Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
07-16-2012, 04:53 PM   #1
Forum Member




Join Date: Feb 2012
Posts: 90
Does AF adjustment depend on distance ?

Hi,

At f2.8 with Tamron 17-50, I have accurate focus with -3 for AF fine adjustment when subject is at 5 ft. When I focus on something at around 200', it needs +4 (at f2.8). For both cases lens is at 50mm. I can not understand this, is this normal ?

Thanks.


Last edited by kamisu; 07-16-2012 at 05:26 PM.
07-16-2012, 09:16 PM   #2
Pentaxian




Join Date: Jun 2009
Location: GMT +10
Photos: Albums
Posts: 10,585
At 200 feet, at 50mm and f2.8, you would have a depth of field of about 80 feet on the near limit and infinity at the far limit.
Online Depth of Field Calculator

Your adjustments at the long end therefore make little sense.
07-16-2012, 09:28 PM   #3
Forum Member




Join Date: Feb 2012
Posts: 90
Original Poster
That is why I do not get it, I constantly get out of focus results at f2.8 when subject is far, while LV or larger aperture values like f8 give sharp results.
07-16-2012, 09:49 PM   #4
Pentaxian




Join Date: Sep 2006
Location: Kaunas
Posts: 1,452
QuoteOriginally posted by rawr Quote
At 200 feet, at 50mm and f2.8, you would have a depth of field of about 80 feet on the near limit and infinity at the far limit.
Online Depth of Field Calculator

Your adjustments at the long end therefore make little sense.
Wrong. They make very much sense. I remember having same issue as kamisu has with wide open 77mm limited. Focus spot on on for near objects and off focus for far objects. If adjusted to be spot on on far objects, it would misfocus on near objects. I thought this is a lens issue. It seems that it's not...

07-16-2012, 11:20 PM   #5
Pentaxian




Join Date: Jun 2009
Location: GMT +10
Photos: Albums
Posts: 10,585
In the OP's case, my point was that with such huge DOF at 200 feet distance (ie with an accompanying acceptable level of sharpness of maybe 80 feet either side of the focus target) how would one even be able to confirm BF/FF with any precision?
07-17-2012, 03:47 AM   #6
bxf
Pentaxian
bxf's Avatar

Join Date: Jul 2010
Location: Lisbon area
Posts: 1,041
I've had a hell of a time trying to perform AF adjustment on my lenses, simply because at times, I found it very difficult to identify the best image. That is, I found the differences in the images between the different AF settings to be rather small. DOF of a 50mm at f2.8 focused at 200 ft. goes from about 85 ft. to infinity, with a hyperfocal distance of 145 ft. Given these numbers, AF mis-adjujstment is not likely to be the problem causing the perception of soft images.

I don't know the level of experience of the OP, but, speaking from my own limited knowledge, I found that what I used to perceive to be an AF problem was simply a limitation of lens resolution at large distances. These days it's too easy to zoom into a 100% and see all the faults that were invisible to the naked eye in days gone by.

Last edited by bxf; 07-17-2012 at 04:20 AM.
07-17-2012, 08:44 AM   #7
Pentaxian




Join Date: Sep 2006
Location: Kaunas
Posts: 1,452
QuoteOriginally posted by bxf Quote
I've had a hell of a time trying to perform AF adjustment on my lenses, simply because at times, I found it very difficult to identify the best image. That is, I found the differences in the images between the different AF settings to be rather small. DOF of a 50mm at f2.8 focused at 200 ft. goes from about 85 ft. to infinity, with a hyperfocal distance of 145 ft. Given these numbers, AF mis-adjujstment is not likely to be the problem causing the perception of soft images.

I don't know the level of experience of the OP, but, speaking from my own limited knowledge, I found that what I used to perceive to be an AF problem was simply a limitation of lens resolution at large distances. These days it's too easy to zoom into a 100% and see all the faults that were invisible to the naked eye in days gone by.
It's very easy to check. Take image of far object with Live View contrast detect AF. It's always spot on. Then you'll see real lens sharpness. Then compare it with phase detect AF shot.
07-17-2012, 12:34 PM   #8
Forum Member




Join Date: Feb 2012
Posts: 90
Original Poster
As Edvinas said, I have sharp focus with CD focus using LV, but with PD focus, I need to change focus fine adjustment to get same image quality.

07-17-2012, 01:22 PM - 1 Like   #9
Senior Member
tlwyse's Avatar

Join Date: Aug 2008
Location: Waxhaw, NC, USA
Photos: Gallery | Albums
Posts: 208
I don't have the K-5 but instead a K20D...but I've been chasing BF/FF issues with this camera as long as I can remember.

My latest go round (with a used Tamron 17-50mm f/2.8 that I acquired recently) helped me immensely.

Using the LensAlign gadget, I shot at both 25x and 50x the focal length....at 17, 35 and 50mm focal lengths. I shot a series of AF adjustments from the full range of +/- 10 in increments of 2. I was finally able to find a setting that was consistent across the board (+8). One thing I gave up on was shooting at max aperture...everytime I've done it, the results weren't consistent. I finally decided to shoot everything at exactly -1 stop from maximum. The softness I was getting at max aperture with virtually all my lenses was making it really difficult to make a definitive visual assessment and may have been leading me to the wrong conclusions. I also switched from using AF.S to AF.C and forced the camera to refocus at least 2-3 times before taking a shot. It also should go without saying that you need to use a tripod, turn of shake reduction...and use the 2" self-timer to reduce mirror slap.

The day I did this last batch of tests, I pulled out no less than 4 lenses.....rather than each lens ending up with a unique setting like I'd come up with in previous attempts, this time all the lenses indicated an AF adjustment between +6 and +10.

It was a total PITA shooting all the different focal lengths at both 25x and 50x distances...but it was worth it as I got much more consistent results.

Terry
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
adjustment, af, af adjustment, camera, dslr, f2.8, focus, k-5, k-5 ii, k-5 iis, k5, pentax k-5
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Landscape Into the distance Symbio Post Your Photos! 9 06-22-2012 12:34 AM
Palin: Our Country’s Prosperity May Depend On Smearing Obama With Rev. Wright jogiba General Talk 5 05-23-2012 05:27 AM
Flange Distance borsuk Pentax Q 7 11-05-2011 03:38 PM
shooting the distance montreal.eric Pentax K-5 7 06-04-2011 04:27 AM
Cityscape Monument in the Distance jupiterstar Photo Critique 10 08-17-2010 06:55 PM



All times are GMT -7. The time now is 07:46 AM. | See also: NikonForums.com, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top