With digital SLR cameras it can occur that the sharpness is given but does not lay on the focused point. Although it was focused on the right object spot, the focus could lay in an area in front or behind the targeted point – this is termed front focus and back focus.
Front focus / Back focus
SLR cameras use in viewfinder mode the so-called phase detection autofocus for automatic focusing (see article Contrast, phase detection and hybrid autofocus). Here, a separate autofocus module that is placed in the camera bottom, is used. And at this point slight discrepancies may occur. To ensure the automatic focusing, a part of the light information of the subject is directed by an additional mirror to the autofocus module. This in turn determines the necessary focus correction and passes it on to the lens, which sets the targeted object in focus.
Reasons of front focus / back focus
The lens and the autofocus module can be considered as two separate optical systems. Thus, there are two optical paths, one to the camera sensor and one to the autofocus module. To obtain a good focus, these two must be perfectly aligned with each other. If an inexact tuning of both systems exists or the lens or the autofocus module is not adjusted precisely, focus deviations can be the result. Furthermore, an inexact adjustment of the camera sensor also can cause the focus shift.
In a photo where the focus point lies on the center of the object, the shift of the focus area is noticeable as follows:
Front / back focus by lens
Although the autofocus module works fine and determines a correct focus adjustment, it may happen that the focused point is not in the image plane, but on a point / area in front or behind this. Since the light passing to the sensor can be seen as a separate path, a not exactly adjusted lens could be the reason for a front / back focus. Another cause could be that the autofocus motor of an older lens does not regulate the focus distance precisely. An indication for this would be an alternating occurring front and back focus.
Front / back focus by camera
However, it may also be that the autofocus module or the camera sensor is not properly adjusted. Due to slight deviations of the components, the distance that the light has to cover between lens and autofocus module or between lens and camera sensor, is no longer correctly. As a result of the deviation, the focus area in the image is not exactly on the targeted point.
Checking front / back focus
It can be checked at home if the own camera-lens combination tends to a front or a back focus. For this test a scale is required, with whose help a focus shift can be detected.
a) Focus test chart
Who looks around the net will find test templates with various scale size from different providers. The reason for this can be explained by the depth of field. To be able to read out the focus area afterwards, the depth of field should turn out low. That means the sharpness level will devolve in blur and can be recognized very well. This is important because only by this a front / back focus can be located. Depending on the depth of field, the scale of the test template should cover a corresponding scale range. This may vary with the providers. But of course you do not need to use a different test chart for each lens. One focus test chart is sufficient because the photographer can control the depth of field, too.
Several free printing templates can be downloaded and printed from Internet websites. Appropriately cobbled together, the autofocus check can start. But the self-made test charts are not always very stable or accurate, because print quality as well as construction may be inaccurate. Who appreciates a more professional assessment, chooses a special focus test chart like the SpyderLenscal / Datacolor or the LensAlign / Michael Tapes.
The stable test panels often have a clear scale information and a separate focus spot on and can be set up in an exact angle of 45 degrees. But why 45 degrees? At an angle of 45 degrees, the depth of field extents in both directions. If the camera is positioned in front of this slanted test chart, a good statement can be made, whether the sharpness lies before or behind the central targeted point "0", so a front or back focus exists.
b) Setup and shooting
Set the focus test chart on a table so that it is placed inclined at a 45 degree angle to the camera. In addition, provide a good illumination. Use a tripod and place your camera in a corresponding height in front of the test chart.
As already mentioned, the depth of field should be shallow. For this reason, a large aperture (small F-number) and a lens corresponding shooting distance should be selected. It is important that the distance is not shorter than the minimum focusing distance, so the minimum distance at which the lens can still focus an object.
Select the viewfinder mode. To avoid unwanted blurring by camera shake, it is advisable to keep the shutter speed short. Also, turn off possible image stabilizers. Select the central AF point and focus on the target of the focus test chart. Many templates offer an extra field or an additional focus plane for this. Make some images.
Open your images in the enlarged Live View of your camera (or on your computer). With the help of the scale, now it can be seen whether a front or back focus occurs for the used recording system. Let us have a look at the crops of the sample pictures, which we have taken with the SpyderLenscal (Datacolor):
In case that a recording system shows a front / back focus, it is not clear whether this is caused by lens or camera. But that can be easily checked. Is, for example, a focus shift present, you can repeat the test for other lenses. If those do not show a shift, it is obvious that the lens causes the front / back focus. Does the focus shift occur with all lenses the same, it might be that the autofocus module in the camera is not adjusted precisely or the two systems are not aligned perfectly.
If a camera-lens combination has a front / back focus, it would be possible to send the camera and lens to a corresponding service for correction. In case of slight focus deviations this is not absolutely necessary because these can be corrected with the help of the camera. Therefore, many DSLRs offer the menu option “AF fine-tune” ("AF Fine-adjustment”, “AF Micro adj.”). However, manufacturers like nikon recommend in case of focus problems to send the affected device to the service (source: www.nikonimgsupport.com/eu/BV_article?articleNo=000006300&lang=en_GB&setRedirect=true).
To determine the right correction value, you can use a test template and the monitor display:
- First, the focus test chart should be photographed in viewfinder mode.
- Then, the image can be displayed on the camera monitor. With the help of an enlarged view, here a possible front / back focus can be detected.
- Is, for example, a front focus present, change to the AF fine-tune and gradually adjust the correction value in positive direction. This will move the focus point to the back and so compensate the front focus.
- After that, change back to viewfinder mode, record the test template again and check it on the monitor view.
- Repeat this operation until the necessary correction value is found, until the focus is at the center of the scale "0".
Because one lens normally is not like the other, the autofocus check and the later fine tuning should be performed for each camera-lens combination. Also, it is advisable to repeat the front / back focus test periodically. Among other things, the ambient temperature can affect the material of camera and lens. Depending on heating or cooling, plastics stretch or shrink. This in turn may affect the autofocus accuracy due to different temperatures. Who would like to play safe, checks the autofocus accuracy also for these cases.
Lens fine adjustment
The slight lack focusing can not only be corrected by the camera or the manufacturer service. Sigma and Tamron offer for their own lenses a USB Dock (Sigma) / TAP-in Console (Tamron). With these it is possible to make focus adjustment for the individual lenses. For example, fine adjustments for different focal lengths at various focus distances. Besides, those tools allow making other adaptations like limiting the focus distance or updating the lens firmware.
For more information please visit the manufacturer websites. There you can also find out which Sigma / Tamron lenses are compatible with the USB Dock / TAP-in Console:
Sigma USB Dock: www.sigma-global.com/en/lenses/cas/product/accessories/usb-dock
Tamron TAP-in Console: www.tamron.eu/magazine/blog/detail/719