Originally posted by CTR01 I've already stated my reason for each one but was wondering is there a way to extend the working distance of the 250? The closest I would like to be to some subjects is about 1 (American/USA) foot. Any other thoughts?
Nope, the focal length of a diopter isn't exactly fixed, but it won't focus (much?) beyond its nominal focal length. For a close enough approximation of subject distance, divide the number of diopters into 1000.
For the 250:
1000/8 = 125mm focal distance
For the 150:
1000/4.8 = 208mm focal distance
For a distance of 1 foot, you need a weaker diopter:
1000/(12X25.4) = 3.3 diopters. Of course magnification will decrease as subject distance increases.
With the Tamron set for 300mm and infinity focus, a 3.3 diopter will achieve about 1:1 macro. I don't believe Raynox makes a 3.3 diopter. Others have made achromatic diopters, (
Currently Available 2 Element (Achromatic) Close-up Lenses)**, but possibly not as versatile as a Raynox and likely more expensive. I would discourage you from simple diopters, stay with an achromatic.
** The webpage says Currently Available but I'm reasonably certain most of them are obsolete.