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04-26-2012, 06:45 AM   #1
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How do you autofocus?

Question may sound strange but like to know how do you best use AF in your camera.

I use K-x and often frustrated with out of focus shots and may be I am missing on some techniques. I have calibrated AF and always use center point focus. Today I met a professional photographer who always focuses on infinity before focusing on subject, so was wondering what are the techniques other people uses.

Yusuf

04-26-2012, 07:22 AM   #2
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I manually set the AF point 95% of the time. It really helps to put the AF point on what you're wanting to focus on...
04-26-2012, 07:25 AM   #3
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Since there are no red dots in VF on my K-x I use the AF botton to autofocus and I have disabled AF on shutter botton while using center AF point. After using that technique I have had more keepers because I don't accidentally lose focus while recomposing.
04-26-2012, 07:26 AM   #4
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Low light, I find the center of the subject and AF on that for the entire picture. When the subject is smaller and the other facts are the same, I AF on the ground or nearest to get a lock. In day time the errors are few and due to size of the subject. For example, day time and an object about 6cm wide at more than 10m and lens at 28mm and wider, I'll have possible errors in the AF catching the object... this is with my 16-45 DA lens and sometimes due to the barrel drooping a little. I figure that AF error can come from many places?

04-26-2012, 09:22 AM   #5
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If I'm shooting-from-the-hip with a short lens, I just sort of point and shoot and hope. If I'm actually working my K20D carefully, I either use center focus and center the subject, half-press the shutter, maybe locking the focus with the AF button, or watch the VF closely to see that a red.dot focus point is on the subject. Then I wait for the green confirmation hex before taking the shot.
04-26-2012, 12:04 PM   #6
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Are you using a zoom lens?

Phase detection autofocus isn't as accurate as Live View and tends to be temperamental. One way to get around this if you are using a zoom lens, is to zoom in pretty close to your subject, lock focus, then zoom out to take the picture. Zooming in gives the camera an easier target to focus on, and when you zoom back out you still maintain the same focus point. For example, if you have a 18-55 mm lens and want to take a picture at 18mm, zoom in to 55 first, lock focus then go back to 18mm to take the photo. Try it! I have an 18-250mm lens and I get MUCH sharper results doing this.
04-26-2012, 12:35 PM   #7
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QuoteOriginally posted by sahale Quote
Are you using a zoom lens?

Phase detection autofocus isn't as accurate as Live View and tends to be temperamental. One way to get around this if you are using a zoom lens, is to zoom in pretty close to your subject, lock focus, then zoom out to take the picture. Zooming in gives the camera an easier target to focus on, and when you zoom back out you still maintain the same focus point. For example, if you have a 18-55 mm lens and want to take a picture at 18mm, zoom in to 55 first, lock focus then go back to 18mm to take the photo. Try it! I have an 18-250mm lens and I get MUCH sharper results doing this.

This assumes that your zoom lens focuses the same way at ever focal length. Many do not, and need fine adjustments as you zoom in and out.
04-26-2012, 08:20 PM   #8
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Same as you. I just focus using the center point. You may have to make an adjustment.

04-26-2012, 09:12 PM   #9
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You need to bracket your important shots!!

PD AF is not 100% accurate, so to increase my chances I do focus braketting when I can. Take a few shots, defocus manually or by focusing on infinity, and then refocus and take a few more.

So if PD AF is only 80% accurate (and I tihnk that's generous to PD AF), I've gone from having a 20% chance of missing a shot to only 4% for a 2 shot bracket, or less than 1% for 3. Lately I've been doing that plus doing MF for some shots (as I got a new focus screen which when the conditions are right, is better than AF).

I've missed focus on too many important shots in the past to just trust AF got the focus right on the very first attempt.

