extension tubes and reverse ring, google is not my friend

squirrels

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So I've been playing with el cheapo extension tubes for a while and picked up a reverse mount ring as well. Now I'd like to understand how the focal length I choose for my lens mounted to the extension tubes is effecting magnification and how reverse mounting fits in. I tried google, but could only find information along the lines of you stick it on your camera and "Bam! Macro!" I want to know enough about how it works to know how much. Can someone point me in the right direction so I don't have to spend a day photographing a ruler in all these permutations?

Thanks!
Squirrels
 
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squirrels

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1mm of extension tube for every mm of lens focal length = 1:1. Read more here....


Thanks tirediron, I'd been on a break from Cambridge in Colour after their color managment tutorial left me crying quietly in the corner a few months ago, but this was super helpful especially the calculator. Will google further to tease out the reverse ring thing.
 

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Take a photo of a ruler, and compare the measurement shown in the image to the size of the sensor in the camera. From there, it's just plain old math.
 

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If you had a simple, single element lens, then the way you would focus on something closer than infinity would be to physically move the whole lens further away from the film/sensor plane. The further you move it, the closer you focus the thing. With a simple lens, you could focus very very closely. Basically as close as you can get to the glass when the lens is moved out far enough.

With actual commercial lenses, there are various problems you run into that stop you from focusing this closely:
1) Even if it was a simple lens system, the lens will only be built with some maximum amount of range that you can slide the lens back and forth, since lenses must be mounted in some sort of housing, and cannot just float in an abstract optical theoretical space. If the manufacturer design doesn't allow enough sliding room, then you wouldn't be able to focus on something arbitrarily close.
2) Well why don't manufacturers just allow enough slide to focus on anything? Because most complex lenses with multiple elements (which are necessary to fit on commercial lens moutns and correct for various optical flaws) do NOT focus by merely moving the entire lens forward and backward, but instead focus by moving just one "focusing group" back and forth. Sometimes, this focusing group is sandwiched in between other elements in the lens and cannot physically move far enough to focus on very close things without crashing into other glass elements.
3) Also, lens manufacturers want to avoid vignetting, and the further you slide a lens group forward, the more light is going to hit the walls inside the lens and be lost, unless you have sufficiently large internal lenses to catch it. If you just slide the focusing group forward without large enough lenses further back, then you'd get vignetting. In order to keep costs down and not have us lugging around lenses with 8 lb. pieces of glass in them, the manufacturers impose a minimum focusing distance and only let the focusing group slide that far, in order to help prevent vignetting and guaranteeing quality.

What an extension tube does is to let you get around this by physically moving the ENTIRE lens forward, instead of just the focusing group. This will always allow closer focusing, no matter what the lens design is. All in all, the effects are:

1) You will gain magnification ROUGHLY according to the following equations (these are not exact for a variety of reasons, but are pretty close):
a) Look up the maximum magnification factor for your lens online (you can calculate it yourself too using minimim focus distance and FL, but it's more complicated and involves some assumptions about lens design. Better to use known or empirical values). Let's say you use a 50mm lens and it has 0.1x max magnification. multiply this number by the focal length: 50mm * 0.1 = 5mm.
b) Now add the length of the extension tubes you put on the body to the above result. If we use 10mm of extension tubes, 10 + 5 = 15mm.
c) This new number divided by focal length = new magnification: 15 / 50 = 0.3x magnification.

edit: as mentioned in tirediron's link, this can be simplified I guess down to (mm extension / FL) for the gain only (not the total resulting magnification, must add original for that).

Note that with longer focal lengths, more extension tube will have to be used for the same magnification effect. If a 300mm lens has 0.1x magnification, and we add 10mm of extension tubes, its new magnification would only be 0.13x, a much more modest increase than on the 50mm lens.
Also note that if you add sufficiently many extension tubes, it is possible to end up with a close focus distance that is actually behind/inside your lens + its extension tubes, which means you can't focus on anything.

2) You will lose light directly proportionally to the amount of magnification you gain, at LEAST. So if your magnification at closest focus increases by a factor of 3 as in the above example, you will also get 1/3 as much light. This is due tot he fact that the lens is only letting is as much light as before, but now only 1/3 as much of it is falling on your sensor (because you magnified the image circle and part of it now falls off to the side of your sensor). In reality, you lose even more light than this, because some light will also start crashing into the sides of your extension tubes / camera body. This extra darkening will show up as vignetting.

