Is long exposure noise reduction needed when capturing RAW for astro?

[ahsanford]

I'm off to do something I never do -- shoot some astro on my 5D3 -- and I have a very simple question: does long exposure noise reduction do a damn thing when shooting RAW, or does it only affect in-camera JPGs?

The 5D3 manual does not seem to indicate this in the long exposure NR section.

Thx,
A

 

[Loswr]

LENR is the only non-exposure camera setting that affects the RAW image data – the subtraction of a dark frame is baked in. (HTP affects the RAW metadata but not the image data.)

LENR removes hot/stuck pixels (which most raw converters remove automatically anyway), and it removes fixed pattern noise. However, in terms of random/thermal noise, from a mathematical standpoint subtraction is the same as addition – so LENR actually adds random noise. Probably not enough to matter at low ISO, but at higher ISOs it will be noticeable, which is why Canon recommends against using it at higher ISOs (above 1600, IIRC).

Personally, I leave it off. Hardcore astro photographers will actually create a master dark frame generated by averaging several individual dark frames, then subtract that from each of the actual exposures, before averaging those exposures. Going back to the math, averaging noise reduces that noise (as a square root function, so there is a diminishing return).

Or you could just look at the pretty stars.

Hope that helps, and good luck!

 

[Don Haines]

Also, if you turn on LENR and take, for example, a 30 second exposure, the camera will then spend 30 seconds doing the LENR exposure. During that second 30 seconds your camera will behave like it is dead....

 

[ahsanford]

Done and done. Thanks, gang.

I will keep that feature turned off as I'm stuck with my 16-35 f/4L IS and undoubtedly will be shooting at a very high ISO.

- A