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rjlittlefield

Joined: 01 Aug 2006
Posts: 20449
Location: Richland, Washington State, USA

Posted: Tue Apr 20, 2010 11:45 pm    Post subject:

OK, let's take these things one at a time.

 Quote: Regarding the difference in NA between the two lenses used for comparison to the JML lens, f/3.5 vs. f/2, how significant is this in terms of diffraction? At 7.5x mag on the D700, EF = (1+7.5)*3.5 = f/29.75 vs on the Canon at 7.34x, EF = f/16.6, how significant is this? Does it fully explain the perceived difference in apparent sharpness on the Canon 4.7µm pitch sensor?

No, it doesn't.

Before showing the images, I should explain some theory. The calculation is not as simple as (m+1)*f. There is an additional issue called "pupil factor", which can cause the working aperture to be larger or smaller than predicted by (m+1)*f. For the OM 20/2.0, the pupil factor is about 0.75 (rear pupil smaller). At 7.34X, this means that the working aperture is actually about f/21.5, not the f/16.68 that would be predicted by ignoring it. On a quick measurement (calipers by eye at arms length), the JML lens seems to have a pupil factor of around 1. Assuming that's correct, then on my system the comparison with both lenses wide open would be about f/29.25 versus f/21.5, both effective. Of course the OM can be stopped down. The setting necessary to achieve same working aperture turns out to be almost exactly f/2.8 on the OM 20. Making that change and comparing resolution produces this comparison:

To my eye, it seems clear that the OM 20 has lost some resolution at its f2.8 setting versus wide open, but it doesn't get quite down to the JML.

 Quote: How would the JML lens compare to the Olympus 20mm f/3.5 on the Canon T1i?

We have to speculate, since I don't have a 20/3.5 to test. But in your test the JML is significantly better. I have trouble believing that the results would get reversed simply by putting both lenses on a smaller, finer pitch sensor.

Periodically, people ask me how the OM 20/2.0 compares to an OM 20/3.5. I've always had to say I didn't have a clue. I guess now we have at least suggestive evidence that it's higher resolution, since my OM 20/2.0 beats my JML, and your JML beats your OM 20/3.5. Regarding CA, no information.

 Quote: Regarding the two sensors used, how significant is the difference in pixel pitch, 8.45µm D700) vs 4.7µm (T1i)? Assuming that the two JML copies are optically identical, is it possible that the JML lens could provide near sensor-limited resolution to the D700, even though the results are poor on the T1i?

Yes, that's distinctly possible. If I simply resample my test results down to 50% pixels, kinda sorta making my pixels act as if they were 9.4 instead of 4.7, then the picture looks like this. It's awful hard to see the difference between JML and OM 20/2.0 at this scale.

 Quote: It is clear that smaller dimension sensors, with finer pixel pitch are a good match for high-NA microscope objectives. However, are they an equally good match for true macro lenses which were designed and optimized to cover a 43mm image circle?

The first answer is "of course not", but the situation is not quite that simple.

At 7.5X, the 36 mm sensor images an area that is 4.8 mm on subject. To image the same subject, the 22.3 mm sensor only needs to operate at 4.64X. Assuming a 20 mm lens and making the simplifying assumption that it acts thin, then the angle of view for the large sensor will be about 12.09 degrees, while the angle of view for the small sensor will be 11.28. So the two different sensors are not making dramatically different demands on the covering power of the lens. It's only a difference of about 1.07X, versus the 1.61X that you might think from just considering the sensor size. You have to take all the other scale factors into account also.

 Quote: How much difference in resolving power/apparent sharpness would be seen, on the D700, between the Olympus 20mm f/3.5 and the 20mm f/2? Is it enough to justify the high price (for making large prints, 13x19in or larger)?

Now that's a hard question. The scaled image that I showed earlier in this post suggests that the 20 mm f/2 might be overkill for a D700 at this magnification. So in that sense, it would not be justified.

On the other hand, since we're now talking price, I see that the D700 with 12 MP body is now being listed at B&H photo for \$2470. I don't know what the Nikon equivalents are, but the Canon T1i with 15 MP is currently selling for \$670 at B&H, and the 5D Mark II with 21 MP for \$2500.

So it seems, just looking at the prices, that you have already decided to select equipment based on something other than resolution.

