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Thread: Optics Information Required

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    Glenn NK's Avatar
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    Optics Information Required

    I have a Canon 100 macro f/2.8 lens, a 2.0EX extender, and a set of extension tubes.

    The lens and extender are not compatible (the rear element of the lens conflicts with the extender).

    Just read somewhere else that by using an extension tube on the rear of the lens, the tube can be fitted to the camera body. I tried it and it works.

    What is the formula for the focal length of a lens plus an extension tube?

    I know what effect an extender has - in my case the resultant FL of lens plus tube would be multiplied by 2.0.

    Glenn

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    Re: Optics Information Required

    http://www.cambridgeincolour.com/tut...es-closeup.htm

    Also google 'lens extension tube factor'

    You'll find a couple of PDF's with more detailed calculations. One from Kenko looks promising.
    Last edited by Andrew1; 3rd September 2012 at 08:52 PM.

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    herbert's Avatar
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    Re: Optics Information Required

    Magnification = (Focal length + extension tube length) / focal length

    So extension tubes work better on shorter lenses.

    Alex

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    Glenn NK's Avatar
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    Re: Optics Information Required

    What is the focal length of lens plus extension tube?

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    Re: Optics Information Required

    Focal length of your lens = 100mm + the extension tube = 12mm, 20mm, 36mm etc.

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    Re: Optics Information Required

    Quote Originally Posted by Andrew76 View Post
    Focal length of your lens = 100mm + the extension tube = 12mm, 20mm, 36mm etc.
    Thanks Andrew.

    Glenn

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    Re: Optics Information Required

    No its not. The focal length is unaffected. The only thing that changes is the ability to focus close up.

    HTH
    JC

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    Re: Optics Information Required

    Quote Originally Posted by JonathanC View Post
    No its not. The focal length is unaffected. The only thing that changes is the ability to focus close up.

    HTH
    JC
    Hi Jonathan,

    Focal length is related to the angle of view. If you put on an extension tube you move the imaging plane (e.g. sensor/film) further away from the projected image. The effect is that you will have to (1) refocus the image and (2) you will sample a sub-section of the original image (as it is spreading out from the lens in a cone but your sensor is a fixed size). The result is that (1) the minimum and maximum focus distance are affected due to focusing differences and (2) the angle of view captured is smaller. Since the angle of view is smaller, this can be interpreted as changing the focal length. You are not changing the physical optics of the lens but the effect on the image is the same as if you did.

    @Glenn,

    I've been away for a while so I'm sorry I misread your question. Thanks to Andrew for filling in the blanks.

    Alex

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    Re: Optics Information Required

    Quote Originally Posted by herbert View Post
    Magnification = (Focal length + extension tube length) / focal length

    So extension tubes work better on shorter lenses.

    Alex
    Extension tubes work exactly the same with any focal length ... you just need more extension with a longer lens so to achieve double extension with a 135 lens you need 135mm extension ... in theory anyway ... it is not something I bother about just the resultant image since with TTL metering one doesn't need to do calculations any more. In getting the greater extension it probably means you will be using plain tubes [ once I used a length of plastic drainpipe ] and will need manual control of the iris in the lens.
    It is probably more conveinient with shorter lenses to have full control against that is being on top of the subject. Swings and roundabouts.

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    Glenn NK's Avatar
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    Re: Optics Information Required

    I've checked other sources - the answer by Andrew is correct: new FL = lens FL + Extension tube length. It's that simple.

    However, note that the FL of the lens does not change (John), but the addition of a tube increases the FL of the combination.

    For my purposes, the change in FL with the 12 mm tube was more academic than significant; the tube was put in to allow the use of a 2.0 Extender (note my first post).

    And what I also learned that the FL changes with the Focus distance to some degree - FL is not a constant.

    Glenn
    Last edited by Glenn NK; 8th September 2012 at 05:06 PM.

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    Re: Optics Information Required

    You might have checked but that is still incorrect. It's more easily understood by forgetting the focusing mechanism built into the lens. Take a simple lens and the closer the object is to it the further away the image is from the lens. Google optics magnification and you will find standard formulae for the arrangement. A camera lens arranges for changes of focus to still form an image on the sensor. Set it to some focus point add an extension tube and the focal point will move closer. The focal length of the lens remains the same. In real terms as a camera lens is focused the focal length probably does change but it will be very marginal and shouldn't be of any real consequence.

