The FoCal Target

The FoCal target is specifically designed to work with the FoCal software and the camera autofocus system to allow not only reliable calibration, but also the ability to warn and/or adjust various settings under changing test conditions.

For best results we recommend you use a FoCal Hard Target as your target will then be the same as what is used during the development and testing of the FoCal software. A FoCal Hard Target is available from the FoCal store at http://store.fo-cal.co.uk.

Figure A wall mounted standard size (150mm) FoCal Hard Target

Alternatively, you can print your own target from the image files supplied in the Target Images directory of the downloaded installation package. We recommend you use an ink jet printer and print on matte paper for best results.

Target Distance

FoCal allows you to calibrate the combined AF system of your camera and lens in order to achieve the best possible performance. The result of the testing is a value which is applied to the AF Microadjustment setting (Canon) or AF Fine Tune setting (Nikon) of the camera.

Unfortunately, this number is applied to the camera-and-lens combination for all focus distances, whether the subject is 1m or 100m from the camera, but each distance often requires a slightly different value.

In reality, as you move the focus point towards infinity, the required microadjustment/fine tune value stabilises, so generally the rule is to calibrate at a “far” distance from the camera.

The following graph shows an example of the change. When the distance is close to the minimum focus distance (the far left of the graph), the microadjustment value can change a large amount with small changes in distance, making calibration both difficult and not very useful for general shooting. However, as you focus further from the minimum focus distance, the value stabilises – the shaded region has about the same AF microadjustment value required for all the focus distances out to infinity.

As a rule-of-thumb, testing at around 50x the focal length of the lens gives good results (so for a 50mm lens you test with the target around 2.5m from the camera). However, for longer focal lengths, this can prove impractical, but luckily also unnecessary.

When you start testing with longer telephoto lenses – e.g. 300mm and above – you can generally test at around 20x the focal length as the curve starts to stabilise earlier for longer lenses.

Recommended Test Distance

The Target Distance Tool will help you choose the right minimum test distance, but here are some examples for common lens focal lengths:

Note that Focal length is the actual focal length of the lens (and teleconverter combination if applicable) – the sensor “crop factor” can be ignored.

The image below shows this visually (not to scale):

Lighting the Target

Low light will affect the AF performance of a camera. Phase detect AF works by looking for comparable features in two copies of small sections of the same image which take different paths through the lens (see the section on how AF works at the back of this document for more information). If the light level is too low the two copies can appear very similar and the AF system will not be able to lock. So it is important to ensure you have a good light level.

Ideal lighting

Bright daylight would be ideal, but you want to try to ensure that the light level doesn’t change too much over the course of a test (typically 1 or 2 minutes), so if the sun is going in and out of the clouds then you may have problems. The FoCal algorithm does have features which allow it to produce consistent and reliable results with small changes in light level, and it will also stop the test if the light level changes too significantly while the test is running.

During development of FoCal, it was found that the low-light AF performance of different cameras degrades at different light levels. FoCal has built in detection for the light level and will warn if you are running with the not enough light (specific to each supported camera).

Lighting to avoid

There are some lights which should not be used for testing with FoCal.

• Some mains powered LED lights can flicker at 50 or 60 times per second, and if you are running with high shutter speeds this can lead to serious problem with the analysis.
• Some fluorescent lights can also flicker which can cause the same problems

Whilst the target is black-and-white, the analysis does take into account the way different colours travel through the lens, so it’s a good idea to try and light the target with a light that is relatively close to white. This is not an absolutely critical requirement (any normal household lighting, studio lights etc will all be fine), but you will find, for example, that if you illuminate the target with a purely red or purely blue light you may find the results differ.

Continue on to Camera Setup (Physical)

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