How are front/back focus distances established?

[tom dart - (kitekook)]

I have been playing with the lens align and the distance tool the last two days...

I was wondering how you came up with the distances for the front focus and back focus for a given distance/lens/size of sensor?

My particular 70mm f/2.8 lens at 6feet:

Depth of Field = 2.9 inches
Front = (49.0%) 1.4 inches
Back = (51.0%) 1.5 inches

Are 49/51% the 'ideal' ? They seem to vary a lot between distances...

So am I trying to give just a tick more to the back Focus when viewing embossed images? (Thats what I tried to do anyways)

I was having trouble seeing what I was seeing...so I made a series of exposures...from -10 to +10.
cropped them the same size in camera raw(thanks for RAW without FUD), embossed them in photoshop...and then ran a slide show....

there I could see the movement of the focus point...and finally 'see' what I was seeing.

Anyways, my 70mm/50D adjusted at +3 ...although... up to +5 looked good.(so maybe another test, lol)
the photos I took this afternoon at +3 came out great. :)

thanks for a great product.

[Michael Tapes - (MichaelT)]

Originally Posted by kitekook

I was wondering how you came up with the distances for the front focus and back focus for a given distance/lens/size of sensor?

The DOF front/back percentages are based on math formulas from the text books. They are the theoretical values based on a simple lens design, and will vary by distance. Generally the closer the distance the closer to a 50/50% distribution, moving to 33/66% as it approaches infinity. With the AF Adjust and LensAlign we can choose to adjust to mimic the theoretical ratios, or we can center the focus point in the center of the DOF which gives the most leeway for the sporadic AF "errors" that can move the focus point around.

And we also have the creative license to push the focus point toward the front of the DOF to give more focus percentage behind the actual focus point, which some photographer prefer. But be careful since the setting is only "precise" at the distance you set it at. You would have to test at other distances as well to see if the adjustments is valid for the other distances.

Hope this helps a bit..