Originally posted by vass Thanks for your response. I'm still in the same confusion like, the DOF factor in Macro and a prime(for a given focal length, say 50mm). In my understand, macro photography means, distance less than a feet.But i couldn't understand the lens like 135mm macro etc.!
Cheers,
Vass
Macro means focusing very close, so close a ratio is usually used to describe it, like 1 to 1, which means that for a subject that is 2 centimeters long, an image is projected onto the sensor that is 2 centimeters long. Some older Pentax macro lenses are 1 to 2, which means that for a subject that is 2 centimeters long, an image is projected onto the sensor that is 1 centimeter long. Some 'Macro' zooms have ratios like 1 to 4 (1:4), not really macro, but still close focus.
As for DOF, a 50mm prime will have the same DOF as a 50mm macro at the same focus distance and the same aperture.
Macro lenses move the glass elements inside the lens far enough away from the sensor to allow close focus. With a standard lens, as you focus closer to the subject, the glass elements inside the lens move farther away from the sensor. As you focus on a subject far away, the glass inside the lens moves closer to the sensor. You can do macro photography with a standard prime lens by using extension tubes between the lens and camera body.
I don't know the 135mm macro lens you mention, but there is no reason a 135mm couldn't be made to do macro. I think Sigma makes a 180mm macro.
Hope that helps.