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09-03-2007, 11:48 PM   #1
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Time Exposure - Bulb setting - Read write times

Hi there,

When I got my Pentax *ist DL I went out one night and did some long time exposures, of 30 sec on f22.

After taking the picture, I had to wait another 30 seconds before I could use the camera. My question is....

Why does it take 30 seconds to "process" the image after you have taken it? Or if it is a 1 minute exposure 1 minute to "process" it before writing/recording it to the SD card?

I thought this may have been some quirk with the *ist DL but it happens with my K10D as well. Has it got something to do with the processor? It is annoying taking a long exposure then having to wait and wait before you are able to shoot another shot.

Thanks. :-)

09-03-2007, 11:59 PM   #2
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hi
30 seconds is the exposure time, the camera takes another 30 seconds to do the noise reduction
this can be turned off in the custom settings menu
i have played with this function over the past couple weeks, but i'm not sure whether it's better
to leave it on or turn it off (for close enough to instant review)

i guess it just means you have to do more noise reduction yourself, versus being able to shoot the next pic or review faster

dave
09-04-2007, 12:02 AM   #3
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Past a certain speed there is an automatic noise reduction process happening first image is taken and then a second image taken at the same speed with the shutter closed and the colour of the noisy pixels subtracted from the first image - more or less.

All digital cameras do it when used on program modes- not just Pentax. Not sure about bulb mode or manual modes.
09-04-2007, 10:54 AM   #4
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FYI - When shooting long-exposure firework shots, or whatever long-exposure shots, you do not need to wait for the camera to process the first picture before you take the second picture.

09-04-2007, 06:26 PM   #5
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Noise Reduction for long exposures takes a dark frame (shutter closed) of the same duration as the picture, then subtracts it from the light frame. This subtracts a lot of noise from the photo. It is a process long used by astrophotographers. After all, the night sky is black and shows noise speckles quite well.

If speed is not important then by all means use NR, it is the default. As Tom mentioned when I was shooting holiday fireworks I turned NR off because I wanted faster recycle time and the bright subject drowns out the speckles in the sky.

When my exposures are up in the minute range I need dark subtraction bad! An alternative is take the images with NR turned off and take your own dark frames which you later subtract with software.
09-04-2007, 06:38 PM   #6
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QuoteOriginally posted by Tom M Quote
FYI - When shooting long-exposure firework shots, or whatever long-exposure shots, you do not need to wait for the camera to process the first picture before you take the second picture.
Yes I do have to wait, it will not allow me to use the camera in any way while each shot is being "processed" after taking it. I can't press the shutter, I can't use the cable release or anything.

Believe me I tried, I find it very frustrating. As yet I haven't tried turning off noise reduction, I will give that a go next.

Thanks.
09-05-2007, 05:22 AM   #7
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Time Exposures

As Leo points out, the extra time (matching that of the light exposure) is to produce a noise reducing dark exposure that is subtracted from the light exposure. If you can't wait for the dark exposure, you can turn noise reduction off. That will result in a noiser picture. If you don't want that then you can put on the lens shade and eye piece cover and shoot some dark frames of the same duration when you are finished capturing the event in question. You should do at least 4-5 then average them in photoshop to create a dark frame you can subtract from the light frames you took to clean up the noise. Note that you should shoot the dark frames at the same approximate temperature as the light frames because noise levels are correlated with temperature so don't shoot them later (eg., at home in your house). Save the dark frames you start collecting and name them with the time and temperature. You can then reuse them when shooting using those same parameters in the future and not have to reshoot dark frames every time.
Steve
09-05-2007, 07:38 AM   #8
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Thanks everyone,

I will give it a go.

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