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05-12-2010, 05:08 PM   #31
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QuoteOriginally posted by kacansas03 Quote
I really wish to thank those of you, rparmar and Class A, who have gone to all the trouble to do the research and plot the graphs then share it with the rest of us.
Glad you found this useful.

Your observation regarding the focal length deviating from what is printed on the lens may be explained by the fact that
  • manufacturers often slightly deviate from the specified focal length.
  • the focal length changes with the focus with internal focus designs. The number printed on the lens only applies to infinity focus. An extreme example is the Pentax/Tamron 18-250 zoom whose 250mm at the closet focusing distance of 45cm is more like 125mm.



Last edited by Class A; 05-12-2010 at 08:46 PM.
05-12-2010, 06:42 PM   #32
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QuoteOriginally posted by Class A Quote
Glad you found this useful.

Your observation regarding the focal length deviating from what is printed on the lens may be explained by the fact that
  • manufacturers often slightly deviate from the specified focal length.
  • the focal length changes with the focus. The number printed on the lens only applies to infinity focus. An extreme example is the Pentax/Tamron 18-250 zoom whose 250mm at the closet focusing distance of 45cm is more like 125mm.
I believe there is a misunderstanding. I was saying that Field Of View varies from lens to lens sometimes by quite a lot. Three quarters of a degree greater FOV at 400 mm almost a degree is a lot. I will find the specs and quote them for you when I get a chance.
05-12-2010, 08:37 PM   #33
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QuoteOriginally posted by kacansas03 Quote
I believe there is a misunderstanding. I was saying that Field Of View varies from lens to lens sometimes by quite a lot.
For a given sensor format, focal length and FOV are two ways of expressing the same thing, right (unless hard vignetting is involved)?
I'm not sure what you are getting at.

QuoteOriginally posted by kacansas03 Quote
Three quarters of a degree greater FOV at 400 mm almost a degree is a lot.
A 400mm lens on APS-C should have a FOV of 4.02░. If it has 0.75░ more than that then it really is much more like a ~340mm lens.

Last edited by Class A; 05-12-2010 at 08:45 PM.
05-12-2010, 09:36 PM   #34
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QuoteOriginally posted by falconeye Quote
But does anybody know why everybody else including Pentax non-limited proposes 50mm? What is the history behind this?
I thought it was that our eyes supposedly match the angel of view of a 50mm lens? Don't know where though, at the edges? "half focus"? What? Must depend on eye configuration to =P

05-21-2010, 02:02 PM   #35
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I know it's a little late but wanted to play a little with the graphs and see it for myself. So here they are, all done in gnuplot.

I find the angle of view for the horizontal (wide side) of the film better for comparing since that is what you will most likely want to use in normal compositions.

First, a detail of the wider angles from UWA to WA. Second, a lesser detail until normals. Third, a set of some focal lengths seen in lenses including the FA ltd.
Lastly the previous set plus longer focal lengths until 600mm (in logarithmic fashion and in normal).
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