That decision -- to use a 12 MP 36x24 sensor, as opposed to a 15 MP 22x13, or a 21 mP 36x24 sensor, is now affecting your choice of lenses that cost a fraction of the price of your camera body. Someone with different equipment or goals will optimize to a different point.

 Quote: I’m also a bit concerned that any novice members of the forum, who may have purchased this lens, may see the statements regarding low resolution on the T1i and dismiss it---without ever giving looking at its properties on their unique equipment, for their particular applications.

That is always a risk. Hopefully they will read the entire thread and think about their own applications, as it seems that Pierre has already done.

 Quote: I quickly learned the axiom, simplify, simplify and then simplify, again.

There are some similarities in our backgrounds, but I think some differences as well. My training is in numerical analysis and computer science. I recently retired from 38 years with an R&D lab, where much of my job was developing and validating computational models.

What I quickly learned was a little different: "Everything should be made as simple as possible, but no simpler". I have seen far too many cases where a straightforward end-to-end validation test under realistic conditions of interest showed that something important had been missed by more restricted unit testing.

In the case of lenses, what this means is that I will generally test first using subjects and setups that are typical of my intended use.

It happens, for example, that because I almost always stack and almost never image flat subjects, I don't care very much about flatness of field. I would much rather have a lens that is crisp corner to corner over a curved field, than a different lens that is admirably flat but not quite as sharp. Other people will have different priorities.

 Quote: I haven’t attempted to run all the calculations to quantify the effects of the variables stated above. And frankly, although I have a background in applied mathematics, I’m still struggling to get my head around many of the mathematical relationships in this specialized field. I really need to construct a tabbed spreadsheet containing all of the pertinent equations, so I can run what-if experiments.

I try to model everything, and when the simple models don't work, I try to figure out why not. That's usually a frustrating exercise, though satisfying when it's all over. I've accumulated a modest collection of links to useful math, which I'll be happy to share.

--Rik
rjlittlefield

Joined: 01 Aug 2006
Posts: 20449
Location: Richland, Washington State, USA

Posted: Wed Apr 21, 2010 12:07 am    Post subject:

 pierre wrote: Thanks for your test !

You are very welcome.

 Quote: It seems there is a difference on the net aera on the 1st test: JML seems on the top while OM on the middle. Or maybe it is my screen ?

I don't understand your words. Can you say it differently?

 Quote: For the contrast difference, your analyse suggest the NA # is the possible explanation, then, the lightning must be adapted for each lens regarding the angle sensibility ?

NA is involved, but it is not the whole explanation. I just now did some study with LiveView. With the OM, the false colors become less intense at smaller apertures, but they are still obvious at the same aperture where the JML has few or none. Also I checked whether the false colors depend on focus. They do. It seems that when the reflection is exactly in focus, the colors do not appear, but when it is out of focus they do. The colors are different in front and behind focus. I do not understand what all is going on.

--Rik
pierre

Joined: 04 Jan 2010
Posts: 253
Location: France, Var, Toulon

Posted: Wed Apr 21, 2010 4:40 am    Post subject:

Rik,

 Quote: It seems there is a difference on the net aera on the 1st test: JML seems on the top while OM on the middle. Or maybe it is my screen ?

Sorry.. my english still not goog enought.

On your last picture (JML / OM side by side), the JML frame's focus seems, for me, better on the lower half when the OM seems more focused on the middle of the frame. Is it an impression due the rendering of my screen?
_________________
Regards

Pierre
ChrisR

Joined: 14 Mar 2009
Posts: 8495
Location: Near London, UK

Posted: Wed Apr 21, 2010 9:50 am    Post subject:

 Quote: What Chris has done is to make a 200% blowup of the out-of-focus scale just to the right and below the labels in my illustration. The upper is JML, the lower is OM, neither one reflects best behavior of the lens.

I do apologise. I made it bigger so it was easier to see.
I admit it's not ideal to show out of focus parts of an image in a resolution comparison. But I had no idea there might be any there!

I have tried a similar clip from the middle of the frames, and after tweaking the curve and saturation, there's even less to tell the two apart. Zooming the image down in size on the screen until I can't really tell them apart, and doing some measuring, tells me the whole image would conservatively be about 1.2m / 4 feet wide.

