Fixed aperture lenses - depth of field suffers?


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nsim76

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May 7, 2008
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#1
Hi,
As a newbie, I have a question regarding the fixed aperture lenses like the Tamron 17-50mm f2.8

I understand that bigger the aperture smaller is the focal plane. So, with a fixed aperture of say f2.8, wouldn't it be difficult to take landscape pictures with bigger depth of field? Also, if you want fuzzy motion with slow shutter speed, the exposure will be too much?

If yes, why are such lenses more expensive and popular because it would be less versatile than lenses with variable apertures?

I also noticed a lot of bros here carrying Tamron 17-50mm f2.8 with a 50mm prime lens f1.8. Wouldn't that be overlapping considering f1.8 is generally un-usable?
:dunno:
 

Rashkae

Senior Member
Nov 28, 2005
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#2
Erm. Who on earth said it's fixed at f/2.8?

I think you need to read up a bit more. In the example of the tamron, it means that it is able to stay at f/2.8 thoughout the focal range, but of course you can stop it down all the way up to f/22 if you want.
 

Rashkae

Senior Member
Nov 28, 2005
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#3
Wouldn't that be overlapping considering f1.8 is generally un-usable?
:dunno:
If you ever actually learn how the aperture works, and the fact that it's not a fixed, but just the maximum value, you may one day appreciate the great out-of-focus effects that large aperture lenses can present you.
 

grainpixel

New Member
Oct 10, 2006
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#4
lets say, a photo taken with a tamron 17-50mm at 50mm at f/2.8 will still look different from a prime lens, say canon 50mm f/1.8 set at f/2.8

both lenses are constructed differently, in terms of blades and glasses, and therefore the end results are different.
 

nsim76

New Member
May 7, 2008
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#5
okie... got it... sorry, i took the 'Fixed Aperture' term literally!! lol
Thanks for all your replies....
 

darrrrrrrrrr

Senior Member
Sep 19, 2006
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#6
i think lenses like the tamron 17-50, canon/nikkor 17-55, 70-200, etc are referred to as 'constant aperture' rather than 'fixed aperture'

Mirror lenses are fixed aperture lenses though, commonly fixed at f8.
 

catchlights

Moderator
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Sep 27, 2004
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#7
yes, is constant aperture.

fixed aperture usually only seen on mirror lens.


when you have a lens with a maximum aperture of f2.8 or f1.8, it means you can use f2.8 or f1.8 on this lens, does not mean can only shoot at f2.8 or f1.8, or die die must shoot at f2.8 or f1.8.
 

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