What does it mean when u say a lens is fast?


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Sukoo

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#1
Question as above.

Fast in auto focus? :dunno:
 

billpepsi

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#2
Sukoo said:
Question as above.

Fast in auto focus? :dunno:
Usually it means fixed and big (eg: f/2.8) aperture for all focal length eg: Nikkor VR 70-200 f/2.8 or Canon IS 70-200 f/2.8. As for fast auto focus, that will be USM (Canon) or SWM (Nikon).
 

Ah Pao

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#3
"Fast" as it has a large maximum aperture (e.g. F/1.4, F/2, F/2.8, etc.), so you can use a fast shutter speed to freeze motion.

Large aperture = more light gets captured onto the film/sensor = shorter shutter can be used for proper exposure.
 

Sukoo

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Thanks! U guys are wonderful!
 

sulhan

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#6
Hiee..

The term fast could also refer to the lens being able to let in more light (due to large aperture)....hence allowing the exposure time shorter (allowing...faster shutter speeds) for a given desired exposure.

rgds,
sulhan
 

Oxygen

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The smaller the aperture F number.. the shorter the exposure time required, hence faster the shutter speed...

FAST= Fast shutter speed... very important for handheld !
 

Ah Pao

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#10
+evenstar said:
Fast lens = shallower DOF
...which translate to a smaller margin of error in focussing. For example, the AF logic may choose to focus on the neck when you want a sharp focus on the eye. Because of a shallow DOF, the neck area will be sharp but the eye area will be out of focus.

However, it comes in very handy in low-light conditions where sometimes you can get away with a sharp picture (no handshake blur) without using a tripod.

BUT...fast lenses are usually big, heavy and expensive (perhaps the only odd exception is the 50mm prime lens).
 

jj1987

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#11
A good fast lens is the 50mm 1.8 canon btw...........Im not sure what its equal would be in nikon.

A f/2-3 (somewhere between 2 and 3) is going to be a plus when shooting sports. Most all non brand lens are going to be F5 and up, imposisble to shoot sports other than in braud daylight and 400iso or higher.
 

kingpin

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#13
Think fastest lens is the f1.0 like the Leica 50mm f1.0 nocturnal. But I stand to be corrected. Believe there is a f0.8. Think it is a canon lens. Anyone?
 

kingpin

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#18
zekai said:
there is a canon FD 50mm f0.95
Ya did some checks. There is a Canon 50 mm f0.95 for rangefinder Canon 7. Made in the 60's. Probably fastest production lens today is the Leica noctilux and the Canon EF 50 mm f1.0.
 

Ah Pao

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#19
Adam Goi said:
My example of fast lens.

"Wah! You just bought this lens some days ago and now you're selling it? That's fast man!"
:sweat: Teacher!! Wrong context lah!! :sweat:
Fail for your sentence construction test!! :bsmilie:
 

Jed

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#20
Just to complicate things, I personally reckon that fast is a relative thing.

A 50/1.8 I wouldn't consider fast, it's about the slowest 50mm lens you can buy.

OTOH a 400/2.8 is fast, despite the smaller maximum aperture.

Otherwise if you count in absolute terms, nothing slower than f2 should be considered fast. In which case there would be no fast zooms at all, nor any fast long telephotos.

Just speculation. I'm not saying that everyone who considers it in absolute terms is wrong!
 

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