thoughts on extension tubes and lenses


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toasty

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I've been thinking about how a lens focuses and how extension tubes work to allow you to focus closer, while giving up the infinity focus. I've read that focusing on subjects close to the lens presents optical challenges to lens manufacturers, and yet, it seems that the solution of simply increasing the distance between the lens and the film plane is so simple. I don't suppose anyone here has an insight into why lens manufacturers don't simply have a kind of sliding lens mechanism inside the barrel which can slide the whole lens forwards and backwards thereby enabling the photographer to shoot macro when he desires (giving up infinity focus and maybe a stop or two) and also to shoot at infinity when required, by sliding the glass back. It seems that it would not increase the cost of producing the lens very much, and would be quite a boon to macro photographers.
 

ST1100

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#2
Er, they've already done that. They're called macro lenses. Essentially lenses with extension tubes built in.
 

toasty

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ST1100 said:
Er, they've already done that. They're called macro lenses. Essentially lenses with extension tubes built in.
Are you sure? I was under the impression a macro lens is a lens with specially designed optics (ie: not just a tube) that are capable of focusing closer than a normal lens. If it is just a tube, you will lose the infinity focus, and you also lose an f-stop or two.
 

toasty

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#5
RossChang said:
F-stop?... don't think so... u loose F-Stop using extenders, not extension tubes.
I believe that extension tubes (a tube which goes between the lens and the film) does reduce the maximum f-stop of the lens. Extenders also cost you an f-stop, but extension tubes, will certainly reduce the amount of light (or photons per second) striking the film plane. This is the same as losing an f-stop. A lens which has a maximum aperture of 2.8 will have a maximum aperture smaller than 2.8 when you mount it on a tube. The amount of decrease depends on the focal length of the lens, but it is guaranteed, the aperture will be less. Often you don't worry about it though, because you just set your flash to TTL and shoot. The close distance of the subject means that the loss of the f-stop is normally not serious. You do however lose the ability to focus at infinity. This is why I believe that macro lenses, which can focus at infinity, are not just normal lenses mounted on an extension tube.
 

sumball

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toasty said:
....and you also lose an f-stop or two.
u dun lose any of it since there is no glass in between the tube, it is just a TUBE. :D
 

eawtan

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#7
Calculation on light loss with extension tubes is explained in : http://www.clearsightusa.com/extubecomp.html

toasty said:
I believe that extension tubes (a tube which goes between the lens and the film) does reduce the maximum f-stop of the lens. Extenders also cost you an f-stop, but extension tubes, will certainly reduce the amount of light (or photons per second) striking the film plane. This is the same as losing an f-stop. A lens which has a maximum aperture of 2.8 will have a maximum aperture smaller than 2.8 when you mount it on a tube. The amount of decrease depends on the focal length of the lens, but it is guaranteed, the aperture will be less. Often you don't worry about it though, because you just set your flash to TTL and shoot. The close distance of the subject means that the loss of the f-stop is normally not serious. You do however lose the ability to focus at infinity. This is why I believe that macro lenses, which can focus at infinity, are not just normal lenses mounted on an extension tube.
 

toasty

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sumball said:
u dun lose any of it since there is no glass in between the tube, it is just a TUBE. :D
Although it's true that it is just a tube and does not absorb any photons, the photon density is smaller as you move the lens away from the film plane.

This phenomenon is similar to how the intensity of a torch light will drop as you move the torch further away from the wall. You are effectively doing the same when you move the lens further from the film plane.

You may legitimately ask where did all those photons go? They didn't get absorbed, and they didn't disappear by magic. The answer is that they are now falling outside the area of the film. As you move the lens away from the film plane, the image size gets bigger, but the image intensity drops proportionately. This drop in intensity is the same as a loss of f-stop. You will need to open your shutter for longer in order to get the same exposure at the same ISO, same subject etc...
 

RossChang

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#9
Won't argue on the theorical side of physics... it doesn't not matter... as IF you're gonna take out a pen and paper to do some physics before u mount on ur extension tubes...

all it matters is, when I mount on extension tubes, I still get the same Max Appeture I can use. tat's it.

With extender, say, 2x, ur 2.8 immediately become 5.6.. (any mistake on the figures here? hehehe... )
 

Terence

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Take the MP-E 65mm for example. It's essentially a macro lens with 1-5x magnification. It's really 5 25mm tubes built in (someone correct me if I'm wrong). You do lose a bit of light as magnification increases. Nominal viewing aperture at 1x is about f5.6 and going to f16 at 5x. The viewfinder gets mighty dark at 5x.

