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02/27/2007 04:31:23 PM · #1 |
Ok...say I have a 200mm f2.8 lens. To get the "glass" radius I divide 200/2.8 to get 71.4mm. So is 71.4 the actual "glass" radius of this lens?
If so, what if that lens is an 80 - 200mm zoom and it is f2.8 fixed. How does the radius change to 80/2.8 (28.5mm radius) to the 71.4mm radius at 200mm?
I guess I'm asking how the mechanics of a fixed appature zoom works...
Thanks,
Kenskid
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02/27/2007 04:49:40 PM · #2 |
I don't know for a fact, but it seems that the aperture blades would stop down as you zoomed out? Maybe try your cam/lens at 80/2.8 and do a DoF preview and see what happens?..
Yeah, I dunno.
edit: Maybe this would also explain why zooms are softer at the long end? Because they are, in a sense, stopped down when zoomed all the way out, and wide open when zoomed all the way in?... Yeah, I'm gonna shutup now, and let someone that knows answer ya...
Message edited by author 2007-02-27 16:52:39. |
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02/27/2007 05:24:22 PM · #3 |
not sure really. I just took a look through my 80-200 2.8 and looking through the rear element while zooming i saw no difference. However from the front element there was a drastic change in size of the aperture while zooming. It's really hard to tell if it's some kind of optical illusion though. It also looked as though the aperture opening moved closer and further away from the front element.
edit: not all zooms are softer at the long end.
Message edited by author 2007-02-27 17:24:49. |
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02/27/2007 06:45:47 PM · #4 |
The ratio of the focal length to f ratio does give the minimum required front element diameter, but what it really refers to is the entrance pupil diameter. The reason that, when you look thru the front of the lens and twist the zoom, you see the aperture apparently change size is that optically, the size that the aperture appears to be (form the front) *is* the entrance pupil diameter. So when you zoom a contstant-f-ratio zoom, the apparent aperture size *must* change to keep the f ratio the same.
For telephoto lenses, the front element diameter will be just slightly larger than the FL/f ratio predicts. For wide angle, it will be a lot larger. An extreme example is the 15mm fisheye, which has a front element about 45mm across, but an entrance pupil of only 5.35mm.
Message edited by author 2007-02-27 18:46:54. |
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02/27/2007 07:26:52 PM · #5 |
With that said....does my 80-200mm f2.8 zoomed to 200mm gather more light than my 50mm f1.8 ?
Thanks
Originally posted by kirbic:
The ratio of the focal length to f ratio does give the minimum required front element diameter, but what it really refers to is the entrance pupil diameter. The reason that, when you look thru the front of the lens and twist the zoom, you see the aperture apparently change size is that optically, the size that the aperture appears to be (form the front) *is* the entrance pupil diameter. So when you zoom a contstant-f-ratio zoom, the apparent aperture size *must* change to keep the f ratio the same.
For telephoto lenses, the front element diameter will be just slightly larger than the FL/f ratio predicts. For wide angle, it will be a lot larger. An extreme example is the 15mm fisheye, which has a front element about 45mm across, but an entrance pupil of only 5.35mm. |
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02/27/2007 07:31:47 PM · #6 |
Originally posted by kenskid:
Ok...say I have a 200mm f2.8 lens. To get the "glass" radius I divide 200/2.8 to get 71.4mm. So is 71.4 the actual "glass" radius of this lens?
If so, what if that lens is an 80 - 200mm zoom and it is f2.8 fixed. How does the radius change to 80/2.8 (28.5mm radius) to the 71.4mm radius at 200mm?
I guess I'm asking how the mechanics of a fixed appature zoom works... |
All lenses have a focal length. It is defined as the distance from the surface of a lens to its focal point.
A zoom lens is going to have a variable focal length defined as the distance from its shortest focal length to its largest focal length. (like 80mm-200mm).
Aperature is a hole or an opening through which light is admitted. The bigger the largest opening of a particular lens the lower its aperature rating. That is a good thing. A low aperature rating allows for more light under lower light conditions.
Aperature is entirely separate from focal length. Typically, focal length and aperature rating are features we look for in zoom lenses. Typically, we want a wide focal length rating and a large focall range in a zoom. More expensive lenses have a low aperature rating and a large focal range without severe optical distortions (distortion, chromatic aberation and stuff like that) at the lens extremes.
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02/27/2007 09:20:10 PM · #7 |
| bump....does my 200mm f2.8 gather more light than my 50mm f1.8? |
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02/28/2007 09:23:47 PM · #8 |
Originally posted by kenskid:
bump....does my 200mm f2.8 gather more light than my 50mm f1.8? |
I would guess that yes it does gather more light at the front element, but that less of the light gets through to the sensor. |
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