3) You will lose the ability to focus at infinity. This is because the internal focusing system of your lens again has physical limits placed on it, and the focusing elements cannot move far enough back to compensate for the amount you moved the lens forward with your extension tubes (they're only built to go JUST far enough back for infinity when normally mounted). The further forward you move it, the further your lens will fail to meet infinity focus by. Usually this doesn't matter, because if you're putting on macro tubes, you don't WANT to focus at infinity anyway.

4) Your lens will gain additional chromatic and spherical aberration and astigmatism and coma, etc. (color fringing, softness, blur that increases as you move away from image center, etc.). Lenses are designed to have minimal aberrations when used attached to the lens mount, and by moving it further away, you are moving away from the optimally lowest point of optical flaws, so your image will have lower quality than normal. This may be more or less noticeable, depending on the lens.
 
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Gavjenks

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Macro reverse rings function entirely differently, and pretty much always inferior-ly, with MUCH larger increases in optical flaws, and no control over amount of magnification. And in addition to lower quality and less control over mag, you also cannot control aperture electronically like you can with electronically connecting extension tubes. This makes it annoying to compose or control your shot. Their only advantage is being cheaper.

Bellows allow more fine tuning than extension tubes (they can vary length continuously instead of in a fixed number of combinations, and do so more quickly), but also lose electronic communication. They are also usually flimsier than a solid metal/plastic tube, unless you get really nice ones.

Basically, electronic connection extension tubes are the best, as long as you can get the length you need by combining the ones you have.
 
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squirrels

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Excellent post. That made sense, I learned useful information, and it didn't require any red wine.:thumbup:

If you had a simple, single element lens, then the way you would focus on something closer than infinity would be to physically move the whole lens further away from the film/sensor plane. The further you move it, the closer you focus the thing. With a simple lens, you could focus very very closely. Basically as close as you can get to the glass when the lens is moved out far enough.

With actual commercial lenses, there are various problems you run into that stop you from focusing this closely:
1) Even if it was a simple lens system, the lens will only be built with some maximum amount of range that you can slide the lens back and forth, since lenses must be mounted in some sort of housing, and cannot just float in an abstract optical theoretical space. If the manufacturer design doesn't allow enough sliding room, then you wouldn't be able to focus on something arbitrarily close.
2) Well why don't manufacturers just allow enough slide to focus on anything? Because most complex lenses with multiple elements (which are necessary to fit on commercial lens moutns and correct for various optical flaws) do NOT focus by merely moving the entire lens forward and backward, but instead focus by moving just one "focusing group" back and forth. Sometimes, this focusing group is sandwiched in between other elements in the lens and cannot physically move far enough to focus on very close things without crashing into other glass elements.
3) Also, lens manufacturers want to avoid vignetting, and the further you slide a lens group forward, the more light is going to hit the walls inside the lens and be lost, unless you have sufficiently large internal lenses to catch it. If you just slide the focusing group forward without large enough lenses further back, then you'd get vignetting. In order to keep costs down and not have us lugging around lenses with 8 lb. pieces of glass in them, the manufacturers impose a minimum focusing distance and only let the focusing group slide that far, in order to help prevent vignetting and guaranteeing quality.

What an extension tube does is to let you get around this by physically moving the ENTIRE lens forward, instead of just the focusing group. This will always allow closer focusing, no matter what the lens design is. All in all, the effects are:

1) You will gain magnification ROUGHLY according to the following equations (these are not exact for a variety of reasons, but are pretty close):
a) Look up the maximum magnification factor for your lens online (you can calculate it yourself too using minimim focus distance and FL, but it's more complicated and involves some assumptions about lens design. Better to use known or empirical values). Let's say you use a 50mm lens and it has 0.1x max magnification. multiply this number by the focal length: 50mm * 0.1 = 5mm.
b) Now add the length of the extension tubes you put on the body to the above result. If we use 10mm of extension tubes, 10 + 5 = 15mm.
c) This new number divided by focal length = new magnification: 15 / 50 = 0.3x magnification.

edit: as mentioned in tirediron's link, this can be simplified I guess down to (mm extension / FL) for the gain only (not the total resulting magnification, must add original for that).