 

dkw

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#12
RossChang said:
Won't argue on the theorical side of physics... it doesn't not matter... as IF you're gonna take out a pen and paper to do some physics before u mount on ur extension tubes...

all it matters is, when I mount on extension tubes, I still get the same Max Appeture I can use. tat's it.
Ross, you've got it confused.....
no theoretical physics or calculators required, but if somebody wants to shoot a properly exposed macro with an extension tube, he surely needs to know what he is dealing with. You definitely lose light when you put on a tube. That the camera reports the same maximum aperture on your parent lens is irrelevant to the fact that the tube has moved the rear-most element of the lens further away from the film plane, resulting in greater light fall-off (toasty has a good explanation)


RossChang said:
With extender, say, 2x, ur 2.8 immediately become 5.6.. (any mistake on the figures here? hehehe... )
This is where the confusion arises. The camera may now report that you are f5.6, however, the max aperture attainable by your parent lens is still f2.8(as above). In fact, you can trick the camera into believing that you are still at f2.8 by either taping some pins on a Canon EF mount extender, or using a 3rd party extender which does not report the change in aperture. Your camera may now report f2.8, but that does not mean that you have not suffered light fall-off. This has implications for 1) focussing and 2) metering

Cheers,
 

dkw

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#13
toasty said:
I've been thinking about how a lens focuses and how extension tubes work to allow you to focus closer, while giving up the infinity focus. I've read that focusing on subjects close to the lens presents optical challenges to lens manufacturers, and yet, it seems that the solution of simply increasing the distance between the lens and the film plane is so simple. I don't suppose anyone here has an insight into why lens manufacturers don't simply have a kind of sliding lens mechanism inside the barrel which can slide the whole lens forwards and backwards thereby enabling the photographer to shoot macro when he desires (giving up infinity focus and maybe a stop or two) and also to shoot at infinity when required, by sliding the glass back. It seems that it would not increase the cost of producing the lens very much, and would be quite a boon to macro photographers.
Practically, there are a fair number of zooms and primes which already have a dedicated macro function, some with very excellent optics. Alternatively, it should be easy enough to slap on a tube, wouldn't it? I for one wouldn't want this feature built into every lens, if it means any increased cost.

Cheers,
 

RossChang

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#14
kie dkw... think I get what u mean... but with the lost of light with extension tube, and even the main lens appeture remains at the max 2.8 appeture... won't the meter compensate for that? (tat is, unless u are shotting in full manual lar...)
 

smallaperture

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Ext tube shouldn't be used on short focal length lenses, becos, with the tube, your subject would become too close to the front element of the lens, so that the corners would not be sharp, even if you stop down to F16 or even smaller aperture.

Ext tube used on longer focal length lenses like from say 105 onwards would be better. Even better would be to use on longer lenses like 180 or even 200mm lenses. And if you're an ocassional tube user, even a zoom like 80-200 could be used, being easier to find the focus.
 

dkw

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#16
RossChang said:
kie dkw... think I get what u mean... but with the lost of light with extension tube, and even the main lens appeture remains at the max 2.8 appeture... won't the meter compensate for that? (tat is, unless u are shotting in full manual lar...)
Ross,
here's an interesting article-->
http://www.photo.net/macro/primer

You ask a good question. From my understanding, the extension tube, if it reports the aperture 'as is', would cause the camera to meter incorrectly. However, the article above seems to be ambiguous about it. Anybody else got a better grasp of this?

Cheers,
 

toasty

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#17
dkw said:
Ross,
here's an interesting article-->
http://www.photo.net/macro/primer

You ask a good question. From my understanding, the extension tube, if it reports the aperture 'as is', would cause the camera to meter incorrectly. However, the article above seems to be ambiguous about it. Anybody else got a better grasp of this?

Cheers,
From my understanding of how the meter works, it would be able to select the correct shutter speed. It's just the shutter speed will be slower than if you did not have the tube. Here's how it works: The lens is producing an image of the subject (upside down) on the CCD. When you move the lens further from CCD or film plane, the image size increases, the position of the subject in focus moves closer and the intensity of the image decreases (think of shining a torch on a wall and moving the torch further back, to understand why the first and the third things happen). The meter is sitting somewhere behind the lens reading the amount of light which is coming through the lens. It chooses the correct shutter speed such that enough light has fallen on the CCD to give what it judges to be a correct exposure. When the tube causes less light to reach the meter, the meter will just determine that a longer exposure is needed in order that enough light has been captured. Typically though, you will need a small aperture to get sufficient depth of field on your subject, you'd set it to something like F16 or F22, on the lens, which is actually smaller than F16 or F22 because of the tube. Then you set your flash on TTL and you don't worry about the exposure. The only thing you worry about is the composition and getting your subject in focus. Both these are much more difficult than in conventional photography.
 

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