10-05-2012, 08:07 PM
[quote name='Shamrock' timestamp='1349462108' post='20514']
Hello Klaus!
First of all, thanks for maintaining such a great website!
With the advance of super-zooms and in-camera distortion-correction I think I see an increase in lenses that are released which have a high degree of distortion. What I always wonder with these lenses, is what their actual zoom range is after the image is corrected? Maybe a 18-200 with both barrel- and pincushion-distortion would have an effective zoom-range of only 22-180 after correcting the images.
Do you think it would be possible to include, in the review of a lens, the 'corrected' focal length or range?
If we look, for example, at the Olympus M.Zuiko 12mm f/2 ED, it has a barrel-distortion of 5.4%, which is corrected in-camera. This means that of the original wide field-of-view, quite a bit is lost in the distortion-correction. Would this equate to maybe a 13mm or 14mm lens that doesn't need the distortion-correction? Or, does Olympus take this into account and is it actually a 11mm lens that gives the FOV of a 12mm after the in-camera correction?
Kind regards,
Bart Roskam
[/quote]
There is a proper industry standard. Focal length holds for the centre. It is perhaps most obvious for 15mm equivalent lens, which one can buy in two varieties: fisheye and rectangular. If you correct for rectangular it has 110deg, while when produced as a fisheye it gives you about 180deg. If look for details in the centre, they are same size because the focal length is the same. In short focal length is not defined via the angle of view.
One can actually view this, as if a proper rectangular lens has longer values for the focal length when you move into the corners. To illustrate this, take an old fashioned frame-viewfinder. This you can construct by holding a slide mount (the little frame you put a piece of film in when wanting to project it) in front of your eye. The distance from your eye to the plane of the mount corresponds to the focal length. Obviously this distance is larger in the corner than it is in centre.
So you can view a barrel distorting lens (like the Oly 12mm) as one whose focal length doesn't grow enough when moving into the corner.
I hope this helps.
Joachim
Hello Klaus!
First of all, thanks for maintaining such a great website!
With the advance of super-zooms and in-camera distortion-correction I think I see an increase in lenses that are released which have a high degree of distortion. What I always wonder with these lenses, is what their actual zoom range is after the image is corrected? Maybe a 18-200 with both barrel- and pincushion-distortion would have an effective zoom-range of only 22-180 after correcting the images.
Do you think it would be possible to include, in the review of a lens, the 'corrected' focal length or range?
If we look, for example, at the Olympus M.Zuiko 12mm f/2 ED, it has a barrel-distortion of 5.4%, which is corrected in-camera. This means that of the original wide field-of-view, quite a bit is lost in the distortion-correction. Would this equate to maybe a 13mm or 14mm lens that doesn't need the distortion-correction? Or, does Olympus take this into account and is it actually a 11mm lens that gives the FOV of a 12mm after the in-camera correction?
Kind regards,
Bart Roskam
[/quote]
There is a proper industry standard. Focal length holds for the centre. It is perhaps most obvious for 15mm equivalent lens, which one can buy in two varieties: fisheye and rectangular. If you correct for rectangular it has 110deg, while when produced as a fisheye it gives you about 180deg. If look for details in the centre, they are same size because the focal length is the same. In short focal length is not defined via the angle of view.
One can actually view this, as if a proper rectangular lens has longer values for the focal length when you move into the corners. To illustrate this, take an old fashioned frame-viewfinder. This you can construct by holding a slide mount (the little frame you put a piece of film in when wanting to project it) in front of your eye. The distance from your eye to the plane of the mount corresponds to the focal length. Obviously this distance is larger in the corner than it is in centre.
So you can view a barrel distorting lens (like the Oly 12mm) as one whose focal length doesn't grow enough when moving into the corner.
I hope this helps.
Joachim
enjoy