This is going to get technical, but you may end up seeing my point. The Canon EOS R6 has two UHS-II speed SD card slots. That means you can record your data redundantly on two equally fast cards, and the fact that you are using both slots should not slow you down.
Not so on the R5. The R5 has one CFexpress card slot that provides the kind of data rate needed to sustain high frame rates and longer raw bursts at 45 megapixel resolution – the higher resolution is the reason why the R5 needs a CFexpress slot to do this. That all works fine as long as you leave the second slot empty.
Just as in a complex chemical reaction, the slowest reaction determines the rate at which your target product is formed, in an imaging pipeline, the slowest process also determines the overall speed at which the system operates. If you add a slow write process to your pipeline, you end up with an overall slower operation of your camera.
The CFexpress standard allows writing at faster speed than even UHS-II does for SD cards – that’s why Canon added the CFexpress slot to the R5. Now, if in addition to putting a super fast CFexpress card in the CFexpress slot, you also want to write to an SD card as a backup, then, in the best imaginable case, only your burst shooting clearing time would increase as the camera copies missing files from the CFexpress card to the SD card. This would necessitate Canon to have specifically created this feature in software.
So in reality, it’s likely to be worse. The buffer will probably fill up slightly sooner because it can’t be as effectively cleared because for every image the camera has written to the CFexpress at high speed, it needs to keep that image in memory until it’s also been written to the SD card at a slower speed. Once the buffer fills though, things will get really bad because now, you will feel the slower write on every shot – your frame rate may very well be drastically lower now than if you hadn’t used the SD card slot.
The car analogy
Sometimes a car analogy helps. Picture a car that has two axils, each with an engine. There are no differentials, nothing clever, just two engines each driving an axil. The only feature is that you can choose to disengage one axil’s engine. So the rotational speeds of the two axils need to match for an optimal driving experience. If they don’t match, you’ll be smoking up your tyres in short order. The EOS R5 is like a car where the speeds of the two axils don’t match. You can disengage the slower engine, which leaves you more liable to getting stuck in the mud, but at least then you can drive at full speed. The EOS R6 has two “slow” engines, but at least you can leave them engaged all the time, and because the R6’s body is lighter, you’re still going at full speed. All the time.
Comparing UHS-II vs. even the slowest CFexpress card standard, it’s pretty clear continuous shooting speed after buffer fill will be at least three times slower if you use the SD card slot in addition to the CFexpress one, than if you used the CFexpress slot without any back-up.
So it looks to me like Canon is expecting people to not be writing raw files to the SD card, but rather JPEGs or perhaps HEIF, rendering the R5 a bit of a poor man’s dual card slot camera. Let’s look into how well this could work. A sample DNG file I was able to obtain for the R5 was 53MB in size. Exported as an approximately 90% quality JPEG, it was 15MB in size – I would expect the R5 to export at this quality or better, in which case the JPEG file size could be even bigger – up to 71MB, in fact (JPEG compression is just not that great, as we probably all know); a lossless 8-bit HEIF file would be 31MB according to Apple’s implementation, so might be in the region of 47MB with Canon’s 12-bit implementation. In essence, writing JPEGs at 90% quality as a backup would work without slowing the camera down. Any other format or quality setting will probably create a bottleneck for the pipeline and become the rate limiting step. In the worst case (fingers crossed), you’ll probably see a 2.5 times slow-down. That would be a large 12-bit HEIF file – high quality JPEG should probably be avoided if you are looking for speed – the DNGs seem sufficiently well compressed that you should write RAW to both cards rather than JPEGs close to 100% quality.
Taking the assumption that the slowest CFexpress standard is used in the Canon R5, let’s crunch the numbers. The data throughput on that specification is 1GB/s, so the example raw file would be written in 1/20 of a second, suggesting full speed writing does not need the buffer. Strangely, although I’ve previously said that being able to continue to shoot without worrying about the size of your buffer is what we want, in this case not needing the buffer may be the worst possible case, because it could mean the manufacturer has saved the expense of the buffer, and thus the penalty from using the SD card slot will hit instantly, and knock you down from 20fps shooting to 6fps if you’re writing redundantly to a UHS-II card as backup. Then again, maybe the camera has a buffer, but if it can write at full speed without it, why would it? If it does have a buffer, the reduction from 20fps to 6fps will happen as soon as the buffer is full. How long that might take is a question we won’t be able to answer until someone with hands on a camera tells us how deep the buffer is.
It is my personal opinion that even though CFexpress cards are more expensive than SD ones, and even if the camera overall might have gotten a little more pricey from adding a second CFexpress card slot instead of SD, that small bit of added expense would have made a lot more sense than forcing customers to choose between the safety of their data and decent raw burst shooting.
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