Originally posted by dh4412 how is the raynox dcr adapter used? does it screw on to the cap threads or ? and what does it do? thanks could it be used on a dfa 100?

Last night I posted this

in a related thread:

Originally posted by RioRico: Originally posted by Azzy: Is there any advantage on putting the raynox diopter on a dedicated macro lens (say 100mm pentax / 105 sigma, etc) over 55-300mm ?

That question deserves a test. I mount my +8dpt Raynox DCR-250 on my M42 Vivitar-Komine 90/2.8 macro and extend that fully. Without the Raynox it goes to 1:1. With the Raynox it goes to 2:1. Now I whip out my A-type Tamron 60-300. At 60mm it reaches about 1:1.75 at infinity focus; it vignettes at close focus. At 300mm it reaches about 2.25:1 at infinity focus, and a non-vignetted 3:1 at close focus. My target is a ruler so I can be pretty sure of those numbers.

So the answer is: with a +diopter adapter, you get most magnification with longer lenses at close focus.

**newarts** provided tons of good info. Another way to see the difference between the DCR-15 and -250 is their +dioptre strength. The -150 is +4.8dpt, the -250 is +8dpt. With this we calculate theoretical magnification:

**M= F*D/1000**, where F is focal length of the host lens and D is dioptre strength of the adapter lens.

Mounting a +8dpt Raynox DCR-250 on a 100mm lens at infinity focus gives M= (100*8)/1000= 0.8:1 or 1:1.25. Using the DCR-150 would give M= (100*4.8)/1000= 0.48:1 or 1:2.1. The -250 thus gives twice the magnification of the -150 on a 100mm lens at infinity focus.

This assumes that 1) the lens is actually 100mm, and 2) that it's at infinity focus. Magnification increases when we extend the lens, as we see in my test above. And if it's an IF (internal focusing) lens, the focal length at infinity won't be the same as at close focus. The difference may be profound. Some have measured the DA18-250 as actually being 200mm at close focus. Focal length changes with IF primes too. Exact magnification thus is flexible; your mileage may vary.

IMHO the best way to determine magnification is to JUST DEW IT! Setup the lens system (host lens, adapter, extension, whatever) and view|snap a metric ruler. Our APS-C sensors are close to 24mm wide and 18mm high (exact size varies) so if we see 4.8cm on the ruler, we know that M= 1:2.