Comparing macro lenses using MTF - part II

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Miljenko
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Comparing macro lenses using MTF - part II

Post by Miljenko »

Instead of presenting lens by lens and bring final conclusion at the end, I decided to show important findings in this opening part in order to encourage discussion and shake spirits. As I announced, there will be surprises and discoveries, so here they are from heavier to lighter:

1. All the tested lenses (except for finite microscope objectives) perform better on relay (tube) lenses than via extension tubes!


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At first I thought it could be the particular lens peculiarity, then after couple of such cases I thought I'm doing something wrong or have done lousy tube flocking, or something else. But when it happened on ALL tested lenses, it was definitely clear it's a rule. Differences scale from mild to drastic, sometimes it's more prominent at the edges, sometimes at center. Only CA could get worse, but it frequently gets better as well. Results differ depending on the relay lens; with some it's just a bit better than with extension tubes but with some it's astonishing. Probably the most extreme case is Schneider-Kreuznach Componon-S 50mm f2.8. When mounted on extension tubes, at 2x magnification it produces 1767 LW/PH at picture center when stopped down to f/4.8 and only 750 LW/PH at the edge. It has to be stopped down to f/6.8 to raise edge resolution to 1154 LW/PH. But when mounted on Nikon E 100mm f2.8 lens (at it's best when stopped down to f/4) the resolution jumps to whooping 3138/2988 LW/PH! And if mounted via scanner lens Agfa 107mm f4 the resolution jumps to the highest resolution recorded at 2x ! And CA gets better as well: from 0.83 (tubes) to 0.74 (Nikon) to 0.2 (Agfa). Obviously, when interfaced with compatible relay lens (focused at infinity) most lens give their best. The only negative point is working distance that always gets shorter.

2. I still didn't figure out the criteria for ideal relay lens


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You have seen my tested lenses list in part 1 which included all relay (tube) lenses I have used. There are ordinary camera lenses, enlarger lenses and surplus lenses. Most of them did not produce exceptional figures when measured alone but when used as the relay lenses, they are head and shoulders above all the others. For instance, Agfa scanner lens 107mm f4 (aperture adjustment ring without any markings or clicks) measured at 1:1 1656/965/0.03 at f/4 and 2412/2018/0.06 at f5.6. It is the best 100mm relay lens tried and it actually performs better at f4 than at f5.6. Another peculiar case is Rodenstock Rodagon 150mm f5.6. I have two samples; older, very heavy one with visible lens edge separation and newer one, very elegant with reasonable weight and size that I payed three times the old one. Well, the old tank performs significantly better as the relay lens. BTW, I have few 200-210mm lenses entitled as tube lenses but still didn't reach final part of my test where those will be tried with one of my infinity objectives. I do expect surprises there as well!

3. In general, modern lenses are better than vintage lenses


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Some of the lenses tested are 50 or more years old. Those in general give the worse results and most of them didn't get to Imatesting as visual pre-test showed below average sharpness. As the tested lens is younger, it produces better results. I believe designers and technology were not inferior then, just the demand and criteria was lower. 35mm film had much lower resolution than today's sensors and designers had no reason to do any better. Only special applications like repro photography and microscopy called for max resolution and they delivered. The highest resolution I recorded in this test was produced by modern Fujifilm XF 80mm f2.8 Macro lens at 1:1 and it was well over Nyquist frequency of 4000 LW/PH or 128 line pairs per milimeter, with immeasurable CA. Similar results I've got from another contemporary macro lens; Fujifilm XF 60mm f2.4 but at 0.5x magnification. At the same time, older Rodenstock Apo Rodagon D 75mm f4 delivered „only“ 3345/3477/0.28 at 1:1. Very high readings but not exceptional by today's measures.

4. Surprises can happen, both negative and positive


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In those test results you will find unknown lenses producing very good figures while some established brands did produce mediocre lenses that still have above average prices. For instance, Rodenstock Rodagon 50mm f2.8 barely gets to 2000 LW/PH at 1:1 while relatively unknown Minolta C.E. 50mm f2.8 gets easily to 3201/2319/0.07. Of course, there are no many surprises with enlarger lenses 'cause they are tried over and over again. The joy of discovery lies with industrial and totally unknown lenses. Thats why I've spent hundreds of $$$ on surplus and all sorts of strange lenses in search for „golden sample“. One of those is Bell & Howell funny little lens with just „24x“ marking (probably a microfiche lens). It produces fantastic 2888/2709/0.54 at 3x when relayed to another valuable find, tiny Rodenstock 60mm. Unfortunately, in order to find above average surplus lens, you have to buy 5 – 10 of them. Although a single lens can cost just $20 or $30, if it takes to buy 10 to get one, it is actually a pretty expensive find. Fortunately, some surplus lens Ebay sellers are willing to negotiate the price if you ask for larger quantity.

