7D Mark II USB slow transfer speed

Started Jul 25, 2016 | Questions thread
OP Woodbird New Member • Posts: 7
Re: 7D Mark II USB slow transfer speed
1

Tazz93 wrote:

I haven't tried it on my 7D2, but my 5DS transfers right around that same speed using the USB 3.0 cable. I never transfer with the camera anyway so I immediately just filed it away as "break in case of emergency". I'm not sure where the bottleneck comes in, but I'm curious if the hardware on the camera is really a USB 3.0 port.

Here a site that tested quite a few cards and readers, Camera Memory Speed, see the bottom for its take on the slower than expected 5DS USB 3.

It seems my issue is not uncommon. The following is my take of the issue. I am familiar with system programming but knows little of the software in cameras.

My guess is that the USB 3.0 port is only useful when tethering the camera, when the images are read directly from the buffer to computer (I don't do tethering and thus cannot be sure if it is the case).

When downloading images, the bottleneck is likely how fast the camera read the card. And it is probably has more to do with how the camera does the reading. When a computer read from a card reader, with proper software (such as Lightroom), the reading from the card and the writing to disk happens at the same time. And the later is in fact often buffered. For example, when importing in Lightroom, with a fast SSD drive, I can see peak write speed at 400MB/s, occasionally and most of time 200MB/s, far above the read speed.

On the other hand, the in camera transfer through USB probably happens in two stages,

1) Read one or more images into buffer

2) Transfer through USB. Go back to step 1) until buffer is cleared

If it is done in this fashion, then I won't be surprised to see a 30MB/s rate for 7D II and about 50MB/s for 5DS. The reason that the later is faster is likely due to the larger image size. Without going into detail, it is similar to the situation where copying a thousand 1MB files is far slower than copying one 1GB file in computer.

The camera CPU probably only has one core for general purpose use and the bulk of the die is most likely dedicated to a fast DSP for processing. And the bandwidth is probably limited for reading since it has to prioritize fast write to card speed. Or probably it is just an issue of stupid programming.

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