I'm looking to get a bit more min. focusing distance on my 300 and 500 lenses (on IQ180 / 645DF), how effective will extension tubes be for this? I know with longer lenses they are less effective etc etc, and what are the downsides? Thanks,
Hi Brian,
It depends on the minimum focus distance, or magnification factor, you need.
With the lens focussed at infinity, a 300mm lens requires the following extensions:
128.6 mm to focus at 1 metre (magnification 0.4286x),
52.94 mm to focus at 2 metres (magnification 0.1765x),
33.33 mm to focus at 3 metres (magnification 0.1111x),
24.32 mm to focus at 4 metres (magnification 0.08108x),
19.15 mm to focus at 5 metres (magnification 0.06383x),
15.79 mm to focus at 6 metres (magnification 0.05263x),
etc.
With the lens focussed at infinity, a 500mm lens requires the following extensions:
500 mm to focus at 1 metre (magnification 1.0x),
166.7 mm to focus at 2 metres (magnification 0.33x),
100 mm to focus at 3 metres (magnification 0.20x),
71.43 mm to focus at 4 metres (magnification 0.1429x),
55.56 mm to focus at 5 metres (magnification 0.1111x),
3.876 mm to focus at 6 metres (magnification 0.007752x),
etc.
It depends on the internal focusing groups mechanism how that translates to other focus distance settings on the lens barrel. Some lenses simply provide an extension of all lens elements, effectively a variable extention tube, others also change effective focal length, a kind of built-in zoom.
Cheers,
Bart
