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Saturday, March 1, 2008

March/April 2008 HelpLine

Lindsay Miller Published in HelpLine
March/April 2008 HelpLine

Noise Solutions
Q)
A coworker and I were shooting a fireworks display the other night. This was my first experience using extended shutter-release settings. I have a Sony DSLR-A100 and he uses a Canon; we metered for ƒ/22 at 30 seconds with an ISO of 100. My camera would take a 30-second exposure and then say "processing" on the LCD for another 30 seconds before I could take another picture, whereas my friend would take another picture immediately with no in-camera processing. This happened with RAW, JPEG-fine and Standard. Needless to say, this was quite a nuisance, and my picture capability was cut in half compared to the Canon.

I contacted Sony, and after speaking with my third tech support guy (on a tiered system) over one hour later, I was told that after the extended exposure, the sensor generates a lot of heat, which creates noise, so the camera takes another exposure with no shutter release to create a black picture and the camera combines these to create a picture with less noise. He also stated that all digital SLRs use this same process. I told him about my friend's Canon that doesn't do this. He then conferred with another tech support guy who said that he knows that Nikon does this. My friend talked to a fellow student of his who owns a Nikon, and she said her camera only takes one to two seconds to process after exposure. Sony also stated that this can't be turned off and that the new Sony DSLR-A700 does the same thing and that he wasn't sure about the yet-to-be-released Alpha pro-series camera.

Is this an accurate explanation of my problem, and am I stuck with this disappointing feature?

Mel

Via the Internet

 

During long exposures, a digital camera's image sensor can build up a lot of image noise, which is most noticeable in the dark areas. Actually, the noise is always there, but it becomes more evident the longer the exposure you take. Whenever there's noise, manufacturers implement noise-reduction algorithms to reduce it.
A) While you describe this "feature" as disappointing, it can be useful—it just depends on how you look at it. But first, let's deal with what's going on here and why your camera takes so long to process images between shots.

During long exposures, a digital camera's image sensor can build up a lot of image noise, which is most noticeable in the dark areas. Actually, the noise is always there, but it becomes more evident the longer the exposure you take. Whenever there's noise, manufacturers implement noise-reduction algorithms to reduce it.

One popular way to reduce noise is to try to capture just the noise and then use the captured noise to remove it from the original. If you remember anything about sine waves from school, you might recall that if you invert a signal 180 degrees and add it back to itself, you'll cancel out the original signal.

For example, let's say you walk into a room that has a lot of noise in it—a blowing air conditioner, a buzzing sound coming from a speaker or a humming microwave in your kitchen. If you want to make an audio recording of someone speaking in that room, you would get poor results. But if you could record all the background noise in the room before the person started talking, you can take the noise, invert it and add it back into a recording of the speaker in order to reduce the noise.

A digital camera's noise-reduction process works using the same method. By taking a second exposure immediately after the initial one (this time with the shutter closed), a so-called "dark frame" is created. In essence, the camera is taking a picture of the noise. This second picture is then subtracted from the initial exposure so any noise that appears in the same place in the two images is reduced or removed.

Unfortunately, there are no shortcuts. The only way to accurately capture the level and placement of the noise is to have an exposure that's the same length as the original exposure. When you use long exposures, the doubling of the exposure time is very noticeable. As you experienced, a 30-second exposure becomes a 60-second exposure. As you use longer and longer exposures, not only is time an issue, but battery power is a concern as well.

But the good news is that you can turn off this feature if you have a Sony DSLR-A100. Find the Noise Reduction option in your shooting menu and turn it to Off-most cameras allow you to turn it off. If you have questions, please send them to HelpLine, PCPhoto Magazine, 12121 Wilshire Blvd., Ste. 1200, Los Angeles, CA 90025 or This email address is being protected from spambots. You need JavaScript enabled to view it.. Visit our website at www.pcphotomag.com for the web-exclusive HelpLine Weekly and past HelpLine columns.

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