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09-21-2008, 03:17 PM   #1
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AF Microadjustment, for primes only? (OR more: "Better for primes?")

It seems with zooms, it looks good on a chart, when i take the stuff outside, i am either front or back focusing and i can't seem to find a pattern.
Of course, one thing i have not done yet is to take the focus chart into the sun, so i either have indoors indirect lighting or worse synthetic lighting.

I guess i am wondering if my camera would yield more consistent results with primes if they were adjusted compared to a zoom which is never used at the same focal length.

I am right now tempted to not sell one but both 2.8 zooms and get one long super zoom for whenever, and primes to work with. (3 out of 21/3.2, 40/2.8, 70/2.4, 35/2, 50/1.4 (Or one of 31/1.8 , 43/1.9, 77/1.8 + one of 21/3.2, 40/2.8, 70/2.4, 35/2, 50/1.4 ))

Basically i am wondering if people who actually have a budget larger than mine can attest to adjustments allowing for more consistency on primes than zooms.

Thanks for shedding a little light,

Daniel

09-22-2008, 12:44 PM   #2
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Looks like i have to answer this one myself once AF primes come in.
09-22-2008, 01:21 PM   #3
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I think many might disagree with me but I think trying to calibrate AF using a chart is a mistake. The problem is not the chart itself, but the distance. Back in the manual focus era, focus calibration was done using virtual infinity target (they had a long piece of instrument for this purpose). For AF, what I have found after calibrating my own DS is that AF accuracy is better if done using near infinity target. For example, when I was done calibrating using a 1 metre target and everything seem fine, focus was obviously wrong when tested with distant well defined subject. So I grabbed my FA43 and took a series of shots wide open on tripod with different um in the debug menu, with 2 shots at least for each um value, then inspect the RAW files on computer at 100% without sharpening. I picked the sharpest few and redo the tests until I found the sharpest setting in most cases. Now the interesting thing is, once AF is accurate with distant target, it is accurate for 1 metre too (but vise verse was not true).

Another thing is that because Pentax DSLRs have wide area AF sensors, they can be easily confused as the subject gets smaller, the lens goes wider, or the distance gets greater (same really). AF error might not be an accuracy issue, it just got confused. For this reason, when testing AF, especially with small or distant target, make sure you try it on different targets, not just one or similar targets before drawing the conclusion. Sometimes what we believed to be good target might confuse the camera.

Last edited by wlachan; 09-22-2008 at 01:34 PM.
09-22-2008, 01:27 PM   #4
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I may have fallen for the wrong technique, i will recheck just judging objects taken at a distance and then test focus closer.
I am very tempted to believe this for right now.

Thanks for nudging me into the right/a new direction.

10-02-2008, 10:36 AM   #5
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Below is Pentax's answer to my question, which is included under their reply.

"Dear Sir/Madam,

Thank you for contacting Pentax.

If you suspect a particular lens is showing front or back focus, it is a good idea (if you haven’t already) to download a focus test chart (http://focustestchart.com/focus21.pdf) and shoot a few tests before sending the lens in to be checked out. Also, if you do decide to send the lens in to our shop for evaluation, it is very important to include the camera body as well (since any lens-specific focus adjustment information is stored in the camera body).

The following focus adjustment instructions are based on input from the Pentax engineering department, and our own experiences in house.

1. Use a focus test chart such as Tim Jackson’s “focustest” chart. Do not use batteries or film canisters set up at an angle. The curved surfaces don't allow for accuracy.

2. Make sure the chart is flat and that you are shooting the chart at a 45-degree angle (or close to it).

3. Use back-lit daylight or tungsten light for the best results.

4. Use a tripod. This will prevent any error due to camera movement.

5. Position the camera approximately 1.5 meters (5 feet) from the test chart. (This may vary of course depending on the lens focal length and minimum focus distance.

6. Set the camera to aperture priority or manual exposure mode and set the lens to its maximum aperture.

7.Shoot the first test shot using manual focus. This is important to establish a baseline for judging back or front focus in regards to the normal depth-of-field of the tested lens at a particular focus distance and f/stop.

****IMPORTANT: Lenses will typically have more depth of field behind the focus point than in front. This is true for any lens from any manufacturer, and is the basis of "hyper focal" charts used in landscape and macro photography. The first shot manually focused will establish a baseline for the particular lens in use****

8. For zoom lenses, set the lens at a middle zoom position or at the zoom position where front or back focus is suspected. Note: it is not possible to save multiple custom focus adjustments for different focal lengths of a zoom lens. Only 1 adjustment per lens can be saved.

In response to the question about what distance to shoot at, really anything over 5 feet is fine but you do not want to be in closer then that.

If you are in need of further assistance, please respond to this email or call our technical support center at 800-877-0155.

If you would like to participate in our new Pentax Survey, please call 1-800-350-3891.

Sincerely,

Pentax Imaging Technical Support

Your Email:
Inconsistent AF on K20D.

Only the center AF point is used in AF-S.

The camera seems to generally tend to "front-focus" when using a new lens that does not have any sort of adjusment in the camera stored for it.

The camera allows micro AF adjustments, here is where your info comes in.
To assist in adjusting the focus on my lenses i use focus test charts, you shoot a target at a 45 deg angle to be able to judge the amount of front or back focus.
Once a satisfactory adjustment seems to be found, i take the camera on a shoot and the actual shots seem to be either still front focusing or even back focused.
The amount of adjustment needed seems to vary.

I need to know if the use of a focus test chart at rather close distances is the recommended method to adjust focusing on the K20D with Pentax
lenses.
Or if you would prefer that adjustments are made at distant objects at infinity focus.
Further on a zoom lens, do you recommend making adjustments at the
"wide end", the "middle" or the "long end"?
Thank you in advance for your assistance, i am hoping the info you provide can help me achieve more consistent results, without the need to send in the camera with lenses for factory adjustments.

The lenses used are/were DA* 16-50 f/2.8, DA* 50-135 f/2.8, DA 12-24 f/4.
Thank you in advance for your time,

Daniel Goller"
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