5. Not a discovery but a guide to tests that follow


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In part III I'll start to present the most interesting lenses in order of magnification. In order to understand those measurement results here are few guidelines.
a) Every lens/aperture combination gets triple figure in xxxx/xxxx/x.xx format. First two designate center and edge resolution. I didn't care about corner resolution which is not that important in macro photography the way most of us do. Resolution is measured in LW/PH or line widths per picture height at point where MTF curve drops to 50% of it's max value. This is very rigorous criteria compared to old USAF1951 chart reading where diminishing point was searched by naked eye usually distinguishing between 5 and 10% brightness difference. As I'm using APS-C 24Mp sensor camera, vertical resolution is exactly 4000 pixels. This means that the lens that can resolve the same resolution (4000 LW/PH) will give the max total possible resolution. Of course, as the MTF slope is modifiable by sharpening, lenses within 3000-4000 LW/PH can be „stretched“ to max sensor resolution.

b) Lenses with central resolution of 1500 LW/PH and lower are not perceived as sharp. Those were eliminated just by checking butterfly wing shot by naked eye. You will see couple of lenses tested for MTF that are worse than that. Those are lenses with certain reputation so I just couldn't believe they can be that bad. Unfortunately, they were. Basically, I was testing lenses that start at about 2000 LW/PH as those are really sharp. Lenses approaching 3000 LW/PH are perceived as very, very sharp no matter the magnification. And those exceeding 3000 LW/PH I simply call „insanely sharp“ and they are really rare. Among 50 lenses tested only 7 were that sharp.

c) Third figure segment is chromatic aberration (CA). It is measured as pixel area, pretty imaginary term. The only thing you have to remember is that anything below 0.2 is invisible, 0.2 to 0.5 can be seen by naked eye as slightly colored object edges and everything above 0.5 should be corrected for CA. Excessive CA actually spoils perceived sharpness. Most today's applications that read and process raw files have excellent CA correction tools so this parameter is of no huge importance such as resolution.

Part III with first set of tests will follow shortly, probably in a day or two.
Enjoy!
Miljenko
All things are number - Pythagoras

mjkzz
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Post by mjkzz »

WOW, thanks!!!
Miljenko wrote: 1. All the tested lenses (except for finite microscope objectives) perform better on relay (tube) lenses than via extension tubes!
That is what I thought, too.

The only "drawback" is that you can not change magnification, unless a zoom lens is used as tube lens. With extension tube method, I can use a helicoid to change magnification to match FOV.

Miljenko
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Post by Miljenko »

Well, Peter, if you collect the lineup of tube lenses like I did, you would have almost continuous row of magnifications. Here I have 60, 80, 100, (125 - yet to be purchased), 135, 150, 200, 210, 240 mm. One gets a feeling for the right one very quickly.
Best regards,
Miljenko
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Adalbert
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Post by Adalbert »

Hello Miljenko,
Very interesting test!
If you wanted to test the microscope - lenses you could use the razor blade with a sharp, smooth, straight edge as a test target.
https://www.photomacrography.net/forum/ ... sc&start=0
BR, ADi

mjkzz
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Post by mjkzz »

Miljenko, wow, having that many lenses to choose from makes it much more flexible, of course. Can't wait to see more of your results!

JohnyM
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Post by JohnyM »

Excellent test!
I've been using stacked enlargers to get faster apertures in 2x-5x range aswell (effective aperture = nominal x magnification).
Awaiting more results! Im especialy interested of your finds with micro objectives.
mjkzz wrote:WOW, thanks!!!
The only "drawback" is that you can not change magnification, unless a zoom lens is used as tube lens.
Of course you can. Just add extension behind rear lens, if working distance permits. Some combos are quite tollerant for being quite away from infinity.

Miljenko
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Post by Miljenko »

Adalbert wrote:Hello Miljenko,
If you wanted to test the microscope - lenses you could use the razor blade with a sharp, smooth, straight edge as a test target.
That's exactly what I do for magnifications from 1:1 and up. I believe I've mentioned it in part one. Imatest is very tolerant concerning the edge smoothness so razor blade edge should be ok for up to 60x.

Regards,
Mike
All things are number - Pythagoras

mjkzz
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Post by mjkzz »

JohnyM wrote: Of course you can. Just add extension behind rear lens, if working distance permits. Some combos are quite tollerant for being quite away from infinity.
OK, be honest to you, I am a kind of conservative person when it comes to "following" specifications unless I know why I can break it. For example, as far as tube lens focal length is concerned, I THINK it only affects magnification, and it has little or no effect on CA and other properties. With shorter tube focal length, it has other advantages like gaining resolving power, etc.

