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Film student slightly confused about light meters


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I can't seem the find the answers to these two questions, (the following is based on how my teacher taught us at film school).

  1. Why, when taking an incident reading do you point the dome towards the light and not the camera?

  2. When measuring contrast ratios with an incident dome, why do you cover up the contaminating light?, in other words why do you measure each light separately?

 

expanding on question 2:

If you are measuring the contrast ratio of a subject on a 3 point lighting set-up, depending on where the key light is placed, the key will inevitably spill onto the 'fill side' of the subjects face and so when measuring the fill with a meter, surely you have to account for this 'spill' from the keylight, because it will inevitably affect the final result?

as in if you dont account for that spill then whats the point of having contrast ratios because you will get compltely different looks for each contrast ratio depending on how much the key light wraps into the 'fill side' of the face? (and vice versa with the fill light)

 

Side note: I am asking my lecturer and he doesn't really want to give me any answers, he is just says 'stop overthinking', it helps me to know the ins and outs and not just blindly follow what my lecturer says without knowing why.

 

Wondering if anybody could help me out. Thanks so much!

Edited by imran qureshi
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There's no rule that says you have to point the dome toward the lamp, in fact generally you would point it toward the camera. This would give you a reading of the light that is falling upon the subject from the camera's point of view. It's therefore giving an averaged reading of all the light at that point. Angling the dome towards the light gives a reading that is much more weighted towards that particular lamp. This is useful for determining contrast ratios.

Whichever technique you use, you still have to interpret the readings in order to get the exposure you want.

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If you point the dome at the camera and the key is coming from the side, for example, the dome will average the light reading from the key and fill (or if there is no fill, the key and the shadow). It's better to measure the light by pointing the dome at the light and then decide for yourself how much to adjust for its angle, like deciding that since the light is coming from the side, you should open up a 1/2-stop, if that's what you want.

You don't really need to flag the fill off of the dome if reading the key, since the key is really both the key and the fill in terms of brightness, but you'd want to flag the key off of the dome if measuring the fill alone. You'd want to flag a backlight off of the dome however if measuring the key.

So yes, for contrast ratio, you meter the key + fill and then the fill separately. However, some of this is semantics, if you wanted to know the ratio between the key and the fill LIGHTS, you measure them separately. If you wanted to measure the contrast ratio of the lit and shadow side of the face, the lit side would be the key + fill.  So it just depends on what you want to know. It may be more useful to measure the key and fill separately for your notes for matching or later adjustments, i.e. "my key light was this bright and my fill light was this bright so next time I want to change it to..."  Or if you had multiple keys throughout a set, it might be easier to set them to a certain level without thinking about how the fill would change them.  Plus today, we tend to work at high key-to-fill ratios (less fill) so the fill really doesn't lift the key much anyway.

 

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2 hours ago, imran qureshi said:

Side note: I am asking my lecturer and he doesn't really want to give me any answers, he is just says 'stop overthinking', it helps me to know the ins and outs and not just blindly follow what my lecturer says without knowing why.

Your lecturer is a plonker. 

He probably doesn’t know the answer and is saying that as an excuse. 

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