04-07-2012, 07:15 AM
[quote name='Plochmann' timestamp='1333774268' post='17352']
What I'm trying to say is, if I take that 17mm 0.95 lens and somehow put it on a 135 camera and then go and take a light meter and read a gray card in a dark place, and it says expose to 0.95, so I go and open it all the way up and take a picture, would not it even on 135 film expose the gray card to be 40% when properly printed? I think so. It comes down to that maybe section you mention, the exposure.[/quote]
Exposure wise, a f/0.95 lens is near enough a f/0.95 lens. But in the above scenario, you'd only get a little circle of exposed image on the sensor. Plus that lens probably wouldn't be able to give a focused image at all due to extension, but that's another story...
However, to get approximately the same image as from a m4/3 camera with 17mm f/0.95 lens on it, you would still use a 34mm f/1.9 lens on a full frame body (or nearest). Use this from the same place, giving you the same field of view, the same depth of field. You keep the shutter time the same through using a higher ISO setting, as the increased noise from a higher ISO setting is cancelled out from having lower overall noise per normalised output area from a bigger sensor in the first place.
Short answer: they're not. I'm not aware of any easy way to tell the image circle size from easily available lens specifications, other than of course what the manufacturer says it will do!
Back to the original question, why *can* lenses be smaller for a given specification if you also have a smaller image circle? Look at it this way, you have light going into the front of the lens, this gets refracted around, and you get light coming out of the back of the lens. There are lots of things that are not ideal when passing through lens elements, so you use multiple elements (sometimes of different optical material) to better correct for flaws. The better the correction, the more elements you would tend to use. A bigger image circle is harder to maintain image quality over, so would tend to requiring bigger or more elements.
Note the above only considers the difference due to image circle keeping the lens focal length and focal ratio the same. If you are trying to compare m4/3 to full frame then of course you also have different focal lengths for a given field of view, and lens mount distances factoring into the design considerations.
What I'm trying to say is, if I take that 17mm 0.95 lens and somehow put it on a 135 camera and then go and take a light meter and read a gray card in a dark place, and it says expose to 0.95, so I go and open it all the way up and take a picture, would not it even on 135 film expose the gray card to be 40% when properly printed? I think so. It comes down to that maybe section you mention, the exposure.[/quote]
Exposure wise, a f/0.95 lens is near enough a f/0.95 lens. But in the above scenario, you'd only get a little circle of exposed image on the sensor. Plus that lens probably wouldn't be able to give a focused image at all due to extension, but that's another story...
However, to get approximately the same image as from a m4/3 camera with 17mm f/0.95 lens on it, you would still use a 34mm f/1.9 lens on a full frame body (or nearest). Use this from the same place, giving you the same field of view, the same depth of field. You keep the shutter time the same through using a higher ISO setting, as the increased noise from a higher ISO setting is cancelled out from having lower overall noise per normalised output area from a bigger sensor in the first place.
Quote:How are size of the aperture opening and image circle related, is that the missing link.
Short answer: they're not. I'm not aware of any easy way to tell the image circle size from easily available lens specifications, other than of course what the manufacturer says it will do!
Back to the original question, why *can* lenses be smaller for a given specification if you also have a smaller image circle? Look at it this way, you have light going into the front of the lens, this gets refracted around, and you get light coming out of the back of the lens. There are lots of things that are not ideal when passing through lens elements, so you use multiple elements (sometimes of different optical material) to better correct for flaws. The better the correction, the more elements you would tend to use. A bigger image circle is harder to maintain image quality over, so would tend to requiring bigger or more elements.
Note the above only considers the difference due to image circle keeping the lens focal length and focal ratio the same. If you are trying to compare m4/3 to full frame then of course you also have different focal lengths for a given field of view, and lens mount distances factoring into the design considerations.
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