ESFotoClix Blog

What is the right exposure? This question comes up often, especially when wondering whether the camera’s meter and automated mode has let us down. As Bryan Peterson writes in Understanding Exposure: “…[exposure] comes up most often as part of a question — a question I’ve heard more often than any other: ‘Hey, Bryan, what should my exposure be?’ … And my answer is always the same: ‘Your exposure should be correct!’”

“Correct,” Peterson goes on to demonstrate, depends entirely on what you are trying to accomplish. In fact, right after this quote he shows a “correct” exposure where the subject, a man, is in shadows but well exposed while the background is severely blown out. Oh, my, what would the Matrix-metering detractors do with such a shot if their Nikon D80 or D90 had exposed likewise?

To illustrate the challenges that both photographer and in-camera meter face, let’s look at an example where, with a much maligned D80, we placed the focus point on the subject, we set the aperture to f/8 and ISO 100, and let Matrix metering give us the shutter speed. We snap the shot, and sure enough, just like everybody says, the background is blown.

Maybe we should spot meter on one of our usual suspects, moving the focus point around until it lands on some of that blue water in the background. We set the shutter speed accordingly and press the shutter, only to find that now the background is well exposed, but the vase is almost a silhouette.

Well, maybe we should spot meter on the vase and see how that turns out. We do that and discover the background is even more over-exposed that in the first photo.

You may start to realize that with this lighting situation and others like it, you, the photographer, will have to make a compromise. How you arrive at this compromise will depend on what is important to you in this image. Do you want a beautifully exposed background? Or is the foreground vase more important? In short, what is this image about?

You could say that it’s about the pretty blues and greens in the background. Alternatively, you could reason that the vase is the subject, and its surroundings, though necessary for the overall look of the image, are secondary. There, you made a choice. Now take it a step further: if the vase is what matters, how do you want it to look? You think about and decide that you like how the back lighting and how it shows off those nice amber hues.

With that choice, it looks like the second shot is the best compromise, but you are not satisfied. The bright background is really too distracting. If the point of this image is to portray the vase, you don’t want anything to take away from it. Do you have any alternatives? You do. One would be to wait until the sun goes down a bit and see if that helps us in the background. As you think about it, though, you realize that the vase will also get darker. That gives you an idea. You have to add light to make this work. If you used the exposure setting for the second sample above, but added some light in the foreground to bring out the vase, how would that work? Here’s one try, with bounced flash to keep the light as natural as possible. You see some improvement, but the nature of the lighting (front-lit vs. back-lit) doesn’t quite give you the look you want.

Another option is to select the first exposure, which thanks to Matrix metering’s inclinations, is over-exposed by +0.7EV in the background, and in ViewNX, pull the highlights down with -0.7EV compensation and push the shadows out with shadow protection. The result is acceptable, and very little noise has crept in the shadow areas.

Still seeking that perfect exposure, you decide to give HDR a try. HDR has gotten a bad name thanks to a slew of cooked up samples, but you are very conservative in how you use it, and achieve the following result with a 3-exposure bracket.

You may have noticed that in this discussion, we started with a question about what constituted the right exposure, which we went on to answer by other means, namely what made this a good photograph. And that’s Bryan Peterson’s point. You can’t know whether you have the “correct” exposure unless you first know whether you have the intended photograph. This is why metering often frustrates us. No algorithm in the world has been programmed with our personal intentions in mind.

As a final observation, perhaps you agree the last two images offer the best balance between lighting and the goal we set out to accomplish in this photo. But it took post-processing to achieve these results, and you may find that distasteful because for some reason you believe every shot should come out of the camera perfect and ready for prime time. I advise you to jettison such a view-point. When the lighting is tough, you will have to use all the tools in your kit, and you better have post-processing as one of them. In fact, with experience you may find that as you approach shots like these you will start making them with post-processing in mind.

To master exposure one must first understand that it is all about capturing light — just the right amount of light to get the job done. Key to this understanding is how the exposure triangle determines the amount of light that touches the capture medium, film in the old days, and in more recent times, the digital camera’s sensor.

Aperture, shutter speed and ISO form the three corners of the exposure triangle.

