Hi there,

 

in the past I did some long time exposure shots. I'd like to spent more time in this photographic style. But a common issue in digital photography is the resulting additional noise with long time exposures. With my camera it isn't just noise like higher ISO noise. There are really lot of hot pixels all over the image.

 

I know, a fairly useful method to get rid of the noise is the dark frame subtraction.

 

The camera internal function of this method, Canon calls it "Long exposure noise reduction", is working pretty well. But the drawback is, that this method is very time consuming. The dark frame is taken directly after the original shot, there's no chance to preview the original shot and check if it was taken correctly first. I've to wait twice as long before I can look at the result. So it would be great to take the dark frame after a original shot I'm happy with, manually. So I came up with the idea to take the dark frame separately and doing the subtraction process in post production.

 

I while ago I played around with a possible workflow: I did put a dark fabric over the lens or attached a lens cap. The view finder was covered anyway. I was taking the dark frame with the same exposure settings as the shot before. In theory it should create the same amount of noise but on a black background, which can be used later in Photoshop. But it wasn't the same noise. When I was layering the dark frame on top the original shot in Photoshop, there's was a difference between these "long time exposure noises", that means the hot pixels were appearing in different locations. I wonder why are these differences? I know how to setup subtraction process in Photoshop, there shouldn't be a mistake. Could this be a issue of the raw conversion process? I disable all noise reduction routines in Lightroom before sending the two images to Photoshop. But it wasn't the solution at the end.

 

Maybe someone as some useful hints for me?

 

Thanks a lot,

Peter

Hi!

 

For realized long exposure (2-3min) I just use the long exposure noise reduction of the case.

 

It is only in astro-picture what i achieves night setting to 100iso more 10min. In this case I make a picture with the cache, called "dark" and I use the IRIS software that which allows me to treaty a series directly from RAW.

 

I make be several "Dark" and averaging of all. we must do "dark" before, during and after, because the temperature of the air decreases during the night. Moreover the sensor temperature increases. Thus, a "dark" to another, even if the temperature outside is the same, the response of the sensor will not be the same.

 

But I do not use this technique for landscapes.

For pictures of days, you just do a test without filter and multiply the time by 1000( in the case of a ND1000 filter). So you will avoid having to repeat several times the photo long exposure to find the correct exposure.

 

For night shots you can augment the ISO and photographing large opening for correctly exposed picture and get a couple (open / exposure time), there with a table (or an Android application) you found the time to pose for photo at 100 iso, and therefore made a single test

 

@+

I'll expand a little on what Victorien wrote since he mentioned astrophotography and IRIS.

 

There are a number of sources for noise and image defects in an image of any exposure length. Dark current is one of them. Others include read noise (introduced by the readout electronics), shot noise (introduced by the random variation of where photons land on the detector), hot or cold pixels (varations in detector manufacture), and cosmic ray impacts (look like hot pixels, but will vary frame-to-frame).

 

You can combat most of these. Software like Pixel Fixer lets you map your hot and cold pixels and remove them from your RAW files. If you make your own darks (which works fine, by the way) you may need to make several of them and take the pixel-by-pixel median to reject cosmic rays. You can do the same while making long exposures: make several, combine them by taking the median of each pixel, and the resulting image shouldn't have any cosmic ray hits in it. This also helps reduce shot noise.

 

IRIS does a really good job of all of this, though it's written from the standpoint of doing astrophotography. It's a little cryptic to use if you haven't seen astrophotography work flow before. I wish Photoshop let you combine multiple frames using a median filter, but I've never seen that. I've used ImageMagick for median combining frames, but as far as I know it won't work with RAW files.

 

Tom

Hi Victorien, Hi Tom.

 

I'm sorry for the late reply. Thank you booth for your very useful comments.

 

As I understood you booth correctly, cause of different influences that's usual that there's a different noise amount/types of the original photo and the manually taken dark frame? That's interesting and good to know! But I wonder why the "Long Exposure Noise Reduction" of the cameras is so successful even with very long exposures (> 10 minutes) and just one dark frame. Maybe there's more than the utilization of dark frame subtraction in the camera internal procedures?

 

I'm going to check Pixel Fixer and IRIS out. Thank you for this. I heard that "Nik's Dfine" can fix some of the noise issues as well. But I never tried it.

 

Taking multiple darks (how much exactly?) seems very time consuming to me, much more than the camera internal method! Has this done every single photo or can this be done just once for several exposures and reused?

 

Apparently, the stacking can be done in Photoshop as well. I found an article about Median Stacking:

 

http://petapixel.com/2013/05/29/a-look-at-reducing-noise-in-photographs-using-median-blending/

 

And there's a Photoshop workflow described:

 

 

Generally, are there more practical information on the web, the article I linked above is informative, but isn't enough in my opinion.

 

 

I've a similar workflow, but sometimes something goes wrong and hopefully I want to . Usually I want to make long exposures of daylight scenes. Depending on the scene or situation I would love the work with exposure times between 3 to 20 minutes. Maybe it's to extreme, but I love the blurred skies or water. I also like the idea of "blurring pedestrian away". :)

 

Cheers,

Peter

 

I don't really do multiple darks. It's pretty standard fare for astrophotography, but for landscapes I either use in-camera dark subtraction or I take one dark for each exposure time I use, run PixelFixer on both the dark and the light images, subtract the dark, then go over the resulting image at 1:1 pixels to look for problem areas. It winds up being a lot less complicated than it sounds.

 

The whole going over things at 1:1 seems like a lot of work, but it's no different than spotting a black and white print for dust.

 

Hey, thanks for the info on how to do median stacking in Photoshop! I'd never seen that before. Now I need to go play!

 

Tom