CF Speeds and performance - Some facts.

There's a lot of guff talked about CF cards and speed. Here are some facts regarding CF cards and the Canon 7D. The same principles will apply to much of the rest of the EOS range, I've no idea about Nikon, perhaps somebody can do the same experiments as I've done.

First off - how does CF speed affect how your camera performs. If you're shooting video then all you need is a paltry 8 MB/s which almost all modern cards are easily capable of. If shooting stills then speed only really has noticeable effects if you shoot in rapid bursts, filling the buffer (the camera's inbuilt memory). With the 7D, shooting raw at 8 fps, the buffer can handle around 20 images. So, regardless of the speed of your card, you'll be able to rattle off 20 images at full speed. What happens next depends on the speed of your card.

Here's a diagram showing how various cards respond to burst mode. (Note that the two Extreme IV cards were stopped at 11 and 12 seconds, respectively, and that the Extreme III is mislabelled as Extreme II). Larger image is here.



With the very slow Digitex card, after the initial burst, it fires two pairs of shots 0.5 s apart then there's a 6 second gap between subsequent pairs of shots. When the buffer is full, after 20 images, this card takes an incredible 166 seconds before the 'busy' light switches off.

With the Extreme IV (45 MB/s) cards they belt out pairs of shots at 0.5 second intervals after the initial 20. The buffer is cleared in around 11 seconds.

So the card speed only affects the rate at which the buffer empties (after 20 raw shots) and how fast you can shoot with a full buffer.

Now to CF speeds and uploading to your PC. Most people are probably using a USB interface with a bog-standard CF reader. That's going to limit transfer speeds to around 30 MB/s - no matter how fast your card is. Using a UDMA card with a UDMA-enabled reader will up that to around 40 MB/s. But to get faster than that you're going to need a Firewire 800, ExpressCard (using PCIe) or SATA card reader.

Here's my results, using a Sandisk UDMA 16 GB, 45 MB/s card writing to a SSD -

• Standard USB reader - 26 MB/s
• Lexar UDMA USB reader - 38 MB/s
• Lexar ExpressCard reader - 46 MB/s

So, before you splash out on a 60 or 90 MB/s card ask yourself some questions -

• Do I often fill my camera's buffer?
• Do I ever fail to get a shot because I'm waiting for the 'Busy' light to go off?
• Do I have a high-speed card reader?

If the answer to any of those questions is 'Yes' then a high-speed card may be for you. If the answers are all 'No' then you'd probably be wasting your money.

Can you resize the image please

I've just bought a Lexar UDMA reader and it is a lot faster than the Sandisk ones I had been using. Read speed is equally important to me as I hate waiting while the files transfer

That's very helpful - I was shot down a bit in a couple of recent threads when I recommended a card (despite first hand experience of using it for video and stills in a 5d Mk II which is near-ish the top end for file size) - good to have some info to point people towards!

If I'm uploading from a card to the computer, I leave it to it while I do something else - browsing from a laptop, answering calls of nature, feeding the cat etc.

To the OP. You say "Standard USB reader", do you mean USB 1 or USB 2?

Nod said:

If I'm uploading from a card to the computer, I leave it to it while I do something else - browsing from a laptop, answering calls of nature, feeding the cat etc.

Click to expand...

Yup, me too. Especially as I upload intoLightroom and have to wait for it to render previews. But I keep reading comments on how a fast card will improve upload speeds while the truth is that it often will make no difference at all.

Nod said:

To the OP. You say "Standard USB reader", do you mean USB 1 or USB 2?

Click to expand...

USB2. I'd rather listen to three hours of Celine Dion than try to upload 8GB via USB1.

I did do some read write tests when i got the lexar card reader but got a bit bored of it and didn't quite finish it lol.

Anyway.....



Sandisk 4Gb ExIII

To write 3.71Gb to Lexar UDMA reader 3min 23 secs
To read 3.71Gb back to hard drive 2min 43 secs

To write 3.71Gb to Sandisk 12 in 1 reader 3min 48 secs
To read 3.71Gb back to hard drive 3min 44 secs

To write 3.71Gb to Sandisk basic CF reader 19min 1 secs
To read 3.71Gb back to hard drive 18min 50 secs

Hama 16Gb 200x

To write 3.71Gb to Lexar UDMA reader 6min 50 secs
To read 3.71Gb back to hard drive 2min 03 secs

+++++ not done

To write 3.71Gb to Sandisk basic CF reader 29min 1 secs
To read 3.71Gb back to hard drive (not done)

I'll not be completing it either. The Hama card has given me a couple of read errors so it is on its way back for refund/credit.

My new 8gb Sandisk card would only allow about 20sec of video in my 7D before buffering !
I guess it's a fake card with a sandisk sticker, but how can you be sure when buying them?

Very hard to spot a fake without seeing it. Best bet is to avoid e-bay sourced cards since that seems to be where an awful lot of fakes are bought. I've bought a fair number of SanDisk cards from 7dayshop over the years and never had a fake. Probably spent a few extra quid but IMO it was worth it for the peace of mind.

There's an article HERE on how to spot fake SanDisk cards once you can see them in real life.

Thanks for that response, it's exactly what I was looking for in response to the question I asked in another thread...