MACRO: test of extension tubes on magnification of macro lenses

[troypiggo]

I've been meaning to do this for months. Finally took the opportunity. Wanted to take shots of a measurable subject at different magnifications to:

- show what 1:1 really means (even though I know)
- measure the real magnification of extension tubes on my macro lens, other than saying it's about 2:1
- see what the magnification of extension tubes is at infinity focus on my macro lens.

Lately I've been shooting with full set of extension tubes on, and I realise you lose infinity focus, but I've been wondering what the minimum magnification really is.

All shot with Canon EOS 30D (sensor size 22.5mm x 15.0mm), EF 100mm f/2.8 Macro USM lens. Extension tube set was Kenko 68mm full set. I didn't bother testing individual tubes because I have never used less than the full set since I want them for maximum magnification, and also because testing the various combinations of 3 different length tubes would drive me nuts.

I realise it's probably not the most accurate test. If I'd shot with the ruler across the diagonal it'd be a little more precise because when scaling things, the longest measurable distances are the best. I also could have used a tripod and made sure the scale was dead level. But I figure it's near enough for my purposes and still helpful for others.

So here we go.

Bare 100mm macro lens, 1:1 magnification. Near enough to 22-22.5mm on the scale, 22.5mm sensor width, so the 1:1 mag seems correct.


100mm macro lens with 68mm of tubes, focused to minimum distance. Near enough to 11mm, 22.5mm sensor width, so 2:1 mag seems about right.


100mm macro lens with 68mm of tubes, focused to infinity. Near enough to 34mm on the scale, 22.5mm sensor width, so around 2:3 or 1:1.5 magnification.

[LordV]

Good demonstration. One of the slightly annoying things about using tubes is I know what the max magnification is but don't know what the magnification is when I'm not at minimum focus.

Brian V.

[Jon]

Interesting. That you're getting around 2:1 with only 68 mm additional extension, bears out the reports that the 100 f/2.8 macro's actually a vari-focal lens and is closer to 65-68 mm at 1:1 magnification. So someone would need to run a series of tests at various indicated magnifications/ranges to determine actual focal lengths at those settings.

One other thing that this raises the possibility of - if you're metering externally, or manually calculating flash exposure, the difference between a 100 mm and a 68 mm lens at constant physical aperture is about 1 stop; this could throw off all your calculations. If you're using TTL metering, either of available light or E-TTL flash, this won't be an issue (unless, of course, you meter a separate subject at different magnification). If the lens is really "constant aperture" and maintains a "real" f/2.8 with respect to the actual focal length, this won't be any kind of problem.

[troypiggo]

Thanks Brian and Jon.

Jon - I was of the understanding that the reason 68mm gives 2:1 is to do with crop sensor. On a FF camera you calculate magnification by ratio of extension to lens focal length. But on crop camera you need to multiply extension by crop factor. 68mm x 1.6 is 108mm. The maths seemed to work. Are you saying there's something else going on?

Couple of weeks ago I started looking into manual flash for macro, but decided that relative to the sort of distances macro is shot at, the variability or standard deviation of focusing distance is quite high and would cause flash to be quite over or underexposed. E-TTL makes it much easier.

Not sure I fully understand all of what you're getting at in the second paragraph. I have been reading a little recently also about "effective apertures" in macro, where some optical effects that are negligible at normal working distances become more significant at macro distances causing, for example, an f/2.8 lens to behave as a f/5.6 aperture at 1:1. Is that what you mean?

[LordV]

Originally Posted by troypiggo

Thanks Brian and Jon.

Jon - I was of the understanding that the reason 68mm gives 2:1 is to do with crop sensor. On a FF camera you calculate magnification by ratio of extension to lens focal length. But on crop camera you need to multiply extension by crop factor. 68mm x 1.6 is 108mm. The maths seemed to work. Are you saying there's something else going on?

Couple of weeks ago I started looking into manual flash for macro, but decided that relative to the sort of distances macro is shot at, the variability or standard deviation of focusing distance is quite high and would cause flash to be quite over or underexposed. E-TTL makes it much easier.

Not sure I fully understand all of what you're getting at in the second paragraph. I have been reading a little recently also about "effective apertures" in macro, where some optical effects that are negligible at normal working distances become more significant at macro distances causing, for example, an f/2.8 lens to behave as a f/5.6 aperture at 1:1. Is that what you mean?

Troy - think that is just a coincidence - AFAIK the same magnification change is seen on a FF camera.
Jon is correct- the focal length of macro lenses changes at minimum focus and approximates to the min focus distance/4 which for the 100mm macro is about 75mm. I'm not sure of the accuracy of that estimate but as Jon indicates you need to get to a focal length of about 68mm for this lens for the magnification calculation to work properly. Interestingly the mag calculation does work very well for non macro lenses and ext tubes presumably because their focal length does not change that much at min focus.

Brian V.

[troypiggo]

I see. I must dig around and try to find a site somewhere that explains all this stuff. I find the maths and physics fascinating, but am only discovering bits and pieces as I get more experience with my photography. This stuff with manual flash also fascinates me.

[Jon]

Well, it's not all macros that exhibit the focal length change, and it's not only macros either. I have a Tamron 90 mm macro that is a true 90mm across its full range; OTOH my 70-200 2.8 IS changes focal length as you focus closer. It's really a characteristic of internal focusing lenses, not lenses designed for any specific purpose, such as macro. It all falls out of the basic lens formulas:
1/fl=1/i+1/o and M=i/o where fl is the lens' focal length, i is the distance from the lens' nodal point (which would be the center of a simple lens; may not be with multi-element lenses) to the image plane, o is the distance from the nodal point to the subject, and M is magnification.

If you'd put your macro+tubes on a 5D, you'd have gotten a FoV of about 18 mm where you got 11 mm with your 30D,but that's still twice life size, just you're getting more subject area due to the larger sensor. "Crop factor" only affects the angle of view that's captured, not the image magnification. You may have to enlarge a "crop" photo more in printing, but that won't get you any more detail than was originally captured; enlarge a FF shot by the same amount and you'll have the same level of detail, just a bigger picture.

[macro junkie]

i like the mpe-65 because it has what mag your shooting on the side of the lens. expensive but worth every penny.

[troypiggo]

Thanks Jon.

MJ - the 100mm macro lens, and other macro lenses I've seen, do have mags on the side of the lens. Only difference is that their max is 1:1, but they do have 1:2, 1:3 etc markings on the side.

What I was testing is with extension tubes on.

[Super-Nicko]

i just got my 100 macro and had been wondering what 1:1 meant and i was pretty sure of exactly what you showed here but now im dead sure - thanks for sharing - i know it might sound simple to others but you just dont know something until you seek it and learn -

Luckily memory retention seems to holding still so i should only have to learn this once!

Ill leave all that techie stuff to learn another day... when i need it perhaps

Cheers,
Nicko

[johnkermit1]

very good Troy...i was asking about this a while back, and JUST today looked to see my sensor size (poor lil thing) 5.76 wide...(thnsk LordV for the info on how to do this also..)

so today ill break out the ole metric scale, and see about what mag factor i am getting with the raynox on it.

[number six]

A while back I bought a reversing ring and Nikon adapter so I could reverse mount my Nikkor lenses on my 300D for macros. Reversing a lens on the body is a good way to get quality macro shots.

I got the best result from my 35-105 zoom. Interestingly, the zoom didn't do much at all to the magnification but it worked beautifully for focusing.

I got about 1.55:1 magnification from it:

nikkor_f16.jpg



Depth of field is a function of magnification and aperture - and it can get pretty thin:

nikkor_dof.jpg

-js