When reading some reviews on lenses in cons or pros are noted speed of lens. What does it means when we are talking about analogue cameras without autofocus?


The speed of the lens (independently if its about analogue or digital photography) is the aperture. When you have wide aperture (small number) we talk about fast lens because on the same scene you can use faster shutter speed. For example f2, f1.4 are apertures of fast lens. For other side if you have not so open apertures like f5.6, f8 you have slow lens because on the same scene you must use slow shutter speed.

We talk about maximum aperture of the lens.

And speed of the lens do not refer to how quickly it can autofocus

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  • 3
    good answer, but just to state it explicitly... "speed" of a lens does not refer to how quickly it can autofocus. – osullic Dec 12 '18 at 20:07

The lens acts like a funnel in that it gathers light. The larger the working diameter of the lens, the more light it will gather. In other words, if you are imaging under feeble light conditions, you likely will benefit if the lens you are using has a large working diameter. Conversely, if you are imaging under bright light conditions, you would likely choose to work with a lens with a lesser working diameter.

In the jargon of photography a lens that passes a massive amount of light is termed a “fast” lens. Such a “fast” lens permits the use of a very quick opening and closing shutter. A quick opening and closing shutter is also termed a “fast” shutter.

We must add a qualification: Just because a lens has a large working diameter doesn’t always translate to being a “fast” lens. This is because the amount of light a lens passes to film or digital imaging chip, during the exposure, is also dependent on how much the lens magnifies. In other words a powerful telephoto lens magnifies greatly but this action pays a price as considerable light will be lost. Contrariwise, a wide-angle lens images with low magnification and this action results in a bright image being passes to film or sensor.

In other worlds the power of a lens to magnify is based on its focal length. Long focal length lenses are classified as telephoto and short focal length lenses are called wide-angle.

Because the speed (image brightness) of a lens intertwines the working diameter (aperture) and focal length, we must do some math to make sense of the speed of a lens. We divide the focal length by the working diameter to find what we call the “f-number). Suppose the focal length is 100mm (about 2 inches) and the working diameter (aperture) is also 100mm. We divide 100 ÷ 100 = 1 stated as f/1. This is a very fast lens in that it passes tons of light. If we stop down (reduce) the aperture to 12.5mm the math is 100 ÷ 12 = 8 stated as f/8 (a lens with moderate speed. Stop down to 4.5mm, the lens is now operating at f/22, a slow lens that only allows a petite amount of light to pass to film of sensor.

How about, you study the f-number system and gain some insight into how cameras expose under different levels of light brilliance.

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