01-27-2013, 10:42 PM
[quote name='nuke12' timestamp='1359321977' post='21564']
I'm not going to argue with you. I'm no lens expert. I just did simple trig on what I seen and used trig to generate the numbers from a well known lens formula. I figured it would be all very practical because the numbers generated, came from using the actual lens. No need to worry about nodal points, etc.
My picture shows a 128.5875mm (horizontal) ruler at a distance of 1000mm (MFD). I used a focusing rail to find MFD. The horizontal image size will be 36mm.
When I pump my numbers into the calculator you provided, I come up with the same AOV (7.3574 degrees) as your calculator but the calculator says the focal length is 218.72mm. My calculations give me 279.97mm but your saying it is 168mm.
I'm not saying your wrong. I simply don't know. Many of them are using this formula, based on the thin lens and I've not found it correct when dealing with a thick lens.
D = focusing distance (distance between subject and sensor)
F = focal length
M = magnification
F = D / ( 1/M + M + 2 )
[/quote]
The calculator says 218.72mm when you take the object distance as being the distance from the object to the sensor. Not the distance from the subject to the front nodal point (apparently the correct way to calculate a lens' focal length).
Thats the difference between the 215.7 I derived from the lens specs and the wrong distance (the one to the sensor), your 218.72mm (measured to the wrong distance,(to the sensor)), and taking the front nodal point into account with the object distance (168mm with my numbers, and 170mm with your measurement) (assuming the front nodal point to be 22cm away from the sensor).
I'm not going to argue with you. I'm no lens expert. I just did simple trig on what I seen and used trig to generate the numbers from a well known lens formula. I figured it would be all very practical because the numbers generated, came from using the actual lens. No need to worry about nodal points, etc.
My picture shows a 128.5875mm (horizontal) ruler at a distance of 1000mm (MFD). I used a focusing rail to find MFD. The horizontal image size will be 36mm.
When I pump my numbers into the calculator you provided, I come up with the same AOV (7.3574 degrees) as your calculator but the calculator says the focal length is 218.72mm. My calculations give me 279.97mm but your saying it is 168mm.
I'm not saying your wrong. I simply don't know. Many of them are using this formula, based on the thin lens and I've not found it correct when dealing with a thick lens.
D = focusing distance (distance between subject and sensor)
F = focal length
M = magnification
F = D / ( 1/M + M + 2 )
[/quote]
The calculator says 218.72mm when you take the object distance as being the distance from the object to the sensor. Not the distance from the subject to the front nodal point (apparently the correct way to calculate a lens' focal length).
Thats the difference between the 215.7 I derived from the lens specs and the wrong distance (the one to the sensor), your 218.72mm (measured to the wrong distance,(to the sensor)), and taking the front nodal point into account with the object distance (168mm with my numbers, and 170mm with your measurement) (assuming the front nodal point to be 22cm away from the sensor).