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Zoom range regarding corrected distortion
#1
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
#2
[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
enjoy
#3
[quote name='joachim' timestamp='1349467675' post='20515']

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

[/quote]



Hello Joachim,



Thank you very much for your extensive answer! It certainly helps, but I do not yet fully understand it. I had some physics long ago, but not enough to easily understand this. Please bear with me...



Your example of the angle of view with a rectangular and fish-eye shows indeed that there is a different angle of view depending on how the lens is corrected, which is more or less independent of (but limited by) the focal length. Still, with a 12mm lens that is perfectly rectangular-corrected, the field of view on a m43 would be 84deg, and all lines in view would be perfectly straight.



Is it true that when you correct an image from a lens like the Oly 12, which has a field of view of 84 degrees, that after correction, this lens, which of course still has a focal length of 12mm and an fov of 84, will generate an image that has a smaller fov? As you can see [url="http://www.opticallimits.com/olympus--four-thirds-lens-tests/414-pana_1445_3556?start=1"]here[/url] the correction of an image can take quite a bit of the fov.



Then, hypothetically speaking, this corrected image, with a fov of (lets say) 80 degrees, could be generated by a perfectly-rectangular-corrected lens of a slightly longer focal length. This would not be the same image, but would cover the same scene... Does this make any sense or am I way of the mark? Lets say you have to choose between a 17-55 lens with heavy distortion and a 18-55 lens with no distortion, and all you do is take architecture shots? My question is which lens would give a bigger range in corrected fov. Subsequently I wonder whether it is possible to measure this corrected fov and somehow include the corrected fov in reviews.
#4
A 12mm lens is a 12 mm lens.



There are two things to regard.



The focal length does not describe the field of view, all on its own. A 14mm corrected lens gives a MUCH less wide view as a 14mm fisheye lens. A lens that is not well corrected will give a wider view than a well corrected lens.



As such, you can assume that a corrected photo from a not well corrected lens gives about the same field of view as a photo taken with a well corrected lens. The Olympus 12mm lens will probably be a 12mm lens, and the view angle given by the manufacturer will probably be based on the in-camera correction. Lens manufacturers anyway are very creative sometimes with published lens data...



The other thing to regard is that focal lengths are given for infinity focus. Especially with zoom lenses with big zoom ranges, like the 18-200mm type you mentioned, the focal lengths at close focus can be a lot different. Most 18-200mm lenses do not reach past 135mm at close focus, for instance. This is called focus breathing.



Another, more complex, thing to consider is that field of view and focal length are not 100% "linked". If the focal length remains the same (iow, the focal length does not "breath"), the field of view will get more narrow the closer by you focus.
  


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