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Old Jul 19, 2006, 12:55 PM   #21
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ejbrusselsprout, I understand perfectly now, thanks
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Old Jul 19, 2006, 3:06 PM   #22
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nlp239 wrote:
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The macro function of the 70-300 is at 1:2 but it isn't really macro is it?


just checked out 3 of my photo books and those three indicate that a "macro" lens usually goes to 1:2 by itself without other aids (tubes or supplementary close-up filters) but at least one of them (written by John Shaw) adds that real macro lenses are designed (optically) toperform well at very close distances and to do a good job on flat subjects like stamps or documents where regular lenses are optimized for 3 feet or more subject to camera distances. Shaw goes on to say that for photography of stuff that isn't flat it's hard to see any differences between real macro lenses and regular lenses used with extension tubes or bellows.

i kinda think that's Roy's initial point; you don't need a macro lensto do macro work.

my suspicion (based mostly on price!) is thatmy sigma 70-300 is not optimized for flat field work like a 50mm or 100mm prime macro lens, but it beats my last camera (s602 fitted with close-up filters) all hollow!

ira, neat how some things work out. seems like i remember some posts about turning a teleconverter into an extension tube in which the operative word was "hammer". :-)

all the best, eric


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Old Jul 19, 2006, 3:45 PM   #23
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it was me that told harriet about it, as i'd done the same thing with an old POS converter i had.

it's always been my understanding that a TRUE macro did 1:1 reproductions. some how the 1:2 snuck in there. if you look at the first pic i posted is a millimeter ruler, it shows 53mm in the pic. if you take this image witha 1:1 macro lens then it would only show 23mm because that's how wide the sensor in the cam is. that's why i said , almost a 1:2 macro. actual size of a 1:2 would be showing only 46mm. and , yes a dedicated macro is designed for a flat field but it works just fine for everything. i forgot the actual formula for what you get with different lens and amount of ext.
but i seem to remember that to get 1:1 using tubes you need approximately the same amount of ext as the focal lenght of the lens. ie; a 50mm lens needs 50mm ext, or a 100mm lens would need 10mm of ext.

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Old Jul 19, 2006, 4:31 PM   #24
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robar wrote:
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to get 1:1 using tubes you need approximately the same amount of ext as the focal lenght of the lens. ie; a 50mm lens needs 50mm ext, or a 100mm lens would need 10mm of ext.

roy
That's why I need a 500-600 mmext tube! (Another shot out of frustration that the kite didn't show up, although I could hear him/her scream on the other side of the tree tops.)

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Old Jul 19, 2006, 7:21 PM   #25
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A true macro lens has a flat field of view so documents can be photographed at very shallow DOF and be sharp from edge to edge. Other so called macro/regular lenses have a curved lens element and will not be completely sharp from edge to edge on a document with very short DOF, although the crop factor takes some of that away, but not all of it. This is especially evident with slide copy work where a macro lens shines compared to a curved front element lens.

That is why they still make macro lenses instead of replacing them with extension tubes or bellows assembly's.

Tom
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