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06-01-2010, 06:50 PM   #1
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Question involving focus of 50mm

Hello,
I just received a used 50mm f/1.7 Pentax-A lens in the mail and am kind of confused with the focus. Looking through the viewfinder, my target seems to be in focus but when I snap the picture, the target looks soft. I've been using f/2 to take pictures on AV mode and well, is it just me? Do I have to make some kind of adjustment? I've had a friend try to take pictures with it and focus on objects but still no go. Both of us are inexperienced though.

06-01-2010, 06:59 PM   #2
Ole
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How do you determine if focus is spot on? Do you get focus confirmation in the viewfinder?

Make sure that you have adjusted the diopter of the viewfinder to suit your eye-sight. Otherwise it is impossible to determine when the image actually is in focus.

Most lenses are somewhat soft wide open so try to use f/5.6 or f/8. If you still get soft photos it is either a problem with your focusing technique or the lens.

When you have perfected your focusing technique you can go back to using f/2.
06-01-2010, 07:08 PM   #3
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Yeah that's what I figured. I should probably just play around with it more. Beginners jitters. Thanks.
06-01-2010, 07:49 PM   #4
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First, post pictures, that will help us figure out what is going on.

Only part of the light is directed up to the viewfinder, so effectively you are seeing a much smaller aperture than 1.7 or 2. More will be in focus than when you take the picture, so you need to go by focus confirmation. The 50mm f1.7 is pretty sharp at 1.7, so I doubt that is the issue.

If you have LiveView, use that and optical preview to get a real idea as to what will be in focus. Or just take pictures while adjusting the focus reel.

Also make sure to have a fast enough shutter speed.

06-02-2010, 10:30 AM   #5
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I can't emphasize the above enough - the viewfinder *will* lie at large apertures like f/2, showing more in focus than actually will turn out to be. So it's unfortunately completely normal for some parts of the scene that appear in focus to turn out not to be. Do some testing by shooting newspaper or similar text on an angle, paying attention to which lines appear in focus in the viewfinder versus in the final picture. Some lines will appear in focus in the viewfinder and turn out to be; others will appear in focus in the viewfinder but turn out not to be. With practice, you can learn to focus in such a way that the part of the image you care about is in the region that really *is* in focus.
06-02-2010, 03:12 PM   #6
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What Marc Sabatella said. Focussing a little back and forth will usually give a good indication of where the plane of focus really is.

For another suggestion, try putting the camera in AF mode instead of MF. Hold down the shutter button, aim the center focus point at the subject, then turn the focussing ring (somewhat slowly), trying to get the subject in focus.
The camera will release the shutter (only) when focus is achieved. It's somewhat limited as you can't recompose (the subject would have to be in the middle AF point), but it does the trick in a pinch (tracking something with manual focus isn't easy if you are not used to it), and it's just a matter of switching to MF mode to be able to take pictures without focus confirmation.
06-02-2010, 04:09 PM   #7
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QuoteOriginally posted by jalexyep Quote
Hello,
I just received a used 50mm f/1.7 Pentax-A lens in the mail and am kind of confused with the focus. Looking through the viewfinder, my target seems to be in focus but when I snap the picture, the target looks soft. I've been using f/2 to take pictures on AV mode and well, is it just me? Do I have to make some kind of adjustment? I've had a friend try to take pictures with it and focus on objects but still no go. Both of us are inexperienced though.
If you want to test if the focus is off, put the lens on a tripod. At 1.7 DOF is still pretty shallow. Also, I got a A 50mm 1.7 for my brother and even though I've already had a lot of experience with shallow DOF on my Takumar 50mm 1.4, i think out of 10 shots I only got 1 in focus with the A 50mm, so I guess something about that lens is a little tricky.
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