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12-02-2012, 12:36 AM   #1
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Pentax F 50mm f1,4 focus accuracy

I bought this lens a couple of months ago. I noticed early on the focus accuracy using this lens was unusual. This is my first faster-than-f2.4 AF lens, so I'm wondering, is it because of the über thin DoF, or did I buy the cat in the sack regarding this particular sample?

It doesn't feel like a classic case of FF at all.

Ideas?

12-02-2012, 12:44 AM   #2
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QuoteOriginally posted by Zafar Iqbal Quote
I bought this lens a couple of months ago. I noticed early on the focus accuracy using this lens was unusual. This is my first faster-than-f2.4 AF lens, so I'm wondering, is it because of the über thin DoF, or did I buy the cat in the sack regarding this particular sample?

It doesn't feel like a classic case of FF at all.

Ideas?
Well, if you're shooting wide-open, you can't expect much to be in focus if you're shooting close-ups. Try shooting a test chart to see if the AF needs adjustment - that could well be the case.

I once used my 50mm f1.2 lens and at 1.2 there was so little DOF that I basically haven't shot wide-open with it since (haven't used it at all since then, for that matter).

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12-02-2012, 01:10 AM   #3
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I think the behavior is very weird and I can't make any sense of it.

I have a battery tester laying on the other side of the room, about 2,5 meters away. I've tried 20-25 times and havent had any luck getting it in focus. I always get FF by either 7ish or 15-20ish cm. Not front AND back focus. There's nothing surrounding it that should confuse the AF.

There are some cords about 15cm behind it - if I aim for them I get accurate focus.

I have a large bottle sitting also about 2,5 meters away on another part of the room. I can achieve proper focus on it as well most of the time. Then another but smaller bottle sitting by the window with nothing in front or beside it. Hit ratio is 50% perhaps more. Some times it's a bit off other times it's off by something like 5cm.

I tried aiming flat for my monitor from different distances and it has nailed it each and every time (didn't try too many times though). Farthest distance was 2,5m and even though the lens is rather soft wide open, I can still read the text from this forum on the preview of the photo I took, at the back of the camera.
12-02-2012, 01:15 AM   #4
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Trying it on my second K-30. Same behavior.

Tried again with the battery tester. If I try to focus on the edge closest to the me, the focus lands about 7cm in front of it. If I try to aim for the farthest edge then the actual focus lands even closer - weiiiird.

12-02-2012, 05:51 AM   #5
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In good natural light and with the lens wide open and the camera on a tripod or stable surface, half-press the shutter to focus on a target item using liveview Zoom in if necessary to confirm focus is where you want it. Take the camera out of liveview and half-press the shutter to focus again (no need to take a photo). If the focus ring moves, make a micro adjustment. Keep doing this until there is no focus ring movement when switching from liveview. You should be at least 50 times the focal length of the lens from the subject, so with a 50mm lens make sure you are at least 2.5 meters away.
12-02-2012, 06:53 AM   #6
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Ah yes, I do not know why I forgot about that testing method. I'll try it once I'm home again.
12-02-2012, 10:25 PM   #7
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I believe the FA 50/1.4 with its spherical aberrations is quite challenging for an AF system.

I recommend using a proper focus chart for your AF micro adjustments or at least an unambiguous target that your camera manages to focus on (c.f., my AF adjustment hints).

There is no point in messing up the AF adjustment for a weird target the camera obviously struggles with.
12-03-2012, 03:53 PM   #8
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Something in Zafar's description tells me that this could be an issue of poor collimation (such as could result from a mechanical shock).

Now, focus is determined with the lens wide open and the FA is notoriously weak in contrast at f/1.4. This was one thing that "worried" me when I bought mine, so I took it through some quite rigorous test shootings as soon as I received it. My findings were and are that only in some special low-light situations do I get better results using manual focus. Normally autofocus is fast and reliable as demonsrated in this litte test made tonight with a small doll suspended under a lamp.

Test target shot at f/8, (click to see larger image):



As expected, the target is sharp all over at f/8 as the following details (100% non-resized crops) show:



Shooting at f/1.4 and focusing at the bottom gives this result:



While shooting at f/1.4 and focusing at the head gives this:



This is just one example. As said, I may get poor autofocus once in a while, but never in fair lightning situations. So, I think this lens works as one could reasonably expect and quite differently from what Zafar has experienced.

12-03-2012, 11:58 PM   #9
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Thanks for the inputs. I have not had time yet to do proposed chart tests, but I did try something quick.

Cam mounted on tripod and shot @ f/2. I was aiming for Pentax logo on my dummy K-x model. I do not know why, perhaps I was influenced by some of the writing in the suggested test approach, but I tried several times first by focusing from min. focus distance to target and then from infinity to target.

Min. to target: Didn't do the math but I'd say 80% or slightly more were spot on.

Infinity to target: The results reversed. 20% or less were spot on (felt like lesser than 20%).

The misses from the minimum focusing distance to target were closer than from infinity to target. This fit's well with earlier finding/impression about the focus either being this much or that much off. I always only had FF going on but once again, I do not believe this to be a classic FF issue.
05-13-2013, 04:35 AM   #10
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Is there any chance the lens can be improved, either on my own (yeah, right) by disassembling it, or by sending it in for repairs?
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