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Old Sunday 8th May 2016, 16:36   #1
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PF80ED-A focal length


What’s the real focal length of this scope? In some places I see that’s 518mm, in others 520mm and in others 522mm. What’s more, the magnifications specified in Ricoh’s XW eyepieces webpage don't help clarify:

XW7 -> 74x
XW10 -> 52x
XW14 -> 37x

The most confusing is XW20 -> 27x. With neither of the focal lengths, 27 is obtained. 518/20 = 25.9, 520/20 = 26 and 522/20 = 26.1.

The same thing happens with the smc PENTAX Zoom XL 8-24mm specs ( With the shortest focal length (8mm) the specs say 63x, but if you divide 518 by 8 you get 64.75.

I know that the differences are negligible, I'm just curious to know what's the real value, and why the depending on the eyepiece, the scope's focal length seems to be different...

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Old Monday 9th May 2016, 16:25   #2
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With regard to the listed:
XW7 -> 74x
XW10 -> 52x
XW14 -> 37x
Those agree with a 518 mm FL for the PF80EDa. (52x = 51.8x to the nearest whole power.)
Not sure why they list the XW 20 as 27x, since 518/20=25.9x or about 26x. You are right, however that this small difference is negligible. For astronomy uses, where all objects are at infinity, one can specify magnification with some certainty by dividing Objective FL by Eyepiece FL as was done above.

I suspect that some birders do not even realize that the above calculations are not exact for nearby objects. The scope objective FL is the distance from the near principal plane of the objective to the principal focus of the lens. This is a fixed distance for each objective and is often published (except Pentax PF80EDa which seems to be 518 mm). This principal focus is where an image forms for an object at infinity. Everyone knows that for closer objects, the image is formed farther from the objective lens. If you want a more accurate value for magnification, it is the distance from objective lens (near principal plane) to the actual image location that should be divided by eyepiece FL to give magnification. Thus as the distance to the bird decreases, the actual magnification increases slightly. This might be a topic of conversation when viewing hummingbirds at a feeder with the scope at its closest focus, but most birders simply ignore this increase in magnification. HTH - Bill
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Old Monday 9th May 2016, 23:36   #3
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Great thanks Bill!

So the scope's objective length is not even specified in the user's manual? That's strange...

Thanks for clarifying the differences in magnification for objects at infinity vs close ones.
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