What is Depth of Field (DoF)?
Simply explained, if such a thing is possible, DoF is an area (field) of sharpness (focus) that exists in front and behind the subject you’re trying to photograph. More precisely, it is the nearest point in front of your subject and the furthest point behind your subject that are in focus. This field, is very exact, and is very measurable. I should also mention that this field is perfectly perpendicular to your lens. So that if you tilt your lens, the focal plane shifts with your lens - hence the term "tilt-shift" but that's a theme for another blog.
Circle of Confusion
When you play with DoF you will come across what is known as the "Circle of Confusion" this is an area that will appear to be in focus even though it is slightly out of focus. What happens is that our eyes cannot tell the difference.
Don’t worry, I won’t get into the math of DoF. Fortunately for us, some awesome math geeks, have come out with great DoF Calculators so we don’t have to spend all our time doing math instead of making photographs. However, if you are part photographer and part math-geek, I will have a link at the bottom where you can learn everything you wanted to learn and more about lens optics and all the math that goes into photography. You'd be surprised how much math and science actually goes into photography.
DoF is a by-product of aperture, it is also affected by other factors such as,
Why does DoF occur?
Why it occurs is a function of lens optics, Gauss' Lens construction, physics and many other principles that are beyond the scope of this blog. Check this cool DoF applet that will let you play with the factors and see exactly what happens and why.
For the purpose of this blog, what's important for us to know is that DoF occurs and we can manipulate it by changing one or more of the conditions listed above to achieve different results. Let’s take a look.
All these images were shot on a full frame camera from the same distance - roughly 2 ft. with an 85 mm prime lens. This distance was definitely pushing the limits of the minimum focal distance of this lens. I purposely shot with a prime lens so we wouldn't have to deal with compression from a telephoto. The only thing that has changed is the lens aperture and the shutter speed which was adjusted in order to get the proper exposure.
If you want to calculate the DoF yourself, you can use this calculator from DoF Masters. Here's the link DoF Masters Calculator
There will be times when you want to use a shallow DoF to isolate your subject and guide the viewer's eye to where you want them to focus. A shallow DoF is especially useful when you're trying to eliminate clutter, busyness or anything that distracts from your subject. Since our eyes naturally gravitate towards things that are in focus, you need to give your viewer's eye a place to stop. You must be careful not to use too shallow a depth of field or you may get too much of a blurry subject. When everything is out of focus, our eyes wander, searching for a place to rest - that place is anything that is in focus.
The opposite is true, there will be times where a wider DoF is called for. For example when you're shooting a landscape or when you need some context in your picture.
In the photograph below, I have certainly isolated my subject, but I am so close to the subject that some of the subject is blurry. If you look at the calculator next to the example, my depth of field at 2 feet is .01 feet. That means that the area of focus in front of and behind my subject is 1/8" deep (which includes my subject). Well, 1/8" is not very deep. No wonder there is a very small area of my photo that is in focus.
On the second image, shot at F16, the DoFis much wider - more of the picture appears to be identifiable. We can see some rocks, buildings, the lake. By changing your DoF, you can control exactly where you want your audience to look.
Now let's see a few examples. Each photograph has been shot at different depths of fields. You can see thee different levels of blurriness in each picture. You can click on each photo to make them larger.
While my use of a shallow DoF is more utilitarian when I'm photographing shelter animals, I usually have to hide all sorts of distractions. it doesn't always have to be done for this purpose. Shallow DoF is also an artistic choice and a creative way to pop your subject out of the photograph for greater impact. What DoF to use is up to the creativity of each photographer. Do you have preferred DoF? I usually adapt my DoF to the circumstances.
This blog was created as part of a weekly challenge. Each subject encompassing a new focus or photographic technique. Stop by to visit us next week.
To see more examples of this week's theme, please visit: Ashley Siemon Photography, Serving the San Francisco Bay Area and Northern California.