« on: August 08, 2012, 03:07:21 PM »
There is some great information here. What I am not reading is that these adjustments have made a noticeable difference in IQ. Is that just implied?
One unit of AMFA is 1/8 of the depth of focus for the lens at max aperture, so an adjustment of units means your combination is off by a full depth of focus. Whether or not that makes a difference depends on how you shoot. If you only shoot stopped down to f/5.6 or narrower and your subjects are always relatively distant, AFMA likely will not matter. If you shoot at wide apertures, with close subjects, basically any time the DoF is shallow, it makes a big difference. Obviously, the amount matters, too - a 2 unit adjustment on an f/4 lens isn't going to make much difference, but a 4 unit adjustment on an f/2 or faster lens will be noticed.
Absolutely. The increase in IQ by calibrating AFMA is staggering. It is both quantifiable and easily noticed when shooting is the field. If your AFMA is off by even a little bit your image IQ will drop dramatically. I have attached a chart that plots IQ for one of my lenses through the AFMA range that was generated by foCal. Before I calibrated this lens I was always complaining that my images looked soft at 100%. As you can see the optimal AFMA point for this lens is +8. I was shooting at the default of 0 and my IQ was suffering.
Right - a big difference because you were off by a full depth of focus. But it really is lens/camera dependent. If you look at your plot, you can see that if you set a value between 7 (or perhaps 5) and 13, there would not be a meaningful, real-world difference. If the lens with your camera had been in the -3 to +3 range, you'd likely not have needed the adjustment at all.
But...when it's needed, it makes a huge difference.