Kenko 36mm tube and KM100 Macro

[Greg Beetham]

It was only a quick test shot nothing marvellous, it really needed f16 or f22, but as I was changing the ISO and f/stop the subject departed. What I wanted to know was just how thin was the DOF at 1:1 with a 36mm tube (I'd bought a set some time ago and never got around to using them).
I got my answer..at f11 it's mighty thin, looks like f22 territory, I'm wondering if using say the Sigma 180mm Macro from further away at f16 would give the same subject size but better DOF, maybe David or Olaf might know...? and the Sigma image quality..?
Greg

[01af]

... and f/stop ...

It's "f-stop," not "f/stop."

... at 1:1 with a 36 mm tube ...

That's a magnification of approx. 1.4:1, or 1.4×.

I'm wondering if using say the Sigma 180 mm Macro from further away at f/16 would give the same subject size but better DOF ...?

No.

The Sigma 180 mm macro lens, at minimum focus, yields a maximum magnification of 1:1, just like most modern macro lenses. On a 36 mm extension tube, maximum magnification would be 1.3:1, or 1.3× ... i. e. slightly less than with a 100 mm macro lens set to 1:1 on the same tube (but at a longer working distance). For the same magnification, the Sigma 180 mm macro lens would require approx. 46 mm of additional extension. At the same magnification, however, the depth-of-field would be exactly the same, no matter what the focal length or the working distance are.

To increase depth-of-field without stopping down, reduce magnification and then just crop the final result. This approach, however, will reduce overall image quality.

-- Olaf

[Greg Beetham]

Thanks Don, Olaf,
I had a look at the DOF calculator Don, interesting, but not directly applicable for a macro lens with a tube I'm afraid, and the open forum discussion was informative but my eyes started too glaze over after about the first page and a half, with so much stuff about formats and view angles, image distance and magnification, focus extensions vs focal lengths etc. various focal length lenses with and without tubes or extenders....I sort-of lost the plot.
The only thing about macro lenses that I do understand is that they are the opposite of a normal lens...ie. something like reversing a standard 100mm lens (or 50mm) around backwards and putting it on your camera, the description I saw once was that a normal lens is constructed to operate with a big distance from the lens to subject, and a small distance to the film/sensor plane, wheras a macro lens is constructed for a big distance to the film/sensor plane and a small distance to the subject.
I gather that the addition of a tube doesn't actually change the focal length of a given macro lens, but does change/boost the magnification, and I assume has an exposure factor (light loss), I looked in the Kenko box but didn't see any exposure correction tables, not that I'm going to shoot macro in M mode (these tubes have the eight contacts, thankfully), but the exposure correction factor should give a slight boost to the apparent f-stop (<<look Olaf, happy now?) and hopefully a corresponding slight increase in DOF....maybe.
With DOF so hard too come by at high magnification I might have too investigate stacking some more I think...stacking is all well and good of course but not much use if the subject moves during the process, or worse, departs for greener pastures...
Greg

[01af]

The only thing about macro lenses that I do understand is that they are the opposite of a normal lens ... i. e. something like reversing a standard 100 mm lens (or 50 mm) around backwards and putting it on your camera ...

Umm ... it's too bad that's the only thing you understand---because it's wrong.

... the description I saw once was that a normal lens is constructed to operate with a big distance from the lens to subject, and a small distance to the film/sensor plane, wheras a macro lens is constructed for a big distance to the film/sensor plane and a small distance to the subject.

There's truth in that but it doesn't apply to the kind of regular macro lenses we're discussing here. It's true for macro lens heads which are designed for use on a bellows unit at magnifications beyond 1:1.

I gather that the addition of a tube doesn't actually change the focal length of a given macro lens, but does change/boost the magnification ...

That's right. The longer the tube, the higher the magnification. And the shorter the focal length of the master lens, the higher the increase in magnification for the same tube length.

... and I assume has an exposure factor (light loss) ...

That's right, too. The light loss is basically proportional to the magnification ... no matter how you achieve the magnification, be it a long lens on a long tube or a short lens on a short tube.

... I looked in the Kenko box but didn't see any exposure correction tables ...

That's because the magnification achieved with the 36 mm tube---and thus, the exposure correction factor---will vary greatly, depending on the gear you use the tube with and how you use it. When used with the fully extended Konica-Minolta or Sony Macro 100 mm 1:2.8 lens then the light loss caused by the 36 mm tube will be approx. 1.4×, or about half an f-stop. In combination with other gear (close-up lenses and/or more tubes), or with the lens not fully extended, or with other lenses, the factor will be different.

... the exposure correction factor should give a slight boost to the apparent f-stop [...] and hopefully a corresponding slight increase in DOF...

