Part 4 - So How Do I Determine a Correct Setting?

Considering the number of variables that come into play when taking a picture, this is a logical and very good question.

First, here are the answers to the exercise from my Part 3 post.

Question 1: What is the exact equivalent exposure to: f/11 at 1/60th of a second if you wanted to change the shutter speed to 1/500th of a second to stop motion?
Answer 1: f/4 at 1/500th of a second. This exposure will allow exactly the same amount of light to strike your camera's sensor while greatly increasing the camera's ability to stop motion.

Question 2: What is the exact equivalent exposure to: f/4 at 1/1,000th of a second if you wanted to change the f/stop to f/16 to expand the depth of field?
Answer 2: f/16 at 1/60th of a second will allow exactly the same amount of light to strike your camera's sensor but result in greater depth of field than f/4.

If you answered these two examples correctly, congratulations. That's as complex as photographic math needs to be for first-time DSLR owners. The rest is an exercise in logic and the infusion of your personal artistic taste.

The CORRECT Exposure

There are two parts to determining the correct exposure setting for any picture.

1. Which exposure setting(s) will give me the best detail in the highlight and shadow areas of my picture?
2. Which exposure setting gives me the proper amount of depth of field and motion-stopping capability required for the scene I'm shooting?

This post will cover the first of these critical questions.

In the first two posts of this series, Exposure is Everything, it was evident that the camera will faithfully provide you with a suggested exposure based on its sole mission of producing a color that has a density of middle gray.

Realize first that this middle gray is not an arbitrary shade of gray. It is a measurable shade of gray. That density is 18% -- a photographic given!

So how do you and the camera’s light meter become photographic partners? And why 18% gray?

If the world contained colors that were all 18% in density, then every picture you metered would be correctly exposed in auto mode. Everything in the scene would be reproduced at exactly 18% gray density. The result would be a perfect picture but a very boring world.

As it turns out, if you merge all the elements that make up a “typical” outdoor, sunlit, landscape scene and calculate the average density for the total scene, the resulting density number is approximately 18%. That’s why the average family outdoor pictures result in acceptable prints. The exposure set automatically by even the simplest camera with a built-in light meter determines the correct amount of light to produce an 18% gray density image. The exposure is at least close enough that it can be easily adjusted by a photo lab or on a home computer.

But what if the scene you are shooting isn’t “typical”? You might have guessed there’s a strong probability that the camera’s automatic choice will be incorrect. The resulting picture will likely be overexposed (very light due to too much light entering the camera) or underexposed (too dark because not enough light was allowed to enter the camera).

The real challenge is to select an exposure that’s “just right” under any given light condition -- an exposure that will produce a print that matches your vision and yet has detail in both the white areas of the picture (highlights) and in the dark areas of the picture (shadows).

There are several ways of determining the correct f/stop and shutter speed combination. Professionals use these techniques while employing our most well-guarded secret. Once you determine the correct exposure of any single element in a typical scene, then all other elements that make up the image will be correctly exposed.

Try working through and understanding these popular professional techniques for determining exposure.

Method 1: Give the meter what it is looking for!

18% gray cards can be purchased in any camera shop. They are inexpensive. Want to make an impression at your local store? Just ask for a gray card. They will recognize that you know photography’s most well-kept secret and that you are a force to be reckoned with. Imagine all that respect for less than $15.

Now you own photography’s most valuable accessory. Take a few minutes and read the section of your camera’s instruction booklet that covers “manually setting your exposure”. That’s right, you are taking control of the exposure away from the camera.

• Now, when you are ready to take your picture, grab the gray card.
• Put your camera into auto, aperture or shutter priority mode.
• Place the gray card in front of your camera under the same lighting conditions as the scene you are shooting.
• Move your camera close enough to the gray card to fill the entire viewfinder. (Don’t let your body or the camera cast a shadow on the gray card.)
• Make note of the exposure setting your camera is recommending (let’s say, f/5.6 at 250th of a second).
• Set your camera to manual mode.
• Dial in those same recommendations. (In this case, f/5.6 at 250th of a second.)
• Set the gray card aside and compose your picture.
• Then “click”.

The reason you entered the f/stop and shutter speed manually into your camera is because, when left in auto mode, the meter will re-evaluate the total scene once you take the gray card away. Manually entering the correct exposure ensures that the camera is forced to take the picture based solely on the gray card reading.

Why does this work? Because... once you determine the correct exposure for one component of your picture, everything else in the picture will also be correctly exposed.

You can prove this by making one more picture of the white, gray and black cards. This time only one picture will be taken. First, take a meter reading from the gray card and manually dial in the recommended f/stop and shutter speed into the camera. Now place all three cards next to each other. Arrange your camera so that all three cards can be seen in the viewfinder. Take the picture. Now when you process this digital file, you will see the picture below.

This single picture containing the white, gray and black cards was possible because the meter was only allowed to meter the gray card. Now that the gray card will be exposed correctly, everything else in the scene (the white and black cards) will be rendered in the correct shade/density.

Method 2: Nature’s gray card

Using the same procedure described in Method 1, substitute grass or the green leaves of trees for the gray card. The photographic gray density of average grass and trees is approximately 18%. So, filling your viewfinder with only grass that’s in your scene and using it like the gray card in Method 1 will result in approximately the same exposure. Close enough. Again, get one thing right and everything else falls into place.

