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06-30-2009, 01:40 PM   #1
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FA 50/1.4 - Front focus up to f/4?

I took my K200D with FA50/1.4 to a wedding of my friends to take some unofficial photos. Back home I realized that most of the images were out of focus, and objects a bit closer to the camera were perfectly in focus. Most of the images were taken with large apertures (f/2) at iso 1600 under dimmed tungsten lighting.

Once back home, i performed the test in the Nikon D70 Focus Chart, which gave some interesting results. Light was tungsten, but far brighter than in the original situation.

from f/1.4 up to f/2.8 there was noticeable front focusing. At f/4 the focus is on target and the depth of field is perfectly symmetrical.

Now I'm perplexed. Lens problem, camera problem, combination of lens and camera problem? Autofocus not functioning well?

Anyone with similar experiences?

I would appreciate any info.

06-30-2009, 02:01 PM   #2
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Try redoing the test in daylight.
06-30-2009, 10:26 PM   #3
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I've found that the AF with this lens on my K20D is finicky as well; I've experienced much front-focus. In fact, I tried AF with it for only a short time and found it too hit-and-miss and so I only MF with it now. I've never had this problem with my DA 35, and rarely with my DA 12-24. And yes, I tried doing the AF adjustment to no avail. By the way, I've found this problem in all lighting situations.

I also never use this lens wider than f/2.8 because I've had such unsatisfying results, which actually makes me regret getting it in the first place - I mean, what's the point of f1.4 if everything from there to f/2.8 looks crappy? I'm pretty sure the photographer is just fine, and I know the camera is, and even after sending in this lens with my K20D when it got repaired a few months back, the technician said nothing was wrong with the lens, either. Go figure?
07-01-2009, 07:22 AM   #4
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QuoteOriginally posted by Mirek Quote
Anyone with similar experiences?

I would appreciate any info.
It's not the camera or the lens, IME. It's the focussing system. Apparently not just Pentax, but other brands have this problem. Here's a good thread for you to look through. https://www.pentaxforums.com/forums/pentax-dslr-discussion/42773-front-focusing-done-death.html

Here's what I had to say in the thread:

After reading this thread, I spent a couple of hours with the DIY Nikon focus chart. I noted no significant issues with front or rear focus in daylight across four lenses and two cameras. I found definite front-focussing issues under tungsten light. Conclusions:

1. I did my tests under two separate tungsten lamps, one a trilight. I concluded that front focussing is related to lighting wavelength, obviously, but not to lighting level. I did not test with fluorescent light.

2. Front focussing was present equally whether I used MF or AF.

3. Front focussing is directly related to aperture settings. I had no problem with any lens on any camera under any light at F4. At F2.8, I had slight front-focussing under tungsten light. At F1.4, I experienced severe front focussing.

My final conclusion after all this mucking around is that I am going to get much more familiar with my Sigma flash, Omni-bounce, Puffer, umbrella, and the K20's wireless flash mode. These tests will also influence my future lens selections. The allure of super wide apertures is much diminished for me.


07-01-2009, 07:38 AM   #5
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In the beginning I also thought my FA 50 was too soft wide open but turned out that it was back focusing (or front focusing, whatever) and after ajusting it in the K20's menu it's now good - not pin sharp as the limiteds but definitely acceptable.

People have to keep in mind that 50/1.4 is very shallow DOF and ANY mistake in focusing will result in a soft image. If the photographer is not careful a tiny bit of movement between focusing and shooting is enough to throw shapness out of the window.
07-01-2009, 01:25 PM   #6
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QuoteOriginally posted by kristoffon Quote
In the beginning I also thought my FA 50 was too soft wide open but turned out that it was back focusing (or front focusing, whatever) and after ajusting it in the K20's menu it's now good - not pin sharp as the limiteds but definitely acceptable.

People have to keep in mind that 50/1.4 is very shallow DOF and ANY mistake in focusing will result in a soft image. If the photographer is not careful a tiny bit of movement between focusing and shooting is enough to throw shapness out of the window.
The problem is that the front focus only happens in tungsten light and only at apertures below F4. If I adjust so the camera gets it right in Tungsten light at F1.4, then it's off for natural or fluorescent light and wider apertures.

The misfocussing I saw was not due to shallow DOF. I did a very careful setup with a tripod and 2s delay and a chart. In natural light I experienced no frontfocussing with the same setup, the only difference being the light source.
07-01-2009, 06:49 PM   #7
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QuoteOriginally posted by audiobomber Quote
The problem is that the front focus only happens in tungsten light and only at apertures below F4. If I adjust so the camera gets it right in Tungsten light at F1.4, then it's off for natural or fluorescent light and wider apertures.
Then it's not a lens problem. It's the body's behaviour. The different wavelenght will hit the autofocus prisms differently and result in mis-focus. AFAIK this affects all brands, all bodies, I don't know of any that's advertised as not having this "feature". Maybe the top-of-line pro bodies from Canon and Nikon have a fix for that, the aps-c certainly don't.

So, learn to live with it.

Just to nitpick: it happens in all apertures, at f/4 DOF gets deep enough you don't notice anymore.
07-01-2009, 07:40 PM   #8
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QuoteOriginally posted by kristoffon Quote
AFAIK this affects all brands, all bodies
Certainly it's reported to happen with most brands (not sure about all), but it's not common knowledge as far as I can tell.

QuoteQuote:
Just to nitpick: it happens in all apertures, at f/4 DOF gets deep enough you don't notice anymore.
The reason I didn't think it was happening at F4 is because the DOF was symmetrical at F4 and above (with the chart at a 45degree angle). At apertures wider than F4, the focus was not centered.

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