kcbrown wrote in post #15897521
There's a refinement of this method that is applicable to zoom lenses:
If the MFA ranges do not intersect, then you need to get your lens calibrated.
Example: Suppose you have a 24-70 f/2.8 lens. The first thing you do is perform the DotTune method for the 24 mm focal length. Suppose this gets you a range of -5 to +2. Then you perform the DotTune method for 70mm. Suppose this gets you a range of -2 to +7.
The intersecting range of those two ranges is -2 to +2. The center of that range is, of course, 0, so the MFA value you should use is 0.
One other thing: the DotTune method has you use live view to get the target in as good focus as possible, but there will be a range of focus that has the target clearly in focus from within live view. What you really want is for the focus to be such that what you're focusing on is in the center of the focus field. This means using a focus target that has something that allows you to see the actual focus field, like something angled at a 45 degree incline relative to the target.
Of course, the above presupposes that your camera takes only a single adjustment value. If, as in the newer Canon bodies, it takes a value for each end, then you use the center of the range for the end in question as the adjustment value for that end.
There's a refinement of this method that is applicable to zoom lenses:
- Perform the standard DotTune method for the lens at each end of its focal length range. Write down the range of MFA values for each focal length.
- Find the intersection of the two ranges you got previously. This will tell you what MFA values will work for both focal lengths. This range will be between the largest minimum MFA value you found, and the smallest maximum MFA value you found.
- Set your MFA value to be the center of that intersecting range.
If the MFA ranges do not intersect, then you need to get your lens calibrated.
Example: Suppose you have a 24-70 f/2.8 lens. The first thing you do is perform the DotTune method for the 24 mm focal length. Suppose this gets you a range of -5 to +2. Then you perform the DotTune method for 70mm. Suppose this gets you a range of -2 to +7.
The intersecting range of those two ranges is -2 to +2. The center of that range is, of course, 0, so the MFA value you should use is 0.
One other thing: the DotTune method has you use live view to get the target in as good focus as possible, but there will be a range of focus that has the target clearly in focus from within live view. What you really want is for the focus to be such that what you're focusing on is in the center of the focus field. This means using a focus target that has something that allows you to see the actual focus field, like something angled at a 45 degree incline relative to the target.
Of course, the above presupposes that your camera takes only a single adjustment value. If, as in the newer Canon bodies, it takes a value for each end, then you use the center of the range for the end in question as the adjustment value for that end.
This I believe applies to those bodies that do not have wide and Tele MFA settings.
