Shooting a grey card when using ND filters

[futureal33]

Afternoon all,

Something that has always confused me, shooting a grey card to get the correct WB when using ND filters (10 stopper and grads)

What I want to do is get my WB correct by setting a custom WB off a grey card.

Assuming my correct exposure time with an ND filter is 120secs, would I need to take the same exposure of a grey card to use as my custom setting?

Getting the exposure too bright or too dark changes the colour of grey, so how do you know what to do? Especially when shooting with a 10 stopper in BULB mode (with no EV reading on the camera display)

Any help much appreciated!

Thanks

 

[arad85]

Assuming my correct exposure time with an ND filter is 120secs, would I need to take the same exposure of a grey card to use as my custom setting?

Getting the exposure too bright or too dark changes the colour of grey, so how do you know what to do? Especially when shooting with a 10 stopper in BULB mode (with no EV reading on the camera display)

Getting the exposure too bright or dark changes the brightness of the grey not the colour of it.

What the camera needs is a reasonable exposure that puts the grey somewhere around half exposure (i.e. half way between black and white). When you set the custom white balance, all that is then done is for the camera to alter the ratios of R-G and G-B to balance out any colour offsets.

Whatever exposure you need to do this is the one you should use. To calculate it, just find out what the exposure should be with the filter removed and then add 10 stops more light (usually by x 1000 the exposure time 1/1000 -> 1 second).

 

[futureal33]

The trouble is that 10stoppers add colour casts, hence the reason for wanting to use a grey card. If I took a custom reading before applying the filter, it would be completely different than after the filter was in place.

So I'm guessing I would need a photo with the same exposure as the final shot, to set my custom WB grey card shot?

 

[arad85]

The trouble is that 10stoppers add colour casts, hence the reason for wanting to use a grey card. If I took a custom reading before applying the filter, it would be completely different than after the filter was in place.

So I'm guessing I would need a photo with the same exposure as the final shot, to set my custom WB grey card shot?

Clearly you have to take it with the filter in place The point I was making was that you don't need EXACTLY the same exposure just one that gives you the right exposure for the white balance card.

 

[wavefront]

This interests me as I get what Futureal33 means when he says that the colour changes depending on exposure /shutter speed.

I get different colour casts (or at least that's my perception) at different shutter speeds when using ND's. ie for a 4min shutter speed I'd get a different colour cast to a 1 min shutter length (altering the aperture to keep exposure/brightness the same)

Perhaps it's because over the longer time things move more, so the colouring rather than white balance would be significantly different - water /sky for example?

I think I'll go and do some tests with a static grey card.

Mark

 

[aeronic]

It's because the filter blocks different frequencies of light by differing amounts (red/blue) . Hence time is a very important factor.

 

[StewartR]

It's because the filter blocks different frequencies of light by differing amounts (red/blue) . Hence time is a very important factor.

You've lost me there.

Why is time a very important factor? Does the filter let in more red light in the fourth second of the exposure than it does in the first second, or what?

 

[futureal33]

Mark,
I think what you're getting there is like Steven says, weaker/stronger colours rather than different White Balances - I get this too, depending on exposure time.

What I was referring to was that if I wanted to shoot a grey card with a 10 stop ND filter, to get my custom WB set, would I need to get my exposure the same as the "end" photo I was going to take, because a shorter exposure would result in a darker grey card - which in turn might affect the WB?

 

[arad85]

because a shorter exposure would result in a darker grey card - which in turn might affect the WB?

No. The brightness/darkness of the grey card should give the same balance however long you expose for... assuming there is enough range in the RGB values to lift them appreciably out of the dark and not too much so that one or more channels is saturated.