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nonhorva Jan 14, 2010 3:50 AM

SDHC class speeds
I have an old 4GB class 2 card which I recently upgraded to a 16GB Class 10 for my OLD Canon Power Shot A640

I often take continuous motion shots for sports (racquet / baseball bat swings) for individuals on our team …….

The read speed on the old class 2 card is

The read speed on the new Class 10 card is

Now the class 10 card is obviously faster than the class 2 as shown in the benchmarks

When I tried taking continuous shots in a span of 20 seconds
Class 2 card took 21 photos ~ clicking sound started lagging at the 10 ~ 13 second mark
Class 10 card took 26 photos ~ clicking sound did not lag at all for the 20 second duration

My question is, if the class 10 card is twice as fast, why didn’t it take twice as many photos? Or is it more on the limitations of the camera?

JimC Jan 14, 2010 5:43 AM

Cameras have an internal buffer (fast internal memory). So, using a camera with a fast burst rate, you won't see any difference in camera speed until the internal buffer is full.

Then, the camera will slow down to the speed it's able to write to the card. Most cameras work in parallel (i.e., they're also writing to the card at the same time you're taking photos and they're writing to the internal buffer. They're also processing the data from the sensor at the same time (which adds to the processing workflow).

Camera processing ----> Internal Buffer (faster)------>Memory Card (slower)

From what I can see from tests with your camera using an Extreme III, it's probably able to write to a fast card at approx. 3MB/Second (keeping in mind that processing time for the images is also involved). So, you'll tend to get diminishing returns with a faster card (you may buy a card that's twice as fast and only see a small increase in performance, and once you get to a certain speed card, you won't see further improvement, because the camera is the bottleneck, not the speed of the card).

You can get an idea of the improvement by taking a number of photos in burst mode, then timing how long it takes to complete writing the images to the card (most cameras have an activity light that remains lit until the internal buffer is flushed to the memory card).

nonhorva Jan 14, 2010 7:05 PM

Thanks JimC,

So for me to get the optimum effect, I can
1. get a new camera with better spec .... this camera is kinda old
2. match card speed to camera write speed
Is that correct?

I will try the burst mode you're telling me about 2morrow at batting practice and see how I go .......

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