Kenko extension tubes

mattd

mattd

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Matt
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Hi,

I know that this is going to be a stupid question with a really simple answer but I have recently purchased some Kenko extension tubes for us on my 60D. Problem is that when I attach them (any combination) to my camera and stick my lens on the end (canon 50mm ef) I cant see anything through the view finder, just very vague and fuzzy outlines, exactly the same as I would if I took the lens off and looked through the viewfinder.

So it appears that my camera for some reason isn't able to see the lens which is fixed to the end of the extension tube\s.

What am I doing wrong?
:help:
 
Are they the auto ones or the manual ones?

I had a set of the auto ones which I enjoyed using but my brother only lent them to me for a month :-(

I assume you have used extension tubes before or are they new to you?

They do provide a very very narrow dof so I would just put the shortest one between your lens and body and experiment with that on manual focus, get it as least blurry as possible then move closer and farther from the subject and see if that's all it is, you may not actually be close enough.

Of course I could be teaching you to suck eggs here:bonk:
 
Do you normally set the lens to auto focus? Extension tubes can often prevent the lens auto focusing so it may be just well out of focus? Try manual focusing with the lens?
All the extension tubes do is move the lens further away from the sensor so they shouldn't effect anything other than the auto focus.

Just a thought.

Gareth
 
I use CANON tubes (12mm and 25mm) with my 60D and 28-135 Canon lens, are you taking into account that you need to be VERY close and focus manually.
 
Your working distance is greatly reduced when using extension tubes set to manual focus and move the camera backwards and forwards til the subject is in focus.
 
Donnie said:
Are they the auto ones or the manual ones?

I had a set of the auto ones which I enjoyed using but my brother only lent them to me for a month :-(

I assume you have used extension tubes before or are they new to you?

They do provide a very very narrow dof so I would just put the shortest one between your lens and body and experiment with that on manual focus, get it as least blurry as possible then move closer and farther from the subject and see if that's all it is, you may not actually be close enough.

Of course I could be teaching you to suck eggs here:bonk:
Click to expand...

I think they are automatic as it says Automatic Extension Tube Set DG on the box?

This is the first time that I have tried to use tubes which is why im sure I must be doing something stupid.

In have tried using the smallest ring and focusing manually but I cant get anywhere near focus even if im literally 4 inches from the subject. Through the viewfinder it looks just the same as if I had taken the lens off.
 
I will try focusing manually again when I get home, im assuming though that with a 50mm I should be able to focus on something within 6 inches of the lens?
 
mattd said:
I will try focusing manually again when I get home, im assuming though that with a 50mm I should be able to focus on something within 6 inches of the lens?
Click to expand...

Will probably be even closer. With my 105 macro lens with a full set of tubes I would guess I am less than 50 mm way
 
Just a thought - might be easier to move back and forth with the camera rather than trying to focus the lens.
 
It does sound like you are just too far away. You will find the focus is ultra critical, so as someone else suggested, try setting the focus to infinity and move the camera in and out.

With all 3 Kenko tubes stacked on my D50 with the kit 18-55, I am anywhere from about 2" down to under 1" away from the subject! The shortest tube brings it out to more like 4-6" IIRC.

A focusing rail is really useful given the fine movement needed. I recently bought a Velbon one (nice quality piece of kit) and it makes life so much easier. Pity it doesn't improve my macro photography skills though :lol:
 
As mentioned already - With the 50mm on a Crop body with the full set of tubes you'll pretty much need to be touching whatever it is you're trying to focus on.
At least it is with my 50mm and the 30D - With that lens I tend to only use the 12mm or 20mm on their own.

Wouldn't bother pfaffing about trying to autofocus either, If you can't get a focus rail, try rocking back and forth till whatever you haver is nice and sharp. Also, because of the very very narrow DOF you'll need to keep the aperture up so lighting is important to avoid silly slow shutter speeds.
 
If you're brain works better than mine, you can calculate it with the following formula (not mine but one I found earlier)......

Suppose you have a lens of focal length F which can focus down to distance X when mounted on your camera. Suppose you use an extension tube of length L. Then you can focus down to the distance given by the formula

F(XF +XL - Lf)/(XL -LF +f^2)

In words, you do the following. First multiply the shortest distance by the focal length, the multiply it by the tube lenth and then multiply the tube length by the focal length. Add the first two numbers and subtract the third. Then multiply the result by the focal length. That gives you the numerator.

Next take the product of the shortest distance by the tube length and the product of the tube length with the focal length, both of which you already calculated, subtract the second from the first, and then add the square of the focal length. That gives you the denominator. Divide the numerator by the denominator, and you are done.

Here is an example. Let the focal length be 50 mm, the shortest distance 200 mm and the tube length 25 mm. Then with the tube, you can focus down to

50 x (200 x 50 + 200 x 25 - 25 x 50)/(200 x 25 - 25 x 50 + 50 x 50)

which comes out to 110 mm.


Some other good reading here
 
Last edited:
Thanks for your help everyone, it was just a case of me not being close enough, if i move to within 4 inches all of a sudden my subject comes into focus. :bonk:
 
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