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PostPosted: Fri Apr 09, 2010 7:09 am 
Hi.
i wanted to know if the depth of field of a 100mm f2.8 lens gets more narrow with a x1.4 extender. im aware that x1.4 extender reduces the aperture by 1 stop but does this mean that only to the quantity of light passing through the lens is reduced or also the depth of field is changed.
since if one were to get an effective focal length that is longer than the original with the same distance from the subject the background should be more blurred right?

i found conflicting answers about this if you have experience with this or have a reliable source i would very much like to know.
thanks


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PostPosted: Fri Apr 09, 2010 10:24 am 
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I use this site to calculate this stuff:

http://www.tawbaware.com/maxlyons/calc.htm

For a given distance, DoF decreases with the square of the focal length and increases with the aperture value.
That means you should get a smaller DoF with the extender

The 1.4x extender gives you a 140mm f/4 (roughly)

Example
Distance: 2 m, focal length 100mm, apeture 2.8 -> DoF = 4 cm
Distance: 2 m, focal length 140mm, apeture 4 -> DoF = 2.9 cm

So yeah, you should get a shallower DoF

Anyone, please correct me if I'm wrong though!

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PostPosted: Fri Apr 09, 2010 11:50 am 
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Just a word of caution: The DoF is no indication of the amount of background blur. Here is an article I find useful to help understand the variables involved.

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Canon DSLRs: 7D2, 7D1, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 10-18, 15-85
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PostPosted: Fri Apr 09, 2010 11:54 am 
ic so the FOV for the extender with regards to DOF is still taken with 1 stop slower


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PostPosted: Fri Apr 09, 2010 11:58 am 
@popo
but since we are comparing the same lens with and without an extender it comparison is logical, or at least as far as im aware of, dont you agree?

also very nice detailed and referenced article thanks for that


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PostPosted: Fri Apr 09, 2010 12:17 pm 
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I believe janern answered the original question, that you should consider a 100mm f/2.8 lens with 1.4x extender to be a 140mm f/4 lens. The caluclator then gives you the DoF results at that setting.

But I saw your question in two parts, one regarding the DoF, and one regarding the background blur. Even if you keep the same DoF by changing aperture or subject distance for example, the apparent background blur can vary. That is what I wanted to highlight.

_________________
Canon DSLRs: 7D2, 7D1, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 10-18, 15-85
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 50/1.4A, 150 macro, 120-300 f/2.8 OS
Compacts: Sony HX9V, Fuji X100.


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PostPosted: Fri Apr 09, 2010 1:56 pm 
aaa ok now i get what you meant
thanks both


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PostPosted: Fri Apr 09, 2010 3:00 pm 
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The article was definately an interesting read. Thanks for posting, popo

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PostPosted: Fri Apr 09, 2010 8:53 pm 
Yeah i agree with janern a good read popo


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