Various ways to use an incident light meter
Suppose we have a portrait with two light sources.
When we use an incident lightmeter to give us a correct average exposure for a tridimensional object which is non-evenly illuminated, we use the dome and we point the instrument to the camera taking it in front of the subject. Part of the dome will receive more light, part less. Inside the dome there will be a great shake of photons and the result will be the reading of the light meter, a form of averaging. We will call this method A.
When we want to determine the illuminance ratio, or how you call it, I mean the difference in lighting on let's say the two sides of the face, we use the flat cover, not the dome, and we point the light meter to the light source, not to the camera. First we measure light source A, then we measure light source B, than we see that they are let's say in 2:1 relation. If you want more contrast you diminish light on the "darker" side etc. So no dome, meter pointed at the light source, and two readings in our case. We will call this method B.
A third way is to use the dome, but on two different sides of the face. You first use the light meter on one side, so that the dome is invested only by one light source. You put it "by the cheek" rather than "by the nose" so to speak, but pointed toward the camera. Then we use it on the other side, so that it only takes the other light source. Again using the dome, and pointed toward the camera. So dome used, meter pointed at the camera, two readings in our case. We will call this method C.
My incident light meter doesn't have the flat cover, only the dome. At the moment I am not doing the kind of work that would require method B, but I might begin soon.
Can't I use method C to investigate the illuminance ratio of the scene? Do I really need the flat cover over the sensor?
Or to be clearer: What is the difference between method B and method C?
Which also begs the question: should I buy another incident lightmeter, that has the option of the flat cover?
PS I specify I don't have the flat cover lest somebody tells me to try and find by myself
No need to complicate things. Meter the main light source. Take a reading and take note of it and turn off the main light. Turn on you fill, take a reading. If it's a 2:1 ratio the fill is one stop less than the main light. The fill ideally should be a broader i.e. a flatter light source. Don't get hung up with numbers and ratios. See how it looks to you. A dome type of incident meter works best with 3 dimensional subjects. No need for a flat diffuser for your meter if your doing portraits.
Well, I intend to do some still life, product shots etc. Initially with two light sources or one and a bounce surface. I thank you for your answer but my question was not about the "look" of it (whether 2:1 is good or not, how to judge visually etc.) but about the difference in results between method C and method B, it really was about metering technique so to speak.
Is there any application that requires method B, or, instead, can I just ignore the existence of the flat thingy?
Good question. The two measurements with the dome can then be averaged or weighted to give a value you can use to make an EXPOSURE. Whereas the two values obtained with the flat disk, pointed right at the light source, will only give you a RATIO. The absolute numbers won't easily compute to an exposure value you can use.
You'll do just fine using method B with the dome. It is actually preferable to using the flat disc IMHO. It gives you both an exposure and a ratio quickly and easily.
I go through the so-called "trouble" of using an incident meter to avoid many of the pitfalls of reflective metering, one of which is that reflected meters average things. If I am trying to avoid averaging, why average? The other reason I want to use an incident meter is that reflected meters are subject/composition-based, not light based. If I am trying to avoid composition-based measurements of reflected light by using a hand-held incident meter, why would measuring the light coming from the direction of the camera make sense as a general rule?
There are instances in which I do want an average reading, and in those instances (primarily uncontrolled lighting), I take an averaged reading. But pointing the dome at the camera as a matter of course just makes little sense, even though it is in all the instruction manuals. The dome gives you 180 degrees of coverage to point any way you would like - not just at the camera. Choose where to place those 180 degrees wisely, and you are set.
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If all you want to do is establish a lighting ratio between the two sides of your subject (using lights), you don't need to mess with the meter: just move the light on the less lighted (darker) side further from the subject.
Originally Posted by Diapositivo
For example, twice as far away will give you a ratio of 1:4. Work it out from there.
Originally Posted by Diapositivo
Mark Barendt, Ignacio, CO
"The mind that opens to a new idea never returns to its original size." Albert Einstein
Use the dome for A and B. I don't see a need for C. Just my opinion.
You would appreciate the need for the flat receptor if you were using it in a studio and you wanted to measure the contrast level between the various lights which by pointing it at each light from the subject position you can, or photographing flat copy or artwork.
Originally Posted by stillsilver
Ok, I distill from your answers that if I want to take a measure of the ratio, I can use method C (separate measure with the dome at each side of the object, pointed at the camera) and it will work just as if I had used method B (separate measure with the flat thingie pointed toward the light source).
I can ignore the existence of the flat thingie, and live happy with my incident meter which only has the domed thingie.
PS Stillsilver wanted to give me this answer I guess but inverted method C and method B. Otherwise it is clear. I'll do as Mark Barendt says. I just use method A for average exposure, and C for checking ratios, and that's all. And yes, I can skew final exposure toward one of the two readings with method C if I want to expose in a non-average way.