Understanding the Exposure Triangle
by eNoBlog on Jul.30, 2009, under Techniques
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.
July 30th, 2009 on 1:22 pm
The examples are useful. Thanks.
July 31st, 2009 on 5:05 pm
After reading article after article about this very subject, your explanation along with the photographic illustrations, have made it MUCH clearer for me. You should have actually posted this first article to Flickr, because there are many folks who could benefit. Maybe it was too basic for the “experts”, but there are many newbies lurking around (like me!), and are too intimidated to ask basic questions for fear of being embarrassed. I’m sure some of them, like me, have been taking photos for months and even years, and still can’t wrap their heads around the concept
. Off to read part 2! Thanks, again!
August 2nd, 2009 on 7:15 am
many thanks for a well written and best part being the three picts , which clarify quite a few things, infact I am a new learner with nikon d5000, and yr example of taking picts with flowing water is helping me to experiment with my camera, cheers !!
August 6th, 2009 on 1:27 am
Very good short article so please consider the below to be nothing but minor niggling criticism.
Though it’s standard practice it’s a bit misleading to call it a triangle as there is one other thing that greatly influences exposure and that is zoom. You’re constantly trading off between all 4 (unless using a prime).
The other thing to note is that with reduced light comes reduced saturation. I still remember the Eureka moment I had once while photographing a flower when I saw vibrant colour at f/4.5 and washed out colours at f/16.
August 6th, 2009 on 5:08 am
Interesting. I haven’t experienced this myself. But related to color rendition, I’d say WB is the fourth corner of the… square. If I’m able to convince myself that what you claim is true, we’ll be up to a pentagon. So much for keeping things simple.
August 6th, 2009 on 7:10 pm
It makes sense when you think about it carefully. Colour rendition and sensitivity depend on how well the sensor responds.
If you use a smaller aperture, you either have to increase the ISO or the shutter speed (or both). If you picked higher ISO (and you probably will if shooting hand held), of course your saturation won’t be as good as colour response of the sensor decreases with higher ISO.
Or to put it more simply, more light = more colour.
As for white balance I haven’t thought about it in depth but it’s definitely something to ponder. Does it change the overall exposure or just the interpretation of the data?
August 8th, 2009 on 2:22 pm
Sammy,
Faulty White Balance does not contribute much to the exposure trinity but it definitely waste dynamic range when converting to JPG and so does high contrast. If you are going to shoot only JPG it is best to custom WB and lower the camera contrast. It is easy to restore a white point in levels or Shadow/Highlight but not if the high end data has already been eliminated by the JPG process in the camera. There is safety in low contrast but danger in high contrast in the camera. Both D80 and D90 have contrast controls but very few people use them. Contrast tells the JPG process how much of the captured data to keep and process. The Holy Trinity should be 5 pillars to include WB and contrast.