Do we really need Sandisk Extereme Pro 95mbps read & 90mbps write speed?

Started Dec 26, 2013 | Discussions thread
Allan Olesen Veteran Member • Posts: 3,391
Re: What is the A77's actual/real speed for writing from Buffer to SD card?

Sam_Oslo wrote:

I can't find it in Soy documentations, or on the net, but if I'm not mistaking A77's can transfer at about 50 MB/s in reality.

On average, yes. But as I have mentioned a few times earlier in this thread, that does not necessarily mean that the peak transfer speed is that low.

Anybody knows the exact speed for writing from Buffer to SD card?

If we assume that the camera can't transfer from buffer to card continuously, because some part of the process is serial, or some of the hardware needed for writing to the card is busy doing other stuff some of the time, we can try make some sense out of the data we saw in the video earlier in the thread.

13 JPG+raw is approx. 390 MB.

With the 45 MB/s card which according to you has a real write speed of 38 MB/s, it took 14.48 seconds total. If my assumption is correct, this could mean that the camera used 390/38 = 10.3 seconds for actual writing. This leaves 4.2 seconds used for other stuff. These 4.2 seconds are probably independent of card type.

With the 95 MB/s card it took 9.14 second. We subtract the 4.2 seconds which I assume is used for other stuff and end up with a real write time of approx. 4.9 seconds. This gives us a write speed of 390/4.9 = 80 MB/s.

So my best guess (including a lot of possible error accumulation) is that the a77 can write with at least 80 MB/s _when_it_actually_writes_.

This means that the 95 MB/s card is not much overkill. According to you, the real write speed of this card is 88 MB/s. The calculated 80 MB/s uses almost all this speed. And since my calculation is rather sensitive to errors, it is entirely possible that the camera actually writes at a speed of 88 MB/s, meaning that the card could still be the bottleneck.

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