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Focal Length, Aperture and F-Stop summarized


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Learn digital photography - a summary of

  • Focal Length
  • Aperture
  • Focal Ratio (f-numbers or f-stop)

Related Articles

Digital photography exposure

Shedding the light on F-numbers

Angle of view, magnification and crop factors.

Depth of field explained

What equipment do I need for my style of photography

Taking creative control over my digital photos

Photo by Gendel Khan

Focal lengths, Aperture and F- stops summarized

Your camera will either have integral or interchangeable lenses.

In either case, you will come across information about the focal length and aperture.

E.g. Canon 70-200mm F/4 or Sigma 18-55mm F/4–5.6

  • But what does this information mean?
  • What does this information tell you about how the lens will perform and what type of photography it’s best suited to?
  • And why is this so?

This and the related "learn digital photography" articles will explain all, in simple easy steps.


Learn digital photography - Focal Length


E.g Canon 17-55mm f/4

The focal length of a lens (in relation to the image sensor size) determines angle of view and therefore the apparent magnification of the image. The focal length also affects the perspective of the image. (The wider the angle of view the greater the perspective distortion)


Focal lengths are traditionally expressed in mm.

As focal length and sensor size are proportional, cameras with smaller sensors sometimes quote focal lengths as “35mm” equivalent instead of actual focal length. This refers to how long the focal length of a lens on a full frame camera (based on the traditional 35mm film format) would have to be to provide the same angle of view.

Photo by Velo Steve

Learn digital photography - Aperture

photo by Nayu Kim


E.g. Canon 70-200mm f/4

The aperture is an adjustable approximately circular hole within the lens that determines the amount of light that can pass through to the images sensor.

Aperture also affects depth of field. (The larger the aperture the shallower the depth of field)

Focal Ratio Number - (also known as F-number or F-stop

The degree by which a particular aperture controls light reaching the image sensor, is in proportion to the focal length. This is the Focal Ratio - but is more commonly called The f-number or f-stop

F- Numbers are expressed as Focal length mm/Aperture Diameter mm

Therefore lenses of different focal length but equivalent F-numbers will provide the same exposure.

E.g. Exposure from a 200mm f/4 lens and a 50mm f/4 lens will be the same.

photo by C H Meredith

There are two formats for expressing F numbers . For example f/4 can also be written f 1: 4

Because apertures expressed as F-numbers are fractions - the larger the f-number, the smaller the aperture and therefore lower the exposure.

E.g. An aperture at f/4 is considerably larger than an aperture at f/16


Learn digital photography - Article Summary

This article provided a quick summary of the key facts and definitions of focal length, aperture and focal ratio.

But dd not fully explain WHY the focal length and aperture work in this ratio, nor how to APPLY this knowledge in a practical manner when taking photographs.

To gain a more in depth understanding of how the focal ratio works please continue by reading

shedding the light on F-numbers

For advice on how to apply this knowledge and understanding effectively, both in taking photos and in the choice of equipment best suited to particular photography styles and themes, please read

What equipment do I need for my style of photography and

Taking creative control over my digital photos




Useful links from "learn digital photography"

go to JARGON BUSTER digital photography terms explained

Digital photography exposure

Shedding the light on F-numbers

Angle of view, magnification and crop factors explained

Depth of field explained

What equipment do I need for my style of photography

Taking creative control over my digital photos

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