18% luminance

christulk · July 15, 2006

Hi,

I was wondering if someone could explain 18% luminance to me.
Also, is there a way to 'find it'? (Can you by a card or something?)
I will put the question in context. I took a photo and asked how I could correctly meter for it. (The photo I took is overexposed for the outside scenery). I was told to use fill flash. The method described was to meter for the external light, then point the camera (20D) to a 18% luminance surface, pop the flash up, press the FEL button, then compose and take the shot.

Here is the shot I took.

Any advice on 18% luminance would be helpful.

Thanks

Shay Stephens · July 15, 2006

christulk wrote:

Hi,

I was wondering if someone could explain 18% luminance to me.

To start with, gray cards do not emit light, only reflect it. So a gray card reflects 18% of the light that hits it and has 18% reflectance, not luminance. Minor, but may help in understanding none-the-less

Also, is there a way to 'find it'? (Can you by a card or something?)

That is the prefered way as opposed to trying to find something in nature or the surroundings. You can buy paper or plastic gray cards. I have a 4x6" plastic gray card in my camera bag (I cut it myself). You can also get a lens cleaning cloth that is 18% gray. I also carry this in my bag. They are handy for exposure and white balance.

18% gray card
18% gray micro-fiber cloth

I have never tried the FEC method you mentioned, so you would just have to try it and see if it worked. If I were stuck without a flash meter, my initial try would be to expose for the bright outside background and set the FEC to -1EV and see where that got me, adjusting the flash output as needed.

Another option would be to set the camera on a tripod and take two exposures (not using flash), one for the outside and one for the inside and blend them later in post.

christulk · July 15, 2006

Thanks

Shay Stephens wrote:

To start with, gray cards do not emit light, only reflect it. So a gray card reflects 18% of the light that hits it and has 18% reflectance, not luminance. Minor, but may help in understanding none-the-less

That is the prefered way as opposed to trying to find something in nature or the surroundings. You can buy paper or plastic gray cards. I have a 4x6" plastic gray card in my camera bag (I cut it myself). You can also get a lens cleaning cloth that is 18% gray. I also carry this in my bag. They are handy for exposure and white balance.

18% gray card
18% gray micro-fiber cloth

I have never tried the FEC method you mentioned, so you would just have to try it and see if it worked. If I were stuck without a flash meter, my initial try would be to expose for the bright outside background and set the FEC to -1EV and see where that got me, adjusting the flash output as needed.

Another option would be to set the camera on a tripod and take two exposures (not using flash), one for the outside and one for the inside and blend them later in post.

Thanks very much for the advice. Much clearer now.

Cheers

devbobo · July 15, 2006

Hey Chris,

Do you have something to do with 'Mr Tulk' at the State Library ?

Cheers,

David

18% luminance

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