[kamisu]

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.

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Last edited by kamisu; 07-16-2012 at 05:26 PM.

[rawr]

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.

[kamisu]

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.

[Edvinas]

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...

[rawr]

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?

[bxf]

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.

[Edvinas]

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.

[kamisu]

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.

[tlwyse]

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