Using Photoshop
to Simulate Camera Aperture & Depth of Field

Photographers create dramatic effects by using the right camera settings…
Camera Aperture is at the top of the list!

Known as “Depth of Field”, adjusting the aperture of a camera lens allows the professional photographer to control what part of a photo is in perfect focus, and what parts of the image he wants to be out of focus.

Since it is human nature to be drawn to what is clear, photographers use this technique to turn a regular picture into one that is spectacular!

Photography & Adjusting Camera Aperture
How a Photographer Controls Depth of Field

Aperture is the size of the opening of a cameras lens. The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor. The size of the aperture is measured in f-stops. In combination with variation of shutter speed, the aperture size will regulate the film's degree of exposure to light.

Small Aperture = Wide Depth of Field

Wide Aperture = Narrow Depth of Field

Using Camera Aperture to Effect Depth of Field

Aperture also has a big effect on "Depth of Field", or how much of the image is in focus. When ALL of an image is in focus it has a wide depth of field. If some parts of an image are out of focus is said to have a narrow depth of field.

Small Aperture
large f value = everything in focus = large depth of field

Large Aperture
small f value = limited focus = shallow depth of field

Simulating Camera Aperture in Photoshop

Photo “As Shot” with Wide Depth of Field

Simulated Narrow Depth of Field using Photoshop

In Photoshop we can “borrow” the concept of Depth of Field (DoF) from photography and simulate aperture adjustments with Photoshop's Lens Blur Filter. In the top menu select FILTER > BLUR > LENS BLUR.

To use the Lens Blur, an alpha channel (in the channels pallet) is created to control how the filter will be applied. Wherever the alpha channel is white, the filter will be applied. Where the channel is black, the image is protected from change and the filter won't be applied. Various shades of gray are used in the alpha channel to apply the filter at reduced strengths.

Make a copy of the image:
Layer > New > Layer via Copy

or by using the keyboard shortcut:
Control + J (on a PC), or Command + J (on a Mac),
or by simply dragging the
background layer to the "create new layer" icon.

Delete the "eyeball" in the layer pallet
of the original background layer
(the one you just copied) and keep it locked.

Then, make sure the "background copy" layer
you just made is the one that is selected
(highlighted in blue).

In the channels pallet (WINDOW > CHANNELS)

Click on the icon to create a new channel.
Once you create the new alpha channel, the entire image will be covered with black, and a new layer will appear in the channels menu called “Alpha 1”

After the new alpha channel is created, reselect the “Eyeball” in the top RGB icon, and a red mask will now be covering the image instead of the black box. This will become the image map used by the lens blur tool to create the depth of field effect.

With the new alpha channel selected, delete the contents of the alpha channel by Select > All Delete.

If the instructions were followed correctly, the red mask over the photo should now be gone, and the black box in the Channels Menu called “Alpha 1” will be WHITE instead of BLACK.

Now it is time to get creative, and decide what you want to be in full focus, and what should be blurry by making adjustments in the new Alpha 1 channel.

What you want to STAY in focus is tinted red.
What you want to be more out of focus is less red.
In the example above a selection was made in front of the front of the bricks to the background and a gradient was drawn.

In this tutorial, the objective was a blurring behind the bench setting area towards the top of the image, so the house and sky in the background would be out of focus. The reason the gradient was used was so the fading would be gradual, as an aperture adjustment on a camera might look. To also fade towards the foreground in front of the bench, choose Select > Inverse and draw another gradient going the opposite direction to blur the foreground.

The two gradients were combined to create a gradual blur, with a focal point around bench and the interior of the brick-in area. To finish it off, fill in that entire area that you want to be in FULL FOCUS with 100% black. You could use a brush, a lasso selection, or your other favorite tool. In our case the lasso tool was used to select the desired area for full-focus.

When finished, this Alpha Channel is known as an Image Map that controls the Depth of Field, simulating camera aperture. You can right-click on the alpha channel (option-click on a Mac) to see only the image map (as seen above) to help you with any fine-tuning of your alpha channel. Continue filling in those ares that are to be in full focus with 100% black, slightly out of focus with a dark gray, etc.

Using Your Image Map
to Simulate Camera Aperture

When your image map is ready, open the Lens Blur dialogue box: Filter > Blur> Lens Blur.

Select the RGB channel (in the channels pallet) to make the image itself the subject of the filter.

Then select your alpha channel in the source menu & begin to adjust the “blur focal distance” slider until you are satisfied with the effect in the preview area.

You can also simulate different lens iris configurations, adjusting the brightness of the highlights, etc.

1. In summary…
   When creating an alpha channel to use as the image map to use with the lens blur filter, remember that:
2. The part of the image you want to remain sharp and in full focus should be painted black in the alpha channel
3. Whatever you want to be blurry should be painted white, typically that which is in the background, far away (although it could include the foreground as well if you want)
4. Areas between the black and white should be covered with a gradient if you want the blurring effect to be gradual
5. Anything the same distance as the subject should also be painted black if you want it to look realistic, ie the way a camera aperture setting would have looked.

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