Luminance or Illuminance ?

Started Mar 28, 2013 | Discussions thread
GeorgianBay1939
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Re: Luminance or Illuminance ?
In reply to Tom Axford, Mar 28, 2013

Tom Axford wrote:

GeorgianBay1939 wrote:

Swamp Duck wrote:

It has been my thought that Luminance was the light reflected from a given subject, Illuminance was the light on the subject?

Yes, that is what I am concluding also. But, contrary to most photo essays, I see no reason to care about the light on the subject.  I really, really care, though, what my camera's sensor is sensing.

That was a significant reason why I decided on a MFT camera with a live view EVF instead of a DSLR with an OVF a couple of years ago.  Of course there were other reasons, especially the ability to use good small diameter, short lenses.

Thanks,

tom

I think one of the historical reasons for caring about the light on the subject was in a method of metering that was more common in the days of film: that of using an incident light meter (measuring illuminance) instead of using a reflected light meter (measuring luminance). These days, metering is nearly always by the camera which is essentially a reflected light meter.

The advantage of using an incident light meter was that white snow came out white and a black cat came out black, whereas using a reflected light meter both would come out mid-grey (if they more-or-less filled the frame).

Thank you, Tom.

That is very helpful.   I suppose if one is lighting a scene on a set or in a studio, an incident light meter would also be useful.... when changing the intensity of the various lights to change their luminance.

Would the white snow / black cat situation be an issue if you used spot metering to give the highest "exposure" of the sensor (ETTR)?

In my case, when shooting RAW I always load the sensor up to white clipping by surveying the scene with the spot meter, half pressing, recomposing and then releasing the shutter.  (with due regard to DOF, motion blur and art.)

The past couple of months I challenged myself with trying to capture "sparkles" in snow, like this scene:

301302 twig with snow crystals

This was also a delicate image to expose properly:

201302 birches Skerryvore

I suspect that the above images aren't large enough to show the delicate sparkles in the foreground snow.  In those cases I did get the little spike in histogram showing the specular reflections.  In both cases I used spot metering, moved it around to let the spike move to the right, and made the exposure.  I then checked the exposure in R,G,B and Y on the postview histogram and had no clipping (except for specular), although IIRC the blue was close in the birches against the sky shot.  Didn't have to bracket or re-expose.

I was having a devil of a time with this challenge until I decided to continue expose the scenes as much as possible and then to normalize them in post processing to what they would look like when using neutral gray sunglasses!!!

Sometimes cheating is helpful, eh?

Yes, I would appreciate your advice re the black cat/ white snow issue as I don't think that I properly understand it.

Many thanks,

t.

PS  Plse forgive my wordiness....  and the imagery.   I try to relate what I am learning here with how I use my GH2.   Old habit!

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