Start with maximum aperture
A fast lens usually means a lens with a large maximum aperture. The name relates to using a shorter exposure under suitable conditions; it does not describe autofocus speed or editing pace. Nikon's lens and focal-length guide connects large apertures with low-light photography.
At the same focal length, f/1.8 is a larger maximum aperture than f/4. Maximum aperture describes an available limit, rather than a requirement to shoot every scene wide open. Consider light, subject movement, the area that must remain clear and the intended image together.
For video, choose frame rate and exposure time for the action first, then consider aperture, lighting and the other exposure conditions. Changing exposure time simply to use a fast lens may give adjacent shots different motion characteristics. A larger aperture does not automatically add usable detail to an already correctly exposed image.
How f-number works and what limits aperture
The basic relationship is N = f / D, where f is focal length and D is entrance-pupil diameter. Printed page 13 (PDF page 16) of Canon's lens white paper distinguishes the entrance pupil from the physical stop. It is the image of the stop seen from the front of the lens, so filter size or front-element diameter is not a substitute.
For simplified examples, a 50 mm focal length and 4 mm entrance pupil give N=12.5; 85 mm and 50 mm give N=1.7. These calculations explain a ratio. They do not establish that a lens of a particular external size has that aperture or delivers usable image quality.
Maximum aperture depends on the optical design, mechanical construction and imaging requirements. Removing aperture blades does not establish unlimited aperture. With finite positive focal length and pupil diameter, the formula does not give f/0. An extremely small calculated ratio is not proof of a manufacturable lens.
Check the specific model's official aperture, focal range and dimensions. One lens cannot represent a manufacturer's entire range. For a zoom, also check and test whether the available aperture changes across the range.
Compare the image and the operating conditions
With other conditions held constant, a larger aperture reduces depth of field. Focal length and focus distance also matter; see Canon's depth-of-field explanation. Separating a person from the background can help, while product text, hand movements or two faces may no longer remain clear together.
Compare weight, operation and cost as well. Assess wide-open edge detail, coloured fringes, flare and focus changes with the actual lens and footage. Neither “all lenses are unusable wide open” nor “one stop down is always best” is a general conclusion. Aperture controls, focus support, rigging and lens-change time also affect the shoot.
Imagine a presenter walking while showing a product. Shallow focus may emphasise the person while losing the product. Compare stopping for the demonstration, repositioning the subjects, reducing aperture or changing camera distance before deciding whether a larger maximum aperture is needed. This is a hypothetical example, not a project or equipment test.
Use a test shot to make the choice
Keep the scene and main shooting conditions comparable. Record a short sample containing the intended action and log the lens, focal length, aperture, exposure time and focus method. Include the start, movement and end; a still frame cannot establish usable focus during movement.
Check the presenter, product and necessary text first, then assess whether background separation serves the message. Inspect focus and outlines frame by frame, and play continuously to review action and exposure changes. Compare wide open with other usable settings and identify affected moments.
If lighting, camera position, performance or support must change, test again with those adjustments recorded. Maximum aperture adds an option. The working aperture should serve the information and operating conditions of the shot.
Sources reviewed 3 October 2026. Optical definitions follow the linked official material; video planning and testing steps are editorial guidance, not a lens review or outcome guarantee.