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Frame rate vs. Buffer

ericbowles

Atlanta, US
10530 posts

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ericbowles Moderator Awarded for his in-depth knowledge and high level skills in various areas, especially Landscape and Wildlife Photoghraphy Writer Ribbon awarded for for his article contributions to the community Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015 Nikonian since 25th Nov 2005
Thu 21-Feb-13 11:58 AM | edited Thu 21-Feb-13 07:56 PM by ericbowles

With the D7100 announcement, we've had some discussions about the frame rate and buffer. I wonder if everyone is aware of the distinctions and the impact on shooting.

For the sake of discussion, I'm using Rob Galbraith's data as a reference.
http://www.robgalbraith.com/bins/camera_wb_multi_page.asp?cid=6007-12454

Frame rate refers to how quickly the camera can release the shutter. The D7000 and D7100 are at 6 frames per second, but the amount of data captured and processed in that time is different. The D7100 spec has 6 24mp images at around 50MB each while the D7000 has 6 frames at 35MB each. Frame rate does not truly consider write speed.

Frame rate does not mean you capture the frame you need. At 1/500 sec 6 fps means you have 6/500 of the time. While you might capture small movements, you still miss 99% of the time. So moment of action still requires some skill and luck. It depends a lot on the subject. With golf, the instant when the club makes contact with the ball is so short that frame rate makes very little difference. With a large bird landing on a branch or nest, frame rate can give you several very nice images to choose from.

The buffer is the place that images are stored until they are written to the card. Larger buffers are better, but you still need to process the image into the buffer and write it to a card. The former is a function of processor speed and camera settings. The latter is a function of your card selection.

Write speed on faster cards run around 20-26 MB per second (D7000) - or less than one frame. So if you are shooting at 6 frames per second, and only writing at one frame per second, the buffer fills. But it takes 24-39 frames (edit - this number is for medium JPEGs - Raw files are much lower) to fill the buffer depending on your card. That means you can sit there and fire the camera for 5-8 seconds before the buffer is filled. That also means that once filled, the buffer takes 20-30 seconds to clear by writing all the images to the card. As camera image files get larger and without major changes in cards, a faster frame rate just fills the buffer quicker and takes longer to clear, so there is a trade off.

So in practice, for some subjects frame rate is very important but its not for everyone. And card speed is another important factor. Depending on your style and subject, you may find a fast card is needed to use a camera with a fast frame rate.

If someone is considering a D7100, getting a fast SD card is important. The big advantage of CF cards is write speeds that are nearly 50% faster than the best SD cards. And XQD has potential to be much faster than CF.

EDIT - Even a card 2-3 times as fast only allows 1-2 extra RAW files. The write speed is just under one file per second, so the buffer may fill in 11 frames instead of 10 with a card that is twice as fast. But a fast card clears the buffer faster so you are ready to shoot another burst.

Eric Bowles
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