Is the Write Speed the Right Speed?

I have been aware that SD cards, and particularly MicroSD cards, can have read and write speed limitations, however, I only recently have had two separate issues that turn out to have been due to slow write speeds.

Though I didn’t realize it at the time, write speed was likely the issue that caused some videos taken by my little DJI Mavic Mini to fail. I started it recording and flew around for a while. Later, the video was only about a minute long; I had definitely intended to record more than that. I now think the slow SD card write speed caused the high resolution video to simply overwhelm the card and the camera just shut off. I presume there was no notice, but I will look for some kind of on screen warning in the future.

I also had troubles with a recent astrophotography capture. I was getting 100 subs of 30 seconds each. The length of the capture doesn’t affect the size of the file, but when you are going to capture for nearly an hour, you don’t want to wait any longer between shots than necessary. Most DSLR cameras will capture to an internal buffer then write that image to the memory card between pictures. Generally, the write time of the camera is hidden from the user because we tend to take a picture or two then put the camera down while we wait for something else to take a picture of to come around. However, with astrophotograhy, you are taking dozens or even hundreds of long exposures in a row. In the example above, I had the camera set to pause for two seconds between exposures. That pause time accounts for nearly four minutes in the whole capture process. I noticed that a little while into the capture, the busy light was staying lit past the time for the for the next picture to take. Because the intervalometer just sends a 1 second signal to the camera and the camera was using it’s internal shutter timer, when this busy event would happen, the camera miss a shutter event, which would then allow it to catch up on the write process then sit idle while the 31 or so second wait on the intervalometer would time out. It would then capture 5 or 6 images before the write busy would add up enough for it to miss another shutter event. So, my 100 captures would have turned out to be 90 or so without intervention.

I changed the delay between shots on the intervalometer to 5 seconds instead of two. This helped it get to 10 or 12 shots before the camera was busy and missed a shutter event. I set it to 8 seconds for the remaining 40-50 shots, the busy light did not miss any more shots.

Had the 8 second delay been in place for the entire 100 shots, it would have added 14 minutes to the entire process. It’s not like that is a huge part of one’s life, but after you capture 100 lights, then you need to capture 30-50 darks at the same shutter speed and 30-50 flats. The flats will be at a shorter shutter speed, but that actually makes the write speed problem worse.

I found, not surprisingly, that a) the read and write speed on memory cards is rarely specified and b) when it is, write speed has a bigger affect on price than capacity. 64GB cards with 250MB/S write speed cost more than 128GB cards with 130MB/S write speed and c) anything slower than about 100MB/S will probably not show the spec and those will pretty much always be inexpensive.

To address both problems, I ordered four 64GB cards that specify 250MB/S read and 130MB/S write speed from B&H Photo.

By the time they had arrived, I found some somewhat questionable data that indicated that the write speed of the particular card I had used in both the DJI Mavic Mini and the EOS Rebel T6i probably has a write speed more along the line of 30MB/S. I found a simple disk benchmark program and tested the new and old cards.

The old card, as expected, was pretty slow:

The new card was much faster, exceeding the write spec, assuming they specify the best spec rather than the average:

No SD card does well with random reads.

For perspective, here is the report on the 250G SSD in my laptop:

In practical terms, I set the camera to it’s fastest shutter time of 1/4000 second and set it to continuous shooting. Press and hold the shutter button, and with the new card, it takes 7 pictures at the max speed of 5 frames per second, then it slows down to about 1 per second. Release the shutter button and it takes about 5 seconds for the busy light to go out. With the old card, you still get the 7 shots buffered in the camera, but the catchup is more like 1shot every 2 seconds, then it takes nearly 10 seconds for the busy light to go out. This card should definitely be an improvement.

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