Re: Plus lenses effect on stereobase calcs.

[T3D john bercovitz <bercov@xxxxxxxxxxx>]

Steve Owsley writes:

> What effect does using plus lenses have

Adding a plus lens increases the strength of the lens, as you might 
expect.  The strength added has to do with the strength of the new and 
of the original lenses and the distance between them.  Since the 
strength of the new assembly is greater, it has a shorter focal length.  
That means it will have to focus on a closer object since the film 
plane is still in its original location.  

The addition also moves the position of the effective front of the lens 
a little.  So the distance from the lens barrel to the object changes 
slightly.

Typically the plus lenses are pretty weak compared to the lens they're 
added to so I think that the effect will be small but I haven't 
calculated it.

The formula for calculating the new position of the front of the lens 
is a bit involved but calculating the new focal length of the lens is 
not:

f = (f1*f2)/(f1+f2-d)

where:
f = combination focal length
f1 = one lens' focal length
f2 = other lens' focal length
d = distance between them

So taking your example of a 50 mm lens and a plus 1:

f1 = 50
f2 = 1000
d = let's guess it's 15 mm

f = (50*1000)/(50+1000-15) = 48.3 mm

So that effect is small.  I haven't got time to calculate the new 
position of the front of the lens right now.  Maybe later.  I hope it 
is also a small effect and suspect it is.

John B


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