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35mm equivalent focal lenght?????????

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Old Friday 4th April 2003, 22:12   #1
Paulyoly
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35mm equivalent focal lenght?????????

someone asked me what the focal lenght of my lens was, i have no idea? I have the pentax 80ed spotting scope with the xl-28mm eyepiece which is 18x (18.5 according to pentax), i saw someone say the scope focal lenght was 500mm, so how do I calculate the 35mm equivalent?
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Old Friday 4th April 2003, 22:17   #2
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I should add i have the cp990.
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Old Saturday 5th April 2003, 09:06   #3
Andy Bright
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Although not strictly true to life (but often used by companies and others) you usually take 50mm as 1:1, so you would get 18.5 x 50mm to give 925mm, then you multiply this by whatever your camera lens does.... so a cp990 gives just over 100mm at max zoom, which is 2x... therefore you can get a maximum of 925mm x 2x = 1850mm.
Like I said, this isn't accurate but is usually the figure quoted by companies (even Nikon in their digiscoping blurb).... big numbers for focal lengths are always popular in a sales pitch!
I think the physical size of the ccd has a bearing on it, and confuses the issue when comparing pro-sumer digicams to 35mm.

btw. Glad I'm not the only one whose fingers work faster than their brain when typing 'length', unless I think very hard, it always comes out as 'lenght' .... my surname causes similar problems..
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Old Saturday 5th April 2003, 17:16   #4
Dennis Hoffman
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It is kinda hard to be exact with the angled version of the scope. 28mm x 18.5 = 518mm. That "should" be the focal length of the scope in mm. Although, if you go to the Pentax website and download their .pdf on the scope, you will find that half of their eyepieces (in mm) times the magnification factor they show, give you a scope focal length of 525mm, while the other half give you 518mm. (Example being: 50x reported mag for their 10.5mm eyepiece = 525mm scope focal length) Coincidentaly, the straight version shows a steady 504mm scope focal length for all eyepieces. Must just be a math problem on Pentax's end, but I noticed it when I still had the angled verson and tried to figure out focal length, before switching to the straight.

I'm probably wrong but thought the conversion was like this.

Multiply the camera zoom focal length by 4.75. (for 3x = 114mm)

Devide scope focal length by eyepiece focal length (518/28 = 18.5x)

Multiply the two = 2109.

([24*4.75]*[518/28])=2109mm

Again, I'm probably incorrect, I just thought this was how it is.
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