Reproduction ratio - What and how?

I was going through the details of the lens Sigma 70-300 APO DG Macro

in one of the photo.net articles and it was mentioned:

"It is a Tele-Macro Zoom lens, capable of focusing down to 1:2 (half

life-size) reproduction ratio at 300mm focal length (an optional

accessory 58mm achromatic close-up lens allows even closer focusing

and reproduction ratios up to 1:1 or life-size) with superb optical

quality"

 

1. What is 1:2 reproduction ratio? I have no idea of what it is?

2. What is ahcromatic close-up lens?

3. How does it make the ration 1:1?

 

I have no idea about reproduction ratio, it would be great if someone

explain it to me in a very basic manner.

 

Thanks for all the response.

 

cheers

Sachin

1. What is 1:2 reproduction ratio?

 

1:2 reproduction ratio means that if you take a photo of a coin with a diameter of 2 centimeters, the coin will only take up 1 cm of the space on the cameras sensor. True macro-lenses will have a reproduction ratio of 1:1, which is like putting the coin on a copying machine.

 

2. What is ahcromatic close-up lens?

 

http://www.daviddarling.info/encyclopedia/A/achromatic.html

 

3. How does it make the ration 1:1?

 

I guess the lens will work like a 2X converter when you combine it with the Sigma 70-300mm, so the lens will become a true macro-lens.

If a lens has a 1:1 ratio it means that if you stick it onto a 35mm film camera (which has an

imaging area of 24x36mm) then you can take a photo of an object as small as 24x36mm

in size and have it fill the frame. The lens in question has half life size macro capabilities,

which means an object 48x72mm in size will fill the frame.

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The lens also has an add-on lens, basically a sophisticated magnifying glass, which allows

you to get a 1:1 ratio. You probably won't be able to focus to infinity with the add-on lens

installed, though.

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<A HREF="http://photonotes.org/cgi-bin/search.pl?input=achromat&which=d"

> Achromatic </A> refers to a design aspect of the add-on lens - the lens is supposed to

keep colour fringing to a minimum, basically.

Bear in mind that while a film frame is 24x36mm, the sensor on a crop DSLR is much smaller, at around 15x22.5mm (it varies slightly according to the camera model). So on a DSLR, you'd get just 30x45mm at 1:2 filling the frame. What is attractive about this capability of the lens is that the maximum magnification is achieved at a focus distance of 95cm, making it an ideal butterfly lens and allowing the use of a regular on shoe flash rather than a dedicated macro one (e.g. a ring flash that attaches to the front of a lens) or an offshoe cord. Macro lenses typically require you to get much closer to the subject (which is fine and even an advantage if it is inanimate, like a coin).

In an over simplified form, achromatic close up lens are two elements close up lens. Most close up lens are single element. Canon's 500D & 250D, Nikon's 5T & 6T are dual element achromatic that are widely available. Dual element provide much better quality then single element close up. BTW, you could get very close to 1:1 with a Nikon 5T on petty much most 70-300mm zoom. The sigma does allow infinity to 1:2 without close up lens. Regarding the "superb 1:2 macro optical quality" claim, I wouldn't read that marketing stuff too much :-)