Q & A : What's the truth about memory cards

To ask a question of The Photo Video Guy, send an email with your question to ross@thephotovideoguy.ca John writes (yes it's his real name :) ) "I swear I have heard every possible story about what memory card I should use, what the numbers mean, and why the most expensive card is the ONLY answer, or why it doesn't matter at all so long as the card is Class 10, whatever that means.  I don't think most salespeople know what they are talking about, and I believe that most of them are making it all up.  Can you tell me the real story?  And don't try to sell me something please."

Ok John.  Deal.  No selling.

Full disclosure, I do work part time in a camera store and I do sell memory cards to clients.  I always give a straight-forward answer and I have encountered enormous amounts of customer frustration given the amount of fabrication and outright falsehood they have been fed from computer stores, big box stores, the convenience store and "pro" camera shops.

So let's cut to the key details of multiplier (the number in front of the X), Class, and MB/s transfer rates.

What is the X Number?

For some readers, the idea of measuring the performance of a CD-ROM drive will sound like ancient history.  The first decent drives delivered READ performance of 150kbps.  That is kilobits per second.  In today's world, think snail speed.  That reality was never impacting on some marketing types so we still see 133x, 400x, 600x used as advertisements on cards.  What does this really mean?

What about the other numbers?  And all the letters? And the little pictorials?

133x = 19.95 megabytes per second or an approximate READ performance of 20 megabytes per second.  That translates into about ten 2 megapixel image read from the card per second.  Fascinating but pretty irrelevant because that is the data transfer rate for getting the data OFF the card.  As photographers and videographers, we must be most concerned about getting data ON to the card, to free the buffer for the next shot.  So this means that for any camera using an uncompressed image storage format, this card is pretty much useless.  The Write speed for these cards is usually undocumented so unless otherwise specifically stated, assume Write speed is about half of Read performance.

The bus type used by the card manufacturer, and even the card architecture has an impact on performance.  Bus types include Default, High Speed and UHS-1.  Faster cards need faster busses.  But to take advantage of the faster bus, the host (your camera, your card reader) also needs to have the same bus capability.  A UHS-1 bus is backward compatible.  A high speed bus will never leverage a card with a UHS-1 bus properly and will always treat it as a high speed bus card.

Accessing the memory on the card uses Direct Memory Access aka DMA.  Newer cards offer UDMA at different levels.  The higher the level the greater the maximum theoretical performance.  Today UDMA 7 has a theoretical cap of 256MB/s but there are no cards presently performing near that level of throughput.

But wait!  There's more bafflegab!

As noted in the 133x example, card manufacturers have an annoying tendency to quote READ performance in big type and WRITE performance in no type or ½ point typeface that requires a microdot reader to see.  In fact, in my research, I found only two cards where the READ and WRITE performance match and are guaranteed minimums.

Class?  Who has class?

Class was created to make it easy (hahahaha) for video camera users to get a card that would work in their cameras.  Most consumer video cameras today shoot at 1080p resolution, so 2 megapixel frames on average at 30fps.  But they also do VERY aggressive compression using H.264 and AVCHD amongst other methods.  The Class number is a number and basically guarantees a megabyte per second rate based on the number, so a Class 10 card guarantees 10MB/s read/write performance.  If the camera were doing a less compressed video stream, such as DNX, ProRes 4:2:2 or any of the other more serious video formats (less compression = better footage), a Class 10 card could not promise to keep up.  The brings up the new VPG graphic, a little film frame with a number in it that sort of guarantees a level of READ/WRITE throughput beyond a Class number.  It's a bit spurious.  For example, the Lexar 1000x cards are one of the very few that guarantee 150MB/s READ and WRITE performance.  I can run them continuously in my Canon C300 that pushes video onto storage at 50MB/s in ProResHQ 4:2:2.  Yet the cards only hold a graphic that says VPG-20.  Huh?  Sandisk's Extreme Pro cards read at 150MB/s, write at something slower but carry a VPG-65 marking guaranteeing 65MB/s write speed for video.  No wonder sellers and buyers get confused.

Here's a little table that might help...

	Bus Type	CF / SD Naming	Memory Architecture	Megabytes/s (READ)	Megabytes/s (WRITE)	Video Class Assignment CF/SD
133x	Normal	SD-XC	DMA	20	6	6
200x	High Speed	SD-XC I	DMA	30	15	10
400x	High Speed	SD-XC I	DMA	60	60	10
600x	UHS-I	SD-HC I	DMA	90	45	10
800x	UHS-I		UDMA 7	120	<=120	VPG-20 / 10
1000x	UHS-I		UDMA 7	150	<=150	VPG-65 VPG-20 1 Class 10

About the Memory...

CF and SD card memory is known as flash memory.  It holds content for a very long time without constant power so is called non-volatile.  This probably sounds like the flash storage used in Solid State Drives in laptops (SSD) or flash storage built in to computers, such as the flash storage found into the recently announced Mac Pro.

