I've started experimenting with longer night time exposures and star trails. Currently, I have my 5D set to long exposure noise reduction. So far the maximum time I have done is 45 mins + 45 mins in camera processing although I think I could push this up to the hour mark.

Is there a way to replicate Canon's in-camera noise reduction processing in PS so I could double my current exposure times and use the battery capacity to the fullest?

Thanks,

Ross

You can use various noise reduction programs after the fact, but they will probably not do as good a job subtracting the specific noise signature of your camera as the built in correction. Here is an idea which might save you some time over the long run:

1) Turn off the noise subtraction feature in the camera
2) Take a 45 minute dark exposure (lens cap on) in the field, after the camera has reached temperature equilibrium.
3) Take a 45 minute real image as usual.
4) Subtract the dark image from the real image later in Photoshop.
5) Compare to automatic camera method, to see if noise reduction is just as good.

So far, you have gained nothing, except an understanding of what the dark image noise looks like and how the correction works. However, if you take more than one 45 minute exposure per night, you will save time, since you can use one dark image to correct all real images. You might even be able to use the same dark image days or weeks later, at least if temperature is similar.

Also, this technique might be useful for cameras which do not have the automatic subtraction feature.

Thanks Eric,

I'll give it a try. Mainly I want the best quality so I'll see how the comparison looks.

How do you do this process of subtraction in PS? Had a quick search in help but nothing obvious came out. Is it a blending process?

Ross

PS ignore my second posting

The best way to do this is not in Photoshop but rather to do the noise subtraction while the image is in the raw state. Raw conversion mixes up the individual pixel data via the process of interpolating the colors from the Bayer array of color-filtered photosites; in addition, the gamma correction is a nonlinear curves adjustment that distorts the original raw data and makes the subtraction even more imprecise. The noise signature of the camera is different from pixel to pixel, and so what you want ideally is to subtract the noise from individual pixels before the raw conversion is done. This is what your camera does when it performs long exposure NR; it holds the image you took in a buffer while it records a dark frame, then subtracts the latter from the former and records the result in the raw file.

The freeware raw converter dcraw has the capability to subtract a dark frame taken separately from a raw image file; astrophotography programs such as IRIS can do the same and more. However they require a bit of manual labor since they are not as sophisticated as ACR (dcraw is a command line application; IRIS doesn't have color management and the user interface is a bit basic).

What would be ideal is if standard raw converters had the capability to do these sorts of noise subtractions that subtract a noise template from a raw image, then one would have better noise reduction and one's usual workflow together, rather than having to make a choice between sophisticated noise processing and less capable raw conversion, or less sophisticated noise processing and more capable raw conversion -- which is the current state of affairs.

Making a separate dark frame shot (don't forget to cover the viewfinder as well!) and substracting it in PS is preferrable to the in-camera long exposure noise reduction, for the latter is a brute action. I tested this with the 20D and 40D: it creates "holes" in the image by the blind substraction. When doing it in PS, one has several tools to soften the effect, for example by opacity.

It would be the best to do this in raw format, because the effect of hot pixels gets spread over the neighbours by the de-mosaicing, but this is not supported yet. (It is coming...)