Last edited by twitch; 04-26-2012 at 09:17 PM.
04-26-2012, 09:26 PM   #10
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Well, the best way to AF is to MF, so when you can MF there is no need to AF - why AF when your AF lens can MF so much better? Does any of that make any MF'ing sense? :~
04-26-2012, 09:36 PM   #11
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QuoteOriginally posted by MSL Quote
This assumes that your zoom lens focuses the same way at ever focal length. Many do not, and need fine adjustments as you zoom in and out.
Are you sure about this? That doesn't make sense to me. If I zoom in and focus on someone 2 meters away, it should be in focus on the same spot when I zoom back out because I haven't changed the focus point. After all, the the 2 meter point on the manual focus ring is 2 meters whether at 18mm or 250mm. If what you're saying is true, then the 2 meter spot on the manual focus ring changes depending on the focal length.

Zooming in for the purpose of obtaining better focus then zooming out to take the photo has worked across the Nikon and Canon cameras I've tried as well. I still haven't tried that many cameras or lenses, so feel free to correct me if I'm wrong.
04-26-2012, 09:41 PM   #12
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QuoteOriginally posted by sahale Quote
Are you sure about this? That doesn't make sense to me. If I zoom in and focus on someone 2 meters away, it should be in focus on the same spot when I zoom back out because I haven't changed the focus point. After all, the the 2 meter point on the manual focus ring is 2 meters whether at 18mm or 250mm. If what you're saying is true, then the 2 meter spot on the manual focus ring changes depending on the focal length.

Zooming in for the purpose of obtaining better focus then zooming out to take the photo has worked across the Nikon and Canon cameras I've tried as well. I still haven't tried that many cameras or lenses, so feel free to correct me if I'm wrong.
Yes, I'm sure. It is a lens design issue and not specific to one camera system. For one thing, I have lenses that behave this way. I can't remember if it is one or both of them, but my Pentax SMC-A 35-105/3.5 and/or A 70-210/4 operate like this. Folks on the forums who know more about lens design than I do may be able to say if the vaifocal nature of these lenses is connected to their having a constant minimum aperture across the entire zoom range.

From wikipedia (Varifocal lens - Wikipedia, the free encyclopedia)

A varifocal lens is a camera lens with variable focal length in which focus changes as focal length (and magnification) changes, as compared to parfocal ("true") zoom lens, which remains in focus as the lens zooms (focal length and magnification change). Many so-called "zoom" lenses, particularly in the case of fixed lens cameras, are actually varifocal lenses.[1]
04-26-2012, 09:44 PM   #13
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Most of the cheaper consumer zooms will change focus when you zoom. Sometimes the expensive constant aperture zooms will as well. I have seem pro quality prime lenses lose focus on change of aperture. You can't assume anything, when you get a new lens you have to get to know its quirks first, you don't want to find out the hard way.
04-26-2012, 09:44 PM   #14
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Parafocal lenses are the exception. I tihnk only IF (internally focusing lenses, like the DA*50-135) lenses can be parafocal, and even then I'm not sure if all are.
04-26-2012, 09:50 PM   #15
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QuoteOriginally posted by MSL Quote
Yes, I'm sure. It is a lens design issue and not specific to one camera system. For one thing, I have lenses that behave this way. I can't remember if it is one or both of them, but my Pentax SMC-A 35-105/3.5 and/or A 70-210/4 operate like this. Folks on the forums who know more about lens design than I do may be able to say if the vaifocal nature of these lenses is connected to their having a constant minimum aperture across the entire zoom range.

From wikipedia (Varifocal lens - Wikipedia, the free encyclopedia)

A varifocal lens is a camera lens with variable focal length in which focus changes as focal length (and magnification) changes, as compared to parfocal ("true") zoom lens, which remains in focus as the lens zooms (focal length and magnification change). Many so-called "zoom" lenses, particularly in the case of fixed lens cameras, are actually varifocal lenses.[1]
I stand corrected; I guess my trick only works with parfocal lenses. Do newer lenses tend to be parfocal?

Edit: oops I see others have already posted
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