Yes they are both flash memory.  No they are not the same flash memory.  Flash memory used for "hard disk storage" is much more stringently tested and is much more fault tolerant than the flash memory found in memory cards.  That's why SSDs are so expensive.  This does not mean that the memory in memory cards is not good.  Lexar and Sandisk would not do lifetime warranties if they weren't very sure they would not get uptake on the warranty.  Because we know that a warranty is only offered when the manufacturer expects never to have a claim.  Did someone just call me cynical?

Hoodman tells me that their memory cards use SSD quality memory.  Ok.  I know that their cards are great and they do offer much tougher casings to help protect the card if you drop it and walk on it.  I don't know that the memory is really any better, but they say it is and they are a good company.

There are very few manufacturers of flash memory in the world.  Samsung in South Korea is one of the world's largest manufacturers.  The other is Micron Technologies based in the United States.  Lexar is the card brand of Micron.  There are lots of card OEMs though so how do you know which ones are good.  Look at the warranties, read reviews and read the specs, or observe the lack thereof.  I have never had an issue with either a Lexar or Sandisk product.  I do like that Lexar gives their recovery software with all their serious cards for free.  Sandisk gives a 12 month time bombed edition with their serious cards.  That's enough to be annoying.

This means that every other card is buying memory from someone and repackaging it.  Not necessarily a bad thing, but in my research I have found that memory comes to OEMs at different prices based on anticipated reliability levels.  100% reliability costs a lot more than memory rated at 85%.  Because these cards (mostly) use the old DOS style FAT file system, the format process automatically creates extra areas so bad sectors can be marked bad without data loss.  If you are a snap shooter, you need a card only fast enough for your most important project.  If you are a serious photographer or videographer, your requirements and standards might be much higher.  Personally, I have replaced every third party card with either Sandisk or predominantly Lexar.

How many....

It's also pretty common to get the question "well how many pictures can I get on a card?" I've put together a table with some popular cameras, along with the uncompressed RAW file size and the largest JPEG file size.  These are maximums based on the camera sensors and also assume 12 bit images.  8 bit images would take less space.  Some cameras let you capture lower bit count images.  I have yet to understand any logic in that since there's usually a JPEG export in the workflow somewhere and more than half the info gets tossed in the trash at that point so why capture at a lower bit rate...  The math is done for a 32GB card.  Divide by 2 to get approximates for a 16GB card, multiply by 2 fro a 64GB card.  Video is a very different question because it depends greatly on the compression type, the bit depth and the quality level.  Consumer camcorders are typically 8 bit devices with high levels of compression.  DSLRs can be 14 bit for stills but be 8 bit for video and then use H.264 compression to further reduce file size.  Pro cameras are 10 bit or higher and may deliver anything from RAW video (you must own a storage company) through a variety of professional codec models.  I created a sample chart for the Canon 7D for stills and its native video and added a column for what the video consumption would look like if using an external recorder and ProRes 4:2:2 like an Atomos Ninja.  Pro video is VERY demanding of storage.

Popular Camera File Sizes and Capacity

Camera	Canon 1Dx	Nikon D800	Sony A99	Canon 5D Mk III	Sony A7	Nikon D5200	Canon  T5i	Canon 7D	Nikon D7100
Rated MP	18	36.3	24	22	24	24	18	18	24
Pixel Dimensions	5202x3533	7360x4912	6000x4000	5760x3840	6000x4000	6000x4000	5184x3456	5184x3456	6000x4000
RAW File Size (uncompressed)	27.5	54.2	36	32.2	36	36	26.9	26.9	36
JPEG Fine File Size	12.25	24.1	16	14.75	16	16	11.94	11.94	16
RAW shots on 32GB	1,164	590	889	994	889	889	1,190	1,190	889
JPEG Fine on 32GB	2,612	1,328	2,000	2,169	2,000	2,000	2,680	2,680	2,000

Comparing Still Consumption to Video Consumption

	# RAW 12 images	# JPEG Fine Images	Hours of 4:2:0  1080P Video (H.264) max 21GB / Hour (Canon 7D)	Hours of 4:2:2 1080P Video (ProRes HQ) max 77.36 GB / Hour
16 GB	595	1,306	0.76	0.21
32 GB	1,190	2,612	1.52	0.41
64 GB	2,379	5,224	3.05	0.83
128 GB	4,758	10,449	6.10	1.65
256 GB	9,517	20,898	12.19	3.31

I have to thank the SD Card Association, Lexar, and Sandisk for producing such useful materials that helped in the creation of this article.  I did go to the Compactflash Association website as well but they were more interested in selling me their specification books than providing education.  I hope that this helped reader John and the rest of the folks who are regularly confused or misled on this subject.  It's more cumbersome than it should be but you now have enough information to make better decisions.

When I first moved to Ontario as a child, there was a fellow called Sy Sims, a clothier in Buffalo, who advertised on the local TV station.  His slogan was "an educated consumer is my best customer".  Some retailers don't want you to be educated it seems.  Fight back against that attitude.