I’m recommending that you train your eye to recognize elements of approximately 18% density in your scene. It does not matter what color the object is – just how light or dark the object is. Once you identify something that’s near 18% in density, meter that object and manually enter those results into your camera.

Both methods produce good images in nearly every shooting condition. I consider the time required by these manual methods to be well spent because it requires me to slow down and evaluate what I’m doing as I compose and expose each image.

Method 3: The Histogram: Digital Photography’s Magic Bullet

A little melodramatic, but the histogram is an important new camera tool. If you could measure the light densities of all the elements in the scene you are shooting and plot their number of occurrences from darkest to lightest on a piece of graph paper, you would have created a histogram for that specific scene. As a manual exercise and considering the millions of densities that make up a photographic scene, this task is nearly impossible. Enter the wizardry of modern digital cameras.

The picture and graph below contain the histogram for a well-exposed landscape image. The far left border represents the darkest components of the scene (black) while the far right border of the graph indicates the lightest densities of your picture (pure white).

The histogram and picture below show that the majority of the densities are pushed to the left of the graph. A photograph containing this histogram is underexposed and lacks ANY detail in the darkest parts of the picture. The selected exposure (f/stop and shutter speed combination) did not allow enough light into the camera to record detail in the dark areas of the picture.

If there is NO detail contained in the image data, then no matter what magic you perform in a computer image editing program NO detail will be reproduced in the final print!

The histogram and picture below show that all of the densities have been pushed to the right of the graph. The resulting print is overexposed and will have little or no detail in the lightest areas of the image. With NO detail in the lighter parts of the image, no detail will be achievable in the final print!

The goal then is to select an exposure where all or most of the densities fall within the borders of the graph – like the first example. Once this exposure is identified, all the dark and light values -- as well as everything in between -- will be recorded to produce a print that’s well exposed and contains acceptable highlight and shadow detail. It’s printable.

In most of today’s moderate to high-priced DSLRs, you can set the camera to display this histogram visually on the LCD monitor. (Quick, get your camera manual and look up histogram display. Set up your camera to display the histogram after you take each picture.)

In reality, viewing the histogram immediately after you take the picture is more important than looking at a tiny preview of your picture. You saw what you were shooting through the viewfinder. What's important is the EXPOSURE! Did the densities fall within the center portion of the histogram? Will it make a good print? Do I need to make an adjustment and shoot it again? Now you're talking professional.

In practice, you can now take a picture at your camera’s automatic exposure recommendation and immediately see the histogram for that image. If the distribution is incorrect, you can override your camera to produce an exposure that brings the densities back to a printable range. You guess it. Camera manufacturers have included a way for you to temporarily override the meter’s recommendations and “tweak” them to adjust for your shooting situation. (Back to the camera manual. Look up “exposure compensation”.)

Now take the picture, evaluate the histogram, and apply “exposure compensation” to adjust the exposure. (Letting more or less light into the camera.) When you move the “exposure compensation” control and take a second picture, you will see the histogram densities move in tandem to the left or right. Digital film is cheap. So take as many pictures as necessary to get the histogram density distribution correct. Eventually, this shooting routine will save hours of frustration in front of your computer.

Some thoughts:

1. This probably seems like a lot of work. The truth is that creative and technically accurate photography is work. Taking the time to consider the composition and exposure components of each photograph will result in the most gratifying images.

In reality, most of this process becomes reflex action over time and as second nature as brushing your teeth. There is also a silver lining to the process. When you are taking many pictures of similar scenes in the same vicinity and the lighting conditions stay relatively constant for a period of time, you’ll find that the same exposure settings remain constant. So, shoot away. You don’t have to go through the process again until the location, scene or lighting conditions change significantly.

3. Hopefully you are approaching an “I get it” moment. There are other considerations in setting the final exposure, but the steps we have already taken together should provide that “warm and fuzzy” feeling that you’ve left your shooting session knowing the pictures you envisioned are those on your data card. I will touch on those other considerations throughout the remainder of these posts.

4. Making these radical changes in the way you look at photography and take your pictures demands one critical discipline. Repetition. Shoot lots of pictures. Commit these actions to muscle memory by taking pictures every day. Take copious notes when you shoot. Download the camera files into your computer and review each image on your large monitor. You don’t have to print every picture to learn from all your hard work. But do take the time to, at least, digitally review your work. What worked? What didn’t work and why? Each digital frame has a lesson to teach.

5. Not all things are possible in photography. There are traditional as well as new digital “tricks” that can be applied to make almost every picture possible. But there will be times when the answer is: “It can’t be done”. Technology always has limits. Some of these situations will be discussed later.

Bottom line: I highly recommend you take advantage of technology and make your exposures based on the histogram results (Method 3). Take it from an old photographer, an understanding of Methods 1 and 2 is important to understanding the logic of exposure, but in today's evolving digital technology environment you will quickly learn to appreciate Method 3 and your camera's histogram.

Above is a histogram as superimposed on the Nikon preview screen after the picture was taken. The distribution of the densities on this graphic indicates that the darkest and the lightest portions of this picture will contain detail that will appear in the final print.

My next post will walk you step-by-step through my normal shooting routine using the histogram method of determining proper exposure. As always, let me know your thoughts or questions.