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rbelyell
Joined: 13 Oct 2009
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Location: somewhere in the mountains of central NY
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 Posted: Thu Dec 13, 2012 3:24 pm Post subject: how DOF works with progressive focal lengths-question |
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rbelyell wrote:
so i,m curious about how in fact DOF works as we increase FL of our lenses, relative to each other. what ive concluded is, as we double the FL of our lenses we have to double the aperture to achieve the same DOF.
for example, the DOF achieved by a 25mm lens at 2.0 is approximately the same as a 50mm lens at 4.0 (a 75mm lens at 5.6) and a 100mm lens at 8.0. etc etc. is that approximately correct?
tony
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Himself
Joined: 01 Mar 2007
Posts: 3242
Location: Montreal
Expire: 2013-05-30
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| Posted: Thu Dec 13, 2012 6:06 pm Post subject: |
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Himself wrote:
Did you check with this one
http://www.dofmaster.com/dofjs.html
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rbelyell
Joined: 13 Oct 2009
Posts: 4269
Location: somewhere in the mountains of central NY
Expire: 2014-01-31
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| Posted: Thu Dec 13, 2012 8:12 pm Post subject: |
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rbelyell wrote:
interesting, that scale shows it is aprroximately 4x aperture to each doubling of FL. so a 25mm at f2 equals about the same DOF as 50mm lens at f8! and a 100mm lens at f32! wow! is that true?
tony
_________________
Epson RD1 + Elmarit 21/2.8; Summarit 50/1.5; Summarit 75/2.5; Elmar-c 90/4; Sankyo Komura 135/2.8, Hektor 135/4.5; Braun Paxina 29 6x6; Photax Boyer Paris; Holga 120 Pano
GREAT STUFF FOR SALE:
Contax T
Hasselblad XPan + 45/4, 90/4
Kodak Retina Reflex IV + full set of Schneider Krueznach lenses
Mercury 2 half frame 35mm
Kodak Pro slr/n
Fuji GM670+100/3.5+65/8!
Praktisix 6x6 medium format + ZeissBiometar 120/2.8
Bessa T 101 Anniversary Edition in Navy Blue
Mamiya Six Folder with Zuiko 75/3.5
Adaptall: Tamron SP 28-85 macro
Cameras: Canon IX
PM for more complete descriptions/pix. All in great shape!
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sichko
Joined: 20 Jun 2008
Posts: 2475
Location: South West UK
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| Posted: Thu Dec 13, 2012 9:49 pm Post subject: |
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sichko wrote:
| rbelyell wrote: |
interesting, that scale shows it is aprroximately 4x aperture to each doubling of FL. so a 25mm at f2 equals about the same DOF as 50mm lens at f8! and a 100mm lens at f32! wow! is that true?
tony |
The DOF is given by (see Wikipedia, for example) ...
DOF =(2*s*f*f*N*c*[s - f])/(f*f*f*f - N*N*c*c*[s -f]*[s-f])
where ...
s = subject-lens distance
f = lens focal length
N = aperture number
c = circle of confusion
If s>>f and f*f*f*f >> N*N*c*c*[s -f]*[s-f]
DOF ~ 2*s*s*N*c/f*f
For constant s and c ...
DOF is proportional to N/f*f
So if you double the focal length you need to multiply N by a factor of 4 in order to obtain the same DOF.
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rbelyell
Joined: 13 Oct 2009
Posts: 4269
Location: somewhere in the mountains of central NY
Expire: 2014-01-31
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| Posted: Thu Dec 13, 2012 11:05 pm Post subject: |
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rbelyell wrote:
yup, i guess so. a factor of 4 is HUGE. i cant believe i didnt know this before...
tony
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Epson RD1 + Elmarit 21/2.8; Summarit 50/1.5; Summarit 75/2.5; Elmar-c 90/4; Sankyo Komura 135/2.8, Hektor 135/4.5; Braun Paxina 29 6x6; Photax Boyer Paris; Holga 120 Pano
GREAT STUFF FOR SALE:
Contax T
Hasselblad XPan + 45/4, 90/4
Kodak Retina Reflex IV + full set of Schneider Krueznach lenses
Mercury 2 half frame 35mm
Kodak Pro slr/n
Fuji GM670+100/3.5+65/8!
Praktisix 6x6 medium format + ZeissBiometar 120/2.8
Bessa T 101 Anniversary Edition in Navy Blue
Mamiya Six Folder with Zuiko 75/3.5
Adaptall: Tamron SP 28-85 macro
Cameras: Canon IX
PM for more complete descriptions/pix. All in great shape!
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take me as i come 'cause i can't stay long' |
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Esox lucius
Joined: 26 Aug 2008
Posts: 2441
Location: Helsinki, Finland
Expire: 2011-11-18
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| Posted: Thu Dec 13, 2012 11:14 pm Post subject: |
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Esox lucius wrote:
I think it is fairly common misperception. Nature photographers know how big a difference it is between 200mm f/2, 300mm f/2.8, 400mm f/2.8, 500mm f/4 or 600mm f/4, someone who never has tried these don't realise it either. |
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