    Any lens can be used in macro mode with extension tubes and the amount needed for a certain magnification factor depends on how close the lens will focus in the first place. For instance one lens I looked at recently will focus down to 250mm and capture about 80mms on the sensor. In this case the magnification is about 0.2. From that the effect of extension tubes or close up lenses can be worked out. Closest focus with and without tubes / close up lenses may not give the best optical performance. Has to be tried to find out.

    I found this site when looking for a decent macro tutorial for some one

    http://www.cambridgeincolour.com/tut...cro-lenses.htm

    There is also a 2nd page off that with calculators that give the effect of close up lenses and extension tubes. Once the magnification of the base lens is known it's easy to see what it can do. Then comes the subject. The magnification relates to it's size on the sensor.

    It is all worth going through at least once even if to only guide purchasing decisions or see what available kit can do. That way you can find out if you can photograph an ant or an elephant. There is also some scope for cropping. I would be happy only using a 1/4 of the 80mm frame providing that it was well focused. The results might even be better than using a close up lens or extensions tubes to fill the entire frame with a 20mm object.

    The sums are also well worth going through if bellows are used. Without a tape measure they can be very difficult to set up and not even easy with the correct distances being available.

    John

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    Re: Optics Information Required

    Come on Gentlemen stop pulling my leg. The focal length of a lens is only 'as marked' when it is focused at infinity. when focused closer the focal length increases ... it doesn't matter if you focus closer with the focusing mechanism of the lens or by physically moving it forward, increasing the extension. This was one of my photo exam questions back in the fifties which I passed with distinction

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    Re: Optics Information Required

    Quote Originally Posted by jcuknz View Post
    Come on Gentlemen stop pulling my leg. The focal length of a lens is only 'as marked' when it is focused at infinity. when focused closer the focal length increases ... it doesn't matter if you focus closer with the focusing mechanism of the lens or by physically moving it forward, increasing the extension. This was one of my photo exam questions back in the fifties which I passed with distinction
    I'm not pulling your leg (last sentence in Post No.10). I notice this change in FL in practice all the time.

    Perhaps you might offer an opinion on the point of contention?


    Glenn

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    Re: Optics Information Required

    Quote Originally Posted by glenn nk View Post
    i'm not pulling your leg (last sentence in post no.10). I notice this change in fl in practice all the time.

    Perhaps you might offer an opinion on the point of contention?


    glenn
    lol I'm falling of my seat. Alternative physics. We have a new form of zoom lens. Even primes must zoom. Well they do a little but it's rather hard to spot as it will be with extension tubes on as well.

    John

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    Re: Optics Information Required

    Quote Originally Posted by Glenn NK View Post
    For my purposes, the change in FL with the 12 mm tube was more academic than significant; the tube was put in to allow the use of a 2.0 Extender (note my first post).

    Glenn
    I used the 12 mm tube because it was the shortest that would permit use of the extender, and had the least effect on maximum focus distance.

    So I still have a question which I posed elsewhere: if the magnification changes using an extension tube (which it must do, else it would be of no use for macro), then why doesn't the effective FL change (ie: the combined focal length of lens + tube)?

    Glenn
    Last edited by Glenn NK; 9th September 2012 at 12:12 AM.

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    Re: Optics Information Required

    Not sure why you want to know the new apparent focal length - I am a suck it and see type. But with a 100mm lens and a 10mm extension the subject distance for a lens set on infinity is 1.1m. For a 12.5mm extension the maximum subject will be about 900mm from the camera. I do not think you can just multiply by 2 when using the extender. With the 2x tele extender on my guess is you will be able to focus on something up to about 2-3m away.
    Last edited by pnodrog; 9th September 2012 at 02:40 AM.

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    Re: Optics Information Required

    Quote Originally Posted by Glenn NK View Post
    I used the 12 mm tube because it was the shortest that would permit use of the extender, and had the least effect on maximum focus distance.