Note that with longer focal lengths, more extension tube will have to be used for the same magnification effect. If a 300mm lens has 0.1x magnification, and we add 10mm of extension tubes, its new magnification would only be 0.13x, a much more modest increase than on the 50mm lens.
Also note that if you add sufficiently many extension tubes, it is possible to end up with a close focus distance that is actually behind/inside your lens + its extension tubes, which means you can't focus on anything.

2) You will lose light directly proportionally to the amount of magnification you gain, at LEAST. So if your magnification at closest focus increases by a factor of 3 as in the above example, you will also get 1/3 as much light. This is due tot he fact that the lens is only letting is as much light as before, but now only 1/3 as much of it is falling on your sensor (because you magnified the image circle and part of it now falls off to the side of your sensor). In reality, you lose even more light than this, because some light will also start crashing into the sides of your extension tubes / camera body. This extra darkening will show up as vignetting.

3) You will lose the ability to focus at infinity. This is because the internal focusing system of your lens again has physical limits placed on it, and the focusing elements cannot move far enough back to compensate for the amount you moved the lens forward with your extension tubes (they're only built to go JUST far enough back for infinity when normally mounted). The further forward you move it, the further your lens will fail to meet infinity focus by. Usually this doesn't matter, because if you're putting on macro tubes, you don't WANT to focus at infinity anyway.

4) Your lens will gain additional chromatic and spherical aberration and astigmatism and coma, etc. (color fringing, softness, blur that increases as you move away from image center, etc.). Lenses are designed to have minimal aberrations when used attached to the lens mount, and by moving it further away, you are moving away from the optimally lowest point of optical flaws, so your image will have lower quality than normal. This may be more or less noticeable, depending on the lens.
 

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To add to that, you can also reverse a lens onto another lens, not just reverse mount a single lens to the body. Reverse a 50mm on the front of say, a 75-300mm zoom and now you have some control of the magnification by using the zoom. You can figure magnification level by dividing the reversed lens FL into the the FL of the zoom, so a 50mm on front of a 75-300mm zoom will provide from about 1.5x (75mm on the zoom) up to about 6x (300mm on the zoom).

Of course, you can forgo all the fiddly stuff and just get a macro lens, and if you're a Canon shooter, the MP-E65 and get 1x-5x magnification all in one self contained, albeit not easy to use lens (and you really, really need a macro flash to go with it...and a fairly sizable chunk of change to get them).
 

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..........Of course, you can forgo all the fiddly stuff and just get a macro lens, and if you're a Canon shooter, the MP-E65 and get 1x-5x magnification all in one self contained, albeit not easy to use lens (and you really, really need a macro flash to go with it...and a fairly sizable chunk of change to get them).

Why not just pony up, go 'all in' and buy one of these? :er:
 

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Lights:

Sigma, Canon, Nikon, etc. sell their own ring flashes. Usually around $400-500 or:

Paul C. Buff - AlienBees ABR800 Ringflash

If you do studio work, or field work that doesn't require much mobility (able to set up tripods and battery packs and such), then this is an option that is much more powerful than a manufacturer's on-lens ring flash. About $400

Or if you want on-lens ring light, but don't need as much juice as a flash, there are various $30 or so LED continuous ring lights, which have the added bonus of full light while composing and focusing, not just at the moment of capture
 
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squirrels

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..........Of course, you can forgo all the fiddly stuff and just get a macro lens, and if you're a Canon shooter, the MP-E65 and get 1x-5x magnification all in one self contained, albeit not easy to use lens (and you really, really need a macro flash to go with it...and a fairly sizable chunk of change to get them).

Why not just pony up, go 'all in' and buy one of these? :er:


Just think of the whatsit posts you could do with that!
 

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..........Of course, you can forgo all the fiddly stuff and just get a macro lens, and if you're a Canon shooter, the MP-E65 and get 1x-5x magnification all in one self contained, albeit not easy to use lens (and you really, really need a macro flash to go with it...and a fairly sizable chunk of change to get them).

Why not just pony up, go 'all in' and buy one of these? :er:


Just think of the whatsit posts you could do with that!

HA! I was thinking the exact same thing! :lol:
 

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