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Does your camera need a fast SD card?

Posted on 22 Feb 2013 at 17:47

High-speed SD cards can be pricey, but is the speed worth the money? Darien Graham-Smith investigates

Flash memory cards for digital cameras are now absurdly cheap. A 64GB SD card can be bought online for around £30. That’s enough space to store 5,000 raw files produced by a typical DSLR – or upwards of 30,000 JPEGs.

These cut-price cards come with a caveat, though. Their transfer rates are comparatively low, meaning that it can take several seconds to store an image once you’ve pressed the shutter. SD card manufacturers encourage serious photographers to pay more for faster cards to ensure they don’t miss a shot – and aren’t left waiting around when it’s time to transfer their pictures to a PC at the end of the day. But how significant is the real difference between a dirt-cheap card and a premium one? And is it worth the cost?

SD card speed ratings


The first challenge is understanding the relative speeds of different cards. Helpfully, all SD cards are rated with a “class”, which reflects their performance. There are four standard ratings, which you’ll see advertised as class 2, 4, 6 and 10; these respectively guarantee that the card can sustain a write speed of 2MB/sec, 4MB/sec, 6MB/sec or 10MB/sec. (We’ll discuss what this means in practical terms later on.)

The class system makes it easy to distinguish the slowest cards. When it comes to high-end cards, however, it’s useless, since a card that supports 40MB/sec will receive the same class 10 rating as a 12MB/sec card.

Do you need a fast SD card?For this reason, manufacturers may supplement a card’s class rating with an explicit declaration of transfer speeds in megabytes per second. They may also give a speed rating as a multiplication factor, such as “100x” or “200x”. This reflects how much faster the card is than (believe it or not) a standard CD-ROM drive with a transfer speed of 150KB/sec; a rating of 66x or above would thus be equivalent to class 10. A 200x rating would imply a transfer rate of 30MB/sec.

Be warned that these ratings don’t have a standard meaning in the way that class ratings do. Unless the manufacturer explicitly asserts otherwise, the figures quoted on the packaging could reflect the card’s theoretical maximum read speed – rather than its minimum sustained write speed, which is the important factor for camera performance.

You may also see cards marked with a UHS-1 rating. This indicates compatibility with the relatively new Ultra-High Speed SD standard, which raises the theoretical maximum transfer speed from 104MB/sec to 312MB/sec. However, certification on its own doesn’t tell you anything about the write performance of the card – a UHS-1 certified card could be slower than an uncertified one.

How fast is fast enough?

The class rating system has its limitations, but it can be a handy guide to the practical capabilities of different cards. A class 2 rating means the card is guaranteed to be fast enough for standard-definition video recording, while classes 4 and 6 are fast enough for Full HD video (which one you need will depend on the bit rate of the video format you’re using).

Do you need a fast SD card?

The highest rating, class 10, is faster than required for any modern video standard: rather, it’s aimed at stills photographers. The idea is to minimise the time it takes to write a photograph to the card, so you can take multiple shots in rapid succession without having to wait around for each one to be stored.

It may seem counter-intuitive that capturing still images requires a faster card than shooting video, but Full HD footage isn’t as space-hungry as you might imagine. Despite the “high-definition” terminology, each HD frame has a comparatively low resolution of just over two megapixels. Plus, since consecutive frames of a video are often extremely similar, clever compression techniques can be used to store moving images efficiently. A data rate of 4-6MB/sec is ample for continuous shooting.

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User comments

Thanks

I'm not a photographer but I've always wanted to know the differences between all the SD card types, speeds etc and this article explains in really well. Thanks very much

By Chatan on 24 Feb 2013

Enjoyed

Thanks for a good article with very useful info.

By cja4sun on 24 Feb 2013

MJPEG video

My wife uses a Panasonic FZ50 camera for her part-time videography. The camera records in MJPEG format in a QuickTime wrapper.

Class 4 cards are adequate for situations where there's little on-screen motion, but it stutters when she must pan. Class 10 cards eliminate that.

By RonJohn on 25 Feb 2013

Tablet/phone cards used for more than just media

"Many tablets and smartphones feature a microSD slot, enabling you to expand the internal storage – perhaps to accommodate a personal media library, or simply to download and install more apps.

Do you need a fast SD card? In this case, there’s often no need to splash out on fast media. A regular class 4 or 6 card that’s capable of recording HD video will also be fast enough to play it back."

This is fine if you're just using it for media files. However these days, many people supplement their tablet or phone's internal application memory flash drive space with external SD or microSD cards in order to install more applications into this space. In this case, using a slower card will delay the launch of any applications installed on that card. A faster card will give you faster app launch times. So depending on what you put on your external card, it can be worthwhile to opt for the faster card.

By storm311 on 25 Feb 2013

USB 3 Card Reader

What is the min. speed of the SD cards that will benefit from using an USB 3 reader (vs USB 2 reader)?

By Albert on 27 Feb 2013

how about MLC vs SLC?

I must say that your review, despite being 5 pages long misses the most important property of flash memory: namely, whether its NAND cells are single or multi-level cells.
Multi Level Cells are cheaper, but will severely (10-fold, give or take) reduce reliability of flash memory. Moreover the MLC’s have higher power consumption and smaller operating temperature range.
Most manufacturers somehow “forget” to mention whether their product is MLC or SLC. The only one indicator is the speed. The MLC NAND cells are always slower than SLC’s.
So, purchasing the SD cards with fastest transfer rates, will guarantee you the highest reliability as it will be manufactured using SLC technology.
For camera´s, the reliability does not matter so much, at worst, one will lose a picture or two, but if the purpose of SD card is storage expansion, then Single Level Cells are way to go.

By stasi47 on 28 Feb 2013

SD card size and speed querry

I have a 4gb class 2 SD card in my lumix camera and I recently purchased an 8gb class 10.
Why would the 4gb class 2 be faster? It is very noticeable when editing the pictures while still on the card.

By iggysmith on 16 May 2013

SD card size and speed querry

I have a 4gb class 2 SD card in my lumix camera and I recently purchased an 8gb class 10.
Why would the 4gb class 2 be faster? It is very noticeable when editing the pictures while still on the card.

By iggysmith on 16 May 2013

I have a Sony HX300 camera for which I purchased a Sandisk Extreme Pro 8GB 95MB/S CD card in order to speed up write times. After taking 10 frames the write time is the same as aslow class 2 card. Will you explain why please.

By HillClose on 25 Oct 2013

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For more details about purchasing this feature and/or images for editorial usage, please contact Jasmine Samra on pictures@dennis.co.uk

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