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Thread started 28 Aug 2009 (Friday) 10:38

# Extension Tube and Loss of light

Aug 28, 2009 10:38 |  #1

Ok, I need to get something straight with regards to extension tubes and loss of light. I'm getting tired of hearing people say there is a loss of light when you use extension tubes. I just don't understand the logic behind it.

I know the inverse square law enough to know that distance is a key value there, so how could {X}mm of distance make any significant change in the amount of light.

Now before you reply, consider this, I don't care about 1 lumen of light. I am considering this in a photographic setting where I have to bump my ISO or slow down my shutter to get from shot A with no extension to shot with shot B with a 12mm extension tube and get the same exposure -- which I know is an idiotic comparison by itself since you can't ever repeat an exposure in a real world setting.

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Aug 28, 2009 10:49 |  #2

Easy example:

100mm lens at f2.8, 1/100sec 100 ISO = properly exposed shot

100mm lens at f/2.8 with 100mm extension tubes now equals 200mm lens with the same size aperture, but the f-stop (because it is a ratio of focal length and aperture size) is now f/5.6

You now have 200mm lens at f5.6, 1/25sec 100 ISO = properly exposed shot

Got it?

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Aug 28, 2009 11:22 |  #3

I'm not taking pringles can here, we're talking 12mm worth. or 24mm worth.

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Aug 28, 2009 11:34 |  #4

Adding 40% of the lens length will reduce your f-stop by one. So, by putting a 12mm tube on a 24mm lens or a 40mm tube on a 100mm lens reduces the f-stop by one stop.

Also take into consider the magnification and how that will affect your shots when hand held. Magnify the subject; magnify camera shake.

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Aug 28, 2009 11:35 |  #5

And if you aren't doing anything 1:2 or greater, then you probably don't really have to even worry about it. Your camera will meter for you anyways.

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Aug 28, 2009 11:54 |  #6

aram535 wrote in post #8538478
Ok, I need to get something straight with regards to extension tubes and loss of light. I'm getting tired of hearing people say there is a loss of light when you use extension tubes. I just don't understand the logic behind it.

I know the inverse square law enough to know that distance is a key value there, so how could {X}mm of distance make any significant change in the amount of light.

Now before you reply, consider this, I don't care about 1 lumen of light. I am considering this in a photographic setting where I have to bump my ISO or slow down my shutter to get from shot A with no extension to shot with shot B with a 12mm extension tube and get the same exposure -- which I know is an idiotic comparison by itself since you can't ever repeat an exposure in a real world setting.

Aram,
You should understand that the way that extension tubes work is to simply project a larger image circle size at the focal plane of the camera, so as to increase the apparent subject size in the frame. So available photons in the image circle are spread over a greater area, leading to 'loss of light' within the frame itself. So the amount of 'magnification' due to the extension tube has a direct effect on the exposure within the frame. It is an in-camera application of the Inverse Square law!

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Aug 28, 2009 12:19 |  #7

And if you aren't doing anything 1:2 or greater, then you probably don't really have to even worry about it. Your camera will meter for you anyways.

This was my thought, that yes there is a loss, but not in any way shape or form is going to effect me. So is it really a loss of light? I'm still getting what I want at the DOF that I want without having to sacrifice shutter speed or higher ISO.

Wilt wrote in post #8538919
Aram,
You should understand that the way that extension tubes work is to simply project a larger image circle size at the focal plane of the camera, so as to increase the apparent subject size in the frame. So available photons in the image circle are spread over a greater area, leading to 'loss of light' within the frame itself. So the amount of 'magnification' due to the extension tube has a direct effect on the exposure within the frame. It is an in-camera application of the Inverse Square law!

Ok, so maybe it's my point of view, that enlarging doesn't mean loss of light. The amount of light isn't falling off, you're getting less light because you're looking at small amount of space. No?

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Aug 28, 2009 12:32 |  #8

aram535 wrote in post #8539069
This was my thought, that yes there is a loss, but not in any way shape or form is going to effect me. So is it really a loss of light? I'm still getting what I want at the DOF that I want without having to sacrifice shutter speed or higher ISO.

Ok, so maybe it's my point of view, that enlarging doesn't mean loss of light. The amount of light isn't falling off, you're getting less light because you're looking at small amount of space. No?

Yes. It is a loss of light. Precisely how much loss is a function of the amount of additional magnification afforded by the use of the tube.

A really simple way to think about it is to imagine a slide projector casting an image on the wall. If you move the projector away from the wall, the image will increase in magnification. But the total light output remains the same (you have only changed the position of the machine). Therefore, the light intensity per unit area must be lower, which means the image appears dimmer as it grows in size, and brighter as it shrinks (if you were to bring the projector closer to the wall).

The relationship between the distance of the projector to the wall and the light intensity, however, is an inverse square one. That is to say, if you move the projector twice as far away, the light intensity has decreased by a factor of 4.

