What is lens speed?

The speed of a lens is determined by size of the lens opening known as aperture. The aperture controls the amount of light that reaches a digital camera sensor .

F StopsThe diameter of an aperture is measured in f-stops. A lower f-stop number opens the aperture to admit more light onto the sensor. Higher f-stop numbers close the lens opening so less light gets through. A lens with an f-number of f/2.8 has a larger aperture than one with an f-number of f/8.

The aperture, or aperture range, is indicated on the front of a lens.

Fast lens

A fast lens is one with a large maximum aperture. The larger the aperture, the faster the lens.

A lens is called fast because the larger aperture lets more light pass through during a given time span. When more light falls upon a subject, pictures can be shot with faster shutter speeds.

Lens apertureThe aperture of a lens can be reduced if desired by the user of a camera with manual and/or semi-automatic controls. The process of reducing the aperture size is called stopping down.

It’s important to note that a lens is usually not at its sharpest when wide open, nor when stopped down too much.

One interesting effect of using a large aperture is it greatly reduces the Depth of Field in an scene. This is very useful to isolate a subject from the background such as when taking portrait and macro shots. A photographer desiring a large depth of field (for instance when photographing landscapes) will have to “stop down” the lens by using a smaller aperture.

Photographers who do a lot of low light photography prefer fast lenses.

Slow lens

A slow lens is one with a small maximum aperture, such as F/4.5. A slow lens lets less light pass through towards the sensor, and exposure times will be longer.

Longer zoom lenses are generally not very fast. They are slower at the telephoto end of zoom and faster at the wide end.

A slow lens delivers a deeper Depth of Field. The same is true for a fast lens sopped down. A deeper Depth of Field can be desirable depending on the visual effect a photographer wishes to capture in a give scene.

Slow lenses are less expensive than fast ones.

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3 Responses

  1. Gail Bjork says:

    80-200mm lenses are typically used for photographing birds, wildlife and sports. However if you shoot at the shorter end of the zoom (eg. 80mm, 100mm, 135mm), it can be used for portraits. When shooting portraits, use a wide aperture (small f#) to blur the background so the subject stands out. Here is some information on Depth of Field as it relates to the size of the aperture. Getting good photos takes practice, so experiment a lot until you get a feel for settings and their effects on an image. Stand where ever you need to get your photo, within reason of course. I never worry if other people think I look silly when taking shots. You may find this guide on dslr lenses of interest.

  2. smitty says:

    how do i take good pictures with a  nikon lens of 80-200 f2.8 and where do i stand or where and how do i move , and not look silly, and get good pictures

  3. christan says:

    It was helpful for me to learn that the f/stop numbers were really a ratio: the focal length is 1 and the f/stop is a fraction of that 1. For instance, a 100mm lens at f/2 has an effective aperture of 1/2 the focal length, or 50mm in this case.

    Helped me, maybe it’ll help someone else.