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Author Topic: Using Extension Tubes with Long Lenses  (Read 852 times)
Brian Hirschfeld
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« on: March 02, 2013, 10:07:57 AM »
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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,
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BartvanderWolf
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« Reply #1 on: March 02, 2013, 11:49:43 AM »
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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
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Brian Hirschfeld
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« Reply #2 on: March 02, 2013, 05:14:57 PM »
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Thank you for the in-depth response, the 300mm f/2.8's Minimum focusing distance is 3.5 meters and this is sufficient for my purposes, since if I need to get closer I could just add the 2x TC and that would prob do it for me.

However, the 500mm is the problem child here, with its min focus of 5m, I would like to bring that down
a) how can I figure what it would take to get it to 3.5?
b) if this is too much considering light loss etc, what would a reasonable goal be?


*Update*
I used the M645 tubes with the 500mm and with the 2x TC and found the following focused on the min focusing distance (5m) and then what the same focus point was at with the Tube/TC

Focus-Tube/TC Combination
5M - no Tube or TC
10m - N3S Tube
8M - N2
6M - N1
5.2M - N1 + 2x TC (Practically Unusable)
« Last Edit: March 02, 2013, 05:34:19 PM by Brian Hirschfeld » Logged

www.brianhirschfeldphotography.com / www.flickr.com/brianhirschfeldphotography
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Leica / Nikon / Hasselblad / Mamiya ~ Proud IQ180 owner
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