04-06-2012, 07:12 AM
[quote name='Plochmann' timestamp='1333691598' post='17341']
I'm writing this in response to a trollish comment on another site's forum. The post was on the announced Voigtlander 17 0.95. Though many Mft users are happy with the prospect of owning one, someone made the comment that the f-stop was only so fast because the puny sensor is so small that it needs less light. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/huh.gif' class='bbc_emoticon' alt='
' /> I think an f-stop is just an f-stop and sensor sizes are not part of the equation. But maybe I'm wrong. Can 4/3 lenses be faster because the imaging circle is smaller and therefore more focused? But then why aren't point and shoot cameras ridiculously fast? And then we have really fast ff lenses. But I can't think of any super fast medium format lenses. And I also notice that rangefinder lenses tend to be faster, does the shorter flange depth help with this f-stop equation? Any experts?
[/quote]
I'm not an expert... But you gave me the chance to make the experts here confirm the knowledge I have <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='
' />...
To my knowledge, a fast lens is called "fast" because it allows faster shutter speeds due to the maximum aperture compared to the slower ones. And if one wants to make fast lenses one must allow more light to reach the sensor plane. And if the sensor area is smaller, then there's the advantage that with relatively smaller glasses (compared to FF for example), one can produce fast lenses just because the image circle required is smaller. But of course, all these arguments are about the fastness in terms of shutter speeds, and have nothing to do with the DoF you can have on the final image.
F-stop values are calculated as FL / diameter of the entrance pupil. There's no direct relationship with the sensor sizes.
Further reading:
http://toothwalker.org/optics/cop.html (down below the page)
http://toothwalker.org/optics/dofderivation.html#fig1
I find Mr. van Walree a very knowledgable guy... And there's also a windows executable tool he shared in his site for such optical calculations.
Serkan
I'm writing this in response to a trollish comment on another site's forum. The post was on the announced Voigtlander 17 0.95. Though many Mft users are happy with the prospect of owning one, someone made the comment that the f-stop was only so fast because the puny sensor is so small that it needs less light. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/huh.gif' class='bbc_emoticon' alt='
' /> I think an f-stop is just an f-stop and sensor sizes are not part of the equation. But maybe I'm wrong. Can 4/3 lenses be faster because the imaging circle is smaller and therefore more focused? But then why aren't point and shoot cameras ridiculously fast? And then we have really fast ff lenses. But I can't think of any super fast medium format lenses. And I also notice that rangefinder lenses tend to be faster, does the shorter flange depth help with this f-stop equation? Any experts?[/quote]
I'm not an expert... But you gave me the chance to make the experts here confirm the knowledge I have <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='
' />... To my knowledge, a fast lens is called "fast" because it allows faster shutter speeds due to the maximum aperture compared to the slower ones. And if one wants to make fast lenses one must allow more light to reach the sensor plane. And if the sensor area is smaller, then there's the advantage that with relatively smaller glasses (compared to FF for example), one can produce fast lenses just because the image circle required is smaller. But of course, all these arguments are about the fastness in terms of shutter speeds, and have nothing to do with the DoF you can have on the final image.
F-stop values are calculated as FL / diameter of the entrance pupil. There's no direct relationship with the sensor sizes.
Further reading:
http://toothwalker.org/optics/cop.html (down below the page)
http://toothwalker.org/optics/dofderivation.html#fig1
I find Mr. van Walree a very knowledgable guy... And there's also a windows executable tool he shared in his site for such optical calculations.
Serkan