Digital Photography - Aperture Settings

The Aperture is a flexible disk normally located with the lens element, however in digital cameras it may be found inside the camera body.

In older cameras the Aperture was adjusted using a manual ring to open and close the opening prior to making the exposure. The Aperture affects both the amount of light striking the photo sensitive device, and the Depth of Field for focus. In modern digital cameras the Aperture is set and controlled by electronic servos.

Given a standard ISO, 200, the Aperture will affect the values for shutter speed and DOF in capturing a usable exposure.

You may notice from the illustration above the Aperture values and the opening of the disk seem to double as the opening decreases, this is known as Steps (Stops), and Steps apply to the three major exposure values (ISO, Stutter Speed, and Aperture Settings. The "F" numbers are the result of mathematical calculations and rounded off to the nearest 1/10. The exposure effect doubles for each Stop. If you start with an ISO=200, and a Shutter Speed=1/60, and an Aperture=F-16 for a perfect exposure, and then change the Aperture to F-8 you double the amount of light striking the photo sensitive element.

Changing the Aperture value also changes the DOF and HFR for the subject photographed.

Depth of Field (DOF)

Depth of Field is the measurement of acceptable focus from the point of focus forward (toward the camera), and beyond the point of focus (behind the point of focus. It can be measured in Meters, or Feet.

DOF relies on three main factors:

The total focal length of the lens. (measured from the front of the Objective lens to the photo sensitive device inside the camera.
The diameter of the Objective lens. (The larger the diameter the more light can strike the photo sensitive device.)
The Aperture opening as noted in F-Stops. (The larger the F-Number the smaller the opening and less light strikes the photo sensitive device.)
The higher the F-Number the greater the DOF range, and the less light available to the photo sensitive device

Thus begins a trade off in the exposure.

If you change the Aperture from a known good exposure setting to affect the DOF you may have to adjust the Sensitive value, Shutter Speed, or lens type to regain a usable exposure.

Adjusting the Aperture value 2 stops means you must adjust the shutter speed by 2 stops, adjusting the sensitive value 2 stops, or changing the lens to one with 2 more available stops. Any combination of the above will accomplish the desired result so long as the changes add up to a 2 stop value.

Hyper Focal Range (HFR)

HFR is the ability to vary the acceptable DOF from behind the subject to in front of the subject. This is where we create artistic effect in capturing an image.

Normal DOF from the sidewalk to the lit sailboat.

DOF adjusted to soften the foreground

Depth of Field to soften the foreground all the way back to the lit sailboat.

Depth of Field adjusted to soften the sailboats and HFR adjusted to keep the foreground and lit sailboat in sharp focus.

Notice the invisible composition line between the boats and calm water, it leads the eye to the central point of interest, the lit sailboat. The use of Aperture adjustments and changing the HFR helps guide the Human Eye.

The acceptable range of focus is between the first arrow and the second arrow. The Hyper Focal Range is between the first arrow and the last arrow and can be adjusted by rotating the focus ring on the lens to bring the acceptable focus closer to the camera.