Compact Flash CF Card Speed Comparison: How Fast Is Fast Enough?

by Alan Haynes / PhotoCitizen on April 15, 2011

Compact Flash cards keep getting faster. How much speed do we really need?  Can our cameras even take advantage of the fastest CF cards?

I just bought a couple of new CF cards for my SLR camera. After spending $133 on a SanDisk 16GB Extreme Pro and $55 on a SanDisk 8GB Extreme, I had a bit of buyer’s remorse.

Is the SanDisk Extreme Pro 16GB worth the moneyWas the more expensive Extreme Pro card going to perform any better in my three-year-old Nikon D300 or two-year-old Windows 7 computer than the cheaper card?

To find out, I spent the better part of a day testing my two new cards along with another older, slower card I had lying around. I now have a pretty good idea about CF card speeds. And it turns out that buying the Extreme Pro CF card was a bit extravagant.

CF Card Speed Ratings: Lexar vs. SanDisk

There are many companies manufacturing memory cards for cameras, but as far as I’m concerned, there are only two worth considering: Lexar and SanDisk.

Lexar Pro 600x CF cardBoth companies have been around a long time and both have developed a reputation for quality. There may be other fine memory card makers out there, but I’ll stick with the big two.

I’ve used Lexar CF cards in the past, but lately I’ve been buying nothing but SanDisk. The only Lexar cards I had on hand were very old and slow so I didn’t include them in my testing, but I’m confident the newer Lexar products are comparable to the SanDisk cards.

Specifications for compact flash cards generally list what’s called “sustained write speed.” This number represents how quickly data can be written to the card. It’s expressed in two different ways:

  • Lexar uses an “x” multiplier. Multiply the x number by 150 kilobytes-per-second to get the speed in megabytes-per-second. So, 400x represents a write speed of 60MB/s (400 times 150KB).
  • SanDisk skips the math and simply lists their speeds in MB/s.

Each company currently offers Compact Flash cards at speeds of 30MB/s (200x), 60MB/s (400x) and 90MB/s (600x). Storage capacity for both brands ranges from 4GB to 128GB with 8GB and 16GB being most common.

Capture and Write Speed

For photographers, there are two important aspects of card speed: how quickly the card can record images and how long it takes to upload them from the card to the computer.

SanDisk Extreme CF card speed comparisonBecause all digital cameras have built-in buffer memory,  card speed has no affect on the speed of image capture. The camera will capture images at the same rate no matter the speed of the CF card.

My Nikon D300, for example, captures images at a little over 6 frames per second in high-speed capture mode. That speed does not vary with the speed of the CF card.

After the image is captured, the camera transfers the image data from the buffer to the CF card. Here is where CF card speed becomes important. The more quickly the images can be transferred and the buffer cleared, the more quickly the camera can return to it’s full capture speed.

My 12-megapixel Nikon D300 is capable of capturing 14 raw images in a row – averaging 9.4MB each – before the buffer fills up. My ancient 6-megapixel Nikon D70, with its smaller buffer, can capture four with an average size of 4.7MB.

For the chart below, I captured as many raw images as possible before filling the buffer. I measured that time along with the time it took for all of those images to be transferred to the CF card. Both cameras have a green light on the back that flashes while the write operation is taking place and a counter that indicates progress.

Then, with some simple math, I converted that number into images per second. For example, with a SanDisk Extreme Pro card in the D300, it took 2.2 seconds to capture all 14 images and another 11.6 to finish writing them, for a total 13.8 seconds. After rounding, that’s one image per second.

Capture and write speed (images per second)
Camera Maximum
Raw Images

Captured
SanDisk
Extreme III
12GB, 30MB/s
SanDisk
Extreme
8GB, 60MB/s
SanDisk
Extreme Pro
16GB, 90MB/s
Nikon D300 14 .8 1.0 1.0
Nikon D70 4 .5 .6 .6

There is no significant difference in speed between the two higher-speed cards. This is because these two cameras cannot transfer data fast enough to take advantage of the Extreme Pro’s 90MB/s speed.

The lesson here is that the lower priced Extreme card performed as well as the more expensive Extreme Pro. With either of those cards, a 14-image burst on the D300 takes about 14 seconds. After that time, the buffer is clear and another 14-image burst can be captured.

How fast is the SanDisk Extreme III CF cardImages can be captured before the fourteen seconds have gone by, but at a much slower rate of about 1.2 images per second for the capture alone; a far cry from the 6 fps maximum capture speed.

For a newer, top-of-the-line camera, the results will probably be different. I didn’t have a Nikon D3X or Canon 1-Ds Mark III available for testing, but I’ll bet they have faster internal processing which will allow them to take full advantage of the faster Extreme Pro card.

