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04-23-2011, 01:46 PM   #1
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Has my DA 40 a front focus?

Dear all,

two days ago I received my brand new DA 40mm Limited. During some test shots I found some images that are not correctly focused. Because I also have a new K-5 (silver) since a few days, I wanted to investigate my problem a bit further (Is it the new camera? Is it the lens? Is it me?). To cut a long story short, I shot some testcharts with by new K5 and my old Kx.

I can still send the Da40 back, so I would appreciate your comments.

Setup:
Used the following chart: http://www.focustestchart.com/focus21.pdf
Camera on a tripod in a 45° angle.
Distance Lenstip <-> Testchart approx. 60cm.
Metering: Spot.
Fokus point: Center
Mirror lockup with 2sec. timer
Light through a window in front of the testchart (no artificial light!)

Tested:
DA40, f2.8 with contrast af (CAF) and phase-AF (PAF)
FA50, f1.4 with mit CAF and PAF
Sigma 17-50 f2.8 at 17 (K5), 35 (K5) und 50mm (Kx+K5) with CAF and PAF.
I focused always on the "Focus here" line.


The pictures are here:

K5: My Art
Kx: My Art

My interpretation:
-Results with my FA50 look always more or less the same.

-Sigma & CAF produces a back focus on my K5.
-Sigma & CAF & Kx -> no problems.
-Sigma & PAF -> results with my K5 and Kx look more or less the same (With Kx there is a slight back focus, but I think it is still "ok").

-Pentax DA40 produces a front focus with both cameras and either with CAF or PAF.

Now I have two questions:

1. Do you agree that DA40 produces a front focus?
I you shoot the testchart in a similar setup, I would expect to see the focus approx. centered around the "Focus here" line. Further, the results of my other lenses are as expected...

2. How comes that I have a backfocus Sigma and CAF on my K5, but PAF works perfectly fine? Is this an incompatibility between Pentax K5 and Sigma?

Happy easter to you all.

Klaus


Last edited by kdre; 04-23-2011 at 02:36 PM.
04-23-2011, 02:16 PM   #2
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I think you should send it in!

Have you tried portrait distance? Try between 2-3 meters on something the AF cant miss.
04-23-2011, 03:17 PM   #3
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Take your shots under different light conditions with different lenses. I don't have the k5 (yet), but DSLRs tend to have problems focusing accurately under tungsten light. Also, I'm guessing the k5 has a submenu somewhere to adjust focus.
04-23-2011, 03:38 PM   #4
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QuoteOriginally posted by asdf Quote
Take your shots under different light conditions with different lenses. I don't have the k5 (yet), but DSLRs tend to have problems focusing accurately under tungsten light. Also, I'm guessing the k5 has a submenu somewhere to adjust focus.
The shots were taken under daylight which came through a window. No artificial light was involved.

AFAIK focus adjustment works only for phase af, not for contrast af. But the pics for contrast & phase af look the same. So, a focus adjustment would probably improve the phase af, but not contrast af.

04-23-2011, 03:45 PM   #5
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QuoteOriginally posted by the swede Quote
I think you should send it in!

Have you tried portrait distance? Try between 2-3 meters on something the AF cant miss.
I did not do testchart shots or similar for this distance. I will try to do it tomorrow during daylight again. But I did normal portrait shots of friends where the focus was sometimes off. Thats why I started my investigation.

However, if it would work for objects in 2-3m distance, but not for objects at 60cm distance, then the lens is still faulty, isn´t it?

Shouldn´t the DOV be more or less equally before and after the focus line (as it is the case for my other lenses) independet of the distance?

Last edited by kdre; 04-23-2011 at 04:04 PM.
04-23-2011, 04:18 PM   #6
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I couldn´t wait until tomorrow and did another qvery quick´n dirty testchart shot. I printed a fat black line on a piece of white paper and layed it on the floor such that the line is horizontally aligned. Next to it, I placed a ruler vertically ("10" was next to the black line). Distance lens<->black line is about 1-2 meters.

Pictures taken with my FA50 f1.4 in an approx. 45° degree angle look good. DOF is approx. equal before and after "10". Exactly what I expected.

Pictures takes with DA40 show the same front focus than before. More numbers before "10" were readable than after "10".

However, this test was really quick´n dirty (thats why I do not post these pics). Did not use a tripod, the room was illuminated by an energy saving lamp and the shots were taken freehand. I will repeat this test tomorrow with a better setup.

But I think it is more or less clear that something is wrong with this particular copy of DA 40. At least I do not feel comfortable with it. Maybe it is better to return it and try another copy.
04-23-2011, 09:35 PM   #7
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It's totally normal that under tungsten light, you'd get slight front focus, at least on the K-x (with K5, that would depend on the firmware, I guess). If you didn't, *then* something might be wrong with the lens. It's also normal that at 40mm, you'd get close to the same amount of DOF behind and in front of the center of the zone (it's a myth that it should always work out to 2/3 behind, 1/3 in front). Your tests, BTW, show the chart not lying flat, and that's contributing to the perception the focus is in front of where you'd want.

It's also pretty unlikely anything in the lens could possibly cause FF or BF. I know people like to blame lenses for FF or BF, but when you think about how an AF system works, you realize it doesn't really make a lot of sense. It's much more likely to be a camera problem, if there is a problem at all. Also, you should run each test several times in a row, starting with the lens focuses in front of and then behind the target, and also with it starting already focused.

Last edited by Marc Sabatella; 04-23-2011 at 09:42 PM.
04-24-2011, 12:07 AM   #8
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I would run the same tests with the chart against a wall (vertical) and the camera (exactly) under 90 degrees.

If they come out right, you're initial test shots (that made you do the test) might have other reasons why they were not sharp.

04-24-2011, 12:30 PM   #9
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QuoteOriginally posted by kdre Quote
I did not do testchart shots or similar for this distance. I will try to do it tomorrow during daylight again. But I did normal portrait shots of friends where the focus was sometimes off. Thats why I started my investigation.

However, if it would work for objects in 2-3m distance, but not for objects at 60cm distance, then the lens is still faulty, isn´t it?

Shouldn´t the DOV be more or less equally before and after the focus line (as it is the case for my other lenses) independet of the distance?
I sugested this to you because, if they find the lens to focus correct in one end, they can write it of as "within factory spec" and you'll get stuck with a lens that good in one end but off in the other end. They can adjust this, but the "anomaly" will move.

Trust me, i've been through that.
04-25-2011, 08:50 AM   #10
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I find the in-camera AF adjustment capability invaluable for getting each of my lenses to be spot on in AF accuracy.

I use the charts often and for some reason, got one of my lenses tuned perfectly to the chart(in outdoor light) and then went out shooting and found everything to consistently off. I fell back on my low tech solution of shooting a target on a mesh grid fence and adjusting to that and now that lens hits AF accurately. I don't know why the paper chart did not work in that case, but i never rely solely on the testcharts anymore, YMMV.
04-25-2011, 10:12 AM   #11
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Hi Klaus,
I did focus adjustments for my K-7 using Yvon Bourque charts: more convenient, IMO.
Also, I did the test shots at or close to the min focusing distance of the lens, which is just over 40cm.
I'd say try, if you can, 2 or 3 other lenses. If they fall, say, within +/- 5, and DA40 is outside +/-10, then get the new DA40.
Good luck.
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