Wierd Macro focusing phenomenon

[iMacfan]

Hello,

Sorry - this is pretty OT, but I have found this forum to be one of the friendliest ones around, and people actually respond intelligently. I haven't been able to get an answer elsewhere, so I thought I'd ask this question:

I'm fairly new to macro, and wondered if any of you could explain a wierd phenomenon that I have experienced.

I'm using a Canon AE1-P, the standard 50/1.8 and alot of extension tubes - I already had a 25mm, and bought a set, so alltogether I can now do 95mm! When I have all of them on the camera, I have noticed that focusing the lens away from the infinity setting, the focus point appears to move AWAY from the lens. Anyone know why?

Thanks,

David

 

[Sonnar Brian]

Have you taken measurements with a ruler or tape measure? I can extend my Micro-Nikkor-P to 1:1 with its tube; the focus always gets closer! The effective focal length increases and F-stop decreases as you entend out that far. I would be interested in the numbers: subject to film plane; subject to front of lens. As far as I know, the Canon 50mm F1.8 does not use a floating element or anything else that can break the normal rules.

 

[jlw]

Remember that the rules start to get a bit "backwards" once your lens-to-film distance becomes longer than your lens-to-subject distance! What's happening in your case is that as you rack the lens out in its focusing mount, it gets significantly closer to the subject, so you have to move the whole assembly away from the subject to restore correct focus.

When shooting at these kinds of distances, it's a lot easier to work out the amount of extension needed for the reproduction ratio you want, set that up with your lens and tubes, and then move the whole business back and forth until you achieve focus, using a focusing rail.

For extra-crazy fun, try this exercise with a reflex housing mounted on your favorite rangefinder camera...