Wednesday, June 24, 2009

Photo Talk: Exposure Simplified - The Big Three

Whether I am at Yellowstone National Park, a local zoo, or some other type of tourist area I am seeing a huge increase in DSLR users. I am always curious as to how the photographer is shooting the scene, especially when I see a flash being used on a scenic mountain scene. So it is not unusual for me to strike a conversation with a fellow photographer or take a peek as what mode their camera is in.

Sadly many people are shooting in Program Mode and dont know how to change their ISO. Often I find people do not understand f/stops, shutter speed, ISO and how they work together. My intention of this posting is to try and simplify how aperture, shutter speed, and ISO work together. Knowing a bit more can help you create top quality images.

ISO stands for International Organizatoin for Standardiszation. It is bascially a system that establishes standards. The ISO standards are used when refering to film speed. The term film speed is really what we want to talk about when we talk about ISO. I think it is easier to understand and remember. Ah...the good ol film days!!

What does film speed mean anyway? It is bascially a way to decribe a sensitivity to light. A slower film speed means that it takes longer for light to burn through the film. The faster the film speed means it takes less time for light to burn through the film. With that said an ISO or FIlm Speed of 100 would be considered slower than an ISO or Film Speed of 1600.

Of course in digital photography we are not shooting film so what gives?? Well the sensor is your film in the digital world. Setting the ISO, for lack of better terms, sets the sensitivity of your sensor...its sensitivity to light. So again if you set the ISO at 100 your sensor will need to be exposed to light for a longer period of time than if you used an ISO of 1600.

You have probably heard of film grain. That is when the detail of the image becomes less sharp and sometimes even hazy. The lower the ISO the better quality image and less grain you will receive. On the other hand a higher ISO is likely to result in increased grain levels. To some degree grain can be control with proper exposure. Nevertheless, the general rule of thumb is the higher the ISO the less quality image.

Really that is about it. In the digital camera world ISO is refering to the sensitivity setting of your sensor and how much light it will need. So if ISO 100 needs more light than ISO 1600, how do we control how much light gets exposed to the sensor? That brings us to the next two of The Big Three.

In optics aperture is a hole or opening in which light travels. The larger the hole, the more light is allowed through the lens. In this case the more light allowed through the lens to be exposed on your digital sensor.

How is the size of the opening measured? To start, just so you know, I dont know why they number it this way but they do. On a lens the aperture is measured by f-numbers. The larger the f-number the smaller the opening. The smaller the number the larger the opening. Check out the illustration.

You have probably heard that aperture settings determine the depth of field. Depth of Field (DOF) is bascially the portion of the scene that appears sharp in the image. Lets say you are taking a portrait of someone and you desire to have the background blurry but you for sure want your subject sharp. If you use a f/2.8 which is a rather large aperture the DOF will be short and the the background will be blurry. But be careful. Your DOF could be too short and the sharness could start falling off sooner than you would like and the result could be that the subjects hair line is not sharp. The answer? A smaller aperture setting. I tend to shoot portraits between f/5.6 and f/8, depending how much I want the background blurred. The smaller aperture also helps ensure my subject is sharp and completely focused.

So lets say I want the opposite result. Lets say I am taking a photo of a cool rock formation in front of a waterfall that is slighty in the distance. Lets say I do not want the background (in this case the waterfall) blurry, I want it in focus. You guessed it!! Then I want to set my aperture at a rather small aperture like maybe f/18 or even f/22. With the smaller aperture the scene sharpness will be longer. I will have the foreground rocks and the background waterfall all in focus. Awesome!!

So aperture is the opening that allows light to hit the sensor. If we are shooting at f/4 we can see the opening is rather large. How do we control how long that light is exposed to the sensor? That leads us to the next of The Big Three.

Shutter Speed
Shutter speed is the length of time the shutter is open to allow the light being exposed through the lens to hit the sensor. If light is very intense in all likelihood it will need to hit the sensor for a very short period of time. Therefore you will want a fast shutter speed, maybe even something like 1/1000 th of a second. If the light is very low you may need to expose the sensor to light for a rather long time, maybe up to 3 seconds.

Depending on the size of lens and how far you are zoomed in, you can typically hand hold a camera at shutters 1/60th of a second and faster. Slower than 1/60th like 1/10th or 1 second you will need a tripod or other means to support your camera to avoid a blurry image.


Hopefully these simple explanations are beginning to help you see that The Big Three are rather easy to understand and a combination of the three makes for well exposed image which in turn makes for a quality photo.

There can be a lot more time spent on explaining The Big Three. My intention is to shed light on the basics and help you see how they work together.
So lets talk through a couple examples.

Example 1
Lets say I want to take some photos of nightime cityscapes with the city lights illuminating the streets of the city. I am not going to use a flash and I would like to have images of high quality with little or no grain. I set my ISO to 200. Based on what I have learned an ISO 200 is not very sensitive to light and will need to be exposed for a longer period of time. In all likelihood I will need a tripod.

With the ISO set at 200 I need to next decide on my aperture setting. I want to make sure my DOF is not too shallow yet at the same time I do not want all of the background in focus. I choose f/11.

Since I have a less senstive ISO and a somewhat smaller aperture selected I may have to choose a slower shutter speed to make sure everything is exposed correctly. Depeding on the actual amount of light the shutter speed will vary. To follow through with the example I will say I will need to set my shutter speed at 2 seconds.

ISO - 200
Shutter - 2 Seconds
F-stop - f/11

Example 2
It is late afternoon and I am taking photos of animals at a zoo. Not only is is later in the day but the animal I am photographing is in a shaded area therefore I have limited light. Typically during the daytime hours I would choose a lower ISO like 100 but I am not so sure that is going to work today. When taking photos of animals you will want a little faster shutter speed since they move unpredictably and faster than you think. I dont want blurry shots so I will need a faster shutter speed.

To get a faster shutter speed to work in this slightly lower light situation I will most likely need to use a little higher ISO. I set the camera at ISO 800. I know that I have selected a little more sensitive ISO so it will not need to be exposed to light nearly as long. To control this I will need to set my shutter and aperture accordingly.
I do want a faster shutter speed so I set it at 1/500th of a second. Although my ISO is sensitive, I have selected a faster shutter speed. The light will be exposed for a very short period of time so I will have to let more in through the aperture. Again, depending on the actual situation the f-number will vary. To follow through with the example I will say I set the aperture at f/4.

ISO - 800
Shutter - 1/500th
F-stop - f/4

Experiment...shoot, shoot, shoot. Since you are shooting digital you will not waste film. Try shooting the same scene in different ways. In no time you will feel comfortable to turn your camera mode dial off of Program and begin shooting in Shutter Priority Mode, Aperture Priority Mode, or even Manual Mode.


Anonymous said...

Hi Scott, what a fascinating and informative article!

I hope I may add a little bit about f stop numbers; the numbers are really fractions referring to the varying hole size in the lens. So f/16 really is smaller than f/11 because 1/16th is smaller than 1/11th. In fact f/16 (1/16th) is an opening with exactly 1/2 the area of the opening at f/11 (1/11th). Hope this helps.

Gary Beeler

James said...

Great info Scott!

Your photos continue to amaze me!

Fotos For Phun said...

Thank you James. Maybe someday the Lord will open a door where I can exercise my passion for nature and wildlife photography at a higher level.

Until then...I will continue to take Fotos for Phun.