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Lens Speed

Johnmcl7

Distinguished Member
Because they let lots of light in which means higher shutter speeds and therefore 'faster'

*runs before someone beats me down for my overly simplistic explanation*

John
 
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senu

Distinguished Member
Hi all,

Just a simple question. Why do people call lenses with small f stops fast lenses?

Thanks,

Sachin

As above really small f stops mean wide aperture which translate to more light allowed in, and therefore usable in lower light conditions
Im also not totally sure of the origin but at a guess it may refer to " fast" shutter speed being usable with such lenses ( you would need slower shutter speed in order to expose correctly if the light going through the lens to the sensor was limited)
High ISO film ( more light sensitive ) was similarly called fast film
 
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RobDickinson

Well-known Member
Because they let lots of light in which means higher shutter speeds and therefore 'faster'

*runs before someone beats me down for my overly simplistic explanation*

John

As good an explanation as any.

An f5.6 lens is a stop slower than an f4 lens which is a stop slower than an f2.8 lens.

So an f2.8 lens lets in 4 times more light than an f5.6 lens, so you can use 4 times quicker shutter speeds.
 

senu

Distinguished Member
As good an explanation as any.

An f5.6 lens is a stop slower than an f4 lens which is a stop slower than an f2.8 lens.

So an f2.8 lens lets in 4 times more light than an f5.6 lens, so you can use 4 times quicker shutter speeds.

....and achieve similar exposure with the same available light;)
 

RobDickinson

Well-known Member
Yep.

A fast lens also lest you control the depth of field more, f4 to f2.8 also halves the depth of field allowing more subject isolation.


Ofcourse this can work against you as much as for you.... :D
 

Faldrax

Well-known Member
You'll also see your bank balance reducing faster when you buy them :laugh:
 

KyleS1

Distinguished Member
I'm surprised no one has recommended reading 'Understanding Exposure' yet. :)
 

shotokan101

Banned

KyleS1

Distinguished Member
Maybe because it is understanding about ISO and aperture which are fundamentally what makes the lens fast.
 

senu

Distinguished Member
Besides what possible impact could lens speed have on exposure ? :confused:

JIm :devil:
Jim.. behave yourself !:lesson:
Maybe because it is understanding about ISO and aperture which are fundamentally what makes the lens fast.
Exposure is affected by lens speed, ( or at least aperture chosen, and wider apertures are obviously better able in lower light) but the ISO and shutter speed selected are independent variables
Although a high ISO may make it fast, but unlike say, IS . ISO selection is not part of the lens .
Fast lenses are also responsible for overexposure if not used correctly.
Im sure the OP would benefit from reading the book but he simply asked why the lenses were so called
 
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KyleS1

Distinguished Member
I know, I was mentioning it in jest. :)
 

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