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.

[karmlol]

I just showed per practical example that my EF lens hits 40-30 FPS at apertures f/5.6-f/8.

But this is entirely irrelevant. Have you looked at my video? Could you explain *WHY* FPS is lower (around 6FPS) when shooting a darker subject, but MUCH higher when shooting a brighter subject, while exposure, aperture and flash power are manual and IDENTICAL between those three different bursts?

[deebatman316]

Canon did NOT give an exact fps for that lens but it said likely to achieve 20 fps. Flash will slow down your camera. The manual says low lighting conditions could impact burst shooting rates. "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". Also is the flash getting hot because it will reduce fps automatically if it starts getting hot. Can the flash's capacitors recharge fast enough are you using good batteries. I've had weak batteries cause slow recharge performance.

[deebatman316]

I noticed that you were using rear curtain sync. Rear Curtain sync only works with shutter speeds below 1/30th of sec. 1st Curtain sync is automatically applied if the shutter speed is above 1/30th of sec regardless if or if not set to rear curtain sync.

[karmlol]

Interesting, I didn't know that curtain sync bit. Good to know.

The actual cause of the FPS differences in my example are still a mystery though. Even if the manual says those factors can affect it, I understand that only for the cases where aperture, shutter speed and flash power are NOT locked and dialed in manually. For my case, I do not understand why (outside of my hypothesis involving the disabling of exposure simulation).

[deebatman316]

It doesn't matter if you lock your settings in manually. If the flash is getting hot the camera will slow down. If the flash cannot keep up with the camera it will also slow down.

[karmlol]

Again, you are correct in your statements, however the flash getting hot is not the issue in my case, as you can see in my video. Its behavior is somehow *ONLY* depending on the brightness of the object I photograph, not on the amount of time I wait before bursts, not on the order in which I do the bursts, etcetera.

[deebatman316]

The flash needs more power the brighter it is resulting in longer recharge times. Then the speedlite could overheat faster.

[karmlol]

Yes. However in my three bursts, flash power was IDENTICAL because it was locked in MANUALLY at 1/64th power. I really feel like the things I have said many times now are being ignored or not understood. I've clarified this many times now.

[deebatman316]

The EL-5 is the only speedlite that can keep up with the R3. The 600EX II-RT cannot keep with it in electronic shutter mode. I would think the same would apply for the R1. My old 550EX cannot keep up on my 5D Mark IV. But the 600EX II-RT can because it was released with the 1DX Mark II. Which shoots more fps than the 5D Mark IV. 

[karmlol]

I'm not expecting it to keep up. Very surely the 600EX would not keep up with 40fps. I know that, and that is not my issue.

My issue is, why does it happily do 20fps when shooting a bright subject, but only 6fps when shooting a dark, underexposed subject, AT IDENTICAL, MANUAL EXPOSURE AND FLASH POWER settings.