Okay, definitely not going to challenge neuro on a technical concept, BUT...isn't the depth of field greater the further you are away from a subject? So if you have a longer focal length, for the same framing you will have to be further away--is that a wash on DoF then, or exactly what happens?
Exactly - for the same framing, it's a wash that reduces to aperture, i.e. the shallower DoF from the longer focal length exactly cancels out the deeper DoF from the greater distance, so for the same framing, only aperture determines DoF.
But, most people don't frame the same shot at 17mm vs. 40mm. If you're shooting at 17mm, you usually want that wide angle view.
Still thinking...(that's a big taks sometimes! )...you're saying keeping the subject and camera at the same distance, as you increase focal length the DoF will decrease, hence that's where you set your microfocus? That still feels backwards to me. I want to understand this so I can start setting my (wife's) lenses on my new 7D!
In a nutshell, for a given sensor size, DoF is determined by three factors:
- Aperture - wider means shallower DoF
- Subject distance - closer means shallower DoF
- Focal length - longer means shallower DoF
If you change more than one at once, the effects will add or subtract. You're statement, "...keeping the subject and camera at the same distance, as you increase focal length the DoF will decrease
," is absolutely correct (assuming same aperture).
Setting the AFMA at the long end of the zoom lens does two things - it sets the adjustment at the place it's most likely to be needed, and it makes the calibration easier.