Compact Flash cards and Camera Buffer Canon 40D

"Which compact flash card should I buy?"

"Is a faster compact flash card any better?"

These are some of the questions regularly asked on photography forums regarding compact flash (CF) cards and Digital SLR cameras. Although answers are varied, most generally come back with a definitive yes, followed with links to tables and charts.

The lost card
I've been looking at this for a while now, well since I lost one of my CF cards. I bought two 4GB Sandisk Extreme III cards last year at a stupidly low price of about £16 each (RRP ~£30-£40). I can't remember where from now though :o(

The question
My question is basically whether I need to spend more money on a fast CF card or can I make do with slower cheaper ones (still branded though).

Why do you need a fast card?
Thing is that I tend to do some action photography every now and then, wildlife (birds etc) and some sports. I tend to find that my buffer fills up and I have a wait a little while before I can take photos again. Confused?

What am I on about?
Ok let me explain a little. DSLR cameras use memory cards to save photos. However, between the time you press the shutter button and the photo being saved onto the memory card, it is stored (temporarily) on the camera's internal memory (buffer). The only problem is that the buffer on the camera is limited in size. So when this fills up the camera more less stops taking photos and waits for the photos to move from the buffer to the CF card. If you're shooting something that can't wait for this (can be upto a minute) then it can be a problem.

In my case I have a camera that can shoot at upto 6.5fps. If you're trying to take a photo of a bird in flight, a racing car whizzing past, or maybe taking photos at a footy match, you'll find that you might miss the action shot sometimes. Well the more photos you're able to take consecutively within a second the more likely you are to get that brilliant shot.

I've put together a quick diagram to explain it, probably not the best in the world, but hey if it works.


NB Before I go into the specifics of the test, I need to point out something, which most of you are probably aware of anyway but just in case.... Sandisk have among their range of CF cards two models named Extreme III and Extreme IV. At some point in their life cycle Sandisk upgraded the transfer speeds on the cards, so in essence there are two cards currently available in shops. The newer faster cards have 30MB/s and 45MB/s on the label, respectively.


Anyway.... The tests (and in no way was this controlled under lab conditions) which I conducted a couple of days ago.

The equipment:

• Canon 40D + battery grip (flashed with the latest firmware as of April 2009).
  
• Sigma 150mm macro lens

Settings

• Shutter 1/8000,
  
• Aperture f2.8,
  
• ISO100,
  
• drive in high burst mode.
  
• I shot at a dark wall, the image was fairly black without an detail.
  
• Focus was set to manual.

I timed it from when the 6.5fps burst mode had stopped to when the red light went out. I repeated this 3 times for each card and took an average.


Results:

• Dane-Elec 2GB (??MB/s) = 52sec
  
• Sandisk Extreme III 4GB (30MB/s) = 15sec
  
• Lexar Premium 80X 2GB (12MB/s) = 17sec
• Sandisk Extreme IV (45MB/s) = 30sec

What this basically shows is that the slower Lexar card transfers data at more less the same speed as my Sandisk. Of course this only applies to my camera, the Canon 40D.

I've just ordered two 4GB Kingston Elite Pro cards 133X (12MB/s) which I purchased for £8 each on clearance price, which I'll be testing when I get my hands on them.

So it seems like I don't really need to spend more on a card that doesn't make a difference.

EDIT
Ok so I've been thinking about why there's a difference in speed between faster cards, even though only slight. The only thing I can think of is the bus on the CF card. [Here's where I try and remember to my system architechture lectures from about 10years ago]. If the card can handle a throughput higher than the camera, then the camera will carry on sending the data while the cards bus keeps taking in and processing it.

For example if you have a thin pipe which then widens for a section before it connects to a tank. Water will carry on streaming into the wider section leaving while at the same time pouring into the tank. I'll investigate this further, but if there's anyone out there that can correct me or add more information then please do.

As you can see in the table above the Sandisk Extreme IV seems to be half the speed as th Extreme III, not really sure what's going on with that.