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11-29-2012, 03:05 PM   #1
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focus problem (is it my eyes?)

I'm having focus problems. I'm not sure if it's my eyes, or the behaviour of the camera, or the lens or both. I am using a Pentax K-x, and I have a manual zoom (old Takumar A 70-200) that when I use it, often the subjects are out of focus once I upload them and can view the photos on the computer. However, when I'm taking the photo, in the viewfinder the subject appears to be in focus, and if I'm unsure, then I wait until I see the tiny green focus indicator light up (bottom right of viewfinder). The other thing I'm not sure about is how I have my auto-focus set up, and perhaps what the camera is actually focusing on, is not the part of the image that I want in focus. I'm really not sure if using the 5-point, or the 11-point is better, or if I should use the other option where I can just choose the part of the frame I want in focus by selecting (while in the focus settings). Sometimes this same problem occurs even with my kit lens (DA L 18-55) using auto focus - subjects that I am focusing on, that I believe are in focus, are actually not very sharp once I upload the image into my computer and can view it full size.

I appreciate any help or advice that can be given.

Thanks!

11-29-2012, 03:09 PM   #2
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For exact autofocus, I would select the point I want. Preferably the center point. The size of the AF sensor is about the size of the little circle in your viewfinder. If you have multiple subjects within that circle, it can be a crapshoot for the camera to determine which object to focus on.

I find that 11 point is a bit better for sports, but can be very unintelligent when taking static objects.
11-29-2012, 03:22 PM   #3
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If it looks in focus in the viewfinder, but your pictures are out of focus, you may need to shim your focus screen. For your viewfinder to be accurate, the distance from the lens to the screen must be exactly the same as from the lens to the sensor. Check the diagram below. L1 must equal L2
For auto focus to be accurate, L2 must equal L3

11-29-2012, 03:26 PM   #4
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For manual focus make sure the diopter is set correctly. The manual focus system uses the focusing screen rather than the AF sensor so it is a completely separate system. It relies on the focusing screen being properly shimmed and the diopter being correctly adjusted to your eyes.

The AF indicator will come on when manually focusing but it is not particularly accurate especially with fast glass. Try moving the focus ring slowly back and forth through the focus range and you will usually see that on different lenses one side of the range is more correct. Start farther away and move into focus, as soon as the light comes on take an image. Then do the same thing but start close and move into focus, take the shot as soon as the light comes on. Check to see if either of those shots is better than the other.

On AF the AF 'points' are not really points but areas. Anything in that area that the camera decides is the most contrasty will be what it locks onto. So if you are far enough away that the subject's face fills the AF point then anything on their face could be what is focused on. Possibly the tip of the nose or an ear, or part of their hair, and the rest of the face might be OOF. I get the best success using single center point mode. Focus using the center point and then re-compose without letting the focus lock release. I don't use the 5 point or 11 point modes because on the k-x you really have no idea what the camera decided to focus on.

The other focusing system on the camera is Live View. This is often the most accurate but takes too long for most action shots. It does make a good test to compare the other systems with though. Set the camera on a tripod and focus on a target using live view. Then use the manual system and then the AF system. Compare the images and see which one is in focus. The Live View image should be the most accurate and if the other systems are properly tuned they should be identical to the Live View image.

11-29-2012, 04:43 PM   #5
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QuoteOriginally posted by johnyates Quote
If it looks in focus in the viewfinder, but your pictures are out of focus, you may need to shim your focus screen. For your viewfinder to be accurate, the distance from the lens to the screen must be exactly the same as from the lens to the sensor. Check the diagram below. L1 must equal L2
For auto focus to be accurate, L2 must equal L3
I don't think it's the focus screen, as parts of the photo are very much in focus, and very sharp. Sometimes it's not the part that I want in focus though - example a subject's face - but their hair or other things close by like a tree branch or plants will be very much in focus.
11-29-2012, 05:05 PM   #6
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I see in the avatar you wear glasses (if that is you in the picture). Do you keep them on when you shoot? I had a friend way back when in the film days who was about to give up his SLR, because the pictures were always out of focus. I went with him once and watched as he pushed his glasses up and used his unaided eye to focus.
12-03-2012, 09:29 PM   #7
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I recently purchased the magnifying eye cup O-M E53 for my K-r and I am much happier now, at least with the larger view in my viewfinder.
12-04-2012, 05:38 AM   #8
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The viewfinders in today's Pentax dSLRs aren't designed for focusing, but for giving a clear view for a camera designed to autofocus. I have no trouble focusing manual lenses on my Pentax film cameras, but the same lenses on my K-5 are very "iffy" because the design of the screen makes the focus point uncertain, especially with fast lenses. I've run enough tests on tripod with focusing targets to be sure the screen is aligned properly and gives correct focus if you hit it right, but there's quite a range of "almost in focus" that looks the same in the viewfinder. This is one area Pentax could improve a lot.
I recently tried a Nikon D600 with old Nikon manual lenses, and I must say they did a much better job for manual focus. I hit every shot with the D600. (Saving up now...)

12-04-2012, 05:39 AM   #9
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My K-x seemed to have pretty good auto focus, and focus confirmation using my manual lenses was excellent. But the K-x doesn't tell you where it's focusing. If you're using 5 or 11 spot you don't know which one of those spots it picks. I set mine to center spot and got very good results.

You can check it with a tripod and a stationary subject. Set it up and focus as is, take a couple of shots. Then set it to center spot and try again, and try to get a good look at where it's focusing in the viewfinder each time. But use a good stationary subject and tripod to eliminate camera or subject motion. You might also use the 2 second timer just to be sure you eliminate most camera shake.
12-04-2012, 06:33 AM   #10
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QuoteOriginally posted by TomB_tx Quote
The viewfinders in today's Pentax dSLRs aren't designed for focusing, but for giving a clear view for a camera designed to autofocus. I have no trouble focusing manual lenses on my Pentax film cameras, but the same lenses on my K-5 are very "iffy" because the design of the screen makes the focus point uncertain, especially with fast lenses. I've run enough tests on tripod with focusing targets to be sure the screen is aligned properly and gives correct focus if you hit it right, but there's quite a range of "almost in focus" that looks the same in the viewfinder. This is one area Pentax could improve a lot.
You can always change the focusing screen, there are plenty available. A cut down Canon Ee-S is a popular choice for a matte screen, there are also many variations of split prisms. My K100D has a split prism in it, I haven't decided which screen I'm going to get for the K-5 yet but I will be changing it.

No DSLR has a good focusing screen out of the box for manual focus, especially at wide apertures.
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