fast lens

[mdadams]

When someone says they have a fast lens, what exactly do they mean by that.

Mdadams

[Lee Yuan Sheng]

Well, basically, a fast lens is a lens with a large aperture (normally f/2.8 or larger). The number is derived from the ratio of the focal length of the lens to the diameter of the aperture, as such, the number you see is the denominator, and so, the larger the number, the smaller the aperture (eg, f/2 represents a larger aperture than f/8).

Large apertures mean that more light is allowed to strike the imaging sensor at any point in time. This means that the shutter can be opened for a shorter time, or faster time, than it would be needed for a smaller aperture to achieve the same level of exposure. Hence, large aperture lenses are fast lenses, since shorter shutter speeds are needed when using such lenses.

[backpackerx]

You'll also notice that the faster the lens, the higher the price. Fixed lenses are normally pretty "fast" as well as some shorter zooms but where it really pays off is in the telephoto lenses. I have a consumer Canon EOS 75mm-300mm lens that is very reasonably priced but it only stops down to 4.0 and at full zoom it can only open up to 5.6. This means that unless I have fast film (400+) I can only reliably use the full zoom in moderate to bright conditions without a tripod to steady it for the slow shutter speeds. The Pro versions of these lenses can achieve super far zooms with large aperatures but are also $$$$$$$.

[Gary Sheynkman]

Well, basically, a fast lens is a lens with a large aperture (normally f/2.8 or larger). The number is derived from the ratio of the focal length of the lens to the diameter of the aperture, as such, the number you see is the denominator, and so, the larger the number, the smaller the aperture (eg, f/2 represents a larger aperture than f/8).

Large apertures mean that more light is allowed to strike the imaging sensor at any point in time. This means that the shutter can be opened for a shorter time, or faster time, than it would be needed for a smaller aperture to achieve the same level of exposure. Hence, large aperture lenses are fast lenses, since shorter shutter speeds are needed when using such lenses.

In english..... :wink:

There is a hole that lets light to shine on the sensor in the camera that captures the image. The bigger the hole the faster the lens is!!! :lol:

:wink: 8)

[Suhit Gupta]

All the above is correct, however, as a side-note, I have also seen references to speed of lenses in terms of time taken to focus and/or zoom.

Suhit

[Lee Yuan Sheng]

In interchangeable lens SLRs, the term will mean the aperture of the lens, since almost all of the lenses are operated mechanically ie. there's no motor driving the zoom ring. Fast focusing will be referred to as "fast AF".

[Gary Sheynkman]

In interchangeable lens SLRs, the term will mean the aperture of the lens, since almost all of the lenses are operated mechanically ie. there's no motor driving the zoom ring. Fast focusing will be referred to as "fast AF".

here I go again...lemme get the dictionary:

:lol:

courtesy of dpreview.com
(and no...i dont link)
http://home.comcast.net/~sheynkman/aperture.gif