Not All Memory Cards Are Created Equal

When you go into your favourite electronic accessories store to buy a new SD memory card, or Compact Flash card, or Micro SD memory card, or XD memory card, or whatever memory card it is you are after, you are generally confronted with a confusing array of options. There are 8GB cards for $15 and $25 and $56 and they all seem the same. So why the price difference?

Well because they are not the same. If we just look at the normal size SD memory, which is the most popular memory card form-factor, there are three key things that change: the capacity; the card ‘family’; and the card speed.

1. The Capacity

This one is pretty easy. As things were at the middle of 2011 (things change fast in computing and electronics), SD memory cards typically come in the following capacities:

  • 2GB
  • 4GB
  • 8GB
  • 16GB
  • 32GB
  • 64GB

You might still see the odd 1GB SD memory card around, but these are vary rare now.

2. The Family

As far as digital cameras are concerned there are currently three families of SD memory cards. These are:

  • ‘Standard’ SD (standard capacity).
  • SDHC (High Capacity).
  • SDXC (eXtended Capacity)

Typically 2GB cards will be ‘Standard’ SD cards.

Typically 4, 8, 16, and 32GB cards will be SDHC cards although there is a trick here because you can get 4GB (and maybe 8GB) ‘Standard’ SD cards.

Class28GBFinally, 64GB and larger cards will be SDXC cards.

Each of these card families are configured differently so you need to know before-hand if your camera can use the card family before you buy it.

Generally all digital cameras built since about 2008 will support both Standard SD and SDHC cards but be aware that if you buy an 8GB SDHC SD card and your camera does not support SDHC then the card will not work in your camera.

3. The Card Speed

Points 1 and 2 were relatively easy to explain. Now it gets hard. There are two methods for designating the speed of an SD memory card. This is the speed at which the writing device, in our case that would be the camera, can output data onto the card.

Speed Class: The first method of indicating the speed of an SD memory card is the Class specification and the classes are 2, 4, 6, 8, and 10; with Class 10 being the fastest.

Class1032GBThe Class rating of the card more or less relates to the write speed of the card in megabytes per second. Hence a Class 4 card can be written to at about 4 megabytes per second. Thus if you were using a PENTAX K-5 camera and taking 22MB RAW format pictures with a Class 4 card in the camera you can expect the camera to take about 5.5 seconds to write a single picture onto the card when the button is pushed. If you were using a Class 10 card this same picture will take the camera just 2.2 seconds to get the image stored away on the card.

The Class rating of an SD memory card is shown as the Class number with a “C” around it, as can be seen on the blue Class 2 SanDisk 8GB card above and the black Class 10 SanDisk Extreme 32GB card.

The x Rating: While the Class rating system was good up until about two years ago memory cards are getting so fast now that a new rating system was needed (apparently). This new rating system is the ‘x’ rating system. The ‘x’ rated cards basically go:

  • 100x
  • 133x
  • 150x
  • 200x
  • 266x
  • 300x
  • 400x
  • 600x

This ‘x’ rating is supposed to relate to the relative read speed of the card—which makes the whole thing kind of confusing because the Class speed rating system related to the write speed of the card.

As a very rough comparison a Class 10 card has an approximate ‘x’ rating of about 66.

Does speed matter? Yes it does!

Firstly, the faster the SD card the more it will cost. An 8GB Class 2 card is currently selling for about $20 whereas an 8GB Class 10 is around $80. These prices are for SanDisk. Prices vary depending on brand and some Class 2 8GB cards are down to as low as $11.

JenHSecondly, if you have a pro-am DSLR that writes out 22MB RAW images and these take about 10 seconds each to write to a Class 2 SD card then this can become an issue. If you take RAW+JPG and the combined write-out becomes 36MB then you are going to be waiting 18 seconds for that single shot to get written to the card! 18 seconds to write out one shot!

This would not work out that well if you were doing continuous shooting of Jennifer Hawkins modelling lingerie. You would only get in about three shots a minute with your seriously expensive DSLR camera due to the slow Class 2 memory card being used.

Okay … It doesn’t quite work out like that due to most high-end cameras being able to buffer between three and five frames internally when they cannot be written to the card fast enough. But once you get to the three or five frame internal memory limit then you are in big trouble because most cameras then stop until they have unloaded ALL the buffered shots before freeing up the shutter again. That could take something like a whole minute to complete. All this time Jennifer Hawkins is getting really pissed off waiting for you to aim up again—and she is probably costing you around $10,000 an hour.


So. When buying SD memory you need to have some idea of what capacity you want. What family of card your camera will work with. And finally, what speed of card you think you need because this will impact the price the most and can impact the performance of the camera.


If you have a pro-am style DSLR then I would suggest you go for SDHC, 8GB, and at least Class 8. But as the Class 10 card is probably going to be about $10 more then why would you not just get the Class 10 card anyway?

I am sure someone is asking why not get 16GB or 32GB, or even 64GB.

There are two reasons. Firstly the cost increases going to 16GB, then 32GB, and then 64GB get steeper as the sizes go up. Secondly, memory cards are not perfect. They do sometimes malfunction. Admittedly with high quality cards like SanDisk this is relatively rare. But it does happen. So if you are out taking 32GB of shots of Jennifer Hawkins, or your own SCN, then better to spread the shoot across 4x 8GB cards for safety. Then if one of the cards malfunctions and you can’t read the pictures back off it (and it DOES happen) then you have only lost a quarter of the shoot.

And this post has gone much longer than I planned so that will have to do.

Feel free to post questions Smile