This will be a short but very valuable learning topic.  I often get questions on this topic, yet I’m surprised by how many people I run across who aren’t asking questions and really don’t understand why their cameras buffer takes forever to empty, so they can continue shooting.  I know people who were actually planning on buying a new camera because they thought the reason the memory card took so long to clear the buffer was only a function of the camera.

Memory cards for our PDA’s, GPS’s, and other personal electronics can be near any speed or quality and we won’t notice that much.  But put an inexpensive slow card in your camera and the bottleneck will stop you in your tracks!

There are two main variables that affect the speed of data transfer between your camera and your memory card.  The camera and your memory card.  The camera is limited.  There is a maximum transfer rate the camera can handle and it will top at this maximum no matter how fast your card is.  However, perhaps 95%+ of the time, it’s the flash memory card which is the bottleneck and not the camera.  The camera can almost always accept data faster than the memory card can provide.  So let’s concentrate on memory cards.

There are two speeds you need to be concerned with.  Write speed and Read speed.  Most memory card manufacturers rate their cards 150x, 200x, and so forth.. but what does that mean exactly?  Here’s a big hint.  If the memory card manufacturer rates their cards in this way then the chances are they’re not a top quality/speed card.  Write and Read speed should be listed in megabytes per second.  (Mbps)  Here’s another catch.  Write speeds and read speeds will NOT be the same on these cards.  Write speeds will be dramatically slower than read speeds.  This means manufacturers, especially ones of dubious quality/value, will only list their read speed, but won’t tell you it’s only their read speed.  Confused yet?

Write speed is critically important because ‘writing’ is exactly what your camera is doing to the flash memory card when it’s ‘saving’ an image to the card.  It’s ‘writing’ the image to the card.  If you want to overcome that bottleneck and get back to shooting as fast as possible, then its WRITE SPEED you should be concerned with most.

Read speed is what you should be concerned with when it comes to how fast your memory card sends your images to your computer after the fact.  Read speed is important, but frankly I can wait while my computer reads my card and sends my images to my hard disk, but it will cost me valuable photo opportunities in the field if I have to wait too long for my camera to write to my memory card.

I’m often asked how I choose which memory cards I personally use.  First, I am very skeptical of any brand which advertises their speeds in the “150x” format.  Especially if they are inexpensive.  These cards might be great if the speed of the card isn’t your priority, but for me speed is a priority.  The only greater priority is reliability.  Unfortunately the brand which advertises their speeds in the “150x” format tends to be the least reliable and offers the least warranty.  By far.

Instead, look for the brands that offer their speed in mbps.  If you see 30mbps being advertised then great, but do realize that with a 99.9% certainty this speed is the read speed.  The write speed will be considerably less, often by as much as 2/3 or more.

So how do you find the real write speeds?  I search sites that test the cards before buying.  Rob Galbraith’s site has been testing the memory cards of most interest to photographers for years and you can count on him being accurate.  He’s compiled a database of SD and CF cards that you can look at here.  He might not have tested the very latest/newest cards (yet), but chances are if the brand isn’t listed in this database then the card probably isn’t a good choice for photographers.  He actually tests the cards IN DIFFERENT CAMERAS so the cameras speed is also part of the equation.  He even rates the speeds from jpegs and raw files separately because file size affects the speed.  This is the best site I know of for finding such information.

Notice that he only lists the write speed?  This is the main concern.  Also, if you start at the bottom of his list you can see the inexpensive brands and how slow they are.  At the top of the list are the SanDisk and Lexar brands.  These are the two brands professionals use the most because they are the fastest and most reliable.  Notice for instance that the 6th card down, the SanDisk “Extreme III 30mb/s edition 8gb” has a read speed of approximately HALF the speed it lists on its packaging?  The packaging won’t tell you the speed plastered on it is the read speed, or that there’s a difference, unless you read the very small print.  And I warn you its very tiny print.

I’ve personally had bad luck with the Lexar brand and I stopped using them.  The bad luck can be attributed to a certain series of cards in a certain camera, a defect that Lexar cleared up.  Finally.  Still, it was the only time I actually lost images because of a memory card and I didn’t care for the way Lexar handled it at the time.  This was when I switched to SanDisk and have never looked back.  I’d only look back if at some point SanDisk cards caused problems for me.

The price differences between the best and worst brands can be significant.  For instance, a 32gb Transcend CF card can be found online for as little as $89 USD’s.  That seems like quite a deal.  Until you put it in your camera and it takes forever to write a file to the card, and as the card fills it gets slower still.  At the same time a 16gb SanDisk Extreme IV CF card goes for $199.  More than twice as much for half the capacity.  Many shoppers who don’t know the finer points of memory card basics will choose the lesser expensive higher capacity card, never realizing what a huge difference there is in speeds between the other brand.  Not to mention warranty and reliability.

If you examine the cards carefully you’ll also see differences in construction.  Last year I purchased a cheap Delkin card for my PDA because speed didn’t matter.  It was a SD card and after only two months the card’s outer plastic shell split in half and the write protect nub got lost, and the card became instantly useless!  But I saved $19..

If you buy quality Lexar or SanDisk products for your DSLR you will not be disappointed.  But do pay attention to which models within the brands you choose.  Even Lexar and SanDisk sell inexpensive slow cards because there’s a market for them, and people will choose them over the other cheap cards because of their name.  With Lexar you want the Professional Series, and with SanDisk the Extreme series.  You won’t be disappointed.