Originally posted by audiobomber I agree with all that Dave. I was disputing your statement that "the working distance will be about 8.3", a little more than a 100mm macro lens at 1:1". The difference in working distance (lens to subject) is a lot more for a Raynox 150 than for a 100mm macro focussed at 1:1. Actually the working distance of the Raynox 250 on a long lens is pretty close to the 100mm 1:1 macro subject distance.
Thanks for the clarification...
I estimated the 100mm lens working distance from theory. According to thin lens optics, the lens-subject working distance is:
Working_distance=Focal_length(1+1/m), which for a 100mm lens at m=1 gives 200 mm (a little less than the 203mm distance for the Raynox 150.)
In real life, actual working distance can be quite a bit less due to lens design, recess, hood, etc. I've a Macro-Takumar 50:4 in which the lens element is recessed 50mm from the front of the lens. Its useful working distance is only about 50mm at 1:1mag.
I've a simple (non-Internal Focus) 90mm Tamron macro lens which at 1:2 mag has an actual working distance of 9.5" (about 241mm); theory predicts 90(1+1/0.5) =270mm, so it is only off by a little bit when the 18mm lens recess is accounted for.
Internal focus lenses can be considerably different from theory because the actual focal length varies with the focus distance (the thick lens doesn't move, its internal elements move, making it hard to even define where the lens is in space!)
Photozone says the DFA100 at 1:1 has a working distance of 130mm (not 200mm), just like you say. Its effective focal length at 1:1 mag can be estimated from the focal distance equation:
Focus_distance=Focal_length((1+m)^2)/m ---- i.e. distance from image plane
The specs say the minimum focal distance for the DFA 100 is 300 mm which according to the above formula corresponds to an effective focal length of 300*1/(1+1)^2 = 75mm, for a working distance of 150mm from the "lens" (wherever that is!!) to the subject (just like you say.)
It looks to me as if the "100mm focal length" in the DFA 100's lens specification may be valid when the lens is focused at infinity, not when it is focused close.
Sorry for the exhaustive response, I thought it might be useful for others to see some of the reasons why things are less simple than thin lens theory predictions.
Dave
PS I just measured the glass' aperture on my Raynox DCR 150. It is 37mm.
Last edited by newarts; 10-15-2009 at 06:29 AM.
Reason: include Raynox aperture size...