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]

No it can’t the EF 100mm F/2.8L Macro IS USM is only capable of 12 fps. The aperture cannot stop down fast enough to achieve 20 fps. Even if the camera is set to H+ it will slow to match the lens’ speed.

[Waddizzle]

“ The camera is set to one shot AF or manual focus, and the issue occurs on both of those settings. It's not AF related. “

I admit to not having watched the video.  It could be any number of things.  From lens IS to camera AF settings.  

[deebatman316]

But the constant is the lens’ aperture motor. Older EF lenses don’t support the high fps. A lot of them max out at 12. But the OP is trying to shoot 20 fps. Which isn’t possible the camera will slow down.

[karmlol]

The issue occurs even with IS disabled and manual focus (no AF).

[karmlol]

I don't fully understand your claim, because I don't know why a lens would need to stop down "fast enough" during a burst at a consistent aperture (the aperture doesn't open and close, it stays the same). However let's assume that you're correct and the highest FPS in my video is 12 (it could be, I haven't exactly measured). Why then does the FPS drop FURTHER down to 6 fps ONLY when shooting an underexposed subject? With all settings on manual?

My question isn't "why don't I hit 20", my question is "why does burst rate slow down to 6 when taking photos of an underexposed subject, when the exact same manual settings give a MUCH higher burst rate on a well-lit subject".

[deebatman316]

With Canon the lens aperture stays wide open until the shutter button is fully pressed. In between shots it opens back up and then stops back down to the selected aperture. Regardless of what the aperture it’s set to. In burst mode it opens and closes. It NEVER stays closed during burst shooting. That would be stopped down metering. Canon’s EOS line had NEVER used stopped down metering. It’s always used full aperture metering. This is true even on DSLRs.

[karmlol]

Ok, that is interesting information. I will try doing a 40fps burst with that lens (without flash) to see if that is true.

However, my question (why the difference in burst rate between the different cases shown in the video) remains.

[deebatman316]

There’s no need to see if the lens will keep with a 40 fps burst. The camera will AUTOMATICALLY slow down to the lens’ max fps. I’ve tried on both DSLR and mirrorless. The lens’ aperture can only stop down so fast. I did a burst test with an old EF 70-210mm F/4 lens from the beginning of the EOS line. With my EOS 5D Mark IV with it set to burst mode and sure enough it slowed down the camera AUTOMATICALLY to the lens’ max FPS which is 3 FPS. I’ve even tried with newer EF lenses that support 12 FPS. The camera will ALWAYS match the max FPS that a lens can do. 

[karmlol]

I just did the brief experiment. With no flash attached and AF set to manual, I set my R1 up with shutter speed 1/2000, f/5.6 and ISO 6400. I aimed at my lamp and held down the shutter speed for exactly 1 second (or as close as I could). It captured just over 40 separate RAW images. (Also repeated it with autofocus enabled, same results.)

I reduced aperture to f/8 and tried again. It captured about 30 images. So the R1 with RF adapter and EF 100mm f/2.8L Macro lens combination definitely IS capable of capturing 30-40 frames per second at apertures f/5.6 and f/8.

You may be correct that there is a limiting impact of shooting at very small apertures, but that limit is still at most 30fps at f/8, so that cannot explain the drop to ~6 FPS that happened in my video (which was shot at f/5.6 by the way, not f/8, as you can see in the settings I show at the start).

It also cannot explain why that limit wouldn't be the same in the 3 cases I showed in my video.

[deebatman316]

Some EF lenses can support up to 20 FPS in ELECTRONIC shutter mode but NOT mechanical. The EF 100mm F/2.8L Macro IS USM will depending on certain shooting conditions per Canon. But its NOT guaranteed that it can provide exactly 20 FPS. But the EF 100mm F/2.8L Macro IS USM is a lens that likely will support 20 FPS in electronic shutter mode. So it may likely cause the camera to slow down or maintain speed. The 12 FPS limit is for the mechanical shutter mode.

Per Canon 

When the Shutter Mode is set to [Electronic]During continuous shooting, if the following conditions are satisfied, high-speed continuous shooting (maximum burst of 20 shots per sec.) is performed.

The lens is set to maximum aperture except for when using an EF-S lens*.

For details on the lenses, see "List of lenses that the continuous shooting speed of max. approx. 20 fps less likely to decrease".

The continuous shooting speed may change in response to the shutter speed, aperture, aperture status during continuous shooting, subject conditions, brightness (shooting in a dark environment, etc.), type of lens, etc.

List of lenses that the continuous shooting speed of max. approx. 20 fps less likely to decrease This is the list of lenses that the continuous shooting speed of max. approx. 20 fps is less likely to decrease.