The F-number, often colloquially called the f-stop or its more official name, aperture, is one of the most commonly misunderstood aspects of photography. This art form blends creativity with a fair amount of technical know-how, and understanding this element can be a valuable string to your proverbial bow.
The aperture influences not only the light level reaching your camera sensor, but also profoundly affects the aesthetic qualities of your photos or video footage. When selecting interchangeable lenses for your camera, such as a DSLR or mirrorless camera, it pays off to understand the principles of F-numbers as you begin bolstering your skill set or diversifying your existing setup.
Knowing what the aperture or F-number does for your image can help you significantly in your photography journey. If you’re looking to build a collection of lenses, or diversifying your existing kit whilst focussing on value, exploring the pre-owned market could be a good option. Platforms like MPB offer an excellent resource for high quality, compatible digital camera lenses that work across various camera models, providing filmmakers and photographers alike with high quality gear at an accessible price.
However, if you’re currently only in the early stages of fully understanding all there is to know about camera F-numbers and aperture, then you’ve come to the right place.
What do F-numbers actually represent?
F-numbers essentially represent the aperture size, which is responsible for controlling light entering through the camera, and influencing overall image quality. The aperture itself is the lens opening by which light passes through; the opening can be adjusted and is measured by the f-stop.
The lens’ focal length – as well as the aperture – is expressed by the F-number.
The F-number (F) equals the lens’s focal length (f) divided by the lens aperture diameter (D) expressed in the equation F = f/D.
For example, this means an 50mm lens with a 25mm diameter aperture opening operates at F/2.
In addition to the quantity of light, the aperture affects the depth of field and (usually) the overall sharpness of the image. In lower quality lenses, the improvement in sharpness as the aperture is closed can are pretty dramatic, whereas in high-end lenses it may be barely noticeable.
Common aperture and F-number misconceptions
“Lower f-numbers always mean better quality”
Many assume that the so-called ‘fast’ lenses, with f/1.2 or f/1.4 apertures don’t necessarily produce superior images by default. Most lenses perform best when stopped down from their maximum apertures to the f/2.8-f/4 range (for lenses with f/1.4 apertures, for example), as at their maximum, wider lenses can exhibit vignetting, chromatic aberration, and reduced sharpness.
“Zoom lenses with variable aperture aren’t as good”
Variable maximum aperture zoom lenses (like f/3.5-5.6) are often lauded by videographers who are accustomed to using f/2.8 zoom lenses. These lenses can still offer tremendous value and image quality, particularly for documentary photography, videography, or situations where the added bulk and costs of various aperture lenses aren’t easily justified.
“Aperture priority isn’t useful for video”
Whilst video requires consistent exposure throughout a shot, aperture priority mode can be invaluable, especially during setup and when testing camera lenses. Understanding how your camera’s metering system responds to different apertures helps establish the essential, baseline settings you need before switching to manual control for filming.
The exposure triangle and depth of field
F-numbers work alongside shutter speed and ISO to control exposure, with the three settings forming the aptly dubbed ‘exposure triangle’, a term commonly used among photographers and creatives.
In video, shutter speed is typically fixed (typically 1/50s for 25fps, or 1/60s for 30fps to maintain natural motion blur). This makes aperture and ISO your primary adjustable exposure controls.
When days are brighter, it’s not uncommon to find yourself shooting at f/8-f/11 to avoid overexposure, even if ISO remains unaltered. This necessitates neutral density filters for shallow depth of field effects, which many photographers aren’t always fans of.
Higher ISO settings can impact dynamic range, colour rendition, and post-production flexibility. Choosing lenses with wider maximum apertures provides more options for keeping ISO low whilst maintaining desired exposure levels.
Wider apertures, represented by smaller f-stop numbers (e.g. F/1.6) allow for more light to enter, while narrower apertures, represented by larger f-stop numbers (e.g. F/18) don’t allow as much light. Essentially, wider apertures result in a shallower depth of field, meaning that less of the image is in focus.
F-numbers control depth of field, but other factors matter, such as:
- Focal length
- Subject distance
- Sensor size
Another common misconception suggests that f/2.8 on a 50mm camera lens produces identical depth of field characteristics, irrespective of the camera make, model and setup. In reality, an f/2.8 aperture on a Micro Four Thirds camera provides roughly equivalent depth of field to f/5.6 on a full-frame camera when framing the same subject.
Aperture recommendations for lenses
Every lens exhibits varying optical characteristics throughout its aperture range.
- Wide open (max aperture): Maximum light but often softest sharpness and shallowest depth of field
- f/4-f/8: Typically sharpest performance with best depth of field and aberration control
- f/11+: Maximum depth of field but potential diffraction issues, often requiring higher ISO or longer exposures
Most lenses hit their proverbial “sweet spot” between f/4-f/8, as diffraction effects are more or less avoided.
Choosing the right lens
If you frequently work in low light or require shallow depth of field effects, prioritize lenses with wider maximum apertures (ideally in the f/1.4-f/2.8 range). F/4 lenses offer better value if you’re shooting mostly at daytime with an aim to maintain consistent high quality.
Premium lenses typically use 9 or more individual aperture blades for rounder bokeh (out-of-focus highlights). More budget camera lenses may only use between 5 and 6 blades on average, producing more angular bokeh shapes which aren’t always as appealing.
It’s prudent to grab a handful of lenses that support different conditions, particularly if your photography work isn’t confined to one specific environment or setting. Consider additional lenses as part of your wider assortment of accessories and must-haves as you begin exploring and expanding your body of work.
F-numbers aren’t the easiest to understand, but their influence cannot be ignored or overlooked. Understanding the basis for their applications and how they interact with other exposure elements helps you make better decisions as a photographer, both in your individual, day-to-day activities and as you further your portfolio. Mastering aperture control may take practice, but when done correctly, the technical and creative improvements you can expect will make it well worth the effort.
