Light Meter Help Needed

Factual Questions
1

Through no fault of my own, I have acquired a Sper Scientific Model 840021 Light Meter last calibrated in 1994 by Technician JS according to the label on the back of the unit. I just installed a fresh 9V battery.
I have been to their website and looked around a bit, but I need a factual answer.

Just how would this be useful to me?
I do a bit of amateur photography, but I rely on my cameras to set aperture, shutter speed, and even focus at times (since I am lazy that way).

How is knowing ambient foot-candles going to enhance my already charmed life?
I tested my bedside lamp and evidently it is putting out light waves, but my eyes could already recognize that.
I tested my cat Mister Whiskers’ evil stare-down, but get a zero readout because he is…well…he is a cat.
Need answer fast because that is the way I roll.

Mods…feel free to move this to IMHO if need be.

2

It’s an incident light meter, which is the professional way to set exposure in situations where you are in control of your subject. In other words, portraiture.
Unless you are shooting a movie, or photographing models, it’s not going to be very useful, especially with the ability to get instant feedback on your exposure by looking at your camera’s preview screen and/or histogram.

But, it’s a neat toy.
See if Mister Whiskers wants to play with it.

3

Thanks. So it is pretty useless except being a nice toy to play with.
I still don’t get the foot-candle thingy and converting that to F-stops and shutter speed.
I may need to go back to school.

By the way, Mister Whiskers decided to pose for a few shots but is demanding a treat.
You all know I am going to give in. Right?

4

Foot-candles isn’t a very convenient unit for photography, but here are some references to help you:
http://photo.net/bboard/q-and-a-fetch-msg?msg_id=000GWb

http://www.instructables.com/id/Converting-a-Foot-candle-Meter-for-Photography/?ALLSTEPS

5

The meter is useful, and incident light meters were the standard for decades. In some ways they remain superior to other meters.

The point is that you measure the light falling on your subject. A camera’s meter measures the light reflected back off the subject. The trouble with the camera’s meter is that it doesn’t know what colour or shade the subject is. Simple light meters simply assumed that on average a subject was 18% grey. Modern meters take reading at various locations to try to work out a spread of brightness, and use that to calculate an exposure. Which is better. But unless your subject is actually emitting light an incident meter can do a good a job. There is devil in the details.

If you assume that something that is 18% grey in the subject is to be reproduced as 18% grey in the final picture, the incident light meter will get you there. This is a very good initial assumption, and one very close to how a camera’s meter operates. Just measure the light coming in, and set your exposure accordingly.

Where you get in trouble is in a couple of places. Trivially, if you are unable to measure the light at the subject. A landscape may be in different light to you. The subject may be emitting light (say a sunset, or fire). So the meter is good if you are outside in full light, or you can place the meter right on the subject (such as taking a portrait.) But you could take a portrait of someone in black clothes against a black background with an incident meter and get the right exposure, whereas a reflected light meter would be clueless about the right exposure.

The other problem is that you still need to control placement of the exposed image in the target space. Your camera, whether film or digital, can only capture a limited range of brigtness levels. Less than are found in reality. Placement is basically a matter of deciding how the deep shadow and bright white ends of the picture are sliced off, and what part of the middle of the exposure range, the part that your camera can actually capture is chosen. An incident light meter can’t help you here. In reality no meter apart from a spot meter can. But reflected light meters with many zones feeding a modern camera’s processor can make a good guess, one that is better than most photographers could themselves. A spot meter would let the photographer determine the exact set of light levels in a scene, and allow the choice of exposure to be made. (This was how Ansell Adams’ Zone system worked. )

If Mr Whiskers is black, the incident light meter would be perfect for taking pictures of him.

I used to quite often use my spot meter to meter a grey card held by the subject to get an exposure. Essentially making a rough and ready incident light meter.

6

Incident light meters are the best! That is absolutely the way to get better photos! And your camera is not already equipped to do it, either, automatically or otherwise.

Hold the light meter on your subject, or on something that is illuminated like your subject*, so it is pointed between the biggest light sources and the camera. The reading you get is what you base your exposure on.

If your film speed, or your digital sensor’s equivalent film speed, is x, then you will get a nice photo in full sunlight if you set your lens at f/16 and set your shutter speed to 1/x. If you like, this is a useful way to think about how film or sensor speed was defined in the first place. I mean, I don’t actually know the history, but it’s as if this were the original definition (which it might be), in a numerical sense.

Once you know a proper combination of f/stop and shutter speed, you can also get other good combinations by moving up and down in f/ stops, and in shutter speeds, in opposite directions, moving the same number of steps, given that your lens f/ stops go in the usual pattern like f/4, f/5.6, f/8, f/11, f/16 and your shutter speeds go in the usual pattern like 1/125, 1/250, 1/500, 1/1000. These steps all change the amount of light by a factor of two, so changing them both together keeps the light proper and give you control of your depth of field or your ability to stop versus blur motion.

For example if you have a film or sensor speed of 125 and it’s full sunlight, you set your shutter speed at 1/125, your lens at f/16, and you’re good; or you can set the shutter to 1/250 and your lens at f/11, or 1/500 and f/8, which will keep even fast sports motion frozen. Or you can set your lens down to f/32 and set the shutter to 1/64, and get great depth of field for a macro still life.

Now here’s where your shiny new old meter comes in: you use it to measure what factor up or down you are from full sunlight. Go outside and calibrate it in full sunlight! I know, I know, this isn’t very precise as a lab instrument calibration, but it will be within a factor of two, which is just one stop in photography. You can try it on a few different days. Having the sun right overhead is best, but, jeez, this will be close enough.

When you go taking pictures, get a reading, and then cipher how many factors of two you are from full sunlight. Below, typically. If you get a reading of 1000 (whatevers) in full sunlight, and a reading of 1 in your living room, that is ten factors of two. Ten stops, in photography terms.

  • If you are taking a picture of, say, a pirate ship race, you don’t have to swim out there to hold your meter there, provided you and the boat are in the same outdoor weather conditions. Figure out how you would point the meter if you were on the boat, and point it in the same direction right where you are standing.
    If this was all way too messy and involved, well, I dunno, don’t play with scientific instruments. I guess.
7

Actually quite helpful, thanks also to beowolff and Francis Vaughan for enlightenment.

(My ex once made the above comment after I bought a used oscilloscope, and hooked up a Shure microphone to it and sang ‘Strangers In The Night’ to her.
I could only respond with “But it is FUN!”).

Yeah, I am weird like that…