5 replies to this topic

[mackiedlm]

One of my Canon EOS1100Ds is off for Modification (Cut IR Removal) and I've been reading up about how to do Custom White Balance on it when it is back.

 

Everything I've read says to take the CWB image "under a clear noon sky". Where I am in the West of Ireland "clear noon sky's" are a bit like hens teeth. Particularly in October. Even less common than the "moonless clear night sky".

 

So I'm wondering if setting up the pic with the 18% grey card under a daylight lamp would be effective. Anyone with any experience of this?

 

Thanks

 

David

[SteveInNZ]

Don't get too hung up on it as you'll probably be adjusting the colours in post anyway. A sheet of white paper, out in the open on a cloudy day is fine. Leave the bright orange hi-viz jacket at home.

Let the camera pick the exposure and check the RGB histograms to make sure the red hasn't clipped.

 

Steve

[ChristopherBeere]

Yeah just shoot AV under natural daylight and you will be fine.

[t_image]

One of my Canon EOS1100Ds is off for Modification (Cut IR Removal) and I've been reading up about how to do Custom White Balance on it when it is back.

 

Everything I've read says to take the CWB image "under a clear noon sky". Where I am in the West of Ireland "clear noon sky's" are a bit like hens teeth. Particularly in October. Even less common than the "moonless clear night sky".

 

So I'm wondering if setting up the pic with the 18% grey card under a daylight lamp would be effective. Anyone with any experience of this?

 

Thanks

 

David

Lots of ways to do things.

Getting channels balanced is not just WB, but applying distortion of tint properly.

Note to have any colors you need to expose your AP images well, clipped stars have no color info, they are max values in all three channels.....

Easy to consider if you've got a well exposed Sun-class star in your image, just WB/tint off that,

or use things like Pixinsight scripts that solve and take star specific spectral tables and objectively balance/tint one's data so it is consistent with reality......

If you've done terrestrial photography you know that conditions will affect the daylight balance and tint....

A daylight lamp meant to mimic the noon Sun spectral and a gray card will do....

Keep in mind the whole point of calibration.

If one takes a still of a gray card under noonday Sun,

and a still of the same gray card under overcast conditions,

 

one can easily determine the WB/tint offset......

 

The whole point of using a known source like a graycard is to eliminate the variable of lighting that changes things,

thus knowing a neutral gray will have neutral values with even lighting is probably easiestkey.

You can easily use the color picker in PS to test this out. If you balanced to the neutral gray, the neutral gray will have the same RGB values across....

However,

color is more complex and the CWB is just a simple trade-off v. a willy-nilly no-care attitude.....

[jgraham]

I started out using a gray calibration card in sunlight for my custom white balance. It worked great for normal photography, but it gave me a perfect rendition of my muddy light pollution in my astro images. I found that a reference image taken of a relatively blank section of sky worked much better. I still use this for viewing my images in real-time, but since I save my source images as RAW the custom white balance isn't applied. Instead, I use either the automatic color balance function in Nebulosity or DSS to set the white balance after my images have been stacked and calibrated. Soooo, I wouldn't worry too much about setting up your custom white balance. It's easy and you have several options.

[Samir Kharusi]

A clear night sky on Earth below the atmosphere on a moonless night is toffee colored, not grey. In spite of all the astro images you see with a grey background. Here's proof. So basically you have to deal with two color balances, one to get the tint you want in your background sky, most opt for grey RGB=1:1:1 though personally I prefer a greenish blue RGB=0:0.5:1 (your choice/taste) and the actual image as you imagine it "should" look outside the atmosphere (that G2 star business). Fortunately most astro images have something that should be close to white (most stars, galaxies, etc.) so we can white balance on those. A good choice is the halo of an unsaturated star. Eye-drop it white and work your way from there. Most amateur images end up with extreme color saturation anyway. Looks prettier that way. How often do you see deep brown or deep blue stars with your eyes? But as long as some (most?) stars are white we accept, e.g. the Andromeda Galaxy with deep blue and deep brown whorls. Add in a greenish blue sky background and you come right up my taste buds  Ok, I'll also submit to the masses who prefer a grey background