This is why even a seemingly minor increase in the separation of lens and body (via an extension tube) can lead to significantly less light gathering capability, because the effect is squared. With a small tube, it may be less than a full stop, but since the increase in magnification is directly linked to the amount of light loss, it is unavoidable.

You are correct that the total light coming in has not changed, but since the only light that is captured is that which falls upon the sensor--everything else is either absorbed by the mirror box or the interior of the extension tube--the fact remains that at the sensor, the light intensity per unit area has decreased.

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Aug 28, 2009 12:34 |  #9

Yes, there is really a loss of light. Your DOF will change when you focus closer BTW.

Think about it this way (not even macro):

50mm lens with a 50mm aperture is a 50mm f/1.0
50mm lens with a 25mm aperture is a 50mm f/2.0

F-stop = Lens focal length/aperture size

300mm f/2.8 lens = same around of light on sensor as 100mm f/2.8

So if we increase the focal length (by adding tubes), and don't touch the f-stop, the effective f-stop will change because it is a ratio of the focal length to the aperture size which means that you will have less light on the sensor.

Enlarging means loss of light. Have you ever used an overhead projector? Same principal.

No, you are getting less light because you are spreading the same amount of light over a larger area. think of a flashlight with an adjustable beam. Long and narrow is bright, but without moving, if you widen the beam it is dimmer all over. The amount of light is the same, you are just spreading the light over a larger surface.

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Aug 28, 2009 12:39 |  #10

Ok, I understand the tech behind it now at least. Thanks.

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Aug 29, 2009 09:23 |  #11

Yeah, we used to call it bellows compensation on our old view cameras and had a formula to figure out the metering with film. I had the rail on my view camera marked to help me with this.

But I use one or 2 12mm extension tubes all the time with everything from 24mm up to 200mm and to be honest, I've never even thought about light loss or even noticed it. I'm sure if I did a bunch of tests, I might be more aware of this but it's just a non issue for the way I work. I don't even have to think about it.

Go ahead and get the tube or tubes, you'll love them, I use them - a lot. Currently I'm waiting to see if Canon comes out with a 100macro IS and if I like it, I'll get one but still use tubes with my other lenses as I like the different perspective a variety of lenses can offer when shooting close ups and table top. But if they don't make a new macro 100 then I'll probably add the Canon 25mm tube to my 2-12mm tubes I now have. I rarely need to go to say 32mm in tubes but occasionally it happens. Most of the time I use 12mm up through my 135mm lens and then might use one or two on my 70-200 2.8 zoom. They are extremely handy and take so little space in the bag when I'm out or sometimes I'll just carry one in my pocket when I want to go light.

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Aug 29, 2009 09:25 |  #12

If the ratio of extension/FL is in the range of 1/10 to 1/5, the increase in exposure needed is 0.5EV

1/4 to 1/3, increase needed is 1EV

For example, 25mm tube on 100mm lens is 1/4, so giving 2/3EV is not without merit

It does mount up!

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Aug 29, 2009 09:41 |  #13

Jannie wrote in post #8543785
Go ahead and get the tube or tubes, you'll love them, I use them - a lot.

Oh I already have them, a 3 size pack from Kenko. This was an argument I got into with someone and they were saying that "Loss of light" is a Con of using Tubes. Which I disagree on. If you don't have to worry about it than it's not a Con.

Just to get off topic a little ...

Currently I'm waiting to see if Canon comes out with a 100macro IS and if I like it, I'll get one but still use tubes with my other lenses as I like the different perspective a variety of lenses can offer when shooting close ups and table top.

According to canonrumors it's just about a done deal -- and it's going to be an "L" lens according to the info.

I'm half tempted to sell my 100mm macro, again. Not sure if I need IS though. 75% of my macros are on tripod which I would have to turn off the IS on anyway, the rest are in good light which means IS won't even kick in.

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Aug 29, 2009 09:42 |  #14

Wilt wrote in post #8543793
If the ratio of extension/FL is in the range of 1/10 to 1/5, the increase in exposure needed is 0.5EV

1/4 to 1/3, increase needed is 1EV

For example, 25mm tube on 100mm lens is 1/4, so giving 2/3EV is not without merit

It does mount up!

I've been using them on my 70-200mm with a lot of success, I've never had to adjust my settings and that distance I'm in Manual so I would notice a drop off of light.

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Sep 03, 2009 20:13 |  #15

aram535 wrote in post #8543853
Oh I already have them, a 3 size pack from Kenko. This was an argument I got into with someone and they were saying that "Loss of light" is a Con of using Tubes. Which I disagree on. If you don't have to worry about it than it's not a Con.

Ask them "compared to what". A macro lens of constant focal length (the 100 f/2.8 (non-L) macro actually decreases focal length as you focus closer) will suffer the same falloff of intensity as a "normal" lens of the same f.l. and tubes for the same magnification. And "close up lenses" also decrease the focal length of the lens/close-up lens combination.

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