It does not. The apparent aperture becomes smaller indeed but at the same time the magnification becomes greater. So in the end, depth-of-field is even narrower than without the tube.

... stacking is all well and good of course but not much use if the subject moves during the process ...

Exactly.

-- Olaf

[Greg Beetham]

The only thing about macro lenses that I do understand is that they are the opposite of a normal lens ... i. e. something like reversing a standard 100 mm lens (or 50 mm) around backwards and putting it on your camera ...

Umm ... it's too bad that's the only thing you understand---because it's wrong.

-- Olaf

Mate, I got that off the side of a Cornflakes packet in 1975 so it must be true....
Seriously though, as a rough analogy it's not even good enough for the bush then? I must have been AWOL on the day we did optics at school...so could you elaborate a little about the difference between a reversed lens and a dead-set macro lens.
Greg

[01af]

... could you elaborate a little about the difference between a reversed lens and a dead-set macro lens.

A reversed lens is used at magnifications greater than 1:1. A regular macro lens (as opposed to a macro lens head) is designed for magnifications less than 1:1, and up to 1:1, but not beyond.

The reason why 1:1 is the magic borderline simply is this: Consider the thin-lens equation, 1/f = 1/g + 1/b, with f being the focal length, g the subject-to-lens distance, and b the lens-to-image distance. The magnification, m, is the ratio of the size of the image of the subject to the original size of the subject. According to the theorem of intersecting lines (remember junior high-school math?), that's equivalent to b : g. So at magnification 1:1, b is equal to g; at magnifications less than 1, g is greater than b; at magnifications greater than 1, b is greater than g.

So a lens designed for small magnifications, i. e. any regular non-macro lens, is made for large g and small b. When it gets used at magnifications greater than 1:1, e. g. on a bellows unit, then b will be greater than g, so the lens must be reversed so its front element will face the greater distance. It doesn't matter where subject and where image are; just their relative distances matter. Lens heads which are designed for magnifications greater than 1:1 are already built in reversed mode, i. e. they expect a small g before the front element and a large b behind the rear element.

Macro lenses, i. e. regular lenses able to focus from infinity to 1:1, are not designed for b greater than g. They are made for b less than or equal to g and thus resemble regular non-macro lenses more than bellows lens heads.

-- Olaf

[InTheSky]

Let push a little bit the limit :

100mm 2.8 Macro + 36mm + 20mm + 12m + 2X Tamron SP. Lens at F16

When I came home on Sunday, I was sure that what I was looking at was a spider in a web, but after a close lookup I realized that this was group of hundred of baby spider.

[Birma]

That's one hell of a rig you've got there Frank! Nice shot of the baby spiders - makes my skin crawl a bit, but a great shot none the less

[David Kilpatrick]

And those spiders managed to get all the way from Poland for a family party. Amazing!

David

[01af]

Frank, with a shorter lens on the same set of extension tubes you'd get about the same magnification at a better effective lens speed (no tele converter required), and still maintain about the same working distance. Or, with tele converter, you'd get an even higher magnification.

Long focal lengths (i. e. longer than 50 - 60 mm) don't make sense at magnifications significantly beyond 1:1---and you are at approx. 3.5:1 which clearly is way beyond life size!

-- Olaf

[Greg Beetham]

... could you elaborate a little about the difference between a reversed lens and a dead-set macro lens.

A reversed lens is used at magnifications greater than 1:1. A regular macro lens (as opposed to a macro lens head) is designed for magnifications less than 1:1, and up to 1:1, but not beyond.

The reason why 1:1 is the magic borderline simply is this: Consider the thin-lens equation, 1/f = 1/g + 1/b, with f being the focal length, g the subject-to-lens distance, and b the lens-to-image distance. The magnification, m, is the ratio of the size of the image of the subject to the original size of the subject. According to the theorem of intersecting lines (remember junior high-school math?), that's equivalent to b : g. So at magnification 1:1, b is equal to g; at magnifications less than 1, g is greater than b; at magnifications greater than 1, b is greater than g.

So a lens designed for small magnifications, i. e. any regular non-macro lens, is made for large g and small b. When it gets used at magnifications greater than 1:1, e. g. on a bellows unit, then b will be greater than g, so the lens must be reversed so its front element will face the greater distance. It doesn't matter where subject and where image are; just their relative distances matter. Lens heads which are designed for magnifications greater than 1:1 are already built in reversed mode, i. e. they expect a small g before the front element and a large b behind the rear element.

Macro lenses, i. e. regular lenses able to focus from infinity to 1:1, are not designed for b greater than g. They are made for b less than or equal to g and thus resemble regular non-macro lenses more than bellows lens heads.

