How do Extender Tubes work?

[gearsNcogs]

Are they better, worse, or just different from macro lenses? i also just dont understand how they work exactly.

[RioRico]

There are only two ways to increase the magnification of a lens: add another lens to it, or extend its distance from the frame (film or sensor). Add-on close-up lenses and magnifying teleconverters do the first, but with some loss of image quality (IQ). Extension does the second, with no loss of IQ, but with a loss of light.

A 'macro' camera lens is built so that it can be extended on its own body. So, my Vivitar 90/2.8 macro or Macro-Takumar 50/4 can extend out to double the normal length of a 90mm or 50mm lens, and thus can attain 1:1 magnification. Other macro camera lenses may not extend as far, and may only reach 1:2.

With tubes or bellows, you push the lens objective further from the frame. One very popular combination is the Industar-50/3.5 lens, which may cost about US$25, put onto a set of M42 macro tubes, which may cost US$7. The 50mm lens body, placed on 50mm of tube extension, can now reach 1:1.

Tubes are simple and cheap but the only way to change magnification and focal distance is to change tubes. A bellows is much more flexible, and may only cost US$25-40 on eBay. I like to use cheap sharp enlarger lenses on bellows. The lenses often cost US$10 or less, and if they are longer than 80mm, they will reach infinity focus and so can be used for non-macro general photography too.

My Vivitar-Komine 90/2.8 macro is a fine M42 lens, very sharp and easy to use. It is also rather heavy. More often, I use a light-weight combo of inexpensive 90mm or 100mm or 105mm or 110mm enlarger lens on small Bellowscope bellows; the combination weighs half as much, and costs considerably less than a camera macro lens. Both enlarger and macro lenses are built for edge-to-edge flatfield sharpness. A good enlarger lens on bellows is every bit as good, or better, than a camera macro lens.

[newton]

Why did they ever invent magnifying teleconverters then since IQ is affected so much?

I remember in the 70's and 80's these were popular sales items by camera stores.


My Vivitar 100 mm 2.8 seems crappy to me, btw.

[visualopsins]

TC doesn't affect lens focus distance, can focus infinity. Add tube to focus closer, but lose ability to focus infinity.

[gearsNcogs]

Thanks for that fantastic info RioRico!

[adityap]

Very informative post Rico, thanks a lot

[Arkku]

Actually the third way to “add macro” is to reverse the lens. =)

The reason why this works is that the point of focus is then where the film/sensor would normally be, which is much closer than the minimum focus distance of a typical lens. Since the lens is the same but focused closer, magnification will increase…

The underlying reason why extension tubes work is also that they simply allow you to focus closer. With “zero extension” the lens is focused at infinity, and the more extension you add (by moving the lens farther from the film/sensor) the closer it focuses. With extension equal to focal length (e.g. 50mm tubes with 50mm lens at infinity setting) magnification will equal 1:1. Not much higher magnification can usually be reached with tubes (alone) because the focus distance will soon become impractical.

You can combine reversing and tubes, though, or you can reverse a shorter lens in front of a longer one to get magnification approx. equal to the ratio of the focal lengths (e.g. 200mm on camera with 50mm reversed in front of it gives 200mm : 50mm = 4:1 magnification).

[RioRico]

Well, actually, reversing a camera lens brings the focus much closer, but does not magnify. Magnification comes from extension. Camera lenses may have front insets of various depths. Reversing a deep-inset camera lens DOES provide some extension and thus magnification. Reversing a shallow-inset camera lens, does not. (And newcomers complain about this!) For increased magnification with a reversed camera lens, I add short macro tubes or even just some step-rings.

Camera lenses are built so that one side (the object field) can focus out to infinity, while the other side (the image field) sharply projects an image a short distance to the frame (film or sensor). The distance from the lens mount to the frame is called the register or flange focal distance, and it's essentially the working distance of a reversed prime camera lens. Register can be very short, like 20mm for m4/3 systems or around 28mm for old Leica lenses, to fairly long, like around 46mm for Pentax or Nikon, and even longer for medium-format camera lenses.

Yes, primes can be reverse-stacked, where a shorter lens is reversed on a longer lens. This is the same sort of lens supplement as if you had added a close-up adapter, but the reversed lens is better-corrected for optical aberrations than are most such adapters.
________________________________________

I found a neat trick: Cheap zoom lenses improve when reversed! Exact results will vary with the lens, but here is my story. I reversed an SMC-A 35-80/4-5.6 zoom, arguably one of the crappiest lenses ever sold by Pentax, onto my K20D. I used only a simple 49mm-PK mount-reversal ring (US$5). I found that at 35mm, I could reach 2:1 magnification at a working distance of about 50mm / 2in; at 80mm, I could focus as close as 150mm / 6in, and out beyond infinity. And the images are sharp!

I also tried that with a 'better' zoom, a Tamron 28-70. To mount it, I used a 55mm-PK mount-reversal ring and a 52-55mm step ring. That combo, and the slightly deeper front inset of the Tamron, provided enough extra extension that 35mm magnification increased to abound 2.5:1, but the long end could not focus to infinity. And I tried a Tokina RMC 35-135/3.5-4.5 with the 55mm-PK ring. but found that the old one-touch push-pull zoom design was a bit too loose for careful macro work. So, when reversing a zoom, Your Mileage May Vary.
________________________________________

One last note: No non-reversed lens can focus closer than its focal length. No matter how much extension is put on a 50mm lens, it can never focus closer than 50mm. Great extension may result in great magnification, but you can't get any closer! The result: Shorter lenses are great for working very close, for easily filling the frame. Longer lenses allow (or force!) you to work from further way. 35-55mm may be ideal for studio work. 90-200mm may be needed for field work, lest the bugs be frightened away by a lens poking too close to them.

[Arkku]