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Author Topic: SD card min. write speed?  (Read 673 times)

Marsu42

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SD card min. write speed?
« on: March 07, 2013, 02:30:52 PM »
I'm currently using a 2010 32gb SankDisk Extreme with "30mb/s" on the sticker. Since the prices have dropped a lot since I'm wondering if getting a newer card would improve my raw+jpeg continuous burst rate significantly - currently I'm using a 60d but will get a 6d sooner or later.

* Does the *minimum* write speed depend on the size of the card, i.e. is a 32gb card better than a 16gb one except for the obvious storage difference?

* Does my older 60d still profit from the improved write speed of the current Sandisk "Extreme Pro" with 95mb/s on the sticker, or is the a newer uhs-1 sd controller like on the 6d required?

Thanks!

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SD card min. write speed?
« on: March 07, 2013, 02:30:52 PM »

Mt Spokane Photography

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Re: SD card min. write speed?
« Reply #1 on: March 07, 2013, 03:04:50 PM »
The issue with minimum write speed comes when you are overwriting data.  The card first erases data in blocks and then writes to the now empty block.  SD Cards all slow way down doing this, while CF cards are much faster.  There is a technical reason for this, but I'll skip it.
 
The only way to overcome the slow write is do do a card erase or low level format.  This actually erases all the memory blocks so that the camera does not have to erase them before writing.
 
Thats why you usually see a fast maximum write speed, but a low mimimum write speed.
 
Get a large card, and you can use it a lot before having to do a low level format.
 
For example, here is the fine print for a Sandisk 128GB SDXC UHS-1 45mb/sec card.
 
"The 128GB SDXC Memory Card Extreme Class 10 from SanDisk is a high-performance memory card. It improves upon the previous version's 30 MB/s max read / write speed by offering faster 45 MB/s performance. Minimum write speed is still 10 MB/s. These data rates get you ready for the next shot faster, and cut down on wait time when you're transferring your work from card to PC".
 
Unfortunately, not all the listings for memory are forthcoming with the facts and only list maximum speeds, which users only get when the card is new.  We see people telling them to format their card in the camera, but only a low level format brings it back to new condition.  You can select low level format in newer cameras, only use it after you've used the card enough to fill it.
 
Cards try to distribute the usage evenly over a card, so with a 128GB card, you could likely write 8gb of data, do a regular format 15 or 16 times before needing to do a low level format.  With a 8GB card, you'd need to do a low level format every time.
 
UHS-1 Cards if used in a UHS-1 compatible camera may be up to twice the speed of a fast SDXC Card, so you might get up to 20 MB/sec on a card that is being overwritten.  They basically do this by doubling the bus speed.
 
If a card is Class 10, it has a minimum write speed of 10 MB/Sec  If its UHS-1 speed class, its going to be faster, but how fast the minimum speed is, is not published.
Thats why a CR card is a much better way to go if you need fast in camera write speed.
 

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Re: SD card min. write speed?
« Reply #1 on: March 07, 2013, 03:04:50 PM »