Focus Failure warning

[Mjfoto]

I keep getting this "focus failure" when focusing at high magnification. Using a 100mm macro, extension tube, 2x converter and finally a Raynor 205 on front. Since focus stacking needs to be done using auto focus, it doesent want to grab the focus where I want, it will jump to another spot in the image? I'm using DPP4 in Live view and when using the focus magnifier to nail the focus, it jumps when I touch the shutter button on screen. Can Anybody figure out what is happening. I am photographing sand grains. Thanks for anyones imput

[Tronhard]

You have a lot of magnification to enhance the performance of a macro lens.  First, we need to know more about what gear you are using.
1) What version of the 100macro lens are you using: the L version or non-L version
2) What camera are you using?

Has this type of gear been successfully used to photography something as small as a sand grain. Normally one would expect the use of a microscope with a camera attached to be used for this.

While focus stacking is to be expected, setting this up using autofocus is, to me, a risky process.  I would normally expect to set the starting point manually.  I used to photograph circuits on closed circuit boards which, while larger than sand grains, are dealing with the same issues

[Mjfoto]

Yes this rig has been used before by others. I'm using a R6 with 100mm2.8L, IS usm

What you said about starting the first couple shots manually then let the stack begin sounds like the best option. The puzzling part is why it won't grab the focus point that I set it on??

[Tronhard]

Autofocus is an algorithm that depends on several characteristics being there, including contrast, tonal (brightness) values etc.  If I am not mistaken, at high resolutions, grains of sand have many different facets and that is likely to confuse the autofocus system.

[Mjfoto]

Not so much the facets, but the smooth surfaces on many grains and the lack of contrast across a smooth quartz surface

[Tronhard]

I wasn't sure of the right characteristic, but the result is the same.  I think starting with manual focus override might be a good place to start.  Looking at the image you have shared, you will likely have a razor thin DoF, but you must want a fairly massive one compared to that, so a lot of focus stacking.

A good place to go for some more advice would be a site called cambridgeincolour.com (it's a UK-based site so colour is the right spelling).  There are some highly technical photographers in this genre there.  It's free to join and an excellent source of general photographic advice.

[Waddizzle]

You seem to be using an supported combination / configuration of gear, beginning with using extension tubes with a 2x teleconverter on a macro lens.

Are you familiar with the term, “lens groups”?

[Mjfoto]

Did you mean' unsupported' combination? No I am not familiar with "lens groups". Unless you are referring to the element groupings in the lens?? I am interested