Grainy Night Photography

[maortizjr]

Hello Everyone,

I was out shooting some night shots (long exposure) last night, for the first time with my Canon XSI. In many of my shots I got a great amount of 'graininess' and color aberration. They look like white and color specs. See the examples below:



360 Bridge, Austin, TX
ISO 100 / F20 / 360 Seconds

Here is a 100% Crop:



You can see more here.

What is it that causes this problem? Under/Over exposure? Is it camera or lens limitation?

When shooting film (a few years back) I don't remember dealing with this problem, so I'm not sure how to handle it.

Also, once it is there, how do I deal with it in post processing?

Thank you,
Miguel

[Car2n]

I believe (and I could be wrong) that the spots are a result of the sensor and a long exposure. You have a 6 minute exposure there and the sensor heats up.

[private]

It might be alighting issue - I would have made the aperture number much smaller - it would have let in more light and you would not have needed to have such a long exposure - you will would have gotten the cool light trails... It looks like plain old noise to me - I could be wrong tho'

There are free products on the market to help you get rid of the noise - Noise Ninja and Noiseware

[OsmosisStudios]

Its from having the sensor exposed for so long. You'd get it with film: its just less noticeable.

You can try to correct it with a bit of noise reduction, but that will affect image quality a bit.

[maortizjr]

I see how less time could of help, but by going wider I wouldn't have gotten the great DoF I was looking for.

[BryanC]

Here's a pretty good explanation...

http://webpages.charter.net/bbiggers...ot_pixels.html

[navcom]

The best way to counter it is by taking a second exposure with the lens cap on (black imge) and then using Photoshop or a dedicated software package to blend the two together, subtracting the noise.

One package that can do it....www.startrails.de

It's free.

[oxfordy]

Yes, sensor noise without a doubt - the clasic solution is to produce a stack of shorter exposures and blend them together, but I am not sure you could do that successfully with the light trails (unless this was a very busy road with lots of traffic). Does your Canon have a "long exposure noise reduction" feature? I know my Nikon does, but I'm not familiar with the Canon.

[navcom]

Originally Posted by oxfordy

Yes, sensor noise without a doubt - the clasic solution is to produce a stack of shorter exposures and blend them together, but I am not sure you could do that successfully with the light trails (unless this was a very busy road with lots of traffic). Does your Canon have a "long exposure noise reduction" feature? I know my Nikon does, but I'm not familiar with the Canon.

In-camera noise reduction can help but is usually not a long-term solution to the problem. As Osmosis says, NR will affect image quality. From my experience (which includes Pentax, Nikon, and a little bit of Canon), NR will remove some of the sharpness from the image...not something you want to do when trying to get pictures of stars.

On most digital cameras, NR also takes a lot of time to process. If you take a 10-minute exposure and then close the shutter, the camera will then proceed to process out the noise by taking another exposure with the shutter closed (black image) and then combine the two together in-camera. This can take several minutes depending on how long of an exposure your shot was. If you are doing star trails, this is unacceptable, as you will get gaps in your stars. Instead of having really long star trails, you will get an image with twice as many little short trails. It's cool as well, but usually not the end result most are looking for.

What you have is classic sensor noise, no doubt. All digital cameras suffer from this during long exposures to some degree. The sensor heats up and the heat affects how certain pixels are exposed. During the middle of winter (0 deg. F is my winter!), you will notice you can increase your exposures quite extensively as the sensor stays cool longer.

The best results I've had with star trail images are by turning off all NR featuers and taking 5-10 exposures, approximately 10 minutes each, one right after the other...and then stacking them together with one black image using the software I included in my first post.

I generally use the lowest ISO your camera will allow...usually ISO 100 or lower depending on the camera. I use f/8 for an aperture on night skies. I never open more than f/5 as image quality on long exposures may drop off with wide-open apertures. F/11 or smaller requires just too darn long of an exposure to get anything to develop properly noise-wise.

To find the best exposure, temporarily bump your ISO up as high as it will go and take shots at different shutter speeds until you get one that is properly exposed. Exposures will be MUCH faster at this extreme ISO so you will get quicker, albiet grainer, results. Then you can count how many "stops" it is back down to your low ISO. This is how many stops you will need to increase your shutter speed by to get a proper exposure.

Hope that helps!

[westx]

Take a look at this link for some help. Exposure didn't need to be that long and apature could have been wider using hyperfocus.http://www.learnslr.com/slr-beginner...nded-exposures