The purchase of a lens for most photographers is a whole event, which, due to the high price of "glass", does not happen so often. Special attention should be paid to the inspection of the lens before purchasing, because often there are problems in the form of front and back focus.
Necessary
- - camera;
- - the lens under test;
- - tripod;
- - a special scale.
Instructions
Step 1
A problem called back focus manifests itself as a shift in depth of field when focusing back. Correspondingly, in front focusing the lens "misses" forward, i. E. towards the camera. In this case, the subject being shot falls into the blur area. To test the lens for front and back focus, buy a special scale from a photo accessory store. You can also make it yourself, just download the scale image from the Internet and print it. For strength and stability, stick the scale on a piece of cardboard and cut out the retaining “legs”.
Step 2
Place the camera on a table or tripod, the main thing is to achieve stability without wiggling. Set the white balance on a piece of paper, in the camera menu, select the spot AF mode. Set the mode to Av (aperture priority) and adjust the exposure compensation within the range of +1, 3 - +1, 5 Ev. Take all shots at the widest aperture possible, this will allow you to achieve maximum accuracy. If your camera is equipped with image stabilization, disable it.
Step 3
Select the distance to the target so that all divisions fall into the frame. Set the camera so that the plane of the focusing target is perpendicular to the optical axis of the lens. Set the autofocus mode (AF position) and focus to one side. Then aim at the center of the target and take a picture. Bring focus to the other side and take another shot. Take at least 10 pictures for easier checking.
Step 4
View the results on a computer monitor. If the center of the target (your focal point) systematically "flies" over the allowable value (from about 1/3 to 1 depth of field) and turns out to be out of focus, then there is a clear back / front focus. Please note that not only the lens, but also the device itself can "miss". If autofocus "smears" with different lenses, then the problem is most likely in the camera alignment.
