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Opening the Black Box: Aperture and what you need to know about it.

David Hayden Photography

Black boxes, holes and image projection!

The awareness of focusing images on a surface has been around since at least the 4th century B.C. Aristotle noted this and later it was the basis for the camera obscura (c. 1708), an Italian term meaning dark chamber. Renaissance painters used the principle of focusing images on paper in a dark chamber through a pinhole to improve their paintings.

All cameras from the earliest camera obscuras to the most expensive professional digital camera today at their simplest level are black boxes with a hole / lens for focusing an image. That image may be focused on walls, tracing paper, ground glass, chemically treated plates, film, or digital sensors but regardless, the principle and result are the same.

It is not always necessary to have a black box. For example, you can focus the light of a lamp on a piece of paper using a magnifying glass.

Simplifying Apertures, F-Stops and Ratios

In our world, the world of photography, the hole that focuses the image is known as the aperture. Camera apertures are measured in F-stops.

One of the most important concepts about F stops you should know is that F-stops ARE NOT a measure of absolute size of the focusing hole. F-stop is a descriptive term for the ratio of the size of the hole to the focal length of the lens in use.

Ratio as we know from grade school math is a term synonymous with fraction. We use fractions every day. ¼ tank of gas, ½ cup of sugar and so on. As the denominator (bottom number) of the fraction gets larger the thing being measured gets smaller.

A practical example might be, would you rather have ½ ton of gold or 1/8 ton of gold.

F-stops are denominators of a fraction where the focal length is divided by the aperture. Therefore, F/1.4 is a much larger opening than F/16.

When we need to let more light into the camera for proper exposure, we use F-stops with smaller denominators like F/1.4, F/ 2.8, F/3.5, etc. And when we need to cut light to prevent over exposure, we use F-Stops with larger denominators, F/11, F/16, F/22 and so on.

Remember the light triangle.

Faster shutter speeds will require larger apertures (low numbers) and/or higher ISO settings. Higher ISO settings and slower shutter speeds allow us to use smaller apertures with the higher numbers


Who cares, if the image is properly exposed, life is good, right? Not necessarily!

A case where size matters

A concept photographers are acutely aware of is depth of field. Depth of field is a description of the perceived depth of focus in an image. Controlling depth of field is very important for isolating a photographer’s subject, drawing the viewer’s attention to the subject, and making more aesthetically pleasing images.

There are four factors that all work together to determine depth of field. They are:

  1. The focal length of the lens. Long lenses have shallower depth of field at any given distance and aperture than their wide-angle counter parts. Very wide-angle lenses have extreme depths of field even at large apertures and close distances.

  2. The aperture of the lens. When the aperture is wide open (smaller F numbers) depth of field can be very shallow depending on the length of the lens and the distance from the subject. Your use of aperture to control depth of field can have a profound effect on the aesthetic quality of your pictures. By using shallow depth of field you can isolate your subjects and reduce distractions. You can also use a very large depth of field to enhance landscape photos where the image appears very sharp front to back corner to corner. It also helps to create layers that give a sense of dimension. The foreground is a layer leading to a stand of trees that are a layer between the foreground and the mountains in the distance, and so on.

  3. The distance from the subject to the camera. The closer any subject is to the camera, the narrower the depth of field. When the subject is at the minimum focusing distance for the lens, the depth of field will be as narrow as possible for any chosen aperture. This really becomes apparent with close up photos. A medium length lens, with a large aperture at a close distance can create a portrait where one eye is in focus, but the nose and ears are not. On the flip side, when the subject is very far away from the camera depth of field is insignificant. Take for example this photo from the eclipse on 10/14/2023. The sun is 93,000,000 miles away and the moon is 238,000 miles away. Even though the effective focal length of the lens is 690mm, and the aperture F/6.3 (wide open) both the moon and sun are in focus. Because of the distance from my camera to the moon is so great, the depth of field is, for practical purposes, infinite. This combination allows the sun to remain in focus despite its being roughly 400 times farther away.

Now compare the moon/sun depth of field to this image of a horse about 150’ feet away. In this case the effective focal length is 900mm, and the aperture is much smaller than used for the image of the moon (F/8). Because the distance to the object plays a large part in the depth of field, the front fence and the horses face are in focus, but the rear of horse and the back fence, just a few feet behind, is not in focus at all.

4. The size of the image relative to the viewing distance.

Small images up close appear to have a larger depth of field than the same image blown up to a much larger size. But view a giant image on a billboard from 30 yards away and it can appear to have a greater depth of field despite its size.

A picture is worth a thousand words.

I can write text until the cows come home and not make the point so clearly as I can with a few images. So, let’s see what I am talking about. Because the point of this article is discussing aperture the following images will reflect, for the most part, the effects of aperture on depth of field.

300mm focal length, distance to subject 25’ +/-, F/11. Despite the small aperture (F11) the depth of field is shallow because of the long lens and nearness to the subject.

280mm focal length, distance to subject 75’-100’, F/6.3. Background is more in focus despite the wider aperture because the distance to the subject is farther and the lens is slightly shorter.

420mm, distance to subject 15’, F/6.3. Combination of long lens, close distance, and widest possible aperture for the lens renders a very shallow depth of field.

75mm, distance to subject 3’, F/10. The cheesecake and strawberry have the sharpest focus, but the focus gradually falls off in front of and behind the cheesecake.

27mm, distance to branch in foreground 6’, F/11. Because of the combination of focal length and F-stop, the depth of field runs from the front of the scene to the back.

Projects to help you master aperture.

Assignment 1

  1. Place an object on a table.

  2. Set your camera in aperture priority mode and set it at the minimum focusing distance to object.

  3. Without moving the camera take several pictures starting with the lowest F-stop available to you for that lens (F/1.4, F/2.8, F/4,etc). For each successive picture increase the F-Stop until you reach the highest possible with the lens (F16, F/22, F32, etc.)

  4. Study your pictures paying particular attention to the depth of field and notice how it changes.

Assignment 2

Repeat assignment 1 but this time take pictures of a landscape where you set your focus point somewhere in the middle of the scene.

What’s in store for future articles?

Next week we will be discussing shutter speed, the second leg of the light triangle / exposure triangle.

May the images in your mind become your inspired creations!

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