    So I still have a question which I posed elsewhere: if the magnification changes using an extension tube (which it must do, else it would be of no use for macro), then why doesn't the effective FL change (ie: the combined focal length of lens + tube)?

    Glenn
    I think reading up on optics will clarify a bit, even though there are rather fuzzy semantic concepts flying around here.

    So, no, the focal length does not change when you focus a macro lens (of the kind that becomes longer when focusing). Adding an extension tube to a lens and putting that combination on a camera will have no effect on the focal length of the lens, but alas, there's a fuzzy concept of "back focal length" that has indeed increased.

    Now we get to your extension plus a tele converter, which indeed does change focal length. Now if you add an extension between the converter, which is a negative Barlow system, you effectively decrease the focal length of the system, making it focus at a closer distance, without adding so much length to the system as if you had focused the longer lens resulting from adding the barlow lens without an extension between them. Your "back focal length" always remains the same as long as you have the negative system mounted directly on the camera. So when you focus a lens that is on a tele "extender" (thingamajig, whateveryouwannacallit) which is a negative optical system that projects an enlarged image onto the sensor, you effectively decrease the focal length of the system, while maintaining its total length shorter for any distance you set, than if you had extended the whole system, by moving it farther away from the sensor.

    And macro lenses come in various flavours. Some of them extend when you focus them, some don't, the inner focused ones. The lens that is inner focusing, focuses by decreasing the focal length, without altering the total length. If a 100 mm macro lens will focus down to about 200 mm for 1:1 reproduction, its focal length will be 50 mm at that setting.

    I don't think that I have removed much of the confusion, maybe rather added to it, but in essence, an extension tube will not alter the focal length of a simple lens, but an extension added between lenses in a complex system, will alter its properties. In your example, the focal lentgth of the combined optical system will be decreased, effectively focusing it at a closer distance.

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    Re: Optics Information Required

    Thanks Urban - You are making me ponder. A Barlow lens does decrease focal length but as a tele extender is a negative Barlow lens I had assumed it would increase focal length. I think I am going to have to play around with my 1.4X and an extender to suck it and see. It is not a combination I have ever tried. At least you tend to confirm my understanding that the shape and refractive index of a lens (simple) relative to the refractive index of the medium it is in is responsible for the focal length and other factors such as focusing distance or magnification simply relate to how it is used.

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    Re: Optics Information Required

    I think there could still be some confusion here relating to Barlow. This should clear that up. A Barlow lens is a negative lens

    http://en.wikipedia.org/wiki/Barlow_lens

    Some years ago these could be freely moved around in telescopes to vary the increase in focal length they gave. Now just like similar items for cameras they tend to be fixed. I have seen macro adapters on the web. Pass on those, never used them. I suspect that they are just converters and double the size of the image and that's it.

    How a lens magnifies things according to it's distance from it is shown here without the usual maths and confusion. The lens can be dragged about with the mouse and the size of the image varies accordingly. All lenses behave like this. The distances between the object, lens and image vary according to the focal length of the lens for the same magnification. A camera lens also has to keep the image on the sensor.

    http://micro.magnet.fsu.edu/primer/j...ify/index.html

    With macro lenses the working distance increases with focal length for the same magnification. If a 100mm macro lens is persuaded to give 1:1 it will still have twice the working distance of a 50mm lens working at 1:1. Actually I just checked that at 2:1 in case camera manufacturers had done something strange. I am trying to sort out a macro lens for my pen. I have a 100mm Pentax one that I can use on an eos but it's a bit of a problem on a pen so I bought a 50mm one - and even that one is still a problem. The ultimate macro lens is probably the old 200mm nikon with it's kit. Maybe others do something similar now but they are likely to be incredibly expensive.

    The magnification still changes in the same way when a lens is turned round. That's why there are reversal rings about. Using the lenses the wrong way round will produce better optical performance when very short working distances are involved for high magnification.

    John

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    Re: Optics Information Required

    I do not think there was any confusion about a Barlow being a negative lens. As I said I think I will play with my 1.4X with an extension tube and then support whatever theory matches my results.
    Last edited by pnodrog; 9th September 2012 at 12:05 PM.

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