Imagine a terrible situation: you bought a DSLR camera, and its images come out fuzzy. However, if you look closely at the pictures, you will find that the camera has focused on just a completely different focus point, not the one that you shot as the main one. For example, In the next image, everything seems to be fine but focused on the left (relative to me) eye.
The focus, in such cases, “creeps” back to the background. For example, further away from the object, its name is “back focus if it is closer to the foreground, it is called the front focus).
Combining these phenomena with the term “back focus” is customary because, even if we consider them in opposition, they remain one phenomenon because autofocus is missed.
Of course, such problems can also be from the hands: it is worth turning on manual focus and forgetting or focusing on the nearest object 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 an adjustment procedure.
What is Back Focus?
Back focus is a consequence of the inconsistency in the operation of different camera systems caused by the presence of a mirror and a separate autofocus sensor in the camera. These are usually problems with the electronics, which can cause some errors or inconsistencies in operation over time or initially. Even for DSLRs in live view mode using the main matrix (Nikon, Canon) and for mirrorless cameras, such errors should not occur when sighting on the screen.
Usually, the problem can be hidden in the camera, lens, or their combination – sometimes, the phenomenon is noticeable with one lens but not with the other, and errors don’t need to cause a bad one. It often happens that a specific lens with a particular camera causes the 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 returned for adjustment. And it also happens that the camera back-focus, that there is no hanging on it, and from the very beginning. If it came from Japan on its own, you would not wait for warranty obligations from the manufacturer, so you will have to pay for the adjustment of the device personally.
When Do You Start to Be Afraid of Back Focus?
As I already said, first, you need to understand that the problem comes from the camera or lens, and not from the hands: turn on autofocus with focusing on the central point, shoot and know that the problem has not disappeared.
The primary indicator of back focus is your own eyes (provided, of course, that the vision is normal and the diopter correction was made for its features): if you see that the camera is focused correctly (and you have an excellent optical viewfinder), and the frame then came out again unsharp, and you were shooting at the correct shutter speed and your hands did not tremble, which means that there is a problem.
How Do I Determine Back Focus?
There are many techniques for determining back focus, but shooting a test sheet with a ruler applied to it remains the most proven. You don’t even have to use a particular target for shooting, but take a sheet of A4 paper, draw a bold black line across the sheet and put a ruler along with the sheet so that its middle falls just on the line that you just drew.
This sheet needs to be shot at an angle of 45° to its plane (by eye, accuracy does not matter here), and then view the image on a computer. However, even the camera screen will often provide 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 should be precisely in the center of the frame.
Below are some tips for testing our model:
- if your vision differs from “one,” check the diopter adjustment of the viewfinder for your vision
- be sure to turn on the focus on the center point
- set the desired zoom position (zooms can be checked in different positions to beckon; 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 be cut off
- try different orientations, both landscape and portrait
- set the aperture as wide as possible for the lens and the given zoom position (it is written on the lens through a line after the FR or in front of it); otherwise, it will take a very long time to interpret the results
- it is more convenient to hang the target on a wall or door – the main thing is that the surface is just flat
- do not torment yourself with a tripod – increase the sensitivity to the maximum (there is no need for a frame without noise, but jitter can make it difficult to see the results)
- turn off the flash – it will illuminate the sheet unevenly, try to keep the sheet evenly lit (turn on the light or come closer to the window, avoid sharp shadows directly on the sheet)
- do not take a single shot with a test shot – take several, each time focusing on the bold line in the center of the sheet according to your own eyes
- do not wait between the moment of focusing and shooting – this can cause your body to move and affect the purity of the test
- do not use live view in cameras that are sighting on the primary sensor and in contrast mode (now Canon, Nikon, non-reflective Olympus, Panasonic), you can use live view mode with “fast” (phase) autofocus (Sony, Olympus)
- forget about holivar RAW-JPEG, shoot in jpeg; it’s faster and easier to view
- you can test several lenses or cameras at once – use the columns “camera” and “lens,” having previously printed several copies by the number of objects or combinations – you need to fill in the columns, of course, before the test, remembering to change them before changing the lens
Interpretation of Results and Guidance for Action
Actually, without interpreting the test results, there is no sense in it either, so that I will explain. The ideal option to strive for is when the centerline is in focus, and on both sides of it, blurring gradually occurs towards the edges of the target.
Behind the line, blurring may even begin a little later than in front of it. It is a normal effect of lenses (the farther away from the lens, the depth of field at the same aperture increases).
An utterly unacceptable result is a fuzzy line in the center and clear in one direction or the other – if it was not your hands that lied, the camera and/or lens is a direct way to service.
Intermediate cases with sliding of the depth of field forward or backward are borderline and therefore acceptable. However, if the DOF zone begins at the focusing line and stretches only to one side of it, drive the camera-lens bundle still to get an understanding of the phenomenon. For example, if it was shot in a wide-angle position zoom, rotate to a telephoto position and take off the test on it.
In the end, even if you are seriously concerned about it, you can safely take the printout to the service center to make it easier to explain the problem.
The warranty for the camera and lens flies out after a year from most manufacturers (there are, however, individual cases), but issues of back focus can come out at any stage of the life of photographic equipment – from time to time, you hear back focus problems in new models of some cameras,
Remember camera warranty is required to be adjusted free of charge at authorized service centers. However, within the standard timeframe for warranty repair (see the latest version of applicable law, now it is 20 days because spare parts are not required for this).
The latest models of excellent DSLR cameras have become the norm to add the AF adjustment function manually. Therefore you can fix the back focus in them on the fly within a couple of minutes, saving time and the need to go to a service center.
This feature is available on the menu very profoundly. Still, it is very convenient – you just run along the scale with arrows: you clicked the target, looked at the result, twisted in the direction opposite to the direction of focus sliding.
Remember, if the problem is in the lens, the back focus will not be automatically cured. It is only a factor by which autofocus is corrected during shooting and stored in the camera database automatically. When the software is updated, these settings will also be canceled automatically.
Today lens alignment costs around $50-100. Usually, this includes testing and, in fact, adjustment. The required period is about three days, or as the service center says. However, there is no need to despair – when I last adjusted the lens, they took it for a week and gave it back the next day for only $35.
Back focus is not such a criminal problem as it is portrayed on the Internet. Good cameras now can adjust for themselves, and not the best ones justify a lot with their price.
Even in the most extreme case, you only find yourself adjusting this or that degree of complexity. Nevertheless, avoiding or detecting possible problems in time is often enough to be fully armed at the right time, rather than swearing after the fact that you are stupid and short-sighted.
Check the camera and lenses for back focus upon purchase. It applies to the entire range of cameras and economy-class lenses, without exception. They printed out a sheet, clicked, looked directly on the screen.
After causing mechanical damage to the camera, be sure to check for back focus at home – in case of noticeable changes, immediately bring it to the service for repair without waiting for the next commercial orders.
When buying a camera, one of the mandatory measures is to identify the back focus and check the camera and lenses for this glitch. It will relieve the conscience of torment later. Naturally, get used to the idea that you need to know your instrument and not just be able to press the right button.