| Author | Thread |
|
|
01/17/2006 12:10:18 AM · #1 |
| Have recently purchased a second-hand Pentax FA 50mm F1.7 AF lens, which is the first glass I've ever owned with such a large aperture. Most of the time I'm finding it a sweet lens to use, but at it's max aperture I find that the focus (auto and manual) can be a little hit and miss. This is irrespective of focal distance and even with very fast shutter speeds and 'spot focus' on auto. Is this common with large apertures, simply because of the shallow DOF, or does it sound like a problem with the lens? |
|
|
|
01/17/2006 12:24:32 AM · #2 |
if your subject isn't moving, I can't think of a single reason why something wouldn't be in focus...9 out of 10 times that is.
|
|
|
|
01/17/2006 12:30:39 AM · #3 |
Hard to tell. If it's hit-or-miss in both directions, it's probably just variation in the AF system. If it consistently focuses in back of the subject or in front of the subject, then it may be a problem either with the camera or the lens. Such a problem won't show up as readily with slower glass, so it could be the cam, even if you don't notice the problem with other lenses.
There are other reasons that focus might seem hit-or-miss. If you're pre-focusing using a half-press, just in the short time between focus lock and pressing the shutter you or your subject may move enough to affect focus.
|
|
|
|
01/17/2006 12:35:16 AM · #4 |
Originally posted by AdrianQ:
Is this common with large apertures, |
Yes, especially in complex subject situations. The DOF is so short at that aperture that it only has to be off by a few inches at times to throw the image out. f/8 is much more forgiving.
|
|
|
|
01/17/2006 12:35:48 AM · #5 |
| Also, with a 1.7 50mm wide open, you've got about 1 inch of sharpish DoF at a distance of 4ft. Closer in it gets smaller. If you are focusing then recomposing, that's enough to mess up almost every shot, every time. Same if you are swaying/moving after you focus. |
|
|
|
01/17/2006 01:08:20 AM · #6 |
| thanks for the feedback. i had kinda figured it had something to do with the shallow DOF, especially a close range. i'm using the lens for some macro work (with a 3X filter for small stuff) and this is when the problem has been at it's worst. i think i have to accept the need for hauling out the tripod for these shots, because i suspect i am moving slightly between focussing and shooting. |
|
|
|
01/17/2006 01:09:17 AM · #7 |
Originally posted by nsbca7:
Originally posted by AdrianQ:
Is this common with large apertures, |
Yes, especially in complex subject situations. The DOF is so short at that aperture that it only has to be off by a few inches at times to throw the image out. f/8 is much more forgiving. |
F8 & Be there!!
Love that old saying.
|
|
|
|
01/17/2006 01:29:35 AM · #8 |
Originally posted by Brent_Ward:
Originally posted by nsbca7:
Originally posted by AdrianQ:
Is this common with large apertures, |
Yes, especially in complex subject situations. The DOF is so short at that aperture that it only has to be off by a few inches at times to throw the image out. f/8 is much more forgiving. |
F8 & Be there!!
Love that old saying. |
So if you're there, but you miss the shot, was it simply f8? ;-)
|
|
|
|
01/17/2006 09:52:49 AM · #9 |
Originally posted by AdrianQ:
thanks for the feedback. i had kinda figured it had something to do with the shallow DOF, especially a close range. i'm using the lens for some macro work (with a 3X filter for small stuff) and this is when the problem has been at it's worst. i think i have to accept the need for hauling out the tripod for these shots, because i suspect i am moving slightly between focussing and shooting. |
Yup, you have very little DoF to play with. Basically you can't do good macro work handheld, unless you are going to stop the lens down and have plenty of light.
This shot is with a 100mm lens (i.e., less than your 50mm with 3x diopter) so you'll have less depth of field. It is also shot at f2.8 - again at f1.7 you'll have quite a bit less depth of field than this. Combine the 1.7 and 3x features and you'll be lucky to get much of anything other than a real sliver in focus, without stopping down. If you stop down, you'll need a tripod.
Message edited by author 2006-01-17 09:53:36. |
|
Home -
Challenges -
Community -
League -
Photos -
Cameras -
Lenses -
Learn -
Help -
Terms of Use -
Privacy -
Top ^
DPChallenge, and website content and design, Copyright © 2001-2025 Challenging Technologies, LLC.
All digital photo copyrights belong to the photographers and may not be used without permission.
Current Server Time: 09/22/2025 10:56:27 AM EDT.
