[DblArrow]

Not sure if I should be posting here, but -

I just received a new Tamron 70-200 2.8 and would like to do some serious testing for front and back focus. Initially it looks like focus is fairly good at 200 but back focused at 70. Is there a good method for testing the back and front focus of a lens? Have it on the K20 and K100.

Thanks for any assistance.

Chris

[Marc Sabatella]

There are indeed many good focus test charts out there - do a search for the many previous threads here, or just download the chart at focustestchart.com, which is my favorite. But note, if you do find an issue, don't assume it's with the lens - it's at least as likely to be with the camera (and actually, even more likely to be testing error, at least until one gains a lot of experience doing these tests in controlled fashion).

[JohnX]

Firstly, I'm pretty sure that you can only set up adjustments for modern Pentax lenses. The camera needs to know which lens is attached so it can apply the appropriate adjustment -there must be communication between the lens and body.

The problem with zooms, as you've already found, is that FF and BF can occur at different points in the zoom range. Choices are (a) check at points through the full range and calculate the average setting, or (b) just make the adjustment for the most used focal length.

Also, when you do the test(s), assuming you're using charts, be aware that you will get different results in different lighting conditions, ie set up the lens for the lighting conditions you most expect to use it in, be it artificial or natural light.

Adusting FF and BF is in the K20D menu [C Custom Setting; 35. AF Adjustment]; you'll need a hack to do it on the K100D giving access to the firmware 'debug' mode.

[DblArrow]

There are indeed many good focus test charts out there - do a search for the many previous threads here, or just download the chart at focustestchart.com, which is my favorite. But note, if you do find an issue, don't assume it's with the lens - it's at least as likely to be with the camera (and actually, even more likely to be testing error, at least until one gains a lot of experience doing these tests in controlled fashion).

Thanks Marc, I did a search but couldn't seem to find the info I was after. Kinda assumed it could possibly be a camera issue also. But a guys gotta play!!

[DblArrow]

Firstly, I'm pretty sure that you can only set up adjustments for modern Pentax lenses. The camera needs to know which lens is attached so it can apply the appropriate adjustment -there must be communication between the lens and body.

The problem with zooms, as you've already found, is that FF and BF can occur at different points in the zoom range. Choices are (a) check at points through the full range and calculate the average setting, or (b) just make the adjustment for the most used focal length.

Also, when you do the test(s), assuming you're using charts, be aware that you will get different results in different lighting conditions, ie set up the lens for the lighting conditions you most expect to use it in, be it artificial or natural light.

Adusting FF and BF is in the K20D menu [C Custom Setting; 35. AF Adjustment]; you'll need a hack to do it on the K100D giving access to the firmware 'debug' mode.

When you say only modern Pentax lenses - does this imply only made by Pentax! Cause as I was playing with the adjustments it did seem to move the focus point. But could still be in the operator error catagory.

Thanks for your information.

[JohnX]

When you say only modern Pentax lenses - does this imply only made by Pentax! Cause as I was playing with the adjustments it did seem to move the focus point. But could still be in the operator error catagory.

Thanks for your information.

The issue is the camera needs to know which lens is attached to apply the appropriate correction.

In the K20D you can apply a global correction (Apply All) or up to 20 lens specific ones (Apply One)

'Apply All' would apply the same adjustment to any and every lens attached to the body. In this instance the camera would not need to 'know' anything about the lens.

'Apply One' enables you to make different adjustments for different lenses which the camera stores in an internal database. The camera identifies each lens as it is fitted, and if it is recorded in the database it applies the stored AF settings.

As far as I am aware, and I'm happy to be corrected on this, the only lenses that the body knows about are Pentax ones.