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Question about 1:1

Posted: June 9th, 2010, 6:52 am
by Jason Mintzer
I remember a while back Chrish posted the minimum focus distance for many macro lenses, which got me thinking...

Tamron 90mm: 11.4 inches

Tamron 180mm: 18.5 inches

Does this mean that you can get more magnification with the 180? I thought that 1:1 was 1:1, I would expect the minimum focus distance on the 180 to be double that of the 90. I am I missing something?

Thank you

Re: Question about 1:1

Posted: June 9th, 2010, 10:54 am
by chrish
No, 1:1 is the total magnification.

Different focal length lenses produce the same magnifications at different distances.

The reason the distances aren't exactly double is because the total magnification is dependent on the amount of extension the lens provides and the internal elements of the lens itself.

Chris

Re: Question about 1:1

Posted: June 9th, 2010, 11:29 am
by Jason Mintzer
Thank you

Re: Question about 1:1

Posted: June 9th, 2010, 5:39 pm
by bgorum
This gets complicated. There are many factors involved. For example the 180 is physically longer than the 90. So if the distance is measured from the front of the lens (working distance) you will lose some because the front of the 180 is further from the image plane than the front of the 90. Also, different macro lenses focus in different ways. Back in the old days a macro lens focused by extension, (moving the elements further from the film). To get to 1:1 with a 105 macro you needed to add 105mm of extension which resulted in a focal length of 210mm at 1:1. Now most macro lenses focus at least partially by internal focusing, which results in the lens actually losing focal length as you focus closer. So a 105mm macro that uses internal focusing would have less working distance than a 105mm macro lens that focused by extension. So, you just need to stick the different lenses on a camera and see if they give you the working distance you need for your pictures.