Luminance Values and Exposure

Greetings, volks. Welcome to Hohenfels Volks, THE place for our place! Spring, being upon us here in Hohenfels, is ripe with opportunities for our craft. Looking around, the trees are bright and clean, the flowers are bursting with color, and the Germany is waking up to a wonderful summer ahead.

Today’s post, delayed by unfortunate occurrences, is about luminance values, and how we can record, refer to, and use those values to improve our image. There will be some math here, but nothing major.

The first thing we need to discuss is the exposure formula. Ansel Adams described it as being the reciprocal of the luminance at a speeds key stop. The key stop for any speed is the square root of the speed. Here is a list of most speed/stop combinations.

Speed Key Stop
100 10
125 11
160 12
250 16
320 17
400 20
480/500 22
620 25
800 28
1000 32

I began the table at ISO 100 and ended at ISO 1000. ISO 100 is the lowest most consumer cameras go, and f/32 is about as high as most consumer lenses get today.

By setting your camera to any of the pairs in the table, you can read the luminance of your metered object, measured in candles/square foot, or c/ft2. For instance at ISO 125, f/11, metering 1/125, then the luminance value is 125 c/ft2. If your meter shows 30 seconds, then the luminance is 1/30 c/ft2. That will put you at 18%, or neutral gray reference values. This is what I refer to as M, or an M exposure.

When photographing normal scenes, we often encounter a scale of values that for any given exposure will run from approximately M-5 and lower to M+5 or higher. By learning to look at luminance values as described above, you learn to tame that range, creating an image that will make you justifiably proud.

An example of exposing for your luminance reading is when you meter the clear sky; you should read about f/11, ISO125, 1/300. This will put your sky into the range of 18% reflectance. We know that a clear Northern sky should fall about M+1, which means we should expose for the sky at f/11, ISO125, 1/150. Of course, this goes back to place and fall exposures. You must decide the key elements within your scene and where they should be placed. Once this exposure and placement is determined, all other values fall where their luminance levels impact your sensor. Knowing these values will help you determine where you wish to place elements and where your remaining elements will fall.

Even metering clouds, you will find a range of luminance values that may run from 200 c/ft2 to 1000 c/ft2. By meter the brightest part, 1000 c/ft2, then adding 3 stops, you bring your 200 c/ft2 up to nearly M+1 ½ in the exposure range, which is slightly brighter than the surrounding blue sky, and significantly darker than the brights in the clouds. Your brights will print out with slight tonal variations, and darker areas will have adequate or better textural ranges. You also bring areas metering at 50 c/ft2 up to the equivalent exposure of 400 c/ft2. These equivalent values are in relation to your original metering from the bright part of the clouds.

Recording your luminance values across the range of your scene will help you learn to recognize values, and expose with more confidence. It will also help in editing your images and preparing them for either printing or display. I would suggest metering across the range of M-4 to M+4 after determining your priorities, and noting the results. Even if you’re shooting at some combination not listed above, take the luminance readings using a combination. After reading the values, switch to your desired settings, compose, and make your image. Values for M+ and M- can be extrapolated by halving or doubling the values obtained with your meter. This is less accurate, but still of considerable value in perfecting your images.

Another great advantage of knowing your luminance and place values, is the ability to tell anyone about your exposure without giving the ISO, f/stop, EI, shutter speed, and so on. Tell them you metered this at 250 c/ft2, and gave M+1 to get the exposure where you wanted it. Regardless of your settings, they will know that to get that exposure from 250 c/ft2, then they give M+1 at whatever aperture and speed they desire, for instance ISO 125, f/8, 1/250 or ISO 400, f/5.6, 1/1600. Both will give the same results, and use the same values, 250 c/ft2 +1 stop, and make exposing a scene more consistent. It’s also useful when referring to edits performed. Taking the last example, you may have shot at M+1, and then edited it to M to decrease contrast. In this case, you can say this was shot at 250 c/ft2 +1 stop, then edited to M for the final effect. This will help others understand and help you remember your shots without having to know every little bit of detail.

There are other ways of expressing luminance, for instance EV or c/m2, or lumens, and as long as you find a consistent method of evaluating and expressing that exposure, your images will be consistent with your vision. Don’t get hung up on what I use, try it and if it doesn’t work, try something else. You’ll still have a working system, and you’ll still be enjoying what we love to do!

Time to get out and do some shooting, I hope you’ll be out today getting your shots in, too!

I’m looking forward to seeing your results and hearing from you. Let me know what you think and how you’re using your camera! Share your thoughts here or at the Hohenfels Volks Facebook page. Of course, commenting on both Facebook and here is always appreciated, too! Don't forget, we're on Google+, too!