Is it true that the Canon EOS R5 does not have twin Card slots?

This is going to grow more complicated, but perhaps by the end, you’ll get my argument. The Canon EOS R6 includes two SD card slots that are compatible with the UHS-II speed standard. This indicates that you may record your data redundantly on two cards that are both equally quick, and the fact that you are utilizing both slots should not slow you down.

Not the case with the R5. The R5 has one CFexpress card slot, which delivers the kind of data rate required to maintain 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 be able to do this; it is also the reason why the R5 has one CFexpress card slot. Everything will function correctly provided that the second slot is left unfilled.

In an image pipeline, the slowest process also affects the overall pace at which the system functions. This is analogous to the way that the rate at which your desired product is generated is determined by the reaction that takes the longest to complete in a complicated chemical reaction. The functioning of your camera will become noticeably more laborious if you include in your pipeline a procedure that takes a long time to write.

Because the CFexpress standard enables writing at a quicker speed than even UHS-II does for SD cards – which is why Canon included a CFexpress port in the R5 – UHS-II is a marketing term. Now, if you want to write to a super fast CFexpress card in the CFexpress slot, but you also want to write to an SD card as a backup, then, in the best-case scenario that you can imagine, the only thing that will take longer to clear is your burst shooting clearing time. This is because the camera will copy any missing files from the CFexpress card to the SD card. Because of this, Canon would have had to design and program this capability into their software from the ground up.

Therefore, in point of fact, it’s probably going to be even worse. Because the camera needs to keep each image it writes to the CFexpress card at high speed in memory until it is also written to the SD card at a slower speed, the buffer is likely to fill up slightly more quickly than usual. This is because it is more difficult to clear the buffer completely after it has been written to the CFexpress card at high speed.

When the buffer is full, however, things will go from bad to worse since you will begin to experience a slower write speed on each and every shot. As a result, your frame rate may become noticeably slower than it would have been if you hadn’t utilized the SD card slot.

The parallel of the automobile

An analogy with an automobile might be helpful at times. Imagine a vehicle having two axles, each of which is equipped with an engine. There are only two engines, each of which drives an axil, and there are no differentials or other sophisticated components. The only feature is that you may deactivate the motor of one of the axes if you so want. Therefore, it is necessary for the rotational rates of the two axes to coincide in order to get the best possible driving experience.

If they aren’t compatible, you won’t have to wait long before your tires start burning. The Canon EOS R5 can be compared to an automobile in which the speeds of the two axes do not correspond to one another. You have the option of disengaging the slower engine, which will make it more likely that your vehicle will become bogged down in the mud, but it will allow you to go at full speed.

The EOS R6 has two “slow” engines, but at least you may leave them engaged all the time. In addition, the body of the R6 is “lightweight” (lower res), so you are still moving at full speed even if this is the case. Always and forever.

A few of the numbers

Comparing UHS-II to even the slowest standard for CFexpress cards, it is pretty clear that the continuous shooting speed after the buffer is full will be at least three times slower if you use the SD card slot in addition to the CFexpress one, as opposed to if you used the CFexpress slot without any back-up at all. This is because UHS-II has a higher data transfer rate than even the slowest standard for CFexpress cards.

As a result, it appears to me that Canon is anticipating that users will not be writing raw files to the SD card, but rather JPEGs or maybe HEIFs, which will turn the R5 into something of a dual card slot camera for the poor. Let’s investigate how successfully we can implement this plan. One of the example DNG files that I was able to download for the R5 had a size of 53 megabytes.

It was exported as an approximately 90% quality JPEG, and the file size was 15MB; 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 principle, writing JPEGs at a quality level of 90% as a backup would operate without causing the camera’s speed to be slowed down. If you choose a different file format or quality setting, the pipeline will most likely experience a bottleneck, and this step will then become the rate-limiting one. Even if the worst-case scenario occurs, you will probably experience a slowdown that is 2.5 times worse than normal.

That would be a huge 12-bit HEIF file; if you are seeking speed, you should probably avoid high-quality JPEG; the DNGs appear to have sufficient compression that you should write RAW to both cards rather than JPEGs that are near to 100% quality.

Let’s do some number crunching based on the premise that the Canon R5 uses the CFexpress standard that has the slowest transfer rate. The data throughput on that standard is 1 GB/s, which means that the sample raw file would be written in 1/20 of a second, which suggests that full-speed writing does not require the buffer.

Strangely, despite the fact that I’ve previously stated that being able to continue shooting without being concerned about the size of your buffer is what we want, in this particular scenario, not needing the buffer may be the worst possible case. This is because it could mean the manufacturer has saved the expense of the buffer, and as a result, the penalty from using the SD card slot will hit instantly, bringing your frame rate down from 20 frames per second (fps) shooting to 6 frames per second if you’re writing

Another possibility is that the camera contains a buffer; however, if it can write at full speed even without the buffer, what would be the point? In the event that there is a buffer, the frame rate will drop from 20 to 6 frames per second as soon as the buffer is completely filled. When someone has their hands on a camera and can tell us how deep the buffer is, then and only then will we be able to answer the question of how long it might take.


It is my opinion that even though CFexpress cards are more expensive than SD ones, and even though the camera as a whole might have become a little more expensive from adding a second CFexpress card slot instead of an SD one, that small bit of additional expense would have made a lot more sense than forcing customers to choose between the security of their data and the ability to take decent raw burst shots. This is because forcing customers to choose between the two options would have resulted in a lot more negative feedback

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