Macro Magnification

I'm looking into obtaining a macro lens. however one thing is not clear to me. How do two lenes with the same focal length have different macro magnification? e.g. 100mm f2.8 DFA 1x - 100mm f3.5 FA 0.5x. Oh! and what and how much difference does it make.

any help please

Not sure but I think it might have something to do with the brightness of the lens i.e. 2.8 v 3.5 and you get what you pay for.

does that mean the magnification changes with aperture?

Still not sure but it would make sense, to get more light the glass has to be bigger and if the glass is bigger that would mean more magnification, well that's my theory I'm sure someone will put us on the right path
Jack.

It has nothing to do with the speed. A 1:2 lens is not a true macro lens. A 1:1 or a 2:1 are true Macro lenses.
It is all to do with the construction of the lens and the number of elements which affect the magnification the lens provides.
To be a Macro lens the lens must show the subject at at least life size. It has a very shallow depth of field, even stopped down.

So 1:2 is the same as 0.5X and would produce a half size image. Would the image from a 1:2 at 500mm not be the same as a 1:1 at 1000mm for example?

i think i've answered my own question the minimium focus distance for the FA 100mm f3.5 1:2 is 1410mm while the DFA 100mm f2.8 1:1 is 303mm, so you could not move in closer to get the same size image. Am i correct?

Maths was not my best subject so I'm out.

No, a 1:2 500mm is not the same as a 1:1 1000mm. The size ratio is the physical size of the image on the film frame (or sensor)
in relation to actual size of the object. So a 1:1 macro gives the same sized image on the film as the object measures in real life.
A 1:1 macro gives you a life sized image regardless of the focal length of the lens. And the minimum focus distance is largely irrelevant too.

It's very simple. A 1:1 100mm Macro lens will focus down to half the distance a 1:2 Macro 100mm will.

Macro lenses just focus closer. They achieve this with much longer focusing mechanisms, hence the size, weight and cost.

The design is different too, emphasising closer distances and flatness of field.

f/4 Macros are usually relatively simple Tessar type costructions, usually 4 elements at 50mm and 5 elements for 100mm lenses.

f/2.8 Macros are much more complex, maybe 9 elements for a 100mm.

Thanks for the answers. It would seem this one is not a good bet then.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&rd=1&item=220354767380&ssPageNam...

It's a fine lens. I had one of these and apart from being a very strange huge chunk of plastic it produces very good results.

It's a similar optical construction to the SMC Pentax-M 100mm f4 Macro.

That particular model isn't a "true" Pentax lens, it's a re-badged version of the Cosina AF 100mm f/3.5 Macro which is reviewed over at Photozone.

For the money it looks to be quite a good lens - not in the same league as the various 100/2.8 offerings but not too shoddy either. I probably wouldn't pay more than £90-100 for one though.

Thanks hefty1 i'll stick to a new one. tried to speak to SRS Chris yesterday but he is still snowed in.

So what are the differences between a 28mm, 50mm and 100mm macro lens? I am sure it can't be so but wouldn't the best be the longest?

I think it's to do with how far away you have to be to get the shot. the shorter the focal length the closer you have to be. i.e. if your photographing a bee with a 28mm you might get stung.