@Jshemnitz wrote:It's a used lens I just bought on ebay. I was taking shots on the telephoto end and noticed it was back focusing so I recalibrate it to +8 which was the perfect setting to get it right. When I went to try it out on the wide end it was front focusing by about 10 stops on the micro adjustment setting on the camera. So 1 end was was about 10 steps from being correct compared to the other and of the focal length
Was this your first attempt at calibrating a lens with a camera body? As I noted above, first timers rarely get it right on their first attempt(s). Why did I highlight some text in your reply? I think you made a fundamental mistake.
You reported that your camera/lens combo was “back focusing.” Back focusing means the Focus Plane is behind the subject. The Focus Plane represents the distance from sensor to a flat geometric plane of perfect focus.
With nearly ever consumer lens on the market, the Focus Plane is not perfectly flat. It is curved. Theoretically speaking, it should be a perfect circle, which is a uniform distance between the sensor and subject in all directions.
Most lenses contain a number of internal elements that among other things serve to flatten the curved focus plane somewhat. What separates a macro lens from conventional lenses is that it has internal elements that cause the focus plane to be closer to a flat plane than conventional lenses.
Let’s get back to your “back focusing” issue and the value that you entered.
For argument’s sake, let’s assume that your lens creates the theoretical, perfectly flat Focus Plane shown above. If your gear were “back focusing”, then your subject would be located somewhere between the Focus Plane and image sensor. If your gear were “front focusing”, then your subject would be located somewhere “behind” the Focus Plane.
What do the AFMA values that you enter actually do?
The values that you enter effectively move the Focus Plane towards the camera or move the Focus Plane away from the camera. The values that you enter are can be positive or negative values. But what do these values actually represent?
Look at them as a compensation value that is “added” to the distance to the subject. This means a “front focusing” combo would need to increase the distance to the subject. While a “back focusing” combo would need to decrease the distance to the subject.
When you enter a positive AFMA value in a Canon DSLR, then you are increasing the distance to the Focus Plane and moving it away from the camera.
When you enter a negative AFMA value in a Canon DSLR, then you are decreasing the distance to the Focus Plane and moving it closer to the camera.
In other words, if your gear was “back focusing”, then the AFMA value should have been a negative value, NOT a positive value. This is the red flag that got my notice.
Please don’t feel insulted or depressed. LIke I said, almost no one gets it right on their first attempt. There is a LOT more going on under the hood that many people realize. The most common mistakes are not calibrating at the “correct” distance to your test target, only using one test shot, instead of taking several and calculating an average, and calibrating using indoor lighting and then using the camera outdoors in the sunlight.
Here is an excellent video that describes a technique for nailing focus, Dot-Tune. Aside from stressing the importance of calibrating under lighting conditions as when you would use the lens, he does not take multiple test shots and average them, but that is my only criticism of the video.
