Imagine a terrible picture: you bought a reflex camera, and the pictures from it come out fuzzy. However, if you look closely at the pictures, you will find that the camera is focused, it’s just that a completely different object fell into the focus area, not the one you shot as the main one. Well, by the way, if your software supports focus points right on the picture.
Everything seems to be fine, but focused on the left eye (relative to me)
Focus, in such cases, “creeps” back to the background
(ie, further than the object, then its name is “back focus”, from the English back focus) or vice versa, closer to the foreground (ie, in front of the object, then it is called “front focus”, from English front focus) – it’s customary to combine both of these phenomena with the term “back focus” anyway, because even taking into account that they are in opposition to each other, in fact, they all remain one thing, because autofocus missed. Of course, such problems can also be hand-made: it is worth turning on manual focus and forgetting or focusing on the nearest subject, while the person is behind the bushes. Moreover, the autofocus system itself may be wrong. But if the problem is regular, you should contact the service center for a procedure called “adjustment”.
From what moment to start to be afraid of back focus?
As I said, first you need to understand that the problem is from the camera or lens, and not from the hands: turn on autofocus (look for the AF-MF switch – there may even be two in the camera, or one of the items in the menu) with focus on the central point (anyway, in simple cameras the declared tracking focus is either not used, or made so that it’s difficult to use it), take pictures and understand that the problem has not disappeared.
The main indicator of back focus is your own eyes (provided, of course, that vision is normal and diopter correction is made for its features): if you see that the camera focused correctly (and you have a good optical viewfinder), and the frame after that comes out again unsharp, and you shot at the correct shutter speed and your hands did not tremble, which means that there is a problem.
What is back focus?
Back focus is a consequence of the inconsistency of the different camera systems caused by the presence of a mirror and a separate autofocus sensor in the camera. Usually these are problems with the electronics, which over time or initially may cause some errors or inconsistencies in the work. That is, even for DSLRs in live view shooting on the main matrix (Nikon, Canon) and for mirrorless cameras when sighting on the screen, such errors should not occur.
Let it be an official definition
Usually, the problem can be hidden in the camera, lens or their combination – sometimes with one lens the phenomenon is noticeable, but with the other there is not, and it is not necessary that the error will cause a bad one. It often happens that a particular lens with a specific camera causes a back focus effect, while all other pairs with these elements do not lead to such errors. it is possible that one camera does not work well with only one lens, then both must be checked for adjustment. And it happens that the camera backfocuses, no matter what it is, and from the very beginning – moreover, if he came from Japan on his own, you will not get any warranty from the manufacturer, so you will have to pay for the adjustment of the device personally.
How to determine the back focus?
Test table, download below
To determine the back focus, there are a bunch of methods, but shooting the test sheet with the ruler applied to it remains the most proven. You can even not use a special target for shooting, but take a sheet of A4 paper, draw a thick black line across the sheet and put a ruler along the sheet so that its middle falls exactly on the line that you just drew. This sheet needs to be removed at an angle of 45 ° to its plane (by eye, the accuracy does not matter much here), and then view the image on a computer, although often even the camera screen will give sufficient information about the presence or absence of back focus. The focus line should be perpendicular to the optical axis, and the vertical auxiliary line strictly in the center of the frame.
Some tips for testing on our model, I give below:
if your eyesight differs from “one”, check the diopter adjustment of the viewfinder for your vision
make sure to focus on the center point
set the desired zoom position (zooms can be checked in different positions just to be attractive, theoretically they should not lie in some positions and work correctly in others) and aim at the target so that it occupies the maximum part of the frame, non-critical angles can also be cut
try different orientations, both landscape and portrait
set the aperture as open as possible for the lens and this zoom position (it is written on the lens through the line after the FR or in front of it).