This lens may be on a par with some of the cheaper microscope lenses, like a 4x or 5x pushed a bit, or a lower NA 10x. Those aren't quite \$10, but well under \$100. At magnifications nearer the 10x end, the Working Distance would be favourable for a 20mm lens, as long as its performance holds up.
I do have a 20mm f2 and an f3.5, and one of these critters currently delayed behind a pile of ash, so one day I may do a test. What intrigues me is how poor the Oly f3.5 looks!
Bob^3

Joined: 17 Jan 2010
Posts: 287
Location: Orange County, California

Posted: Wed Apr 21, 2010 11:26 am    Post subject:

Quote:
 Quote: Quote: What Chris has done is to make a 200% blowup of the out-of-focus scale just to the right and below the labels in my illustration. The upper is JML, the lower is OM, neither one reflects best behavior of the lens.

I do apologise. I made it bigger so it was easier to see.
I admit it's not ideal to show out of focus parts of an image in a resolution comparison. But I had no idea there might be any there!

Yeah, shortly after sending my response, I realized I’d already seen those scales somewhere! Then Rik chimed in to confirm.
 Quote: What intrigues me is how poor the Oly f3.5 looks!

I agree that the Oly 20 f/3.5 is disappointing. I did not personally purchase this lens---I inherited it from my father who was an advanced amateur mineralogist, specializing in micro-minerals. As a side note of possible interest to the mineralogy folks here, he travelled the south western US deserts (and Mexico) for many years (often with me in tow) collecting some extremely rare micro-minerals, a few of them one-of-a-kind. The University of Arizona would contact him periodically to purchase some of his unique specimens for research. We’ve since sold his collection. But I still have his collection of 35mm slides labeled with the common and scientific names of each mineral. Perhaps I’ll scan them at some point to share in the image area. I think he would have liked that.
_________________
Bob in Orange County, CA
Bob^3

Joined: 17 Jan 2010
Posts: 287
Location: Orange County, California

Posted: Wed Apr 21, 2010 11:47 am    Post subject:

 rjlittlefield wrote: OK, let's take these things one at a time.

Thank you, Rik, for taking the time to provide such a wonderfully detailed response---much appreciated! I’ll provide a follow-up as I get time.

Regards,
_________________
Bob in Orange County, CA
Bob^3

Joined: 17 Jan 2010
Posts: 287
Location: Orange County, California

Posted: Wed Apr 21, 2010 12:13 pm    Post subject:

 ChrisR wrote: and one of these critters currently delayed behind a pile of ash

Maybe this is serendipity at work here. Perhaps you should place a glass side outside somewhere to collect some of that volcanic ash cloud and photographic it under the ‘scope. We could then speculate on why it does such a nasty job on jet engines. And maybe the JML lens will also prove to excel as a volcanic-ash analysis lens!

Regards,
_________________
Bob in Orange County, CA
rjlittlefield

Joined: 01 Aug 2006
Posts: 20449
Location: Richland, Washington State, USA

Posted: Wed Apr 21, 2010 3:02 pm    Post subject:

 pierre wrote: On your last picture (JML / OM side by side), the focus seems, for me, better on the lower half when the OM seems more focused on the middle of the frame.

Thanks, now I see what you are talking about. The bottom two scales, where we can see just the tips, are clearly sharper on the JML side than on the OM side. Also the bright dot at x=166,y=294 is more clearly defined on the left than on the right. There are a couple of other details like that too.

I think there are several things going on here.

First is that the OM lens has wider aperture, so it has less DOF. The two bottom scales are sufficiently out of focus that their sharpness depends more on DOF than on lens resolution. This is made worse by the color fringes that the OM lens puts on OOF edges.

Second is that there may be some artifacts from the resizing. Two details that look exactly the same in the original image can look different after resizing, because one of them ends up centered on a pixel while the other ends up centered between two pixels. Despite Photoshop's best effort to "anti-alias" this situation by sharing brightness across pixels, the two results never look quite the same. I believe the bright dot at 166,294 is benefiting from this effect.

Third is that despite my best efforts, the exact plane of best focus is almost certainly different between the two sides. This difference may be detectable as a gestalt impression that one image is focused "lower on the page" than the other one is, even though the same features appear at best focus in each.

