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How to detect back-focus. Print E-mail
Written by Peleng   

Back-focus means that when focusing on some focusing point the lens misses backward so that the depth of field shifts back, too. Front-focus means that the lens misses forward and the depth of field shifts forward.

To test if the lens has the back-focus we use a special target scale which is easy to be hand-made: you should print it with a laser printer, stick it to the piece of thick carton and make some slits to make it more stable. You can download this scale from here (dimension 300 dpi, size 24x15cm).

back-focus compensation for Lens adapters

Target for back-focus compensation for Lens adapters

Focus testing scale with the target

Bend page corner for 45 degrees and set it on the table. You may install a distant piece in the form of a square with 46mm sides. It can be cut from the carton so that it will be of 45 degrees angle.

The camera should be set on the tripod or on the table. Settings are following: set the white balance (WB) by the piece of paper, choose one-shot autofocus mode, set EV around +1,3...+1,5. AV-mode is suitable to this test. Shots should be done with wide open aperture. If the lens is “soft” you can slightly stop it down (e.g. for 50mm-lens choose f2,8 instead of f1,3). If the lens has image stabilizer (IS mark on it) disable it. Choose the central focus point and turn the camera in such a way that focus target plate was perpendicular to the optical axe of the lens.

Distance from the lens to the target should let scale divisions be in frame. That's how we will check if focus is working properly. Focus mark in the viewfinder need to be set on the target with some reserve which should be of the same size as the very mark. The fact of the matter is that focus sensor blocks are a little bigger than they are seen in the viewfinder. In other words, if some more contrast object is out of mark limits (red in the picture) but in sensor limits the camera will focus on this detail. It is pretty common to complain of it while in truth the focus is working well.

Target for focus compensation for lens adapters
Back-focus test: camera - Canon 300D, lens - EF 24-70 , focal length – 50 mm, aperture – f/2,8, distance to the target – 61 cm. Focus mark in the viewfinder is in the red frame, real square of sensors is green. You can see that the lens hit the aim and the depth of field is evenly spread around the scale center.

Turn the switch to the manual focus mode (MF), put out focusing and turn the focusing ring until you get the focus confirmation. Then press the shutter release.

It would be better if you will examine the results on the computer monitor not on the camera screen. Keep in mind that there is some tolerance about focus accuracy. Cameras like 10D, 300D, 350D are focusing in the limits of the depth of field. Cameras like 1D, 1Ds, 20D, 30D, 400D, 5D have more exact focusing system. It is set in motion in case you mounted the lens with diaphragme f/2.8 or less on it (concerning sensitivity of four-way sensor). The reserve is 3 times less for 1Dxx cameras so it can be measured as one third of the depth of field. It is two times more accurate in 20D, 5D (30D, 400D) and comes to the half of the depth of field.

Calculating the depth of field, the reserved circle of unsharpness makes 0,035 mm for 24x36 mm frame (in cameras like 1Ds, 5D). This is about 1/1000-1/1500 of the diagonal of the frame if the size of printed image is 5x7 inches and the distance of vision is 25-30 cm. The coefficient of image increasing also influences its sharpness. As the consequence the circle of reserve of unfocused image will be less for cropped cameras than for full-framed. This happens because you will have to increase the picture in the proportion of crop-factor to get the print of the same size. So, for 1,3-cropped cameras (like 1D Mark II, 1D Mark III) the accepted reserve circle will be of 0,027 mm, for 1,6 – cropped cameras (like 30D, 400D) it will be 0,022mm.
The information about how accurate the autofocus is and about the tolerant circle is brought to you from the book called “EF Lens Work III. The Eyes of EOS', Canon EOS manual guides and Bob Atkins' (he is a well-known journalist, Canon apologist )and Chuck Westfall's (he heads the technical information department od Canon) works. You can use a special program to calculate the depth of field for your own comfort.

If the lens missed, you should check out how much the depth of field is shifted and how often it misses (can be seen if a series of shots is taken). In line with tolerance described above a little miss can be considered as normal. Example of miss for 100 mm macro-lens with the front-focus in is shown further. Despite the fact that Canon doesn't consider it like a mistake, practically you may get unpleasant result. For example, while shooting full-length portraits. From the distance like this (7-10m) it is impossible to focus manually. Because of little front-focus highest possible sharpness shifts to the foreground and the very object of shooting looks a little bit soft.

Front focus sample for lens adapter
Back-focus test: camera – Canon 300D, lens - EF 100 f/2.8, focal length – 100mm, aperture – f/2,8, distance to the target – 1,39 m. The depth of field is marked with red. It's obvious that the front-focus took place, however it is in the range of the depth of field. In Canon's opinion it is admissible for this kind of cameras.

If the focus point regularly breaks the accepted value (1 … 1/3 of the depth of field in dependance of sensor accuracy) we have a deal with back- or front-focus. But it has to be mentioned that cameras can also miss, not only lenses. That's why it is pretty reasonable to adjust the very camera if the focus misses with different mounted lenses.

n example of back-focus. Focus point is evidently out of the depth of field