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Old Nov 29, 2007, 11:49 PM   #1
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Can any one explain me the mechnism 0f the "Macro" in P&S cam..??

Like in dslr we do have a lens which allow us to focus the subject at closer distance. thus increasing the magnification, bcoz it has a floting element in it

But in P&S cam we do not have as such.. Somebody said , it is b'coz of the "software in P& S cam", not the opticess. Is it so..??

Liu
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Old Nov 30, 2007, 9:57 AM   #2
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mcliu wrote:
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But in P&S cam we do not have as such.. Somebody said , it is b'coz of the "software in P& S cam", not the opticess. Is it so..??

Liu
Short answer - no. Every zoom lens, whether intended for DSLR use or built into a P&S achieves its macro function (close focusing range) by moving the floating elements as all zoom lenses have to have floating elements anyway.

Single focal length (prime) macro lenses intended for SLR/DSLRs achieve their close focusing by having an extra long focusing mount capable moving all the optics further away from the film/sensor plane.

You may now apply the appropriate grade to "Somebody."
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Old Nov 30, 2007, 2:52 PM   #3
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ac.smith wrote:
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Single focal length (prime) macro lenses intended for SLR/DSLRs achieve their close focusing by having an extra long focusing mount capable moving all the optics further away from the film/sensor plane.
I know that this is true -- I learned on an old Crown Graphic, and when I double-extended the bellows, the focal point changed in a way that made the image size on the ground glass the same as the image IRL. However, I honestly don't understand that -- the lens was a solid piece of optical glass. How can it have two different focal points, just by changing the distance to the ground glass? How does moving the bed change the way that the lens bends light? I just don't get it.
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Old Nov 30, 2007, 3:49 PM   #4
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For simplicity we'll eliminate zoom lenses and keep this discussion to prime lenses only. A prime lens is closest to to the sensor when it is focused at infinity. To focus on an object at 25' rather than at infinity we move the lens further away from the sensor. As a corollary an object is magnified (larger image) at 25' than that object is at "infinity". To focus on an object at 2.5' instead of 25' we have to move the lens even further away from sensor. Again the magnification the object is greater when we're only 2.5' away compared to 25'. Getting into the "macro" range, say for example .5' we simply continue moving the lens further away from the sensor and again the image of our object is much larger on the sensor than it was at 2.5'.

The mechanism for moving the lens away from the sensor can be the helicoid focusing mount on a DSLR prime macro lens, extension tubes, bellows as an add-on to an SLR system or built in as on a view camera, my Mamiya C330 or your Crown Graphic.

Hope this helps.
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Old Nov 30, 2007, 8:39 PM   #5
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It works very much like a slide projector, in that, as you move the screen farther from the projector, the image becomes larger, though still in focus. This is why extension tubes and bellows work to increase image size. Note that you also lose some light in the process, as you are spreading the image over a larger area. A teleconverter between the lens and the body does the same thing.

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Old Dec 1, 2007, 7:28 PM   #6
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thanks ac.smith ,VTphotog for explaination & tclune for taking part in discussion


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