pmrid
12-11-2014, 07:09 AM
Does anyone know how to measure the backfocus distance for the WO Reducer 4? The schematic WO provide shows the distances to the top of the T-thread to the lens surface and also, to the lens frame. And it shows both these measurements for the 3 different positions the device is capable of providing - i.e 66,76 and 86 mm.
And the external thread on the CCD end is also given at 4.5 mm. So calculating the correct distance from the face of the device seems to be a matter of deciding on the reduction factor you want first and then some subtraction of the thread length from the provided internal measurements and from the backfocus setting chosen.
But which one? The last glass has a deep meniscus of 7.5mm - an appreciably large meniscus. So, do you start with the distance to the bottom of the meniscus or to the edge of the lens frame?
The difference would seem to be, on my simple maths, and working on the 76mm setting as an example:
Either 76 - 24.1 - 4.5 = 56.4 or
76 - 16.6 - 4.5 = 63.9 mm.
Appreciate any enlightenment.
Peter
And the external thread on the CCD end is also given at 4.5 mm. So calculating the correct distance from the face of the device seems to be a matter of deciding on the reduction factor you want first and then some subtraction of the thread length from the provided internal measurements and from the backfocus setting chosen.
But which one? The last glass has a deep meniscus of 7.5mm - an appreciably large meniscus. So, do you start with the distance to the bottom of the meniscus or to the edge of the lens frame?
The difference would seem to be, on my simple maths, and working on the 76mm setting as an example:
Either 76 - 24.1 - 4.5 = 56.4 or
76 - 16.6 - 4.5 = 63.9 mm.
Appreciate any enlightenment.
Peter