How can the focusing distance manually set on a lens influence the firing of a flash?

Question

Situation: I use a Canon R6 Mark II with Godox flashes (TT350c or TT685c) for macro photography, flash monted directly on camera.

Problem: when the focusing distance on the lens is less than a critical distance of ca. 1m the photos are completely underexposed as if the flash did not fire. There is some flash, but it seems to be the pre-flash only. Slightly above the critical distance the exposure is OK and TTL-measurement works, i.e. the exposure remains right when changing the aperture on the camera with time and iso kept fixed.

Experiment: I checked by setting the focusing distance on the lens manually: when a little over the critical distance the exposure is OK, when a little below the photo is underexposed (of course the photos are more or less out of focus). For one of these experiments the recorded focus distances were:

• Photo 1: MakerNotes:FocusDistanceUpper:1.09, MakerNotes:FocusDistanceLower:0.96
• Photo 2: MakerNotes:FocusDistanceUpper:1.29, MakerNotes:FocusDistanceLower:1.09

I did the experiment with three different lenses.

However when I use the wireless transmitter (X2Tc) the exposure is OK even for small distances. Also using a completely manual lens works OK even with the flash monted directly on the camera. (Of course I changed the flash settings between normal flash and wireless flash.)

Question: Any idea why the distance setting on the lens makes a difference for the flash?

Answer 1

Another possibility may be that the pre-flash reveals to the TTL algorithm that even the minimum power the flash can be set to will be too bright. So it doesn't fire the flash at all.

You can test this hypothesis easily. Stop down a couple of stops and see if the narrower aperture decreases the distance at which the flash does or does not fire. Then open up to maximum aperture and see if the distance increases.

Answer 2

That sounds like your flash does not fire the main flash if it comes too closely after the preflash, and your camera does longer processing between preflash and main flash for larger distances it knows about.

This can be due to short distances changing autofocus aids or method, redeye preflash, light metering zones/algorithms, ISO adjustments for movement compensation and other things.

Other reasons may be different choice of electronic/mechanic shutter, different exposure time and a few other things.

Your best bet is likely shooting shots in close succession with just enough of a change to cause the difference, then compare the full output of tools like exiftool on the images for differences that could indicate a relevant difference in the process.

The good news is that for macro photography, your distances are more often than not constant enough that manual flash exposure will work out dependably anyway. Also it isn't fun to photograph subjects like jumping spiders with a pre-flash.