But focusing to infinity is one of the key feature for infinite objectives and I believe a lot of design consideration went into making that happen. So I do not know what would happen if we do not focus the tube lens to infinity and I would NOT venture into NOT following the design spec regardless UNLESS there is some definite explanation.

Having said that and in addition to the fact that a lens on helicoid provides "stepless" FOV adjustment and without frequently changing tube lenses, I still prefer helicoid. This is my personal preference, of course.

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Post by ray_parkhurst »

I think the improved IQ when used with tube lens ("stacked lenses") probably means the lens was better optimized for higher enlarging ratio. Typically enlarging lenses are designed for 2x-20x ratio, and indeed larger prints require a larger ratio, so you would expect them to be best optimized at larger ratios.

I assume when you test such lenses in stacked mode, you focus the tube lens to infinity. Have you attempted to focus closer? It's possible there is an optimum focus point that puts the enlarging lens closest to its best design magnification, improving IQ vs infinity.

I would not expect lenses designed for repro work to do as well with tube lens, since they are for sure not optimized at infinity. But I expect your data to confirm.

Did you test all the lenses stacked, or did you do it both ways?

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Post by Miljenko »

mjkzz wrote:I THINK it only affects magnification, and it has little or no effect on CA and other properties.
Peter, with all due respect, my tests proved that tube lens focal length/aperture/model influences both resolution and CA. It is valid for extension tube length as well when it comes to finite objectives. I can dig deeper into my measurement data and find some ilustrative samples. It seems there is an optimal tube lens focal length where most of it's parameters click into the sweet spot. If you are interested in proof for some particular lens among those I've listed earlier, I'll be glad to retest with parameters you prefer.
Regards,
Miljenko
All things are number - Pythagoras

JohnyM
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Post by JohnyM »

mjkzz wrote: So I do not know what would happen if we do not focus the tube lens to infinity and I would NOT venture into NOT following the design spec regardless UNLESS there is some definite explanation.
So i propose, you try refocusing your tube lens and checking results with your own EB MkII testing apparatus.
It's not like it will explode or anything.

It's somewhat intuitive, if you have some understanding of optics.

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Post by mjkzz »

Miljenko wrote:
mjkzz wrote:I THINK it only affects magnification, and it has little or no effect on CA and other properties.
Peter, with all due respect, my tests proved that tube lens focal length/aperture/model influences both resolution and CA. It is valid for extension tube length as well when it comes to finite objectives. I can dig deeper into my measurement data and find some ilustrative samples. It seems there is an optimal tube lens focal length where most of it's parameters click into the sweet spot. If you are interested in proof for some particular lens among those I've listed earlier, I'll be glad to retest with parameters you prefer.
Regards,
Miljenko
Thanks Miljenko, but that is a lot of work, so please do not take time to do this just because I (just me) am lacking knowledge in optics, I take your words for it :D

mjkzz
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Post by mjkzz »

JohnyM wrote:
mjkzz wrote: So I do not know what would happen if we do not focus the tube lens to infinity and I would NOT venture into NOT following the design spec regardless UNLESS there is some definite explanation.
So i propose, you try refocusing your tube lens and checking results with your own EB MkII testing apparatus.
It's not like it will explode or anything.
Hahaha, sure, I will try, but I do not think I have enough knowledge, know-how, nor equipment to make conclusions, even for myself.
JohnyM wrote:It's somewhat intuitive, if you have some understanding of optics.
Exactly, I have little knowledge in optics! So I'd rather listen to experts unless something is against my "intuition" :D[/quote]

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Post by ray_parkhurst »

The conclusion of stacking enlarging lenses giving better results has me thinking...I have owned several Nikon 105mm Apo-EL-Nikkors, but was never very impressed with them at the lower mag ranges where I usually work. Like most enlarger lenses, they were not optimized to operate in the 2:1 (reversed) through 1:2 (forward) range. I wonder how well they would work if put in reverse onto a good tube lens in this range? In general how well do short tube lenses (75-80mm) work in this stacked arrangement? Would I see significant vignetting?

I guess better than asking is trying. Yet another project, but an interesting one.

elimoss
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Post by elimoss »

ray_parkhurst wrote: In general how well do short tube lenses (75-80mm) work in this stacked arrangement? Would I see significant vignetting?
I tried the SK 50 f2.8 reversed on the Rokinon 135 f2 without vignetting on APS-C. I tried same on an old 50mm lens and there was too much vignetting.
Didn't try anything between.

I also wonder whether we should try focus distances on the 'tube' lens other than infinity. We are using the SK 50 (or your favorite enlarger) as a diopter or close up filter; I don't think we should necessarily expect infinity focus to have the best IQ. But clearly it works alright.

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