Ironically the difference in focus placement is easier to spot when more attention is paid to out-of-focus regions than to in-focus. Perhaps I can illustrate by diagrams.

Suppose we have two lenses, one sharper but with less DOF. Then near the points of best focus, we might have this situation. It is clear that each lens will be at or very nearly at its best for the same features, and that the blue lens is significantly sharper.

On the other hand, when we expand our attention to include OOF regions as well, then it becomes clear that the lenses were not focused at the same depth. Notice how asymmetric the crossing points become.

I used to wonder, when I went to have my eyes tested, why the last question the doctor would ask was always Which is better, #1 or #2? when both of them were pretty bad. The answer is hinted by the picture shown above. The doctor is deliberately testing two points that are fairly far on opposite sides of optimal. When the client cannot say that one is better than the other, then the doctor knows very accurately where the best point is: exactly halfway between them!

--Rik
pierre

Joined: 04 Jan 2010
Posts: 253
Location: France, Var, Toulon

 Posted: Wed Apr 21, 2010 11:07 pm    Post subject: Hello Rik, This very clear now. Once again this is a great demonstration. Thanks Pierre_________________Regards Pierre
ChrisR

Joined: 14 Mar 2009
Posts: 8495
Location: Near London, UK

Posted: Thu Apr 22, 2010 2:31 am    Post subject:

 Quote: What I quickly learned was a little different: "Everything should be made as simple as possible, but no simpler".

That's one from Albert Einstein, I believe.

Another of his goes something like
"A perfection of means, and confusion of aims, seems to be our main problem."
Pertinent to pixel-peepers ?
elf

Joined: 18 Nov 2007
Posts: 1386

 Posted: Thu Apr 22, 2010 6:38 pm    Post subject: Fortunately this thread is already 3 pages long so I can post this image and hardly anyone will see it (maybe I should get another alias) It was done with the JML 21mm lens @ somewhere close to 5X (extension was 110mm). It's a two shot panorama and post-processing was only sharpening and cloning out two hot pixels. Lighting was with two Ott lamps above the flower, so the bottom was quite dark. The exposure time was increased a bit for the bottom. The top was exposed at .25 seconds and the bottom at .5 seconds. I think the flower is related to the blue bottles as there is some strange Morse code in the stacked results. p.s. If you're not familiar with the insideous blue bottles, details can be found by searching this site.Last edited by elf on Thu Apr 22, 2010 8:39 pm; edited 2 times in total
Bob^3

Joined: 17 Jan 2010
Posts: 287
Location: Orange County, California

 Posted: Thu Apr 22, 2010 8:18 pm    Post subject: Hi Elf, Beautiful image...heck of a lot prettier than my resolution slide! Was this done with your automated x-y-z rig? How many focus slices? Did you mean JML 21mm? Regards,_________________Bob in Orange County, CA
elf

Joined: 18 Nov 2007
Posts: 1386

 Posted: Thu Apr 22, 2010 8:57 pm    Post subject: Yes, I was using the spherical macro panorama head for these images. There are around 250 images in both the top and bottom frames. The focus stacking was done by moving the subject 10 microns per step. Here's a similar blossom shot with the Nikon BD Plan 10 0.25: The BD Plan 10 image looks sharper than the JML 21 image when viewed at 100% in Photoshop. I wouldn't rate either of these images as very good, but I can't blame the lens for that. .
rjlittlefield

Joined: 01 Aug 2006
Posts: 20449
Location: Richland, Washington State, USA

Posted: Thu Apr 22, 2010 9:26 pm    Post subject:

 elf wrote: I think the flower is related to the blue bottles as there is some strange Morse code in the stacked results.

OK, I'll bite. What's with those dots and dashes, anyway? The last time I saw something like that, it was because I had left some really odd image at one end of the stack I had intended to process.

--Rik
elf

Joined: 18 Nov 2007
Posts: 1386

 Posted: Thu Apr 22, 2010 10:02 pm    Post subject: I've never seen the Morse code effect before. There is nothing in the original images that would show up in the stack like this. I ran a slide show of the original images and there are quite a few tiny horizontal shifts from frame to frame (both left and right). These are probably caused by small variations in the lead screw.
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