01-27-2013, 09:26 PM
[quote name='Brightcolours' timestamp='1359316560' post='21562']
That is an incorrect way to calculate a lens' focal length, no wonder you come to 280mm. A correct way is described here:
http://www.giangrandi.ch/optics/lenses/focalcalc.html
The problem is to know what the front nodal point of the lens is. This front nodal point also "unlinks" the focal length with FOV, by the way...
Using that focal length calculator, and were I to disregard the distance to the front nodal point and instead take the distance to the sensor instead, I would come to a focal length at MFD of 215.7mm for the Nikon 70-200mm f4 VR.
Often, when the front nodal point is not known, people simply use the middle of the front element as front nodal point. Do not ask me why, as I am not that much into optics to understand that.
Now I estimate the front element of that lens to be about 22cm from the sensor/film plane... Which then makes the object distance at 1m (MFD) 78cm.
The focal length will then be 168mm at MFD for the Nikon AF-S 70-200mm f4 VR.
So... The focal length shrinks towards MFD, but the FOV gets noticably wider.
[/quote]
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 )
That is an incorrect way to calculate a lens' focal length, no wonder you come to 280mm. A correct way is described here:
http://www.giangrandi.ch/optics/lenses/focalcalc.html
The problem is to know what the front nodal point of the lens is. This front nodal point also "unlinks" the focal length with FOV, by the way...
Using that focal length calculator, and were I to disregard the distance to the front nodal point and instead take the distance to the sensor instead, I would come to a focal length at MFD of 215.7mm for the Nikon 70-200mm f4 VR.
Often, when the front nodal point is not known, people simply use the middle of the front element as front nodal point. Do not ask me why, as I am not that much into optics to understand that.
Now I estimate the front element of that lens to be about 22cm from the sensor/film plane... Which then makes the object distance at 1m (MFD) 78cm.
The focal length will then be 168mm at MFD for the Nikon AF-S 70-200mm f4 VR.
So... The focal length shrinks towards MFD, but the FOV gets noticably wider.
[/quote]
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 )