[woodywesty]

I've been around Pentax cameras since I bought my Spotmatic in 1971 and also nice birding optics over the years but I don't know how to compare/convert magnification between these optics. I have a spotting scope with a 77mm main objective and a 30 power eyepiece. My binos are 7 power magnification with 42mm objectives. With the camera we measure the distance from the front element to the film plane to identify the power of the lens (as I recall)... So how do we convert say a 300mm lens to magnification for purposes of comparison?


Thanks

[Adam]

Eyepieces play a key role when it comes to spotting scopes and binoculars, so usually you get a lot more reach (5x,10x,20x,etc.) simply because of that. If factor this in to the equation and then look at the actual focal lengths of the optics, you should be able to compare the effective magnification.

[zapp]

50mm lens is like 1x magnification. 300mm / 50mm is 6x magnification. Just as a rule of thumb. But be aware that you get this magnification for the whole image frame. Here you can zoom in digitally to even larger ratios. Boinoculars can be made much smaller/shorter to please the eye with high magnification.

Length from front element to film plane is also just a rule of thumb. For long lenses this length will be slightly shorter than the focal length unless you are looking at the funny design of the Pentax 5.6/560mm.

[JimmyDranox]

The proces of converting magnification is simple. You have a 300mm lens, and a 300mm spotting scope. At the spotting scope you have an eyepiece, with his own focal distance. If this focal distance is, let say 50mm, your spotting scope will have a 6x magnification. I didn't choose this focal distance randomly, but for a porpuse. because this is the focal distance of a FF camera OVF (optical view finder). So, if you put a 300mm lens on a FF camera the magnification will be 6x. For an APS-C camera, like Pentax, the focal distance of the OVF is somewhere around 33.3mm. So, a 300mm lens will give a 9x magnification.

Of course in your case, you don't know the real focal distance of your spotting scope, neither the focal distance of its eyepiece. But you can make some comparison between spotting scope and the camera with a long lenses, since you know how to calculate the magnification for the camera.

[woodywesty]

Thanks guys. Now I know how to make a comparison between my Binoculars and Spotting scope with my camera lens. The whole magnification/power thing I guess is arbitrary scale but is something I'm very familiar with. Now I know how to place camera lenses on that scale.