depth of field

When your big light just ain't so big

Lighting a group

We all know that a large light in close to the subject is a soft light. As we move the light back away from the subject a couple of things happen. First, the light gets deeper. Second, the light gets smaller.

Deeper
When you move your light back the light rays are more parallel and fall-off in intensity slower. At one extreme we have sunlight. The sun is so far away that light rays reaching the earth are almost parallel. The same amount of light reaches you where you are standing as reaches someone else a block or two away from you (or miles away). So we know that if we have to light a group of people with a number of rows we need to move the light back so a similar amount of light reaches the back row as reaches the front row. If the light(s) is too close in the fall off will be rapid and the back row will be dark or the front row will be overexposed.

Smaller
Smaller light equals more defined shadows. A small light cannot “wrap” around the subject to cause a wide shadow edge.

Diminishing Returns

(click to enlarge)

I feel that after a certain distance the size of the light doesn't really matter all that much. A 60-inch umbrella far away is just as hard as a smaller modifier. So, I gathered up the studio staff for a group photo and lit it three ways.

Have you met the staff? An interesting bunch. Over on the left we have our lobotomy patient, next is our exhibitionist, then the lowly photographer, and finally that guy who is just a shadow of his former self. Look for a new post about shadows coming soon to this blog.

The light was set up 9-feet away from the group and flash power was adjusted to give f/7.1 in each photo. The first photo is with the 60-inch umbrella. The second photo is with the 7-inch dish. The third photo is the same 7-inch, but with a sheet of diffusion material taped to the front of the reflector.
Looking at the three photos, the big difference between them is in the shadow density. As I expected, the 60-inch has the lightest tone shadows, followed by the diffused reflector. They each allowed more light to bounce around the room to open up the shadows.
But what about the light on the faces? Surely the big umbrella is going to be the clear winner here. Or is it? I see very little difference in the light on the faces in these three photos. It just took a lot more flash power to light with the umbrella than with the smaller, more efficient reflector. And that can be a big consideration if you are going out on location to make the group photo, especially if using battery-powered flashes. However, the umbrella or the dish with diffuser will cast a wider area of light that you might need for a very large group if you want to stay with a single light source.
Of course, if this was to be a portrait of one or two people, I could have the big light in much closer and could be more creative with lighting patterns, and separating the subject from the background, and on and on, etc., etc. But for groups you need to pull the lights back. And, in doing so, you might find that you don't need as large a light or as much strobe power as you thought you would.

 

 

BONUS MATERIAL

Many aspects of photography are related and many of them deal with distances. Here are some ways you can apply knowledge about one aspect of photography to learning another. First we have depth of field and depth of light. As you move your camera back away from your subject your depth of field (the area of the scene that appears reasonably sharp) gets larger. Taking that to lighting, as you move your light back away from your subject your depth of light gets larger. The closer the light is to your subject, the darker the background will be. The closer your camera/lens/aperture combination is to your subject, the more shallow the depth of field.

Next is perspective. The closer your camera is to your subject, the smaller the background will appear. Conversely, as you move back away from your subject the background gets larger. This is often mis-labled as telephoto compression, but it is the physical act of moving back that causes the change. You just tend to use a longer lens when farther back to magnify the subject to fill the frame, which makes people blame the lens.

Very much related to this is how a face looks in a photograph. The closer your camera is to your subject the more pronounced their features will appear. As you move back away from your subject their features flatten out. In close your subject’s nose looks bigger in relation to their eyes and ears. But as you move back the features flatten out as the nose, eyes, and ears get relatively closer to each other than to the camera. When you move back you can switch to a longer focal length lens again to fill the frame, but like before it is the physical moving back that changes the depth of features of the face. As in the lighting situation above, there is a point of diminishing return. Once you are in the range of 12-15 feet away from your subject I don’t think that moving any farther back will have any more effect.

How do you like to light large groups of people?

How do you apply what you know about one aspect of photography to other aspects?

Thanks again for following along!
John


More about Depth of Field

Your only real control is aperture

It is often understood that using a shorter/wider lens gives more (depth of field) DOF, but that is only half of the story. That is true if the subject is smaller (such as taking both photos from the same camera position), but the tradeoff there is file size and resolution. The 35mm shot has to be heavily cropped to give the same subject size, so resolution (pixel count) is greatly reduced.

In practice, if you want to maintain the same subject size on the film/sensor your best control over DOF is to stop the lens down more. If you don’t need all the resolution of your sensor you can maintain the same camera to subject distance and use a shorter lens, then crop—giving you the same perspective. Or you can move back with the same lens and crop, but that will change perspective—the background will get larger (more compressed) and the subject will flatten out. 

The following animation shows 4 full frame images taken with a 200mm, a 135mm, an 85mm, and a 35mm lens all at f/2.8. The camera was moved in closer with each shorter lens to make the mannequin head approximately the same size in each image. The things to note are that while the shape of the face changes drastically and the size of the pattern in the background changes (both due to the changing camera to subject distance--refer to my last post, "The closer the camera to the subject, the smaller the background elements appear"), the depth of field remains almost identical. Pay close attention to the headpiece beads next to the ear. They look about the same in all four images...

dof-animation.gif

Below are two still images to make it easier to compare and see the similarity in depth of field for two very different focal lengths, but giving the same magnification at the same f/2.8 aperture. Each is shot full frame, moving the camera in closer for the 35mm image to maintain approximately the same size head in each photograph...

200mm-35mm-dof.jpg