DotTune: Video Tutorial for AF tuning without photographs

[Nephi]

I watched the video and performed the DotTune procedure using a D800 with 50mm f/1.4. The tests were done wide open on a USAF test chart, with the camera 9 ft away from the wall. Wireless remote shutter, sturdy tripod, non-fluorescent lighting, camera parallel to the target.

With ISO at 100, I'm getting a valid range from 6 to 11. My midpoint would be 8.5.
With ISO at 200, I'm getting a valid range from 10 to 15. My midpoint would be 12.5.

I did multiple tests at each ISO value to make sure to get rid of any outliers. All tests were done in the same 1 hour time window with no environmental changes.

Any idea what might be causing me to get such a difference in values just by changing the ISO?

Any insight would be appreciated.

Has anyone else tested this? Does ISO change the values (it shouldn't!). Adam, have you tested this variable?

-Brian

 

[Horshack]

I watched the video and performed the DotTune procedure using a D800 with 50mm f/1.4. The tests were done wide open on a USAF test chart, with the camera 9 ft away from the wall. Wireless remote shutter, sturdy tripod, non-fluorescent lighting, camera parallel to the target.

With ISO at 100, I'm getting a valid range from 6 to 11. My midpoint would be 8.5.
With ISO at 200, I'm getting a valid range from 10 to 15. My midpoint would be 12.5.

I did multiple tests at each ISO value to make sure to get rid of any outliers. All tests were done in the same 1 hour time window with no environmental changes.

Any idea what might be causing me to get such a difference in values just by changing the ISO?

Any insight would be appreciated.

Has anyone else tested this? Does ISO change the values (it shouldn't!). Adam, have you tested this variable?

-Brian

None of the exposure settings on the camera (aperture, shutter speed, ISO) should have an effect on the AF since the amount of light entering the camera into the AF system is the same regardless of those exposure settings, including aperture since AF always occurs with the lens wide-open (unless you're using a lens with an aperture control ring and setting its aperture that way, which will definitely affect the AF system by reducing the amount of light it sees). The only exception to all this is if the ambient lighting is being changed by the photographer as part of the exposure change; for example, when increasing from ISO 100 -> 200 if the photographer then lowers the ambient light intensity to compensate to produce the same output brightness @ ISO 200 as for ISO 100.

 

[Randy Stephens]

I think I understand all the Dot method but the need to obtain Critical Focus in Live View or Manual. Does it just create a starting point to speed up the process or does something more important?

I was trying the Dot method on a 800 VR 5.6 w/ 1.25 on a D810 wide open thinking I would have a razor thin focus length. But after setting everything up the confirmation dot would lite over a 10+ range after making the short toggle of the manual focus and re-AF focusing. What am I doing wrong?

 

[Canon2Nikon]

when I have time I'll try to build a scale that converts between body-default AF tune values and lens-specific AF tune values, which should allow us to do a DotTune range using body-default values and then convert them to lens-specific tune values, at least when the midpoint fits within the lens-specific tune scale.

Just wondering, has anybody created a chart of conversions for this yet? - (For Nikon)

Thanks.