-- Olaf

With regards to the fly that I photographed,
I guess the only way to double check the repro size is measure the actual fly, (in the abscence of the aforementioned fly a rough estimate would be about 1cm to the end of the wings), then print out an image that would duplicate the actual size of the sensor when printed...then measure the fly on that image to see what size it was on the sensor, (which I haven't done)....but doing a roughie, measuring the fly on the screen and comparing that too the total width of the image, I get about 3/4 of the width of the image.
Sony APS-C sensor dimensions are 21.5 X 14.4mm so the fly measures approx 16.2mm on the sensor...it is subject too error of course, the length of the 'real' fly is only an estimate from memory and could actually be slightly larger, but still not a bad repro just the same with just a tube.
The approx distance from the subject to the lens was about 5 inches...(estimated because I was busy looking through the camera at the time, but I did look frequently at the 'aim' as the tripod balanced forward on two legs and re-aiming the not completely locked ballhead so the whole thing came together at 1:1 focus with the subject in the middle of the frame at the same time), the thing is the distance from the objective back to the sensor/film plane was greater than the distance from the subject too the objective, especially when you take the 36mm tube into account along with the lens at full extension....at a guess about 9 inches perhaps...so in this instance b was greater than g, (unless of course one must measure from the focussing group), and I don't know where that is located inside the lens.

Olaf one thing that interests me an maybe others is, what is the image quality difference between using a suitable reversed lens and a macro lens to produce an equivalent photo..ie. same subject arranged to be at the same end magnification in both, cases and then comparing the results.
I mean, theory is all well and good but what happens in practise, actually taking photos?
I do rember briefly dabbling with reversing lenses back in my MD days, with in some cases not so bad results, but very shallow DOF....
Greg

[01af]

Olaf, one thing that interests me and maybe others is, what is the image quality difference between using a suitable reversed lens and a macro lens to produce an equivalent photo, i. e. same subject arranged to be at the same end magnification in both, cases and then comparing the results.

Depends on several factors. In most cases, in real life it's only half as bad as you might think from theory.

First of all, if you're stopping down generously then your problem is not the lens being operated outside its intended range ... it's diffraction. Diffraction will equalise the output from lenses of different quality.

Then, if you are exceeding the lens' working range only by a moderate degree then the loss of image quality will be only small or moderate as well. And a small quality loss will show up mostly at the frame's corners, not so much at the center. In most macro images, there's the main subject at the center and blurred background at the frame's edges so the lens' performance near the edges hardly matters.

So while adding an extension tube causes the AF Macro 100 mm 1:2.8 to exceed its intended working range, loss of image quality will be only minuscule and a non-issue in real life. Sitll, with a lens dedicated for magnifications greater than 1:1, or with a shorter lens in reversed mode, you'd get a better image quality (if only marginally better), in particlular at the frame's corners. The difference, however, will increase with the length of extension tubes added.

Greg, with your 36 mm tube you are at 1.4:1 or 1:5:1 which is not too far beyond 1:1 so your quality loss is still neglectible. Frank is at approx. 3.5:1 which is way beyond the AF Macro 100 mm 1:2.8's normal working range so he'd get both better image quality and better usability (less cantilevered rig, and lighter weight too) when he used a 50 mm or 35 mm lens reversed. Umm ... on the other hand, when reversing a lens, in most cases you'll lose the auto-diaphragm feature which in turn will diminish usability.

Depth-of-field is completely independent from focal length or the lens' orientation (reversed or normal); in the macro realm it solely depends on magnification (and aperture, of course). So if you need/want a certain magnification, with regard to depth-of-field it's pointless to reason about different techniques to achieve that magnification. All possible techniques, at the same effective aperture and on the same image format, will yield the same depth-of-field.

And oh, by the way, Konica-Minolta's and Sony's APS-C format is approx. 15.7 × 23.5 mm.

-- Olaf

[NevHi]

01af said:

Long focal lengths (i. e. longer than 50 - 60 mm) don't make sense at magnifications significantly beyond 1:1---and you are at approx. 3.5:1 which clearly is way beyond life size!

Isn't that the real point of extension tube(s) used with Macro lens; to get beyond life size?? Other wise why use them.

[01af]

Long focal lengths (i. e. longer than 50 - 60 mm) don't make sense at magnifications significantly beyond 1:1---and you are at approx. 3.5:1 which clearly is way beyond life size!

Isn't that the real point of extension tube(s) used with macro lens; to get beyond life size?

Sure.

But it doesn't make sense to use a long macro lens for that purpose.

-- Olaf

[NevHi]

Is using a 100mm Macro lens the wrong thing to do on an extension tube if you want larger than 1:1? Clearly I'm misunderstanding some points here. Would a bellows system be better/useless?