Taking Control Over Depth Of Field.pdf

Taking Control Over Depth Of Field

Using the Lens Blur Filter in Adobe Photoshop CS

One of the skills that separates novice photographers from the more experienced is an

awareness and control over “Depth of Field” (commonly referred to by its abbreviation

“DOF”). Sometimes, to get the shot, we need to use a very small aperture. When we do,

not only is the subject in sharp focus, but features in the background and/or foreground

can compete for attention. This tutorial will walk you through the steps in using a new

feature in Adobe Photoshop CS: the Lens Blur filter.

If you take an image where the subject is at a distance, everything in the image at that

same distance will be equally sharp. Features in front or behind the subject will have less

sharpness. The further they are from the subject, the less sharp. DOF is the zone of

acceptable sharpness that extends in front of and behind a sharply focused subject.

Figure 1. Too much DOF allows leaves and stems to compete for attention with this

White Iris bloom.

There are three basic facts to DOF. Holding everything else equal:

• DOF increases as a lens is stopped down ( i.e. , the f/number is increased);

• DOF increases as focal length decreases;

• DOF increases with subject distance.

Experienced photographers attempt to manipulate these factors in order to control how

much of the background and foreground blurs and how strong the blur will be. There are

images that should be sharply focused throughout. Landscapes are usually shot with tiny

apertures like f/11 and f/18 to have everything in focus from a few feet in front of the

photographer all the way to the horizon. At other times, a blurred background or

foreground is preferred. When you take a macro or near-macro shot, the DOF can be as

small as a fraction of an inch. Make a mistake and either the front or the back of your

subject will show evidence of being out of focus.

The image in Figure 1 is one of those situations where DOF works against you, where a

sharply focused subject means competing features in the background or foreground will

also be brought into relative sharpness and detract from the subject.

I had to choose. Did I want a blurry background in order to keep the eye on the flower

bloom? Did I want every feature on that flower bloom to be in relatively sharp focus?

The physics of the situation – 180mm macro lens, Canon 10D, the size of the bloom, my

distance from the bloom (1.5 meters) – all required that I use a small aperture in order to

get the iris blossom in sharp focus. I used f/32 at 1/10 of a second on a sunny day.

f/32? What about diffraction? Fortunately, I was using a Canon 180mm “L” macro lens.

If you have never used one, the quality of the optics will astonish. Even at minimum

aperture, this lens delivers sharp, crisp images.

I would have preferred to back off to something like f/22, but my DOF would have

narrowed so much that features of the iris blossom would lose sharpness. Even at f/32,

the DOF extended a scant 2 inches. At f/22, DOF would extend less than 1 ½ inches. Not

enough. I stood back at 1.5 meters, rather than allowing the blossom to fill the viewfinder

in order to extend the DOF a bit.

When I am out in the field, I do not calculate DOF with such precision. I instead use the

DOF preview button and run my eye over the subject. Especially the petal edges in this

case. The problem you confront with the DOF preview is viewfinder brightness. The

viewfinder is very dim at f/32, even on a bright, sunny day. I bracket for DOF in that

circumstance. For this shot, I bracketed f/22, f/25, and f/32.

I knew from the DOF preview that the background was going to be a challenge. The iris

blossom was in the middle of a bunch of leaves and stems. Some were the same distance

from the lens as the subject. They were not going to blur sufficiently. The solution was to

rely on isolating and blurring the background in Photoshop.

Using the Lens Blur Filter

Photoshop CS added the Lens Blur to the Filter | Blur menu items. In prior version of

Photoshop, you would instead use Gaussian Blur to give the appearance of a narrower

DOF to an image. The Lens Blur filter adds a more realistic blur. It is also a more

complicated filter to master.

To use the Lens Blur filter, you need to be comfortable making selections and converting

selections to masks. The Lens Blur effect relies on a Depth Map to control which features

get blurred and how much blur is applied to them relative to other image features.

Figure 2 is the same White Iris in Figure 1 . The image was cropped from a 2:3 aspect

ratio to a 4:5 aspect ratio. Cropping removed some of the leaves and stalks, enhancing the

white blossom.

Figure 2. Effective cropping enhances the subject.

The background in Figure 2 is still distracting -- especially the leaf at 5 o’clock pointing

straight at the blossom. The first step in using the Lens Blur filter is to create a Depth

Map. A Depth Map is just a grayscale image. Typically, an alpha channel. If you recall

that white reveals and black conceals with a mask, then black protects features from

blurring ( i.e. , black conceals features from the Lens Blur effect), white permits the full

effect of the blur, and shades of gray progressively allow more/less of the blur effect.

I started by duplicating the Blue channel. There was already a lot of contrast between the

flower and the background.

Figure 3. The Blue channel was a good place to begin crafting a Depth Map.

I used the Magic Wand to select the background, making sure the Contiguous box was

checked on the Option Bar. This kept the selection away from the darker details on the

flower itself. Using a Tolerance of 30, it took all of a half-dozen clicks of my Wacom

tablet pen to select the entire background. I then converted the selection into an alpha

channel by selecting the “Save selection as channel” icon at the bottom of the Channel

Palette.

To soften the edge of the selection, I applied a 1 pixel Gaussian Blur to the new mask.

This left me with a well-defined mask for the flower. I had my Depth Map.

Figure 4. Mask of the flower to be used as a Depth Map for the Lens Blur effect.

To repeat, where the mask is black, there will be no blur applied. Where the mask is

white, the image will receive the full blur effect. Shades of gray would receive a

corresponding amount, with brighter tones receiving more blur than darker tones.

I used the Magic Wand for this example. You can use other selection tools, too. Gradients

can be very helpful for reducing DOF in a way that adds a dream-like or romantic aura to

an image.

Once you have your mask ready, select Filter | Blur | Lens Blur. A dialog window will

open. You can toggle the preview on and off by selecting and unselecting the Preview

checkbox.

Be warned! The Lens Blur filter is performing lots of very complex calculations. It can

take a minute or two on even the fastest workstation, especially if you select the More

Accurate option for the preview.

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