a panorama with different corrections, but then why to use a focus test chart?).
thanks for advice
Riccardo

03-02-2012, 10:48 AM   #2
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Lots to consider when dealing with zooms.

First, check and adjust at the focal length you use most. Second is to check at extremes plus the middle, ie 17, 50 and around 30-35. Third is only check at the long end - 50 - because the dof at the short end should be suffficient to cover any issues there.

Next, what distance to test at? One school of thought says minimum of 25x focal length, maximum of 50x. Others say check at the distance you'll focus at most often/matchs your subject.

Then, use a flat target or 45 degrees? Text or objects?

Last, adjust at maximum aperture = smalllest dof, or at lens optimum aperture = f4>f8?

Whatever method you use, however, ensure you (a) are in in the light you expect to shoot in, (b) use a tripod, (c) don't use SR, (d) use a timer or remote release, and (e) have lots of time and patience.

Personally, I found the following method good (courtesy of a guy called Richard Day, who used to shoot Pentax but defected to the dark side);

I setup using a near infinity subject like a car number plate and working in 5 unit steps from -10 to +10, I review the results and work around the best in 1 unit steps. I always recheck the AF from both directions to ascertain there is minimal backlash.

If doing it for a zoom lens I check at the wide, mid and long ends.

I then repeat the above for a mid distance and then a near distance (approx 2m).

I generally find that the results are pretty close and adopt a nominal best setting. Critical focus is fairly easy to see as the text usually becomes very dense and sharp. When OOF, the text tends to loose contrast and small halos appear around the edges. I usually review my results on the rear screen at x11 setting (which is approx 1:1 on a K20D)

When I check my setting at macro distances, i.e. the closest focus point, using a test chart, I usually find that there is more DOF behind the focus point than in front (but the focus point is sharp). If I set for equal balance, I find critical focus on most objects at further distances become more OOF as you approach distance/infinity objects.

I've heard it stated by ex-Pentax engineers, that Pentax lenses are designed to perform in this manner as well as many others.


There's a useful 'sticky' covering AF adjustment somewhere on this Forum.

Good luck. It's worth doing

Last edited by JohnX; 03-02-2012 at 10:53 AM.
03-02-2012, 11:01 AM   #3
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Original Poster
Hi John,
thx for this, new and useful information for me. For example that is normal to see "more DOF behind the focus point than in front (but the focus point is sharp).". This is what I sow and so started compensating until it was right in the middle, but then in real life it seems a little bit out of focus.
Also, I forgot to disable SR (although I think is automatic when using remote or 2sec delay on the tripod, at least on my K5).
Finally, that is better to do test in the real life than on a test chart, and that the results should more or less be the same. If I had to think that a setting ok at 2mt is not ok at 10mt then all this would be useles, but it seems is not the case (hopefully). Will work on it....

BTW this triggers another question I forgot to ask. Is there any way to see on the PC which correction to AF was set on the camera for each single picture?
I go around, do all my tests with + or - 3, 5, 8 etc. then go home and I forgot the sequences I used. I don't find this info stored anywhere in the Exif etc. Do you know how to find it? Or shal I use pen and paper :-(


thx
03-02-2012, 03:29 PM   #4
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I've checked the exif in shots I used recently to set up a macro lens, and unfortunately I couldn't see anything relating to AF adjustment.

I just made a note of the image number against the AF setting. It should be a very systematic exercise, ie start with a shot at +10, then +5, 0, -5 -10. Pick the shot that looks the best, say -5, then take shots at, say, -2,-3,-4,-5,-6,-7 and -8, then select the very best.

I used my PC to review the shots, but don't forget you could use the monitor on the camera together with the image enlargement function on the rear dial.

03-03-2012, 03:44 AM   #5
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QuoteOriginally posted by JohnX Quote
I've heard it stated by ex-Pentax engineers, that Pentax lenses are designed to perform in this manner as well as many others.[/I]
I've never read this before.

Theoretically a lens should have a 50:50 distribution of DOF when close focusing. The further you move away from the subject, the larger the back part of the DOF area becomes.

Can you quote the Pentax engineer?

N.B., I've put together some AF adjustment hints. It is not important to adhere to a 45 degree angle, unless you use a special chart and want to use it to read the absolute amount of misfocus. If you are just concerned about getting the adjustment right, any angle will do.

Personally, I'm a proponent of making sure your adjustments work in the real world with 3D objects and the subject distances and zoom settings you are going to use. No point in getting it right in an artificial situation but not get the camera to perform when it counts.
03-03-2012, 04:41 AM   #6
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QuoteOriginally posted by JohnX Quote
I've checked the exif in shots I used recently to set up a macro lens, and unfortunately I couldn't see anything relating to AF adjustment.
Google for "PHOTOME" free software, install, then load a picture and search (Ctrl-F) for: "AF adj"
03-03-2012, 06:57 AM   #7
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samandal, thanks lot!!
This works perfectly and also looks like a great tool.... now I will play with it a little bit ;-)
Not sure if this is a well known tool in this community, maybe could be published in the link section or similar...
Bye
03-03-2012, 07:39 AM   #8
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Hey Samandal,
I am now looking at the info for few pictures... very interesting, there is a section on AF with field names such as AF defocus, AF predictor, AF points in focus etc.
Any idea if these can be used to understand more if you have a ff or bf issue? I mean, sometime the picture is not in focus, but you never know if you moved, or the subject moved, or were using a speed not correct etc. or maybe is a real ff or bf issue. Reading these values I suspect you can compare good and bad pictures and maybe understand more. Maybe someone studied this topic and has some tought? I wonder if AF defocus is something that pentax can set in fw and what is used for, for example. It is not the same in all pictures. Anyway... nice sw.
thx

03-03-2012, 10:18 AM   #9
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QuoteOriginally posted by Class A Quote
I've never read this before.


Can you quote the Pentax engineer? .
I can't I'm afraid, as it was someone elses quote, but it comes from an ex-Pentaxian called Richard Day. He bought into either Nikon or Canon and I have seem him post on other forums, but can't remember which one(s).
03-03-2012, 10:24 AM   #10
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QuoteOriginally posted by quintinir Quote
Hey Samandal,
Any idea if these can be used to understand more if you have a ff or bf issue?

Anyway... nice sw.
thx
Sorry, I don't know (yet) - I'm a beginner
I've used it only to take note of the values of AF Adj. while checking FF/BF on my brand new k5.
You're welcome.
Ciao paisą

Last edited by samandal; 03-03-2012 at 10:34 AM.
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