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CF Cards: reading speeds and buffer clearing


o_brian

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I don't currently shoot digital but am doing my homework prior to

leaping over from film.

 

Question regarding compactflash card speed and camera burst rates: if

a camera has a burst rate of 5 frames per second and capable of a

rate of 6 continuous shots in RAW (which I believe equates to 6 RAW

shots in little over a second), and a Sandisk Ultra ll CF card can

read/write at something like 9 or 10 mb per second, than won't there

be a significant bottleneck since the RAW file, on say the new Canon

20D, is around 12mb PER SHOT? In other words, shooting 6 RAW shots

continuous in little over a second would generate 72mb of data. The

flash card can only read something like 10mb per second. Won't there

be either a lag or gag?

 

I'm sure there's something fundamentally wrong with this question,

but I don't know that until someone shows me the light.

 

While I'm here I might as well slip in another example of ignorance.

What happens after one reaches the continuous shot limit of say 6 RAW

shots? Does the camera burp, belch, and f@rt? Or does the display

start flashing a signal?

 

First post here. Plenty more to come.

 

Thanks,

David

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David,

 

<p>You have to consider the large buffer that the 20D has that will hold the shots until it can puch them to the CF.

 

<p>Sadly the camera doesn't do anything resembling a bodily function, it merely won't allow any more shots to be taken. I wouldn't worry about exceeding the buffer size of the camera unless you like taking 20+ image consecutive shots of someone walking down the street or something!

 

<p>Good luck,

 

<p>Sean

<br><a href="http://www.stnphotography.com">www.stnphotography.com</a>

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Stephen,

 

So it's just a matter of holding my britches for a few seconds?

 

While I'm on the flashcard topic: is it best to have a number of 512's as opposed to a 1G or two with regard to the concern of loosing too many pictures if something happens to the larger cards? Or is the size of the RAW files such that the 1G's can fill up pretty quickly?

 

I think I already know the answer to that one, but always open to second opinions.

 

David

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David, I have both a 1gb card and a 512mb card, I find that the 512 fills up too quickly for my tastes. I prefer the size of the 1gb card, and will probably get another 1gb card soon (Sandisk 1gb Ultra II's are 99 bucks now!!!). I probably wouldnt go higher than 1gb though, both because its not smart economically (more than double the price of a 1gb card) and as you mentioned, I wouldnt want to put that many eggs in one basket. I guess it depends on how much you shoot, but I would recommend a 1gb card.
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David, your question, as formulated gets to the heart of the matter. It a very intelligent question...

 

The camera pauses, and you can't take any more pictures; additionally a display light blinks. I'm not sure how the Canon cameras handle it now, but it used to be that you could do nothing until the buffer cleared, not even view the pics on the LCD.

 

For Nikons, which I know about, when 1 pic is flushed to the CF card, you can take one more, and so on. You can always review/zoom/scroll pictures on the LCD regardless of the buffer's state.

 

In real world terms, the Nikon D70 uses about 5MB per compressed NEF (losslessly compressed, that is). Write speed to a Sandisk Ultra II is about 4 Megabytes per second or approximately 1 pic every 1.25 seconds. i.e. after the buffer is full, you can limp along about a frame a second, or you can wait 4 seconds or so, and the buffer is fully flushed/cleared.

 

So it does matter, in the example above, how fast the CF card can be written to.

 

What you need to figure out is how often, given your style of shooting, and your choice of camera (with its given buffer and write speed) and CF card (with its write speed for that particular camera) you are actually going to reach the buffer's limit. If you are a sports shooter in motor drive, you will reach a 5 frame buffer (such as that on a lower end camera) quite often (say several times a day). You may never reach a 40 frame buffer (such as that on a D2H) because you may never ever do a 40 frame burst in your shooting life.

 

Similarly if you are a people shooter, you may never reach even a 4 frame buffer. I have a D100 and have never, in almost a year of shooting now, filled up the buffer (with a Sandisk UltraII) in actual practices. There are just too many pauses, as a practical matter of framing and timing, for this to happen, seemingly.

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Mani, thanks for the thoughtful answer. In particular your flattering remark about a "very intelligient question". I've got to do this forum stuff more often. My photography may not be very good, but I'm a sucker for compliments.

 

As for style of shooting.....well, one of them is the toughest of all - taking pictures of an 18 month old. The girl won't follow instruction, won't stand still, won't walk parallel to the shooter....always away or running straight toward (can't focus fast enough normally), won't smile when needed, and usually poops just when the light is best. Consequently, I need a larger burst rate, a much larger buffer, and perhaps most importantly a lot more patience.

 

Thanks,

David

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David,

 

I've been this route a few years ago, and am relearning those old skills with my 7 month old daughter! With my D100, which has a 4 frame buffer, I have yet to hit run into a situation when it fills up. The reason, I think, is anticipation. I'm always waiting for the right expression or moment, and that, as a practical matter, allows plently of time for the buffer to flush. Using bursts of shots has never worked for me with little kids-its much more a situation of playing and goofing around with them, and squeezing off the shots as you talk with them, just when they are reacting to you with that cute expressions. Instructions? They don't work, with all due respect. Not in photography, not for the rest of your life as a parent, I'm told...

 

;-)

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