• Aperture is the size of the lens opening at the time the photograph is made. More light travels through the lens and onto the capture medium with a larger aperture, and the opposite is true of a smaller aperture. For all cameras, but especially for SLR and DSLR cameras, aperture is also one of the elements that determines how much depth of field (DOF) one achieves, namely how much of the image, from foreground to background appears sharp. A larger aperture will reduce DOF while a smaller aperture will increase it, but note that as aperture decreases past a certain point (depends on the application), lens performance tends to degrade. Aperture values are specified as a denominator of f, i.e., f/N, and hence the smaller the N value, the larger the aperture. For example f/2.8 is a larger aperture than f/11 and will thus let more light than f/11 and have shallower DOF than f/11. Aperture is steps, with typical values of f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, etc., where each of these represents 1-stop, or a reduction of light by one half from one step to the next (i.e., f/2 lets in 1/2 the amount of light that f/1.4 lets in, or f/1.4 lets twice as much light as f/2).
• Shutter speed determines how long the shutter remains open. The longer the shutter is open during an image capture, the more light that passes through the lens and touches the capture medium. Shutter speed may also have an effect on how sharp the subject turns out if the subject (and/or camera) is in motion. A fast shutter speed will help freeze motion, and the direction of motion will determine how fast the shutter must be to freeze motion. Typical shutter speed values range from seconds to fractions of a second, and again, full-stop values such as 1/100, 1/200, 1/400, etc., represent doubling or halfing reductions or increase in captured light.
• ISO, analogous to ASA in film, is a measure of capture medium sensitivity. The higher the number, the more sensitive the capture medium will be to light. Unfortunately, this sensitivity comes at a price, grainier images in film, and noisier captures in digital photography. Full stop values of ISO like 100, 200, 400 also represent doubling or halfing of light sensitivity.

We call these three settings a triangle, because adjusting one will often require a corresponding adjustment in at least one of the other two parameters. For example, if the camera’s meter has told us that f/11, 1/200 sec and ISO 200 is a good exposure, if we wanted to slow down the shutter speed to, say 1/100 (1 stop more light than 1/200) we would have to either reduce the aperture to f/16, or decrease the ISO to 100 to keep things balanced. Since most cameras also allow half-stop or third-stop adjustments, we could technically, say, lower the ISO by 1/3-stop and decrease the aperture by 2/3-stop to achieve the same equivalent exposure.

Enough talking. Let’s see all this in a practical example. This first capture was made at f/4 (a fairly large aperture), 1/250 sec (a fairly fast shutter speed), and ISO 100. If you look closely you will notice that the water stream’s motion is frozen to some degree, but very little is in focus in this image.

Now we vary the exposure, leaving ISO at 100, but closing the aperture to f/16 and decreasing the shutter speed to 1/15 sec. The effect is that the water has that nice foamy look, and more of the image is in focus.

What if we wanted the water motion to be frozen, as in the first image, but also wanted to get more of the image in focus? We would have to close down the aperture, but leave the shutter speed fairly fast, and to compensate, we would have to increase the ISO, as in this next image at f/16, 1/250 sec and ISO 1600.

Which of these is the “right exposure”? We will discuss this further in future blog entries, but ultimately, you get to decide based on what effect you want to achieve in your image.

If you have been dreaming about becoming independently wealthy so that you can afford a digital medium format camera, you may want to know you don’t need to keep playing the lottery. Having read Dennis Frates’ article from the August 2009 issue of Outdoor Photographer, I decided to try his technique of generating 4×5 format images by stitching 3 vertical frame panoramas.

At first, I decided that visualizing the right composition in 3 frames instead of just one would require some imagination, so for each scene I tried, I first shot a single 3×2 standard format image. This next sample was taken at 10mm to approximate the wide perspective a 3 frame panorama gives. Incidentally, and not particularly on point, this image also happens to be a 3-stop HDR.

Once I decided the composition worked, I then set up for a 3-frame panorama with vertical frames. For extra credit I could have shot each frame as a 3-stop HDR, but regrettably, I decided against it. Nonetheless, I was able to push the shadows out a bit in the resultant 4×5 composition. Since each frame was shot at 14mm, the final image benefits from reduced distortion while retaining the wide perspective.

A parting note: while the first image is a 10 mega-pixel capture, the stitched 4×5 version clocks in at 16.7 mega-pixels. Not bad for the lowly D80.