What is Aperture in Photography? | DOF, F-Stops, and T-Stops

All amateur photographers with a camera get stuck at this question at some point, what is aperture? Put simply, the aperture is an opening in front of the lens that allows light to reach the image sensor in the camera. Photographers manipulate this opening to produce magnificent photos. Also, it does have some limitations which we will cover later in this article. Let’s see what aperture is in the context of photography, and how it affects your photos.

The Eye Analogy of Aperture

Aperture compares best with the human eye for understanding the concept. The iris in the human eye controls the pupil to control the light entering the eye. When we are exposed to bright lights, our pupil contract to decrease the amount of light entering the eye. And when we are in dark, the pupil dilates so that more light will enter the eye for an effective vision.

Understanding Aperture with Eye Analogy | Photography Guide

We can apply the eye analogy to understand the aperture of lenses. It is the pupil of lens whereas the diaphragm is the iris. A photographer controls the light entering the camera using the diaphragm. Controlling the aperture effectively affects the need to control ISO and shutter speed, which the system can adjust using metering. If you’re shooting in manual mode and you keep the aperture wide open along with high ISO and slow shutter speed, the image could be over-exposed. Thus, the right rhythm among ISO, aperture, & shutter speed is necessary to produce a good image.

Depth of Field & Aperture

The depth of field (DOF) can be defined as the distance between the nearest and farthest points that are in focus. In other words, if your image has a blurred background, then the image has a shallow DOF. Whereas if your image is sharp from the foreground to the background, it has a deeper DOF.

Shallow Depth of Field - Understanding Aperture in Photography
Camera: Canon EOS 7D | Lens: EF100mm f/2.8L Macro IS USM
100mm · ƒ/8.0 · 1/100s · ISO 250

Now, let’s take a look at how aperture controls the DOF. When the aperture is wide open, we get a shallow DOF, and inversely, we get a deep DOF when the opening is small. It is evident that the aperture is directly related to the DOF of an image. Photographers manipulate this relation to make artistic photos.

Deep Depth of Field (DoF)- Understanding Aperture in photography
Camera: Nikon D5000 | Lens: 12-24 mm f/4.0
14mm · ƒ/11 · 1/8s · ISO 100

Aperture & F-Stop

Aperture is measured in f-stops or t-stops. Let’s talk about f-stops first. The “f” in f stops refers to the focal length of the lens. The maximum aperture is always mentioned in the lens model name. Let’s say we are using a Sony FE 70-300mm F4.5-5.6 G OSS lens. The name implies that the lens supports up to f/4.5 stop at 70 mm, and f/5.6 stop at 300mm. Most photographers watch out for the maximum aperture of a lens, as it defines how much light the lens can capture. Note that there are higher-end zoom lenses in the market that are capable of maintaining the maximum aperture throughout the focal range, for example: Sony FE 70-200mm f/2.8 GM OSS.

Now, let’s learn to read f stops. Aperture is mentioned as f/x.x or f.x.x. f.3.5 is read as 1/3.5th the focal length of the lens. This value solves for the diameter of the diaphragm at that focal length. A wider aperture has a larger diameter, and since it is a fractional value lower denominator value refers to a wider opening. In effect, f/3.5 is larger than f/5.6 in the above case. It is confusing as an amateur but it gets better on the way once you start taking photos.

f-stop valuef/2f/5.6f/22
ApertureWideModerateNarrow
Depth of FieldShallowModerateDeep
A general idea on how DOF is dependent on f-stop.

T-Stop vs F-Stop

The “t” stands for transmission in t-stop. Unlike f-stop which is is the measure of the openness of the aperture, t-stop accurately represents the light entering inside the lens. In other words, t-stop is the measure of the transmittance of light through the lens at any given f-stop. Precisely, T-stop value of a lens at an f-stop is equal to the ratio of the f-stop to the light transmittance ratio of the lens.

A great downside to f-stop: Let’s assume we have three “f/2.8” lenses of different focal lengths from different manufacturers. There is a great probability that all of these 3 lenses would open the iris at different diameters at the same f/2.8 aperture. As a result, these lenses would capture different amounts of light even being at the same f/2.8 aperture. Now Imagine you’re working on a big & expensive project where you’re switching these lenses between shots. How big of a nightmare it would be in post-production to correct the inconsistencies between shots in terms of lighting, DoF, etc.

Therefore t-stops are used as the standard for lenses in filmmaking. Note that the t-stop calculation is very expensive and they only reduce light transmittance by 1-3rd stop max. T-stop is predominant in the filmmaking industry, whereas f-stop is the standard in still photography.

Aperture and Lenses

After reading about f-stops and depth of field, you can understand the importance of aperture in producing photos. A wider f-stop like f/2.8 or f/2 is suitable if you have to shoot a portrait or creatively use DOF in landscape photography as well. This allows the photographer to catch the subject with precise focus and blurs out the background. Prime lenses offer a wider aperture than the zoom lenses when compared at the same price. A prime lens, like Sony FE 24mm F1.4 GM, has an aperture that opens extremely wide up to f/1.4. However, we cannot zoom in or using a prime lens. A prime lens has a fixed focal length.

Unlike prime lenses, most zoom lenses offer variable focal length lenses. But, the complexity of zoom lenses limits wider apertures. Due to the large number of elements, these lenses are bulky and cost a lot. Zoom lenses are suitable for sports, wildlife, or other types of photography where you cannot reach close enough to the subject. A photographer should choose the right lens for the right situation, and with the right camera settings.

Things to Remember

Mastering aperture control can help you make extraordinary photos. There are some things every photographer needs to know about aperture.

Loss of Sharpness

Extreme narrow apertures can result in sharpness loss in the image. As the aperture closes, light from different places interacts within the limited opening and ends up producing a sharpness loss due to diffraction. The key is to find the correct f-stop value that delivers sharpness and required depth of field. f/5.6 is defined as a medium aperture.

Relating f-stop to the amount of light

It is crucial to understand how a higher f-stop controls light, or by what amount the light decreases with each stop. If a lens opens up to f/5.6, switching to the next stop will decrease the amount of light by half. Going down each stop reduces the light by half following a pattern. This can help the photographer understand how to tweak other options to capture the right moment.

Aperture is an important aspect of photography. Mastering it, along with other options like ISO, & shutter speed will help you capture extraordinary photos. We recommend you take photos at different apertures in different lighting and different subjects so that you get a better idea at a practical level.

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Adarsh R

A blend of a photographer, a movie lover, and now a writer.