Light intensity (Lens speed) / Maximum diaphragm diameter

A lens’s quality is not only determined by its image performance. Another criterion for quality is lens speed. Lens speed is the largest diaphragm diameter that a lens can have.

The larger the diameter is, the higher the light intensity of the lens is.  Alongside the focal length, it is standard information, e.g. f / 2.8 or 1/2.8. Often only the denominator, i.e. the value after the fraction line, is cited e.g. diaphragm 2.8. The smaller the denominator is, the more light intensity there is. On most zoom lenses you can find two values for the maximum diaphragm diameter, e.g. 1:2.8-3.5.  The first value is the maximum diaphragm diameter for the lens’s shortest focal length and the second value determines the lens speed for its longest focal length. 

How is lens speed calculated?

Lens speed = Largest effective diameter / Focal length

As shown in the formula above, focal length has an influence on the lens speed of a lens. This results in typical lens speeds for certain focal lengths. Here we differentiate between whether it is a zoom lens (variable focal length) or a fixed focal length. The term “fast lens“ is not defined, therefore the following tables should only be used as a reference to evaluate the light intensity of a lens.

Examples for typical focal lengths

Focal length

Examples for fast lens speed

50 mm
50 mm macro

1/1,2 or 1/1,4
1/2,8

100 mm

1/2,0 or 1/2,8

200 mm

1/2,8

Examples for typical zoom lenses

Focal length range

Examples for fast lens speeds

Standard zoom
(3 to 4x)
Focal length range
ca. 28 mm to ca. 135 mm

Fixed light intensity 1/2.8
Variable light intensity 1/2.8 - 1/3.5 (1:2.8–3.5)

70 mm to 200 mm

Fixed light intensity 1/2.8
Variable light intensity 1/2.8 – 1/3.5 (1:2.8–3.5)

Advantages and disadvantages of high light intensity

  • Advantages of lenses with high intensity
    • Fast shutter speeds are available. They could be relevant in e.g. the following situations
      • Capturing motion e.g. sports
      • In poor lighting conditions
      • When using long focal lengths as the risk of blurring is increased and faster shutter speeds are required for hand held shots
    • Whenever a shallow depth of field is required
      • E.g. Portraits
    • Brighter viewfinder on single-lens reflex cameras
    • Fast, precise autofocus in poor lighting conditions
  • Disadvantages of lenses with high light intensity
    • Increased light intensity requires larger lens diameters for the same focal length
      • This results in (for comparable focal lengths)
        • More weight
        • Larger lenses
        • Increased cost
    • The filters for the larger lenses that are required are also generally much more expensive

As you can see, fast lens speed does not only have advantages. The high purchase price is not worthwhile for every photographer. For many a photographer as well as the price, the heavy weight is also a deciding factor. Additionally, for most subject situations a limited amount of light intensity is sufficient. For some photographers, the light intensity (combined with the high price) is a sign of prestige.

Note that…
… with a larger lens diameter the depth of field decreases. This effect is used consciously in e.g. portrait photography.  Especially beginners should be aware of the problematic of a shallow depth of field. On many lenses image performance is reduced (poor resolution, more vignetting) if the diaphragm is opened to the maximum. Therefore it is recommendable to stop down one to two f-stops on lens. 

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