Extension tubes

bionicsherpa

bionicsherpa

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Name
Ken
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Can anyone tell me if there is a way to calculate what effect adding an extension tube of a given length to a lens of given focal length will have? Or will it simply depend on the close focussing ability of the lens to which it is added?

Ken
 
There is and it's complex:-

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.

I looked at that back in my 35mm days. You'll spend more time doing maths than taking pics. I think trial and error is a bit easier.
 
arclight said:
There is and it's complex:-

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.

I looked at that back in my 35mm days. You'll spend more time doing maths than taking pics. I think trial and error is a bit easier.
Click to expand...

Wow. Thanks for taking the trouble to respond with that formula. I have put it into a spreadsheet and it looks as if extension tubes are practical with a 200mm lens but become rapidly less effective as the focal length increases. As you say, its not the sort of calc to do in the field. In my case I was contemplating purchasing extension tubes to allow a closer approach to butterflies etc with my 200 or 300mm lenses. I wanted to calculate how long an extension would be needed.

Thanks again.

Ken
 
coldpenguin said:
Hmm, seems my 70-300 is not worth trying on macro
f=300
l=80
x=1500

answer = 1.2m!
Click to expand...

a true macro lens is a flat field lens
i have had a few...50 or 55mm with tube giving the true meaning of macro
life size on the neg
so on a 35mm neg or tranny you would photo a ruler and the measurement was 35mm across..macro
anything greater is micro
anything less is playing around

i dont know the final definition with digital so tubes arent a necessity
but a true macro lens is for the flat field...

do you need a flat field?? answered here

i apologise but i dont understand the inference of a macro on a big lens although i have had them
 
bionicsherpa said:
Wow. Thanks for taking the trouble to respond with that formula. I have put it into a spreadsheet and it looks as if extension tubes are practical with a 200mm lens but become rapidly less effective as the focal length increases. As you say, its not the sort of calc to do in the field. In my case I was contemplating purchasing extension tubes to allow a closer approach to butterflies etc with my 200 or 300mm lenses. I wanted to calculate how long an extension would be needed.

Thanks again.

Ken
Click to expand...

Yes, my understanding is that tubes decrease in effectiveness as focal length increases. Don't seem to work with wide angle lenses either.

Strangely enough I was out today trying to photograph butterflies. They are not too keen to pose if you get near.
 
arclight said:
Yes, my understanding is that tubes decrease in effectiveness as focal length increases. Don't seem to work with wide angle lenses either.
Strangely enough I was out today trying to photograph butterflies. They are not too keen to pose if you get near.
Click to expand...

lie still and dont breath a lot...for me its just luck
 
arclight said:
Yes, my understanding is that tubes decrease in effectiveness as focal length increases. Don't seem to work with wide angle lenses either.

Strangely enough I was out today trying to photograph butterflies. They are not too keen to pose if you get near.
Click to expand...

Yep. That's exactly the problem. I have a 60mm macro that would let me fill the frame with a butterfly if it would allow it, yet my 200 and 300 lenses give a much smaller image albeit at a greater range. I thought if I could focus my 200 down to about 1m I might be getting somewhere and it looks as if I could with about 25mm of extension tube.

Thanks again

Ken
 
bionicsherpa said:
I make your answer 765. One of us has made a mistake - I will check again.

Ken
Click to expand...

Quite probably me!
Second attempt, I get a different answer again!

[root@mythfb1 backup]# echo $a
300
[root@mythfb1 backup]# echo $f
300
[root@mythfb1 backup]# echo $x
1500
[root@mythfb1 backup]# echo $l
80
[root@mythfb1 backup]# echo "$f*($x*$f +$x*$l - $l*$f)/($x*$l -$l*$f +$f*$f)"|bc
880
 
coldpenguin said:
Quite probably me!
Second attempt, I get a different answer again!

[root@mythfb1 backup]# echo $a
300
[root@mythfb1 backup]# echo $f
300
[root@mythfb1 backup]# echo $x
1500
[root@mythfb1 backup]# echo $l
80
[root@mythfb1 backup]# echo "$f*($x*$f +$x*$l - $l*$f)/($x*$l -$l*$f +$f*$f)"|bc
880
Click to expand...

:thinking:
 
HoppyUK said:
I have a seemless range going down through about half life size 1:2 at 2ft distance with 200mm, down to virtually 1:1 at 70mm at a distance of a few inches.
Click to expand...
how do you gauge what is 1:1...do you mean in the viewfinder..which would correspond to a frame size in film??

i had a set of tubes for my nikon ELW's..cheap ones but there's no glass so it doesnt matter..:)
 
mrcrow said:
how do you gauge what is 1:1...do you mean in the viewfinder..which would correspond to a frame size in film??

i had a set of tubes for my nikon ELW's..cheap ones but there's no glass so it doesnt matter..:)
Click to expand...

1:1 is life size on the sensor, so if the sensor is 22mm wide (Canon crop format) then if you can exactly fill the frame with 0-22mm on a ruler across the frame, that's 1:1.

Nikon's crop format sensor is factionally bigger, almost 24mm wide. And of course a full frame sensor or film format is 36mm wide.
 
HoppyUK said:
1:1 is life size on the sensor, so if the sensor is 22mm wide (Canon crop format) then if you can exactly fill the frame with 0-22mm on a ruler across the frame, that's 1:1.

Nikon's crop format sensor is factionally bigger, almost 24mm wide. And of course a full frame sensor or film format is 36mm wide.
Click to expand...

:gag:
stupid me...i am still getting a bit gagged up on the 35mm idea..but that is exactly what it is...or 94% of it on my nikons..

brilliant!....still glad i dont have film any more:thumbs:
 
HoppyUK said:
I use a set of Kenko tubes http://www.warehouseexpress.com/buy-kenko-dg-nikon-af-extension-tubes-set-36mm-20mm-12mm-/p1007657 for close up with my Canon 70-200. Works well for the kind of things you're trying to do.

I have a seemless range going down through about half life size 1:2 at 2ft distance with 200mm, down to virtually 1:1 at 70mm at a distance of a few inches.

Kenko tubes come up in the For Sale section now and then for about £80, which is where I got mine.
Click to expand...

I've seen Onestop Digital complemented on here >>>> http://www.onestop-digital.com/catalog/product_info.php?products_id=227

Almost the same as a 2nd hand price ;)

Phil
 
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