Aperture & DOF

Depth of field is the area of a photo that is in focus. A photo with a shallow DOF will have only a small portion, just the subject or part of the subject, in focus. DOF is not to be confused with bokeh, which is the area of a photo that is not in focus. You will frequently hear someone comment on a photo saying that it has great bokeh, describing the quality of the blurred area.

Depth of field is created by a combination of aperture setting, focal length, and subject distance. We will start with aperture. If you have already completed the last assignment, you have seen for yourself the effects that the aperture setting will have on the DOF. Below you will see in a few quick examples the effect of just changing the aperture.







These shots were taken at 17mm in a range from 1st to last: f/16 f/11 f/8 f/4 &f/2.8. No flash approx 1 foot away from subject, the 1st silver candle.

Most of us already know a larger aperture will give us a shallow DOF. If you want to get a blurred background you need to open up the lens. But now we need to learn how to control the amount of DOF.

So how can you determine the DOF before taking the photo? Well, I will not complicate things by going into the Circle of Confusion(COF). Many wonderful people have put together a simple DOF calculator. Here is one for example; http://www.dofmaster.com/dofjs.html

Take for example my last shot at 17mm f/2.8:


This now tells me how much distance I will have in focus.It's the total under depth of field on the right; .11ft or 3.3cm. Why do I care? Let's say we're taking portraits. I want a shot of a child where the eyes and facial features are in sharp focus but I want a soft edge to the hair fading into the blurred background. I'll use this calculator to determine what settings I need to achieve this effect.

Hyperfocal distance is very important in landscape photography. This is the point at witch everything from half the hyperfocal distance to infinity will be in focus. Probably a bit confusing huh?

Example, (NOTE:SUBJECT DISTANCE DOES NOT EFFECT HYPERFOCAL DISTANCE)
Say I'm photographing a landscape scene with many subjects at different distances(houses, trees, mountains). With the above settings of 17mm at f/2.8 the hyperfocal distance is 16.8ft. If I focus at 16.8 feet, everything from approx 8ft in the foreground to infinity will be in focus.

Give it a try! Think of an effect you want with the DOF & your subject. How much of a DOF do you want? Choose your focal length and subject distance in the calculator and then look at the effects of changing apertures in the total for depth of field in the caluclator, that will be you area in focus. Find the aperture that will give you the amount of DOF you want, and shoot! Don't forget to adjust the shutter speed to give you a good exposure.

The Big Three: Aperture, Shutter Speed, & ISO

Aperture, shutter speed, and ISO all work together to create an exposure. Understanding how they work gives you the ability to manipulate these and produce a wide variety of effects.

The aperture or f/stop determines how much light is admitted into the lens. The larger the f-stop(eg.f/16) the smaller the opening and the less light entering the lens.

Shutter speed controls the amount of time the shutter remains open. A fast shutter speed will freeze motion and a slow one will allow blur.

Aperture and shutter speed are both measured in stops. These stops have an inverse relationship.

The diagram above represents this inverse relationship. A change in one requires a corresponding inverse change in the other. The sensitivity range represents the ISO. Understanding this scale and it's relations will allow you to manipulate these to produce shallow DOF, freeze action, allow motion and blur, and so on.

For example; say you want to capture flowing water but do not want to freeze the water. You want a low ISO, a slow shutter speed, and high f-stop. A slow shutter speed will capture the water flowing and the high f-stop will allow less light in during the longer exposure to prevent blowout.

The aperture setting also plays a large part in DOF, along with a few other factors we will discuss later. A low f-stop 1.4, 2.8 will create a very shallow DOF giving the photo a blurred background while the subject remains in focus.

The ISO setting determines the camera sensors sensitivity. A higher ISO will require less light to produce a correct exposure, however, the higher the ISO the more noise you'll encounter depending on your cameras ability to handle high ISO.

*This is a brief introduction the the big three and will be covered more in depth from here on.


Assignment:
In this brief intro you've learned a little bit about the relationship between aperture, shutter speed, and ISO. For this assignment you can use a person or object but objects work best as they don't move!

Set your ISO 100-200 and make sure the auto ISO is off if this is an available option on your camera.

Set your aperture to the lowest possible f-stop. Look at the light meter and change your shutter speed so that your light meter is reading 0. Write down your settings.

Take another shot of the exact same subject but this time set the aperture to f/8 and make the necessary compensation in shutter speed. Write your settings down again.

Look at both photos, what differences do you see in DOF, lighting, focus? Submit both photos to the Flickr group pool with the settings in the description(shutter speed, aperture, ISO).

Where to Begin

Learn the language, know your equipment, and know your terms. How can you use something properly and begin to manipulate to your desires if you do not know what it is? We’ll start with the most common as I will cover these first. Throughout this series important terms will be highlighted along with its explanation.
• Aperture- The adjustable opening in a camera lens used to control the amount of light reaching the film. The size of this hole is called the f-stop. The larger the number the smaller the opening of the lens; the smaller the number the larger the opening of the lens. This controls the amount of light entering the lens.
• Shutter speed-the amount of time a camera’s shutter stays open so light can enter the lens.
• ISO- is a measurement of a digital camera's imaging sensor's sensitivity.
• Focal length-The distance between the back lens element and the focal plane.
• Bokeh-used to describe the out-of-focus elements in an image.
• Depth of Field (DOF) - The distance between the farthest and nearest points which are in focus. DOF can also be used to describe the zone of acceptable sharpness before and behind a given focused subject. DOF varies according to numerous factors such as lens focal length, aperture, shooting distance, etc.
Please take the first few days and explore your camera. Find the controls for the aperture, shutter speed, and ISO as you will need them for the first assignment.