Kenko 36mm tube and KM100 Macro

[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?

[David Kilpatrick]

The Minolta/Sony macros are optimised for a reasonable range of distances, performing better at 1:10 than at infinity and improving (relative to a simple Tessar-type lens on a bellows) as you approach 1:1. I'd say that they hold corrections well at 2:1 but beyond this are not likely to much better than a good standard lens used in reversed position. Technically, you could mount the macro lens in reverse for high magnifications but the Alpha mount does not lend itself to this.

I use two lenses on bellows for higher magnifications - a 75mm f4 Apo Rodagon designed for slide scanning and corrected for 1:1, removed from a Leafscan 45; and a Minolta 25mm f2.5 Bellows Micro Rokkor, designed for anything up to 10:1 magnification.

David

[01af]

Is using a 100 mm Macro lens the wrong thing to do on an extension tube if you want larger than 1:1?

Yes.

Would a bellows system be better/useless?

A 50 mm macro lens would be better.

The Minolta/Sony macros [...] I'd say that they hold corrections well at 2:1 ...

With a flat subject, at 1.5:1 you can clearly see the loss of image quality in the corners. With a natural subject, i. e. where the corner show nothing but blurred background anyway, it'll be okay at 2:1.

... but beyond this are not likely to much better than a good standard lens used in reversed position.

Actually, at 2:1 and beyond a reversed standard lens will be better than a macro lens in normal position. A macro lens in reversed position however will be better still.

Technically, you could mount the macro lens in reverse for high magnifications but the Alpha mount does not lend itself to this.

Exactly. But don't try that with a 100 mm lens ... or, worse yet, with a 180 mm or 200 mm lens! You'd gain nothing in working distance; you'd just need more extension length. For magnifications significantly beyond 1:1, a 50 mm macro lens is the right choice, or a retro-focus wide-angle lens, or an enlarger lens, or dedicated bellows lens heads (like the Minolta Bellows Micro 25 mm lens mentioned below).

... a Minolta 25 mm f/2.5 Bellows Micro Rokkor, designed for anything up to 10:1 magnification.

Not for 'anything' but for the range from 3:1 to 10:1 specifically, according to Minolta. However in real life, it's fine for anything from, say, 2:1 to 20:1. Unless locking your mirror up and dunking the lens deep into the mirror box, you'd be hard-pressed to achieve a magnification of less than approx. 2:1 anyway with that lens ...

-- Olaf

[InTheSky]

I'm new to macro, so now I have learn a lot of concept with your discussion.

With insect I have found for my very tiny little experience that the best macro lens is the one giving me the longest focus distance from the lens to be able to not afraid the subject.

In this case what is the best choice ? Is a reverse lens will give you more working distance ?

[Dr. Harout]

I'm new to macro, so now I have learn a lot of concept with your discussion.

With insect I have found for my very tiny little experience that the best macro lens is the one giving me the longest focus distance from the lens to be able to not afraid the subject.

In this case what is the best choice ? Is a reverse lens will give you more working distance ?

As far as I know it will give you a fixed working distance and giving a greater scale of magnification with a wide angle lens than a standard one. Try a reverse 28mm lens and observe the result.

[Greg Beetham]

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

Thanks Olaf,
I had an enforced break, (computer had decided too become uncooperative...RAM contact problem it proved to be), yes it seems that one can find an APS-C dimension for every day of the week if one wants, I got mine from here http://www.dpreview.com/learn/?/key=sensor%20sizes
Where I think some of the disparity arises from is that somtimes the whole physical dimension of the sensor (including the border) is quoted, sometimes the sensel area is quoted and that apparently is even sometimes different to the actual recording area as well...there doesn't seem to be any standard in effect for quoting sensor sizes from what I can see.
Greg

[01af]

With insect I have found for my very tiny little experience that the best macro lens is the one giving me the longest focus distance from the lens ...

Focus distance is the distance from the subject to the image plane. The clear distance from the subject to the foremost part of the lens (i. e. the filter threads usually) is called the working distance.

... to be able to not afraid the subject. In this case what is the best choice?

A long lens, of course ... but not at magnifications significantly greater than 1:1. There, shorter lenses are better.

Is a reverse lens will give you more working distance?

No, not generally ... except when the lens is a retro-focus wide-angle lens. The purpose of reverting the lens is to improve image quality at above 1:1 magnification when the lens is designed to normally operate at below 1:1 magnification.

-- Olaf