06-11-2012, 07:49 AM
(This post was last modified: 06-11-2012, 09:10 AM by Brightcolours.)
[quote name='wim' timestamp='1339358189' post='18830']
And since it focuses down to 31.2.cm, that means at 1:1 its FL is 78 mm <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />.
Kind regards, Wim
[/quote]
Different sources give different figures on how to calculate focal length, but theory says to use front nodal point.
Focal lengths are defined by front principal point (nodal point) distance to the subject. And this is not easy to determine at all, so usually the distance to the front element is used as a crude approximation.
Since the Sigma has a quite exposed front element, we can use the working distance given from the review (0.142m).
And then I get a focal length of 71mm, using
http://www.giangrandi.ch/optics/lenses/focalcalc.html
How did your calculator come to 78mm, only knowing the MFD and not the front nodal point or the working distance?
And since it focuses down to 31.2.cm, that means at 1:1 its FL is 78 mm <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />.Kind regards, Wim
[/quote]
Different sources give different figures on how to calculate focal length, but theory says to use front nodal point.
Focal lengths are defined by front principal point (nodal point) distance to the subject. And this is not easy to determine at all, so usually the distance to the front element is used as a crude approximation.
Since the Sigma has a quite exposed front element, we can use the working distance given from the review (0.142m).
And then I get a focal length of 71mm, using
http://www.giangrandi.ch/optics/lenses/focalcalc.html
How did your calculator come to 78mm, only knowing the MFD and not the front nodal point or the working distance?