Canon R1: flash burst speed lower when non-flash exposure is underexposed

[karmlol]

Firstly, let me describe my issue: I have an R1 and a 600EX II RT flash. When I set everything to manual (shutter speed, aperture, ISO and flash power), it should take photos at the same burst speed regardless of what I'm shooting (some photos would just end up exposed differently). However I'm finding that when my subject would (without flash) be underexposed, the burst speed WITH flash is much lower, despite everything being identically set on manual.

Here's a video example: https://www.youtube.com/watch?v=cH8uXa50IGA

It starts out black (because of exposure simulation, the scene is underexposed without a flash), but as soon as I half-press the shutter, the scene brightens up. The burst that then follows is slower the darker the subject is, even though shutter speed and flash power are identical.

My suspicion is that it has to do with the scene preview exposure behaviour, where it automatically turns off exposure simulation whenever a flash is attached (or whenever metering starts with a flash attached): https://www.youtube.com/watch?v=ZjWVEQbdi40

I suspect it tries to properly expose the scene because there is a flash attached, increasing aperture to f/2.8 (up from f/5.6) and increasing ISO, and then has to swap back to f/5.6 and ISO 200 again for the photo. (And then back and forth again and again in between burst frames??) I cannot think of any other explanation why the burst rate is different with all those settings identical in full manual mode. Burst rate isn't bottlenecked when not using flash, so that rules out processing or card write speed issues.

Has anyone else come across this issue, and is there any possible workaround that does NOT involve taping off contacts and removing the possibility of using E-TTL?

I've read this thread on a related topic, however my use case is different because my flash burst rates are actually significantly reduced because of this issue, whenever relying on flash to fill an underexposed scene.

[deebatman316]

Ambient lighting affects the camera’s metering system. Then you have the buffer too and the flash needing to recharge. 

[p4pictures]

The flash will speed up and slow down depending on how hot it is. While your video shows the faster speed working after the previous bursts, I don't know what you were doing before this video. If you had already done a similar test then made the video, it is possible that the Speedlite was "hot" at the start for the first sequence, and therefore could not keep up so slowed the frame rate to allow it to cool down even while working. But by the third set it was cool enough to go faster again. 

If you manually raise the camera ISO 1-stop, you can drop the flash power by 1-stop from 1/64 to 1/128 and that might be better for fast bursts.

[karmlol]

The flash had been cold for several minutes before that video. It is not related to the flash temperature. It's clear when it overheats: it will hitch slow down to only one or two shots per second, which is very noticeable.

This behavior is not just a one-time occurrence; it is consistently happening in the way that I described, with various (manual) settings. Only when photographing a lit scene is my flash (correctly) the bottleneck for burst rates. When photographing a dark scene, which consistently coincides with the visible disabling of exposure simulation/re-exposing of the viewfinder, the burst rate is significantly lower (down to around 6fps). The order doesn't matter.

It happens the same way at minimum flash power (but 1/64 is already pretty low).

I'm actually curious whether other cameras that have the same flash / exposure compensation behavior (like the R5, or R5 Mark II) can reproduce this issue as well, or whether it's specific to the R1.

Try these steps:

1. Attach a flash to your camera using the hot shoe or smart shoe or what have you
2. Set camera to manual (M) mode
3. Set shutter to electronic shutter (for highest burst rates)
4. Set burst rate to the highest continuous burst setting you have
5. Set autofocus to manual or one-time focus
6. Set aperture to something smaller than your lens' max aperture (set it to f/8)
7. Set shutter speed to 1/250 (or to 1/200 if your body's max flash sync speed is lower)
8. Set ISO to 200
9. Turn on your flash and set it to M mode
10. Dial in flash power to something low like 1/64 or 1/128
11. Turn off your flash and find three scenes: (1) correctly exposed (like a white well-lit surface), (2) slightly underexposed (like a brown lit surface), (3) significantly underexposed (like a black unlit surface).
12. Turn your flash back on. First try the black scene. Focus and wait for the viewfinder to finish trying to expose the preview image, then fire off a burst and make a mental note of burst speed.
13. Do the same for the medium lit and well-lit scene.

Is your burst rate the same in all 3 situations? Manual exposure and flash power settings are identical, so the burst rates SHOULD be identical.

[p4pictures]

Only three cameras can shoot with flash and electronic shutter mode, EOS R3, EOS R5 Mark II and EOS R1, all other EOS R-series cameras will not fire the flash in electronic shutter mode. They will in first curtain electronic and mechanical shutter mode. So only those three models are suitable to test. My EOS R6 Mark II will shoot at 40fps in electronic shutter, but is limited to 12fps with first curtain electronic or mechanical. 

[karmlol]

That may have been true before the Canon R1 came out. The R1 is their new flagship camera, and I can confirm (and you can see in the video I linked) that the flash DOES fire in electronic shutter mode. It even has a flash sync speed of up to 1/400th of a second in electronic shutter, full frame mode.