F-numbers: The Essential Guide to Aperture and its Impact on Photography

How aperture affects depth of field, exposure, and overall image quality

As photographers, understanding f-numbers and their relationship with aperture is essential to capture images with proper exposure to achieve the desired creative effects. In this comprehensive guide, we will delve into the concept of f-numbers, explain their importance in determining aperture size, and discuss how they affect depth of field, exposure, and overall image quality.

A little bit of math

The f-number (also known as the f-stop) is a dimensionless value that represents the ratio of a lens’s focal length to the diameter of the aperture opening. It determines the size of the aperture, which in turn affects the amount of light entering the camera and reaching the film or digital sensor.

The f-number is calculated using the following formula:

f-number = focal length / aperture diameter

For example, a 50mm lens with an aperture diameter of 25mm would have an f-number of 2 (50mm / 25mm = 2).

F-number and Aperture

The f-number is inversely proportional to the aperture size (in the formula above, we divide the focal length by the aperture diameter). This means that a lower f-number corresponds to a larger aperture opening, while a higher f-number represents a smaller aperture opening. Larger apertures allow more light to enter the camera, while smaller apertures admit less light.

A lens is like a pipe whose diameter is determined by the aperture. This means that a larger aperture (lower f-number) will allow more light to reach the film or sensor, resulting in a brighter image, while a smaller aperture (higher f-number) will limit the amount of light, producing a darker image. (A lighter or darker image results when keeping all other camera settings equal).

Aperture settings are commonly referred to in terms of “stops,” with each full stop representing a doubling or halving of the light entering the camera. Typically, lenses feature full-stop increments, such as f/1.4, f/2, f/2.8, f/4, and so on. They also come with adjustments in 1/3 or rarely 1/2 stop increments.

The f-stop scale can be obtained by dividing or multiplying by the square root of two (approximately 1.41). This is because we are decreasing or increasing the amount of light passing through the lens opening by 2. Below is an intuitive explanation where for simplicity, I am using a square. Don’t be scared by the math.

You can do the same exercise for a circle. The area of the circle is equal to the square of the radius times pi (S = r²π).

Depth of Field

Depth of field (DoF) refers to the range of distances within a scene that appears acceptably sharp in a photograph. It is influenced by the aperture setting, focal length, and distance between the camera and the subject. I recently wrote about it.

Depth of Field Demystified: A Comprehensive Guide to the Physics and Applications of Depth of…
Delving into the science behind depth of field

The f-number has a direct impact on the depth of field in an image. A larger aperture (lower f-number) will result in a shallower depth of field, with only a small portion of the scene appearing in focus. Conversely, a smaller aperture (higher f-number) will produce a deeper depth of field, with more of the scene appearing in focus. There are figures showing the way light rays pass in the article above.

Understanding the relationship between f-number and depth of field is crucial for achieving desired creative effects in photography. For example, portrait photographers often use large apertures (low f-numbers) to create a shallow depth of field that isolates the subject from the background, while landscape photographers may opt for small apertures (high f-numbers) to ensure sharp focus throughout the entire scene.

Image Quality

Diffraction

Any time light rays pass through an opening, a phenomenon known as diffraction occurs, which causes light to scatter as it passes through the aperture, resulting in a loss of sharpness in the image. At very small apertures, this effect can become more pronounced, diminishing the overall image quality. You can test it yourself by taking a series of photos at, e.g., f/2, f/5.6, f/11 and f/16. Zoom in to 100% to see the difference clearly.

Optimum f-number

Most lenses have an optimum f-number at which they perform best in terms of sharpness. This sweet spot typically lies within the middle range of the lens’s available apertures, often around f/4 to f/8. Diffraction is least significant when the aperture is wide, however, lens aberrations show up more prominently.

Vignetting

Vignetting refers to a gradual decrease in brightness towards the edges of an image, resulting in dark corners. It can be influenced by the f-number, with some lenses displaying more noticeable vignetting at certain apertures. Vignetting is most apparent at larger apertures and becomes less noticeable as the aperture is reduced. It is most prominent in fast wide-angle lenses where light rays enter at shallow angles.

Lens Design

Lenses with large maximum apertures (low f-numbers) are often referred to as “fast” lenses. These lenses can gather more light, enabling faster shutter speeds and better low-light performance. Fast lenses are generally more expensive and heavier than their slower counterparts, as their design requires more and larger glass elements. Furthermore, there are more aberrations that the manufacturers have to tackle, often by using special materials.

Some zoom lenses have a variable maximum aperture, meaning the largest available aperture changes as the focal length is adjusted. For example, a lens may have a maximum aperture of f/3.5 at its widest focal length and f/5.6 at its longest focal length. These lenses tend to be more affordable than those with a constant maximum aperture throughout the zoom range because they tend to have a simpler construction.

High-quality zoom lenses may feature a constant maximum aperture across their entire focal length range, such as f/2.8 or f/4. These lenses provide more consistent exposure settings and depth of field control, as well as better low-light performance. However, they are often larger, heavier, and more expensive than variable aperture zoom lenses.

Conclusion

F-numbers play a crucial role in the art and science of photography, affecting aperture size, exposure, depth of field, and image quality. A thorough understanding of f-numbers and their relationship to these factors is essential for photographers to achieve the desired creative effects and optimal image quality in their work. By mastering the concept of f-numbers, photographers can gain greater control over their camera settings and unlock the full potential of their lenses and photographic vision.

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