Why does my Nikon D800 take longer to finish writing each RAW shot to an SD card after several photos?
Asked 3/14/2020
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2 answers
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On a Nikon D800, I tested a SanDisk Ultra 64GB 80 MB/s SD card by timing the rear green write LED after each shot. Shooting uncompressed 14-bit RAW, the write-light time increased over the first several images, then later dropped again. Between shots I waited about 2–3 seconds. Using a faster CF card as the primary card, write times dropped to about 1 second. Why would the camera seem to slow down after a few shots even though each image has already been taken and the write light eventually turns off?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
1
This is normal and expected. It's basically queuing theory. Here's an analogy that might help in visualizing this:
If you walk into an empty store (sensor/camera), grab an item (light/data), and get in line (buffer) for the cash register, they'll ring you up right away (write to SD card), and you're on your way.
If you walk into a packed supermarket and grab an item — even if there are n cash registers, if there are more than n people ahead of you when you get in line you're going to have to wait before you leave. All else being equal, the more people in line ahead of you, the longer you'll wait.
Similarly: if the length of time between images captured < the time it takes for the buffer to persist that image to SD/CF card and clear itself, your shot will have to "wait in line". How long is a function of e.g., buffer speed, buffer size, SD card speed, and file size, not to mention other variables like compute power.
Hope this helps!
Originally by user85832. Source · Licensed CC BY-SA 4.0
user85832
6y ago
0
Generated from our catalog & community — verify before relying on it.
This is normal buffer behavior, not a fault.
Each photo is first stored in the camera’s buffer, then written to the card. If you take the next shot before the previous data has fully cleared from the buffer, the new shot joins a queue. As that queue builds, the total time associated with each later shot appears longer, even though the camera already captured the image.
With large uncompressed 14-bit RAW files, an SD card can become the bottleneck. Your 2–3 second pause between shots was still shorter than the time needed to fully drain the accumulated data, so several shots built up in the buffer and write times increased. Once the queue finally cleared, the times dropped again.
That also explains why performance improved dramatically with a faster CF card, and why using the SD card only for small backup JPEGs didn’t show the same slowdown.
So yes: the green light indicates card-writing activity, but the overall behavior you measured is mainly the result of buffer fill-and-drain timing plus card speed limits.
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