Monday, November 29, 2004

Depth of field

Depth of field is the area of sharp focus between the nearest and farthest distance of possible focus for any given f-stop. Roughly, it's a range of focus. It's determined by the amount of light allowed to pass through a constricted opening (f-stop).

As a practical tool, using depth of field increases the thickness (depth) of the focal plane. Depth of field is measurable and predictable although most PJs use alternative methods to "guesstimate" it.

We learn depth of field from practical applications. As the f-stop becomes smaller, the depth of field becomes greater. Eventually, if the PJ gets a small enough f-stop, practically everything could be in focus.

It would take a lot of time and money and possibly some bent coat hangers dangling from the lens housing, but it's possible to create a 0-to-infinity lens. But, it's not practical for PJ work.

Ultimate depth of field works great for someone with a tripod and a lot of time to photograph a bouquet of flowers or a landscape. However, it doesn't apply well to most PJ assignments.

Instead, PJs need to know how to keep two or more objects or layers of the image area in focus. They also need to know how to get the depth of field exactly on these items so the subject could do normal activities (like breathing). They try not to surpass the required f-stop for the subject matter to keep shutter speed useful. In practical low-light terms, consider one stop is the difference between hand-held and a monopod and two stops requires a tripod.

Additionally, PJs need to understand which lens combination is most likely to yield the desired result. Often this is akin to hammering a square peg into a round hole.

Common method
This is sloppy, but it works. For any given f-stop, the total depth of field can be divided into three parts. One-third is in front of the focal plane at the maximum aperture (what's visible through the eyepiece) while two-thirds is behind the focal plane.

For the technical folks out there, it's one-half the distance forward and two times the distance back. It's the same result either way.

In either case, it's important to understand this rule while covering extremely quick subjects (races) - particularly when the PJ is positioned in front of the subjects (near the finish line). If the PJ is concerned about making a mistake, go with caution and actually focus slightly in front of the subject and let the depth of field make up for the difference.

I said it was sloppy. Let's talk about absolute precision after you cover hydroplane races. ;-}

Lens markings
High-quality lenses have depth of field guide marks. These are the strange, multi-colored lines on the lens barrel near the range number guide. If a lens is handy, take a look at it. Some older, manual lenses have a whole rainbow of lines spaced equally from the focus range line (center line). Newer lenses may have two sets of lines or none at all.

If we look at the line colors, we notice they match the color markings of different f-stops. F/11 may be blue and f/22 may be yellow, etc... Newer lenses don't use the color code, but have a small number next to the line (11, 22, etc...). These lines are depth-of-field guide lines. Everything between the depth-of-field guide lines is in focus for a given f-stop.

Meanwhile, some modern zoom lens may not even have the guides. Recently, I saw a new lens without any f-stops on the lens because it was designed to only work with "pro-sumer" digital cameras.

Precise depth of field distance
Below is the method to maximize depth of field. The process takes practice (like everything else we do). As PJs settle into their own style, they use a combination of general and precise measurements to get what they want for different assignments.

It doesn't matter if the numbers align in meters or feet, but make sure not to switch between scales while working (especially if using laser range finders or parabolic mirrors).

Obviously, use a tripod whenever possible both to stabilize the camera and make range measurements more accurate. Next decide which two objects or layers to keep in focus.

1. Focus on the nearest object and note the distance.
2. Focus on the farther object and note the distance.
3. Move the focus ring until both distances fit between the depth of field guides.

As the PJ looks through the lens, most elements appear out of focus because the focal plane is between the two objects while the lens is at its maximum aperture. The PJ could hit the depth-of-field preview button, but it still looks slightly off due to light loss. On a dit, fire a frame and chimp the results.

Making adjustments
Frequently, the two objects or layers don't immediately fit into the depth of field. Then, PJs can either back away from the subjects or change lenses.

As evinced below (hyperfocal distance), a lens with a smaller focal length yields a greater depth of field at closer ranges. Frequently, this determination can be handled without moving because the lens itself because it lets the PJ know which ones can handle the range.

The trick is to choose the correct distance and/or lens for the subject. Decisions are determined by how large and close together the two objects are as well as what's happening in the background. I'll save the lens optics and dot-gain discussions for another day, but PJs need to understand the same visual information is held in both a severely cropped 15mm image and a full-frame 600mm image. However, if the crop is too severe, the image can become useless.

Deliberate background blur
When a PJ wants a background to be muted or lost in circles of confusion, the most common remedy is to set the lens to its maximum aperture (f/2.8). This works fine most of the time.

Occasionally, the PJ needs fill flash during a bright, sunny day and also needs to diminish a "busy" background. This is when the maximum depth of field distance again becomes useful. It also becomes somewhat complicated, but we'll work through it. Below is a step-by-step method.

1. Establish the synch speed of the camera.
2. Make EV calculations to match the film speed and aperture for available light.
3. Calculate the flash-to-subject distance and adjust the distance or flash output until it falls within the range set by the other factors.
4. Focus on the subject and note the distance.
5. Move this distance to the farthest point on the depth of field range.
6. Press the shutter release button.

The result is a focused subject with proper ambient and fill light. Additionally, all other elements are outside the depth of field. The PJ has lifted the sharply-focused subject from a cluttered background while still shooting at around f/22.

If the PJ looks through the lens, the subject appears out of focus because the focal plane is far in front of the actual subject. As long as the range is correct on the depth of field guide, the image will be fine.

Hyperfocal distance
Hyperfocal distance is the distance, at a given f-stop, between a camera lens and the nearest point (hyperfocal point) which yields sharpness to infinity. This distance varies in proportion to the focal length of the lens. Again, this term only applies for depth-of-field to infinity measurements (mostly landscapes).

Each lens has a different hyperfocal distance range. The range is based on mathematics and lens optical corrections.

The equation is: the square of the focal length divided by the product of the f-stop times the circle of confusion. The circle-of-confusion variable changes for each film format.

As practical examples, a particular 50mm lens can handle everything between six feet and infinity at f/22. While a particular 100mm lens handles from 19 feet to infinity and a particular 300mm lens can only handle everything between 150 feet and infinity at f/22.

For those who aren't already confused, a smaller focal length offers more depth of field at a given f-stop, but a wider angle of view (so scene elements appear further away with greater separation). The inverse is true.

Enough for now,

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