Why do I have a 1:1 shutter speed to write time?

Why does it take exactly (or nearly) as long to write an image to my card as it does to take the picture? For example, I went out tonight and took some night shots. If the exposure was 6 minutes, it took 6 minutes to write that image to the card. I can't take another shot or view that image for 6 minutes. Do I have a slow card or are all Canons like this. My Father inlaw says he has never experienced this with his Nikon. Any ideas? It happened when I had my Rebel Xt and Ive since upgraded to 40d, same issue. My card is a 4 GB Kingston Elite Pro (45x).

You must have Long Exposure Noise Reduction enabled. This takes another picture of equal time with the shutter closed. The second dark picture just generates noise, which is subtracted from the first picture. C.Fn II-1 on the 40D, I believe.

T Kubik wrote in post #8283302
Why does it take exactly (or nearly) as long to write an image to my card as it does to take the picture? For example, I went out tonight and took some night shots. If the exposure was 6 minutes, it took 6 minutes to write that image to the card. I can't take another shot or view that image for 6 minutes. Do I have a slow card or are all Canons like this. My Father inlaw says he has never experienced this with his Nikon. Any ideas? It happened when I had my Rebel Xt and Ive since upgraded to 40d, same issue. My card is a 4 GB Kingston Elite Pro (45x).

Are you saying that after the 6 minute exposure is finished (after the shutter closes) that it takes an additional 6 minutes to write to the card?

scottes got it

My money is on Scottes opinion being right - it does sound a lot like Long exposure noise correction which is basically the camera taking another exposure of the same length and then cross matching to eliminate the noise.

Yes – the camera is taking 2 exposures; you take the 1st one, your “light” exposure which includes your subject matter + a heap of (unwanted) thermal and system noise when you expose for as long as 6 minutes, then the camera takes a 2nd in-camera “dark” exposure.

The 2nd “in-camera” exposure is basically a same duration exposure (with the lens cap on as it were) which records all the thermal noise and system noise as “seen” by the sensor. The 40D then does some internal processing to subtract the “dark” exposure from your “light” exposure, giving you a nice, clean final image.

I suspect that if you turned off this function, a 6 minute “light” exposure would show heaps of noise and spurious hot pixels, especially at room temperatures, but less so if the camera were cooled.

Cheers

Dennis

I don't use Long Exposure Noise Reduction because when doing night exposures, that delay is just too annoying. I do get a couple hot pixels (easily cloned out), but I don't see any issues with noise or banding.

Yeah, I do have it on. Interesting, I didn't know that's how it was done. Thanks for the info!

How does this Long Exposure Noise Reduction even work ? If I understand correctly, the Noise are generated randomly due to heat or EMI. So noise pattern from the closeed shutter exposure will be different than the actual exposure, how does the camera know where to subtract the noise ?

Heres one explanation ...

http://albertdebruijn.​com/home/archives/59

and Canon says

http://www.usa.canon.c​om …ArticleAct&arti​cleID=1407

T Kubik wrote in post #8285418
Yeah, I do have it on. Interesting, I didn't know that's how it was done. Thanks for the info!

I just learned something too, so today was not a complete waste and I can go home and cook pork chops and drink beer now.

HYBEagle wrote in post #8285952
How does this Long Exposure Noise Reduction even work ? If I understand correctly, the Noise are generated randomly due to heat or EMI. So noise pattern from the closeed shutter exposure will be different than the actual exposure, how does the camera know where to subtract the noise ?

As you say, this is no help with random noise. But hot and dead pixels are present in every sensor and they become obvious with long exposures. Long exposure NR will eliminate them.

-js

It rather frustrating waiting 25 minutes to take another pic. I think my next session will be without NR.
BTW, the shot was nice, here it is.

That *is* a nice shot.

I'd be very happy if I had taken it, and I would have been frustrated to hell while waiting for the NR frame to expose.

CannedHeat wrote in post #8283310
Are you saying that after the 6 minute exposure is finished (after the shutter closes) that it takes an additional 6 minutes to write to the card?

This is correct. My last exposure was 23 minutes long. I packed up my stuff drove to the gas station and drove home before it was done writing.