INFLUENCE OF LENS APERTURE OR F-NUMBER

INFLUENCE OF LENS APERTURE

OR F-NUMBER

The aperture range of a lens refers to the amount that the lens can open up or close down to let in more or less light, respectively. Apertures are listed in terms of f-numbers, which quantitatively describe relative light-gathering area (depicted below).

Note: Above comparison is qualitative; aperture opening (iris) is rarely a perfect circle,due to the presence of 5-8 blade-like lens diaphragms.

Note that larger aperture openings are defined to have lower f-numbers (often very confusing). These two terms are often mistakenly interchanged; the rest of this tutorial refers to lenses in terms of their aperture size. Lenses with larger apertures are also described as being "faster," because for a given ISO speed, the shutter speed can be made faster for the same exposure. Additionally, a smaller aperture means that objects can be in focus over a wider range of distance, a concept also termed the depth of field.

When one is considering purchasing a lens, specifications ordinarily list the maximum (and maybe minimum) available apertures. Lenses with a greater range of aperture settings provide greater artistic flexibility, in terms of both exposure options and depth of field. The maximum aperture is perhaps the most important lens aperture specification, which is often listed on the box along with focal length(s).

An f-number of X may also be displayed as 1:X (instead of f/X), as shown below for the Canon 70-200 f/2.8 lens (whose box is also shown above and lists f/2.8).

Portrait and indoor sports/theater photography often requires lenses with very large maximum apertures, in order to be capable of faster shutter speeds or narrower depth of fields, respectively. The narrow depth of field in a portrait helps isolate the subject from their background. For digital SLR cameras, lenses with larger maximum apertures provide significantly brighter viewfinder images-- possibly critical for night and low-light photography. These also often give faster and more accurate auto-focusing in low-light. Manual focusing is also easier because the image in the viewfinder has a narrower depth of field (thus making it more visible when objects come into or out of focus).

Typical Maximum Apertures Relative Light-Gathering Ability Typical Lens Types

f/1.0 32X Fastest Available Prime Lenses(for Consumer Use)

f/1.4 16X Fast Prime Lenses

f/2.0 8X

f/2.8 4X Fastest Zoom Lenses(for Constant Aperture)

f/4.0 2X Light Weight Zoom Lenses or Extreme Telephoto Primes

f/5.6 1X Minimum apertures for lenses are generally nowhere near as important as maximum apertures. This is primarily because the minimum apertures are rarely used due to photo blurring from lens diffraction, and because these may require prohibitively long exposure times. For cases where extreme depth of field is desired, then smaller minimum aperture (larger maximum f-number) lenses allow for a wider depth of field.

Finally, some zoom lenses on digital SLR and compact digital cameras often list a range of maximum aperture, because this may depend on how far one has zoomed in or out. These aperture ranges therefore refer only to the range of maximum aperture, not overall range. A range of f/2.0-3.0 would mean that the maximum available aperture gradually changes from f/2.0 (fully zoomed out) to f/3.0 (at full zoom). The primary benefit of having a zoom lens with a constant maximum aperture is that exposure settings are more predictable, regardless of focal length.

Also note that just because the maximum aperture of a lens may not be used, this does not necessarily mean that this lens is not necessary. Lenses typically have fewer aberrations when they perform the exposure stopped down one or two f-stops from their maximum aperture (such as using a setting of f/4.0 on a lens with a maximum aperture of f/2.0). This *may* therefore mean that if one wanted the best quality f/2.8 photograph, a f/2.0 or f/1.4 lens may yield higher quality than a lens with a maximum aperture of f/2.8.

Other considerations include cost, size and weight. Lenses with larger maximum apertures are typically much heavier, larger and more expensive. Size/weight may be critical for wildlife, hiking and travel photography because all of these often utilize heavier lenses, or require carrying equipment for extended periods of time.