How fast is fast enough?

GSDMan · Nov 22, 2006

In my Pentax ist Ds I have a SanDisc Ultra II 1gig SD Card. The advertized write speed of this card is 9 megabytes/second. I can't find the max write speed of my camera and I'm wondering how I can figure it out.

The camera will take 3 pictures, or so, very quickly till the camera's memory is full. From then on, I assume the rate at which it will take pictures is limited by the write speed of the camera/card combination. If I time how long it takes to take, say, 30 pictures and do the math, will I have a fair approximation of my write speed? And if it's around the supposed write speed of my card, can I assume spending money on a faster card is a waste of money or will there be some other benifit of which I am not currently aware?

I can't imagine this really being much of a problem. Odd, the places the mind goes when it's allowed to wander, huh?

bitteraspects · Nov 22, 2006

it has more to do with your cameras buffer speed then the cards write speed. buffer speed differes between models.

GSDMan · Nov 22, 2006

Well, that is the basic question, isn't it? What is currently limiting the speed my camera will take pictures? Is it the camera or the card? Can't do much about the camera, I suppose, and I don't feel like spending money on a faster card without knowing if it will do any good. Of course, then I began wondering how I could actually measure the speed.

bitteraspects · Nov 22, 2006

no matter how fast of a card you get, your buffer speed will remain the same. youre going to get the same reaction from from a regular card as you will with an ultra

GSDMan · Nov 24, 2006

bitteraspects said:
no matter how fast of a card you get, your buffer speed will remain the same . . .

I know this, too. So, how do I find my buffer speed?

Big Mike · Nov 24, 2006

2.8 fps up to 8 images

From here

So your buffer will hold 8 images while it's writing to the card. That's not a lot by current standards...but that's a two year old camera.

If you want to shoot more shots...continuously...then a faster card won't let you do that...but a faster card should help to clear the buffer faster...which should ultimately allow you to get more shots over a period of time (say 30 seconds for example).

Maybe you could find a camera store that would let you test some cards. Just hold down the button and let it fire away. It should shoot for 2.8 per second...up to 8 images...and then slow down. Theoretically...when using a faster card...the slow part should be faster...but it also has to do with how the camera is programmed...so it may not make a difference at all.

GSDMan · Nov 25, 2006

I give up. I'll figure it out myself.

GSDMan · Nov 26, 2006

The card I have now is roughly 3x faster than my camera actually needs. The write speed I measured for my camera/card combination was 2.81MB/sec. So, in SD/MMC card speak, I need a card rated at 20x, or 3MB/sec write speed, and anything faster is a waste of money.

Caution - math follows!

I shot a total of 94 frames in four batches. Since it takes a total of six frames to fill the buffer, and the camera to slow down to a shooting speed limited primarily by how fast the buffer can write to the card, I didn't start timing or counting till the seventh frame. So, the four batches of frames consisted of 16 frames in :58, 21 frames in 1:13, 26 frames in 1:27, and 31 frames in 1:41. Since each batch has 5 frames more the the previous one, and each increases by approx 14 seconds, it seems pretty valid and consistant to me. So, these 94 frames took a total of 5:19 to shoot and be written to the card for a total of .294671 frames/sec.. I then loaded the last batch of 31 frames onto the computer and found the last 31 frames totalled 296MB for an average frame size of 9.548387MB/frame. Multiply the two together and you get 2.813633 MB/sec.

I wouldn't call this a simple or easy proceedure, but it was fairly straightforward. And, as a closet math geek I just couldn't pass up this oppourtunity to strut my geekiness.:greenpbl: