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Reflected meter readings


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#1 David Regan

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Posted 17 September 2007 - 07:29 PM

I'm currently trying to work out some lingering confusion I have regarding light meter readings. When a light meter gives you a reading as an f-stop, what does it base that on? As in when I take my incident meter reading in a room, and it says the 'correct' exposure would be an f5.6, what is that correct for? Is it for 18% gray, so if I put a gray card right there, facing the same direction, and shot it at an f5.6 it would appear middle gray on film? And for a reflected meter, does that also use 18% gray as reference? So if I take a reflected reading of my subjects face and it tells me 'correct' exposure is an f4, I take it if I shoot at an f4 then looked at my film as for only the luminance (b&w) the subjects face would be middle gray.
Is any of this correct, although I understand its all a basic concept, its a weird one for me to grasp, regarding how a meter determines a seemingly artistic choice of 'correct' exposure.
Thanks for the help.
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#2 Laurent Andrieux

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Posted 17 September 2007 - 08:15 PM

All the answers to your question are yes I think...

Meters are calibrated for 18% so then if you shoot a gray card it will look middle gray, and white will look white and black will look black.

White skin is more like 30 % reflectance. So if you aim your reflected meter on a face, it should turn out gray, thus one would open up 2/3 of stop from a reading on a caucasian skin.

You see, it is a matter of artistic for what is about interpreting your readings, and it requires experience, but it also is a question of technics, sensitometry etc.

This is why meters do have a common reference. It's just a meter, it's you to do what you want from the reading.

Hope it may help you.
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#3 Chris Keth

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Posted 17 September 2007 - 08:22 PM

I'm currently trying to work out some lingering confusion I have regarding light meter readings. When a light meter gives you a reading as an f-stop, what does it base that on? As in when I take my incident meter reading in a room, and it says the 'correct' exposure would be an f5.6, what is that correct for? Is it for 18% gray, so if I put a gray card right there, facing the same direction, and shot it at an f5.6 it would appear middle gray on film? And for a reflected meter, does that also use 18% gray as reference? So if I take a reflected reading of my subjects face and it tells me 'correct' exposure is an f4, I take it if I shoot at an f4 then looked at my film as for only the luminance (b&w) the subjects face would be middle gray.
Is any of this correct, although I understand its all a basic concept, its a weird one for me to grasp, regarding how a meter determines a seemingly artistic choice of 'correct' exposure.
Thanks for the help.


Correct exposure is based on negative density and development standards. Whether "correct" is what you need is a whole other thing. Light meter readings given as f-stops are based on three things: the amount of light measured (duh!:)), the shutter speed you set in the meter, and the film ISO you set in the meter. It's like an equation, if you know 3 parts you can figure out the 4th. I promise I will try to be concise and not write a book.


Spotmeters always give you an answer that can be read as "f-whatever if you want the spot you metered to record as middle gray." This means that if you meter a patch of 18% reflectance that is evenly lit, it will tell you what you need to expose it correctly as middle gray on film. The trick comes when you meter an area that should record as something other than medium gray. The meter would tell you what you need to expose it as gray. This means you ahve to do some extrapolating in your head to make things right. Say you read a patch of very black velvet. You probably don't want black to record as gray so you would have to close down the iris to darken the image, about 3 stops. The opposite is true if you read something that should be lighter than middle gray, like the face of a subject like you mention above (correctly). If you were to meter caucasian skin, it would be underexposed if you just believe the meter so you usually open up about a stop in that situation.

Incident meters work somewhat differently. Where reflected meters, by nature, take into account the refective indices of whatever is measured, incident meters do not. They measure only the light falling on the subject, or the incident light on it. The reflective properties of all the things in the scene will take care of whether they are brighter or darker than middle gray. Essentially, an incident meter gives you a reading of what stop you would need to ensure that something that is middle gray will be recorded on film as middle gray. If you want the scene to record darker you just underexpose. If you want it brighter, you just overexpose. The trick with this type of meter is visualizing what various things will look like with the contrast of film. That is where a contrast viewing filter or a spotmeter comes in handy.

Did that do the trick or did I ramble and not answer you well? :lol: :huh:

Edited by Chris Keth, 17 September 2007 - 08:25 PM.

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#4 David Regan

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Posted 17 September 2007 - 09:19 PM

This was mostly brought on by a part of Box's well known handbook, which in his discussion of the zone system, referred to how a DP can take a reflected meter of skin and use it as a reference. So from my understanding of your very helpful comments, skin that is falls in a lower zone say a browner skin in zone V would be correctly exposed if metered with a reflected meter, because zone V is middle grey, so proper exposure would be obtained. However darker skin, that is say closer to zone III, if I metered it I should underexpose about two stops. And finally, what effect does the reflectiveness of skin have, perhaps someone has very dark skin yet if it was very reflective, wouldn't my meter indicate it was much brighter and stop down, hence maybe in that situation, I wouldn't have to underexpose 2 stops. Is there any relevance to this?

Thanks again for the comments, all very helpful
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#5 Chris Keth

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Posted 18 September 2007 - 01:37 AM

Dark skin can be tricky sometimes. You want to show the person's face adequately yet you also want to be representative of what they really look like. I know this comes up with about everything but tests can be invaluable, even very quick rudimentary tests.
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#6 Tim Terner

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Posted 18 September 2007 - 02:14 AM

........... I know this comes up with about everything but tests can be invaluable, even very quick rudimentary tests.


You can always test cheaply with a light meter and a DSLR
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#7 K Borowski

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Posted 18 September 2007 - 12:42 PM

If you are working with film stock, I find Polaroids to be infinitely better, although you have to modify the F/stop to account for the different film speed. Being able to actually evaluate a grey card right next to the finished Polaroid makes it possible to achieve spot-on exposure, whereas an LCD screen is impossible to compare with the actual card because you are dealing with transmitted light on the monitor as opposed to reflected light from the actual grey card. I always use Polaroid's 100D pack film for exposure tests (forget the 3-digit number for it).

Regards,

~KB
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#8 Michael Nash

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Posted 18 September 2007 - 10:38 PM

This was mostly brought on by a part of Box's well known handbook, which in his discussion of the zone system, referred to how a DP can take a reflected meter of skin and use it as a reference. So from my understanding of your very helpful comments, skin that is falls in a lower zone say a browner skin in zone V would be correctly exposed if metered with a reflected meter, because zone V is middle grey, so proper exposure would be obtained. However darker skin, that is say closer to zone III, if I metered it I should underexpose about two stops. And finally, what effect does the reflectiveness of skin have, perhaps someone has very dark skin yet if it was very reflective, wouldn't my meter indicate it was much brighter and stop down, hence maybe in that situation, I wouldn't have to underexpose 2 stops. Is there any relevance to this?

Thanks again for the comments, all very helpful


All of this is what incident light metering is for. It measures the light falling on a subject; expose for that and all of those variables just come out looking "normal."

Spot metering is better used for placing a subject at a specific brightness, or for finding out if a bright or dark area is within range of the film. That said, you can use a spot meter as your main exposure reference if you have a keen eye for the relative brightness of subjects and how you want them to look. Many people meter this way primarily, either out of habit or precision. For the rest of us, incident metering provides a much more solid and reliable refernce for "normal" exposure.

And it probably goes without saying, but since we're discussing it I'll say it anyway: Light meters only measure the light; how you expose is an artistic choice.
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#9 David Regan

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Posted 19 September 2007 - 10:08 AM

All of this is what incident light metering is for. It measures the light falling on a subject; expose for that and all of those variables just come out looking "normal."


So it would seem, incident readings in their own way, have less room for error, as they don't take into account reflectance of an object, which requires compensation. Reflected spot meters on the other hand do require a compensation to require an accurate image (subject to artistic interpretation of course) I recall the first time I shot film, I used a reflected meter on a car, and forgot to compensate for how reflective it was. As you can imagine the footage was quite dark.

Thanks for all the help guys
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