Sunday, 19 July 2009

Light meters: A Throw-Back or Is it Worth It?

_DSC1721 (a new-old Minolta iv F)
In answering the Q myself: worth it!!

Recently got myself a(n old Minolta IVF) light meter after a friend at work brought his meter in. And, I've gotta admit, using a meter and hitting an exposure dead on (mostly) first time is kinda addictive.

But, how do we use a light meter exactly?

On the strobist flickr group, Steve Korn provides a method as quoted below, whereby we meter the light from the subject position and compensate backup from medium grey:
Ok, so here's the most important thing that no one mentions but everyone seems to struggle with.

When you use your flash meter, do you find that you meter your key light, set your camera to that reading, take your picture and then notice that your exposure is 1-2 stops under?

There is something about flash meters that is important to understand. A flash meter is essentially an electronic gray card.

Here's where that becomes a problem. If I point my flash meter directly at my key light, I'm metering what is likely to be the brightest part of my picture (this excludes hair lights etc.). The meter doesn't know if I am metering a bright part of the picture, a dark part of the picture, or a mid-zone transitional part of the picture. Because the guy at the factory who built your light meter doesn't know what you're trying to read either, the meter was designed to measure the middle of the road, middle gray. As a result, the key light is given an exposure value that is in the middle gray zone, 1-2 stops under where it should be for a correct exposure.

From here, you have two choices.

1) You can meter a transitional zone. The point between light and shadow in your image. This will be at approximately the mid-point with regard to exposure and if it is metered correctly, the rest of your image will be exposed correctly.

When you see a photographer place a meter directly in front of a subject's face, point the dome at the camera and fire a test shot, they ae essentially placing the meter at the mid-zone point of the subject, if the key light is 45-90 degrees off axis of the camera. The light from the key is falling on one half of the dome, the other half is in shadow, by averaging the two, your meter should find the mid-zone. The problem with this is that it can be a guessing game as to exactly where the mid-zone is, especially if you aren't using modeling lights. Additionally, there are a lot of variables effecting the shadow side. What is reflecting light from the key back into the shadow side of the dome? Is the shadow side really dark and is there ambient light effecting it, am I really only lighting half the dome? So, this is a little bit of a crap shoot. It will get you in the ballpark.

Another problem is that some light set-ups don't lend themselves to this. For example, if I'm using a light directly over my camera, like beauty lighting or clam shell lighting, metering in the middle of the subject's face isn't going to divide the dome with a balance of light and dark.

That leaves us with the second option:

2) Meter with the dome retracted, pointed directly at the key light. The value of this approach is that it's consistent. You are eliminating a multitude of variables because we no longer have to be concerned about reflection or trying to find the mid-zone of our image. The dome is retracted so that we get just direct light from the key light.

What we must do, however, is to add 1-2 stops to compensate for the meter's attempt at reading this exposure as middle gray. I say 1-2 stops as different people have different ideas about exposure. If you like to "expose to the right" on your histogram, you'll want to be closer to two stops.

If I know I want to shoot my subject at f8, I simply point the dome at the key light and meter it at f11-f16. I set the camera aperture to f8 and shoot away. I double check my histogram.

Balancing ratios:

I meter the key light as described in option 2. I then point the dome at each light individually and figure their ratios in relation to the key light reading. If I want to shoot a picture at f8, and I have compensated my key light to read at f11, and want to make my hair light half a stop hotter than my key light, I meter it to f13. All of the ratios are relative to the key light f-stop, not the camera f-stop. The camera is compensating for all of the lights and they must be in the correct ratio relative to one another.

Most light meters can be programmed to compensate their readings. If you are uncomfortable calculating f-stops, you can program the meter to subtract 1-2 stops from it's reading and it will give you the exact number to set your aperture to, if you are using method 2 from above.

Shooting digitally, we must be really precise with our exposure. Boosting an image 2 stops in post means a significant degradation in image quality, over exposing your subject creates an irreversible loss of data. So, the value of precise exposure can't be overstated.

The other main approach, whereby the meter faces the camera regardless of light position, from contributor Don Giannatti:
Here is the way I use an incident Flash meter. I have never, nor have I ever known anyone to use a flash reflective meter so I will take a pass on that.

The spherical shape of the incident meter is three dimensional. That means the light falls upon that ball as it falls on subject matter. A lit side and a shadow side or direct on or simply ambient.

It is designed to be a direct representation of the full element of light as it falls on a subject.

The ball always faces camera. Always for a 'reading'.

Bear with me for a moment.

Incident meter readings are subject/reflective neutral. it doesn't matter what color the subject is. It doesn't matter what the reflective surface is. (That may be a huge challenge due to specular, but specular is not light falling on anything, it is a reflection of the source.) So whether I am shooting a white dress or a deep red tuxedo, the light is falling on the subject and the correct use of an ambient meter reading will render the exposure necessary. The light falling is read, not the subject. Subject Neutral. Subject independant.

If I am standing on the Golden Gate Bridge facing east in the afternoon and the sun is shining on me, and the entire bay area, the same sun is hitting my back as is hitting the city. A reading of the light falling on the ball of my incident is the same reading as the TransAmerica building. I can simply hold the meter over my head and take a meter reading from the back. Same light - same rendering of tonal scale.

In the studio, I use the flash meter to find:
A - Exposure
B - Main / fill / special Ratio
C. Ambient / Flash ratio

Flash 'ball' faces the camera for exposure. The light falling on the ball renders the side light / transition light and shadow to give me a neutral exposure. I cup my hands around the ball to eliminate all other light sources to get a ratio setting set by reading the light directly (ball facing the light) and shadow (ball facing the fill or shadow side).

That is why the newer ones have the retractable ball. By retracting it, you can aim it at the light source and not get any interference or added light from another source. That is why it is used for ratios.

For ambient, I will take an ambient reading - toward camera - to determine the light that exists and to help me make a determination on where I want to place my flash exposure.

F-16 rule says that with full daylight f-16 at 1/ISO is correct. Add 1 stop exposure for side light and 2 stops added for backlight. So if you are in the studio and you aim your meter at the side light umbrella instead of at the camera you are essentially taking a front light reading instead of a side light reading. We know there is a difference, right?

I will sometimes take a meter reading of the hair light by placing the meter straight up on the head and popping the strobe which is a little behind the subject. I am looking for ratio here. If my front light exposure (from camera) is f-11, I look for f-11 - f11.7 depending on the look I want. Remembering that an even light setting gives me a lovely back light. Did I just say 'lovely'? Damn.

As far as using a gray card. Here is what I find.

The angle of the card can make a lot of difference in the exposure. It is not three dimensional and a slight angle away from the light or toward the light can make a big difference.

If you are in full sun or full ambient and can line that bad boy up with the lens at a full obtuse angle, then it works flawlessly. In the studio I prefer the ambient use of a flash meter to read the subject as related to the light falling on her. What she is wearing is not important as the light is being metered to tell me what middle gray would render at IF it were present. The color may not be present, but the meter doesn't care. I place the meter slightly under her chin and in direct plane to her face. Ball faces camera. That gives me a total exposure. I then go for finding my ratios and follow up with a full reading again.

Hope that helps. That's how I have done it for uh, well, a hell of a long time. I learned from working with 4x5 when I was trying to be Ansel Adams Jr and doing all the zone system work and such. It was very demanding, but man I learned a lot.

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