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#1
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Using extension tubes?
I have never used extension tubes and would appreciate feedback on
some questions I have about it. 1. As I understand it the use of extension tubes is mainly used to convert normal and telephoto lenses to macro lenses -- is that correct? 2. If I were to add a 25mm tube (Canon EF 25 II) between my camera (Canon 1Dm2) and a 300mm lens (Canon EF 300mm /4.0) what would the effect be on: minimum focus distance, image size, image brightness (EV), camera function (AE/AF)? 3. What effect does using different tube lengths have? Thanks, Brian |
#2
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1. Absolutely correct.
2. With the EF300/4, your magnification will be between 0.24 (close focus) and 0.09 (infinity focus*), with focus distances of between 1661 and 3925. (Information sourced from http://www.bobatkins.com/photography/eosfaq/closeup.htm) Now, as you will see from that site, you will get more magnification when using lower focal length lenses. I have been using the 50/1.8 with the Kenko tubes (much cheaper than teh Canon jobs, and as there is no glass, they are the same optical quality - they also feel very secure when holding a 70-200/2.8. However Kenko tubes will not work with EF-S lenses, only the new Canon mkII extenders do.) Anyway, here are some samples with tubes: http://planeguy.mine.nu/pictures/?gallery=6 I also, have more images in my other galleries: http://planeguy.mine.nu/pictures/ Hope this helps. |
#3
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Extension tubes do exactly what you do when focusing a lens. They just do
more of it. This may not be obvious with some modern lenses, but if you look at an older camera, you'll see that in order to focus closer, the lens moves forward. Extension tubes allow you to move it even farther forward, thus focusing even closer. |
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Brian Stirling wrote:
3. What effect does using different tube lengths have? In "macro mode" it's usually easier to work in terms of magnification than distance. If your lens has a maximum magnification of Mmax, and you extend it by 'e', then the maximum extended magnification is simple: Mmax + e/f, f = focal length. The EF 300/4 IS is actually rather nice for macro work alone, or in concert with the 2x teleconverter (manual focus required at f/8 with gumby bodies like a 10D). It's Mmax is 0.25 (min distance is 1.5m), or 0.5 with the 2x converter. |
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Brian Stirling wrote:
3. What effect does using different tube lengths have? In "macro mode" it's usually easier to work in terms of magnification than distance. If your lens has a maximum magnification of Mmax, and you extend it by 'e', then the maximum extended magnification is simple: Mmax + e/f, f = focal length. The EF 300/4 IS is actually rather nice for macro work alone, or in concert with the 2x teleconverter (manual focus required at f/8 with gumby bodies like a 10D). It's Mmax is 0.25 (min distance is 1.5m), or 0.5 with the 2x converter. |
#10
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To focus closer you need to increase the distance from the optical centre of
the lens to the film/ccd. So adding a tube allows you to focus closer and stops you from focussing to infinity. You will see most effect on shorter lenses as the length of the tube is a higher proportion of the focal length. For the light dropoff: 'bellows factor' = extension squared divided by focal length squared (extension = focal length plus tube) 50mm lens with 25mm extension, BF=(75x75)/(50x50)=2.25 (increase exposure by 2.25x or just over 1 stop) 50mm lens with 50mm extension, BF=(100x100)/(50x50)=4 (increase exposure by 4x or 2 stops) 50mm lens with 100mm extension, BF=(150x150)/(50x50)=9 (increase exposure by 9x or just over 3 stops) Exposure Factor 1x 1.4x 2x 2.8x 4x 5.6x 8x 11x 16x Loss in Stops 0 0 .5 1 1.5 2 2.5 3 3.5 4 (might have to TAB this yourself) Another way to look at it: BF = (Magnification +1) squared. At 1:1 you have (1+1) sq = 4x, 2:1 (2+1)sq = 9x etc In camera metering with 'auto' tubes negates you having to calc any of this, but it's handy to know what the relationship is. If you're using a hand-held meter, or manual flash, then the formulae become more useful... Cheers, Jason (remove ... to reply) Video & Gaming: http://gadgetaus.com |
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