Photography 101 - Manual Photography Pt. 2 - Depth of Field, Focal Length, Focus Modes (Patreon)

Let's get this P101 started with one of my favorite concepts, which is most often the hardest to explain without a diagram.

Depth of Field

Depth of field is the field in which shapes will be in focus when using your camera. Focus itself is an volumetric shape, think of it as a cube. How thick or thin the cube is will be based on three key factors; distance of focal point from camera, aperture, and focal length of lens.

Here are some crude drawings I've done to help illustrate how it all functions in the real world.

At an aperture of f/4 the area of items which will be in focus is fairly reasonable, at most distances you can fit an entire human within this area, with some falloff on the front and back extremes of the field. 

If you open up your aperture to a much wider number such as f/1.8 your field at which items are in focus (depth of field or DOF,) will become much thinner. The closer you are to your subject, the more out of focus the background will become, and vice versa. Furthermore, the further you are from your subject, the wider your DOF will become.

In the title post photo, you can see I have my aperture very wide, and the focal point is on one of the water drops on the panel, and I'm very close to the glass panel. This causes a razor thin depth of field; so thin that only the water droplets are in focus and the model who is right behind the panel is out of focus. Further there are large lights in the distant background that are being blurred to a large degree because they are so far out of the focal field. This effect is called "Bokeh" and is commonly highly desired by many portrait photographers, amateurs and pros alike.

In this photo, my aperture is the same, however my focal point is on the model's eye. Due to her being slightly further away from the camera than the glass panel, this causes the DOF to widen large enough to encompass her body. If she had been facing sideways like in the above photo, the field may not have been deep enough and would have likely caused her front shoulder to be blurred somewhat. Not a huge deal, but if it's something that's important to your shot (such as trying to keep a tattoo in focus) you may want to close down your aperture a little. For example f/2.8 might work for this situation, just don't forget to either decrease your shutter speed, or increase your ISO to compensate for the light loss of the smaller aperture.

Focal Length

Focal length is fairly simple to understand. It is the distance of the lens from the camera sensor. Many people would refer to this as "zoom." However, many professionals do not use lenses which zoom, rather they use "prime" lenses. Prime lenses are lenses which have a set focal length. Your zoom becomes your feet!  Lenses with a higher focal length will create a stronger difference in blur when photographing a subject with a wide aperture. This allows photographers to get extremely large bokeh with very long focal length lenses. 

For example, this bulb may have been photographed with a very long focal length, such as 200mm or so, causing the background bokeh to be extremely exagerrated.

Photo by Alex Iby 

Here are some more of my crude drawings to illustrate the way a camera's focal length works.

As you can see the lens with the larger focal length has a more "zoomed in" viewpoint. When your focal length lowers, objects become more distorted; such as cameras with extremely low focal lengths which create a fish eye effect. For larger focal lengths, objects become distorted in the opposite way, things become more compressed in the center of frame and warped in that matter. However, this is often much less noticeable than large focal length images.

Everything is a balance of course, shooting with a large focal length is great for portraits, but it also causes the distance between you and your subject to increase substantially. If you are photographing a subject with a 200mm focal length, you may have some amazing quality, but you will need to be yelling posing instructions through a megaphone! Shooting with something as small as a 24mm is great for tight spaces! However you will notice some distortion, especially when it comes to noses and jawline.

Photo by Anna Wu

Focus Modes

There are a billion focus modes on camera's these days. However, I am only going to cover the two I feel are most important in this lesson.

Single Point AF Mode - the "manual" autofocus

This mode allows you a single dot to know exactly where your focus will land. I recommend this mode highly if your camera does not have the next autofocus mode. The hardest thing to learn is how to use this mode effectively.

Often, there are several focus points on a camera, up to hundreds on many modern mirrorless setups. However, I would recommend if you do not have the next feature to put your focus point in the center of your frame, and leave it there.

The reason for this, is that the center point is often the most powerful focus point, it is the fastest and most accurate. Now I hear you all screaming from the back "what if we don't always want to put our subject dead center in the frame!"

Trust me, I hear you loud and clear, and would never advocate doing that with every shot. The key to this is a technique called "Focus and Recompose"

Have you ever noticed how when you half press the shutter button, your camera focuses, but does not take the photo until you have fully depressed? That is the key to this entire technique!!

So, very simply, place the central dot on your subjects eye that is closest to you. Then, press the shutter halfway until you hear the signal that your camera has obtained focus and often the red dot will turn to green as well (depending on your camera model.) Once you have heard this noise, DO NOT LET GO OF THE HALF PRESS. Move the camera left, right, up, or down to get the framing you want on your subject, then depress the shutter all the way. This will give you the kind of framing you want, along with perfect focus everytime.

Some key factors:

• Do not move forward or backward after focusing. Focus points are ALL about distance to the subject. Once you have created the proper focus, it is essentially a perfect plane in space all the way above, below, and to the left and right of your focal point. If your models hand is in line with their eye, it will also be perfectly in focus. The same can be said of any object which is the same distance from your camera as their eye (assuming you focused on the eye)
• Once you have obtained focus, you can continue to always hold the half press even after taking a photo to keep the same focal point. As long as you or your subject does not move closer or further away from your camera, you will not have to refocus. This is great for a set of burst shots such as a hair flip, spin, or anything of that nature which needs your focus to be locked.
• Many professionals prefer to use "back button focus" in order to more effectively utilize this technique. There is a button which can often be remapped on the back of your camera to obtain focus. Typically with this button, letting off of the shutter entirely will not cause your camera to re-focus on subsequent shots. This is slightly more difficult in my opinion however because you have to remember to refocus more frequently, since your camera will not be making noises or signaling that things are or are not in focus like it will if you re-focus every shot. Simply a matter of preference, I would recommend trying both and seeing what works best for you!

Automatic Eye Tracking - easy mode

Many new cameras have gotten so good at eye tracking, that they can handle all of the focusing for you. This goes for high end Sony, Cannon, and Nikon cameras. When taking photos this can take all the work of focusing off of your hands when working with a single subject. However, I would recommend learning the above technique for when working with multiple subjects, or when going for a different look. As a prime example, the title photo of this post would not have been possible with automatic eye tracking. It simply would put the model in focus every single shot. This is a great feature to have, just don't forget to learn the manual method so that it does not become a crutch and so you can continue to be creative and try new things!

That's it for this week, my hands are burning from all of this typing and I've still got more posts to make! Leave comments with any questions or discussion you would like to have on this topic!