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Fast Lens; What is fast lens?
Topic Started: Feb 29 2008, 02:26 PM (2,372 Views)
yannh
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Alpha Sniper Member
Hi,

I heard about the term fast lens / slow lens rather often. What's the meaning of that? Any definition?
Sony Alpha.
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Birder
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Alpha Master Jedi Mita Memori-san 鳥の男 clkoh

Fast lens is usually associated with lenses that have wide aperture, example F1.4, F1.8 or F2.8. This usually make the lens bright and as a result, make the autofocus fast. Hence fast lens.

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jasahchun
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路人甲
yup yup, it's pretty much like what bro clkoh said..

some classic examples are..
Minolta 50mm f1.4
Minolta 50mm f1.7
Sony 50mm f2.8 Macro (Minolta 50mm f2.8 Macro)
Sony 100mm f2.8 (Minolta 100mm f2.8 Macro) --> it's fast right? correct me if i'm wrong!
Tamron 17-50mm f2.8
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Ming
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90609866
100mm f2.8 where got fast la ... <_< But its super sharp on focus .

To me i see fast lens as f2.8 & below . Big & Bright , Fast & Sharp !
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jasahchun
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路人甲
alpha-ming9866
Feb 29 2008, 02:50 PM
100mm f2.8 where got fast la ... <_< But its super sharp on focus .

To me i see fast lens as f2.8 & below . Big & Bright , Fast & Sharp !

but i suppose f2.8 is all fast ba.. correct me if i am wrong :lol:

i prefer f2.8 and below of coz.. but f2.8 is fast enough for me day and night.. ;)
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Birder
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Alpha Master Jedi Mita Memori-san 鳥の男 clkoh

Macro lens although have wide aperture, is slow in the af due to the gearing design. Macro lenses the gearing is built such a way when in manual focus the you can do minute changes with slight turn. How to explain? .. It basically needs you to turn more of the focus ring, to focus, as compared to normal lens.

So macro lens are slow lens actually.

Some lenses have wide aperture and yet have very fast af mechanism. Example Minolta 80-200/f2.8 or Sony/Minolta 70-200/f2.8. Although there are 3rd party lenses with same aperture and focal length, the af speed difference can vary very much. But of course these "fast' lenses are still fast compared to say 80-200 F4.5-5.6.
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Ming
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My 28-70mm f2.8 got slight problem focusing in the dark lit area like last evening in the Fashion House ... Some disappointment on this lens there . A bit too slow on the focusing ... :sweat:

So fast lens even like this 28-70mm f2.8 may not be that fast afterall ... Wonder how will that CZ24-70mm compared to this again ... :P
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DewaKarma
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Beauty lies in the eyes of its beholder
ahh...cz 24-70 will outrun ur G interms of speed anytime..SSM hor...hehehe..
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Tomato
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fast lens is more associated with aperture, smaller aperture means you can use faster shutter speed hence it is faster in terms of shutter speed. hope this clear out the doubt.

50mm f1.7 is consider normal, but 50mm f1.4 is consider fast
70-210 f4 is consider normal, but 70-200 f2.8 is consider fast
600mm f8 is consider slow, but 600mm f4 is consider fast lens.
like wise 35mm f2.8 is consider normal but 35mm f1.4 is consider fast lens.

It is relative, it is not how fast the lens focus or the focus motor speed which make a lens fast lens.

Regards,

Hart
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Birder
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agetan
Feb 29 2008, 04:04 PM
fast lens is more associated with aperture, smaller aperture means you can use faster shutter speed hence it is faster in terms of shutter speed. hope this clear out the doubt.

Hart, you mean smaller F numbers? Otherwise should be wider aperture. :D

Generally there are 2 things here for fast lens - High shutter speed (Wide aperture) and Fast AF.

Ming, 28-70G is known for it's slow AF. It's actually a wonderful lens for sharpness, colour and bokeh.
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