Ran an experiment last night to find out what is really going on with the focus stacking in my EM-1 ii.  I was under the mistaken belief that the focus differential setting was unaffected by the aperture setting. WRONG. I was under the impression that one would not get soft focus in the composite image when using a high focus differential setting as long as one also used a large f stop. WRONG.

So, what I was actually looking for was the minimum aperture I should use at each focus differential setting within the Focus Stacking feature.  IOW, I expected that maybe at a focus differential of 1, you could use f/2.8, but at a focus differential of 5 or 7 or 9...somewhere f/2.8 would fail to cover the gaps between the 8 shots the focus stacking feature takes. I wanted to know at what point f/2.8 fails to cover the gaps. I wanted to know the same for f/4 and f/5.6 and f/8.  Turns out I was asking the wrong question.

Here's what I found. At f/2.8, the highest focus differential one can use and not have soft spots between the frames and in the final jpeg is 3.  A f/4, the highest focus differential one can use is 3. At f/5.6, the highest focus differential is 3.  At f/8, 3. At f/11, 3.  Noticing a trend here?  But wait, higher f stops mean greater depth of field, right?  Why aren't the higher f stops covering the gaps?  Because as you increase your f stop number, the camera increases the space between the shots!!!!!

So, the difference between using f/2.8 and f/11 at a focus differential of 3 is the total depth of field covered.  If you go to a focus differential of 4, you will start to see some minor soft spots in the final product. Go to a focus differential of 5 and the soft spots interspersed become more apparent. The higher you go on the focus differential scale, the larger and more apparent the soft spots get...regardless of aperture. What the aperture does is increase or decrease the total depth of field.

So, it finally dawned on me that what Olympus seems to be doing here is relating the coverage as much to the number of shots stacked, as with aperture. It may well be that stacking 8 shots works well with a focus differential of 3 or less. If you want to use a focus differential of 4, you'd better stack at least 9 frames . If you want to use a focus differential of 5, better stack 10 shots. If you want to use a focus differential of 2, you may only need to stack 7 shots.  As I don't have a good focus stacking software, I was not able to test this theory. The EM-1 does do focus bracketing up to 999 frames.  I'm guessing that is overkill even when you set the focus differential to 10.

Bottom line...I did the testing and now I know. For in-camera focus stacking, the magic focus differential is 3.  Use the aperture to adjust the total depth of field. If you can't get enough depth of field even with a high aperture...then you need to switch to focus bracketing and take a lot more frames, which allows you to use a higher focus differential and still not get soft spots (I call them spots, but they are really more like strips).

One other note, the place to aim your focus when using the focus stacking feature is at 1/5th of the total distance from the close edge you want in focus.  So, you want your total depth of field to be 5 inches, focus 1 inch deep from the close edge of that 5 inches you've targeted (4 inches from the back edge).