Shooting raw won't hurt, especially if you have wide dynamic range.

I simply start at the distance closest to the camera and focus slightly closer, then advance focus until I get to the farthest distance, then an extra shot beyond the farthest point for insurance.

At 20" your depth of field is 1/3 inch, so if you are a little conservative, I would use 1/6 in with the understanding that at 23" your DoF is .48", which would prompt me to use 1/4" increments.

This is all much easier if you use live view and eyeball it. You'll be surprised at how accurate that really is.

Also, On1 Photo Raw looks promising - it allows you to visually "select" your desired depth of field from your sequence using sliders. It's not quite ready for prime time, though. I am waiting for when they provide access to the layer masks, important for when you need to fine tune a mask to make it look right.

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I have been stacking exactly that sort of stacking for the last couple of months. The closer you are to the subject the more shots will be needed; the longer the focal length the more shots; the wider the aperture the more shots; the deeper the subject the more shots. I am using a Canon 100mm macro lens at f/8 so that is equivalent to what you are using.

This flower is about the size of the one you are asking about, and I filled the frame with it. I think it took about 30 shots. You can always shoot more frames than you need and then pick the sequence you want to stack later.



tTulip on Flickr

Here is a trick I used - put a printed page on a table and shoot it at an angle and at the distance you want to shoot your flower. If there are any out of focus areas in your technique, they will easily be seen.

Mike