How can you tell if you are getting true 1:1?

I just bought a Sigma 105mm Macro. It is suppose to be a true 1:1 lens; but I don't know how to verify that claim. The lens barrel is marked with ratios; but it does not go down to 1:1. The minimum focusing distance is suppose to be 12.3"; but is that measured from the front of the lens or the plain of the sensor? I thought that a 1:1 lens was suppose to make an image on the sensor (or film) the same size as the item being captured; but I don't think the Sigma is doing that.

I'm sure this has been asked and answered. Please forgive a newbee.

Thanks
Ches

yes the EF sigma 105 is a true lifesize 1:1 macro

Take a picture of a millimeter ruler. FOV at minimum focus distance should be the width/height of your sensor.

It does 1:1 don't worry- as suggested above take a pic of the mm scale of a ruler at min focus distance. Assuming you have a 1.6 crop camera you should get about 22.5mm of ruler across the pic.
Brian v.

Thanks a bunch guys. I'll try it tomorrow.

ches

Ok, I took pics of a ruler. This is with the lens manual focus at the shortest distance and moving the camera until the ruler is in focus. Looks like the closest it will focus is about 45mm.

Did I get a bum lens?

Thanks

HOSTED PHOTO
please log in to view hosted photos in full size.

You can tell I'm a newbee. I figured out what was happening. If you have the lens beyond 1:2 and select the limit switch the lens will not focus under 1:2 but; if you have the lens under 1:2 and set the limit switch then the lens will only focus from 1:1 to 1:2.

Just a case where I did not read the manual and did not understand the way the limit switch worked.

Sorry for the mix up.

Thanks
Ches

I'm glad you posted this thread, though. Now the rest of us know for future reference. I had no idea about the ruler trick, for example.

Thanks. i've just got this lens second hand but didn't get the instructions with it and having never used a macro lens didn't know what full and limit meant but I think I do now. Well I will once I've had a play around with it.