I’ve been experimenting with the automated Focus Bracketing feature on my Olympus E-M1. Unfortunately I’m still stuck with the 14-42mm f/3.5-5.6 kit lens, I do not yet have a macro lens nor any other primes. Still, this little kit lens is surprising me with it’s sharpness and worked really well for this test. I will be using it wide open here in order to approximate the shallow DOF of macro photography, but of course in a real world situation it would need better ti stop it down a bit.
Note: you can use any autofocus lens with the Focus Bracketing feature but for in-camera Focus Stacking you are currently limited to the M.Zuiko 60mm f/2.8 macro, 12-40mm f/2.8 PRO zoom and the 40-150mm f/2.8 PRO zoom.
So I set out to figure out how this focus bracketing works and try to come up with some guidelines as to what focus differential to use. Unfortunately that last one proved difficult to do as I quickly discovered: the appropriate focus differential is dependent on focal length, subject distance, aperture used and magnification ratio, all factors that affect your DOF. The number of shots needed depends on your Focus Differential and the length of the subject from front to back. So basically you’ll have to experiment a bit until you get a feel for it.
Tip: I used the excellent and affordable Helicon Focus software to easily and automatically stack the images using all the default settings. Helicon Focus makes it a foolproof operation. Highly recommended.
You first want to set-up your camera by going into the Menu2 -> Bracketing -> Focus BKT and then setting both the number of shots (up to 999) and the Focus Differential from 1 to 9, 1 being the smallest step and 9 the largest. You should also mount your camera on a tripod or some steady base: I do not recommend you do this handheld. Note that turning this bracketing mode on will automatically put your camera in High Continuous mode and Silent mode (electronic shutter).
When setting the Focus Differential the main limiting factor is your DOF. If the differential step is larger than your DOF you will end up with bands that are out-of-focus so you should set the differential to a value that will allow the in-focus areas of each shot to overlap. This will allow the stacking software to create a smooth transition without OOF zones.
The number of shots needed depends on both the Focus Differential you selected and the length of the subject being photographed. If you do not take enough shots the back of your subject will remain OOF in the final image. If you select too many, you simply end-up with extraneous shots that you can discard or if you use them then part of the area behind the subject will be in-focus, which is usually not desirable.
Once you’ve got your subject in place and your camera on a tripod, you can frame your subject for the desired composition and put the focus point on the first part of the subject (in front) that you want to be in focus. When you hit the shutter the camera will focus there, take the first shot, then silently and very rapidly move the focus point steadily backwards and take a photo at each focus step. It all happens very quickly and in complete silence so that at times it feels like it didn’t happen. That is basically all there is to it!
Tip: I find that it’s much more efficient to tether my laptop to the camera using the free Olympus Capture software. This allows me to easily adjust the bracketing settings without touching the camera and to transfer the files immediately to my computer. I can then inspect the files to see if the selected Focus Differential and number of shots worked out well or need adjusting.
For this experiment I set my camera on a mini-tripod in front of a steel ruler so I could easily determine the in-focus area of each image. I ran several sets of 5 shots with the Focus Differential (FD) set at 1, 3, 5, 7 and 9 respectively in order to see the effect it has on the actual focus point. I always set the first focus point on the hole at 29.5 cm. These shots were all taken at 22mm and f/4.1 so all the first shots of each set should be identical. Here’s what the first shot looks like (click on the image to enlarge it):
As you can see if you zoom in, the focus point is at about 29.7cm and the in-focus area is from about 30.2 to 29.2 or about 1cm of DOF. Next let’s examine the second and fifth images of the first set taken with the FD set at 1:
If you look at the second image, you’ll see that the focus point moved back about .5 cm. If you look at the fifth and last image, you’ll see that the focus point is at about 26.7 cm. So for this lens at this distance, looks like FD=1 gives us about a .5cm focus step in between images.
Next is the second image of the set with FD set to 3:
Here you can see that in the second image the focus point has moved about 1cm and the final image about 5 cm. So with this lens and focal length, FD = 1 gives us about 1 cm between steps, which is also about what our DOF is so we probably should not go to higher FD numbers for best results. So to cover the entire ruler we would probably need 30 images with FD=3.
Let’s jump to the second and last images of the set with FD =9 (the maximum):
Here we can see in the second image that the focus point has moved back 3cm. The approximate results so far are FD=1 -> .5cm, FD=3 -> 1cm, FD=5 -> 1.5cm, FD=7 -> 2cm, and FD=9 -> 3cm. Keep in mind this only applies to this lens, focal length and subject distance.
Keep in mind that our DOF is about 1cm, so anything above FD=3 is too high and with FD = 9 giving us a 3cm step it is way too much and will cause bands that are out-of-focus. In the last image, the focus point has moved back about 14cm. Here is the final stacked image, which shows banding as expected. Note that 26.5, 23, and 19cm are out of focus:
Finally, here is a more representative image of an actual subject. This is a small ball about 4 cm in diameter. I used five images at FD = 5, which covers the whole subject from front to back:
I also tried 7 images at FD = 3 and it didn’t quite cover the whole subject. Note that 5×5 = 25 while 7×3 = 21, so looks like simple arithmetics can give us an idea of the coverage. Since 25 covered the subject, I can deduce that 9 images at step 3 (9 x 3 = 27) would also cover the whole subject.
I think it is better to err on the smaller side with the Focus Differential and on the high side with the number of images. This is because a smaller than necessary FD will give you really nice results whereas if it is slightly too long it will affect the sharpness of the final image. And if you have too many images, you can discard the last few that go beyond the subject before you stack them.
I hope you found this article useful. Let me know what you think in the comments below!