|
The designation on the lens (ie. 100mm) is the distance between the sensor and the equivalent simple lens when the lens is focused at infinity.
There's a formula for the focus distance of the same lens when it's moved further from the sensor towards the object.
1/f = 1/Obj+1/Sens
Where f is the focal length of the lens, Obj is the distance to the object and Sens is the distance to the sensor.
So if you have a 100mm lens and add 200mm extension, you have
1/100 = 1/x + 1/(100+200)
so 1/x = 1/100 - 1/300
x = 150mm
That means that with your 100mm lens focused at infinity, an object 150mm in front of the lens would be in focus.
This is just the very basics and it gets more complicated, the further you get into it. It's a lot like astrophotography in that respect.
Steve.
|