Noise at long exposures

[jonathan_renouf2]

Hi there,

I had a go this morning doing some 30 second exposures at 400ASA outside at dawn, seascapes, shot

RAW. The result? Huge amounts of noise, loads of purple speckling in the dark parts of the exposure. I'm

obviously committing a beginner's error here, so can someone tell me the correct technique for avoiding

noise and purple pixels when doing long exposures?

Thanks

J

[mendel_leisk]

Enable Custom Function: "Long Exposure Noise Reduction", likely CF02.

 

When processing the raw, try disabling Auto Exposure if using Adobe Camera Raw, if you're using that combo.

[brian_coy]

I'm not sure which camera you are using, but the 20D has a noise reduction setting for long exposures. It is found in the custom settings menu. If you are using another camera, check the manual for a similar setting. If you are using that long of an exposure, you might as well drop your ISO to 100 if possible, this should reduce noise, even on long exposures.

[doug_landrum]

Assuming that you have the full Photoshop, convert the profile to LAB color. Go to the a channel and reduce noise with a Dust and Scratches filter. For a full 20D shot, use a radius of 7 pixels and a threshold of zero. Do the same in the b channel. Go to the L channel and sharpen. Convert back to RGB. Your noise should be significantly reduced.

[jonathan_renouf2]

Hi there,

Thanks for suggestions. However, I had long exposure noise reduction on (on my 5D) and

still at 30 seconds the results were awful. I'll knock down to 100ASA next time and see if that

works, but I find it puzzling how much noise and purple spotting there was even on 30

seconds. Lord knows what it would be like at 5 mins.

J

[arash khoshghadam]

Hi Jonathan

 

I am not sure I have exactly figured out what your problem is. Are you wondering why it happens or are you trying to seek a way to get rid of the problem post-process?

 

Noise in long exposure is an inherent problem in image sensor literature. They get warm, the electrons overflow into adjacent photosites and the trouble begins. variation in electron level causes the green and purple spots. You can get rid of noise not in the camera

Raw software or even ACR in photoshop which I think is ill-equipped to offer a reasonable remedy. Noise Ninja software is a good remedy to consider post-process. You have to learn to live with this inherent technical short coming in the image sensor design till engineering science can find a radical solution in betterment of the image sensor design. Until then ....

[jonathan_renouf2]

Hi,

Having looked at the images in question again, I think a large part of the problem in this

particular case was caused by considerable under exposure. When I tried to get some

information out of the shadows, all that came up was noise. Next time I'll see what happens

when I get the exposure right. But I'm interested to hear that some kind of noise in long

exposures is something that digital sensors haven't yet fully solved. For some things, film is

still better it seems.

Thanks for your answers.

J

[arash khoshghadam]

Dear Jonathan

 

 

 

 

 

 

The fact is that high-sensitivity high-iso films have their own problems dealing with bigger Silver particles. They become noisy too, and to tell you the truth, I'd rather shoot digital to be able to cope with the noise by software than shooting film with little chemical-process option back there in the darkroom.