(TECHNICAL NOTE: Larger capacity cards, because of their extra circuitry, take a fraction longer than smaller cards. So, for the D300 test above, the 16GB Extreme Pro took 13.8 seconds to the 8GB Extreme’s 13.6 seconds. If they had both been 16GB, the results would have been identical.)

Upload Speeds From CF Card to Computer

After a day of shooting, I usually have hundreds of images to upload to the computer. As a travel photographer, this work is often done in a hotel room late at night when I’d rather be sleeping.

This is where the second significant card specification comes into play: upload speed. The faster we can move data off of the card and into the computer, the better.

For this test, I plugged the three CF cards into my computer’s built-in card reader and clocked the time it took to upload 100 raw images.

Transfer 100 raw images to the Windows 7 Desktop (images per second)
Camera SanDisk
Extreme III
12GB, 30MB/s
SanDisk
Extreme
8GB, 60MB/s
SanDisk
Extreme Pro
16GB, 90MB/s
Nikon D300 1.7 2.2 2.2
Nikon D70 3.1 4.3 4.3

Again, there was no significant difference between the top two cards. Although I have a fast computer, the limiting factor is probably the built-in card reader. It plugs into an internal USB connector, so it is subject to the 60 MB/s limit on USB connections. Performance of my external USB reader was much worse.

Sandisk Firewire CF Card ReaderUntil faster card readers become available, the Extreme Pro card will never reach its full potential for computer upload speed.

SanDisk does make an external Firewire CF reader (the SanDisk Extreme Firewire CF Card reader) which is faster than USB 2, but costs about $120 and requires a Firewire port on your computer. Most Macs have one, but many Windows computers do not.

Best price on Lexar USB 3 card readerAnother option is the newly announced Lexar USB 3 card reader which promises to be much faster than USB 2. Most computers don’t have USB 3 ports yet. They can be added to a desktop computer but not to a laptop.

Best price on WD USB 3.0 PCI Express cardIf you’re handy, Western Digital makes an inexpensive card (around $30) that can be added to your desktop computer to add two USB 3.0 ports. Check the latest price here: Western Digital USB 3.0 PCI Express Card.

You’ll need both the Western Digital card and the USB 3.0 card reader to get USB 3.0 speeds on your computer.

Comparing CF Card Prices: Cost Per Gigabyte

The faster the card – and the larger its capacity – the higher the cost. An easy way to compare prices is to calculate cost-per-gigabyte.

Generally, the higher the speed, the higher the cost-per-GB, but for cards with the same speed rating, large capacity cards cost less per gigabyte than smaller ones.

An 8GB SanDisk Extreme CF card running at 60MB/s, for example, sells for about $55. That’s $6.88 per GB ($55 divided by 8GB). A 16GB Extreme card ($99.95) costs $6.25 per GB.

Best Deal on a CF Card: SanDisk Extreme 32GBAs of this writing, the best deal on SanDisk is the SanDisk Extreme 32GB. The price at  Adorama for this 60MB/s card is $169. That’s $5.28 per GB, the lowest of any of the SanDisk high-speed cards.

$169 is a lot to pay for one CF card. If you like to carry more than one card – and I recommend you do in case one fails – you might be better off buying a couple of the second best cards. The 16GB SanDisk Extreme (60MB/s) sells for $99.95 at a cost-per-GB of $6.25.

Click here to check current prices on all SanDisk CF cards. Or, check Lexar CF card prices here. If you buy a card – or anything else – using one of the links on this page, you’ll help keep the information flowing here at PhotoCitizen. Thanks.

Conclusion

There are a few lessons to learn from this experiment:

  1. Fast cards do not always mean better performance. It depends on the camera and computer you’ll be using.
  2. Unless you have top-of-the-line equipment (and an equally top-notch bank account balance) don’t bother buying a top-of-the-line CF card. For most of us, the 60MB/s (400x) cards are the best bet.
  3. If you have a lot of images to transfer to your computer, make sure you’re using an internal CF card reader. They’re faster than external USB 2 readers. Even then, expect to twiddle your thumbs for a while: at 2.2 images per second, it takes over five minutes to transfer a full 16GB card. With a laptop it takes even longer.
  4. If you’re not a speed shooter and have plenty of time to upload photos, buy the cheapest name-brand card you can get. Stay away from off brands. They might not be reliable and, if they stop working, you’ve lost all your pictures.
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{ 2 comments }

1 Steve September 8, 2011 at 7:30 am

I just bought a camera for a photography class, and this article was perfect in helping me know what CF card to get – I went with a SanDisk 8 Gb 400x. Thanks!

2 Alan Haynes / PhotoCitizen September 9, 2011 at 10:42 am

HI Steve,

Glad to help. Hope you’ll check out some of my other articles. Most of them have something for both beginners and advanced photographers.

Alan

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