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Canon EOS Full Frame for HD
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Old April 29th, 2009, 06:31 PM   #1
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Extension Tubes

Reading this review:
Canon 25mm Extension Tube II (EF 25) Review

it seems that extension tubes not only decrease minimum focus distance (poor man's macro), but they also...

* decrease light (makes sense - the image is magnified)
* add softness (again, magnification)
* show up lens imperfections (magnification again), and
* add vignetting on full frame cameras

It's the last point that puzzles me. If the image is magnified, doesn't that push the corners beyond the sensor? Okay, with a really long tube, if the opening is less than 43.3mm, it could crop the image, but the tube has a bigger inner diameter than the sensor, doesn't it?

Anyway, has anybody here used an extension tube? Were the results good regarding light falloff? What focal length and tube length did you use?

For video, I'm not worried about softness. With a fast lens, I'm not worried about light lost (macros are generally a stop or so slower than normal primes anyway). I would be bummed if the corners have hard crops though. I don't want 940p video.

Thanks!
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Old April 29th, 2009, 06:51 PM   #2
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Originally Posted by Jon Fairhurst View Post
Anyway, has anybody here used an extension tube?
As a kid I was using a 50mm tube on my 35mm SLR -- to shoot close-ups of bugs ;^) The light loss (merely mentioned in the article) was horrendous.

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Old April 29th, 2009, 07:50 PM   #3
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Thanks!

From the article, a 50mm lens on a 25mm tube gets another 0.5x magnification. The Canon 50mm f/1.4 has a max magnification of 0.15x, so add the tube and get 0.65x or so.

Doing the math, that's a size increase of 4.33. Square it and we get a factor of 18.78 - or over four stops of light lost. Ouch. Your f/1.4 lens becomes worse than f/5.6. (I think I got that right.)

The 12mm tube might be more reasonable. Things grow by 2.6 times (from 0.15x to 0.39x.) If my math is right you lose about 2.7 stops. That makes the lens f/3.5 or so. This might not be great for bugs, but should allow closeups of hands and other human-scale things. That could be workable for video.

With a 50mm tube, you probably couldn't tell if there was any vignetting!
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Old April 29th, 2009, 07:51 PM   #4
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Hi Jon,

I routinely use(d) extension tubes in preference to diopter lenses for 35 mm size film or now a somewhat smaller digital sensor. No extra glass means sharper images. By choosing the lens/extension tube length combination carefully you can get very tight close-ups with some working distance. If you need to get super close and need to use a short lens (like a 50 mm lens), you can turn the lens around using a double-threaded adapter so the front equipment on the lens is not in the way and the lens "face" is further away from the subject.

However, I find using a 100 mm lens or longer and experimenting with the extension tube length, you can get superb images. In one case, I used a 3 foot extension tube on a medium format camera to shoot the eggs of a monarch butterfly using a 150 mm lens (for the medium format camera this is a small telephoto) -- worked fine with no vignetting, but this was still photography and elecronic flash.

I currently use extension tubes and an 80-200mm Nikon lens on my Canon XL H1s (1/3" video cam sensor) for sharp magnification. With reasonable light outdoors, I get get quite good f-stop settings.

Alan
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Old April 29th, 2009, 08:25 PM   #5
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Jon you can buy sets of cheap no-brand extension tubes for around $20 - no need to buy the Canon version as you won't be using auto focus anyway.

I have a set which I use for play - comprising 12mm, 18mm and 30mm, allowing you to stack any combination thereof. I've just taken two shots for you using the 30mm tube and a Nikkor 50mm 1/8 to test for vignetting - as you see it's hardly noticeable. You lose a stop or two exposure-wise but nothing drastic.

Also, while it only provides 1:2 magnification, I really enjoy using the Nikkor 55mm f2.8 (it can be had cheaply). The big advantage with this lens is you have infinity focus, whereas with the tubes you MUST take a macro shot when attached - you are restricted to very tight DOF by default.

But for $20, you can't go wrong with a set of cheap metal tubes:
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Extension Tubes-img_3251.jpg   Extension Tubes-img_3254.jpg  

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Old April 29th, 2009, 11:58 PM   #6
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Thanks Josh,

Those shots are close enough for my needs. For most video, we don't need anything closer than, say, a pencil, a hand, a cigarette, etc.

And Alan, a 3-foot extension tube... Monarch butterfly eggs... that's intense!
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