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06-11-2018, 08:11 PM   #1
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Grand Lens Test 9: Pentax 645 A 120mm f/4 Macro

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After looking at the two Ukrainian shift lenses, I had a need for sharpness, and boy does the 120 Macro lens deliver it.

I've read it's not quite as good as the 90, and that it's not as good at distance. I don't have the 90 to compare it to, but my test was at distance, so at least I can explore that. There is already an excellent review of the autofocus FA version of this lens here, which praised it highly, so I won't attempt to test it in the macro range. But I wanted to know how it would do as a general-purpose lens.

Here's my test scene at f/11:


Even this image, most of which depends on the depth of field, looks sharp. The bark of the River Birch at left is quite sharp, and the background is reasonably sharp. This lens can definitely make a 16x20 that never undermines the illusion of endless detail.

I had so much trouble trying to decide what the optimal aperture was, that I decided to show all of them.

I'm only showing center crops, simply because I could tell the difference between the centers and the corners. Okay, I'll show one corner. Here's the lower right corner at f/11 and 1:1:


There is some motion--this was a 1/3-second exposure. But the image is sharp. Remember, 1:1 at 100 pixels/inch of most monitors is part of a print seven feet wide.

So, back to the center. Here's the center of the frame at 1:1, shot at f/4:


Not too shabby, for sure. Sharper than the A* 300.

The bokeh isn't much to write home about, though, so I'm not thinking this is a superlative portrait lens. At wide apertures, you'll be adding gaussian blur to the image to make it tolerable for a portrait of a sitter who will actually expect the image to be flattering, but even at wide apertures, the background will be distracting. But that's not what this lens is for.

Here it is at f/5.6:


In the focus plane, I'm not absolutely sure I can tell these apart.

And at f/8:


F/5.6 actually looks a bit better than f/8. Huh?

F/11:


Worse still.

F/16:


F/22:


It's bee a long time since I looked at an SLR lens that was actually at its best wide open. In fact, I'm not sure I ever have. This may be the first truly diffraction-limited medium-format lens I've ever handled. With this lens, you use the widest aperture that provides the necessary depth of field.

Let's look at that lower right corner again. Here's f/11 again:


And here's the same corner at f/4:


Yup, even the corner is sharpest wide open, as long as it's in focus.

Simply amazing. Even the superb 55/2.8 got sharpened up enough down to f/11 to overcome diffraction.

Oh, you can bet this one's going to Alaska. This will be one of my most used lenses. Now, I have to wonder if this lens with the 1.4 converter will be sharper than the 200/4, which, stopped down a bit, is no slouch.

I'm going to have to adapt this lens to my Canon and put it up against my 70-200f/4L (which I think it will beat) and the 50mm Compact Macro, which is one of the sharpest lenses in the Canon line.

Rick "this one's dangerously sharp" Denney


Last edited by rdenney; 06-30-2018 at 12:16 AM.
06-11-2018, 09:32 PM   #2
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QuoteOriginally posted by rdenney Quote

I've read it's not quite as good as the 90, and that it's not as good at distance. I don't have the 90 to compare it to, but my test was at distance, so at least I can explore that. Now, I have to wonder if this lens with the 1.4 converter will be sharper than the 200/4, which, stopped down a bit, is no slouch.
Rick, Are these shot on film or digital?

On film, my 75mm is the sharpest, followed by the 35mm (not including the outer edges), 120, 150, and then my 200mm. For portraiture, I find my 200mm too long and soft and prefer the 150mm. Sorry, I don't have a 90 to comment, although I suspect it would be sharper than the 120 macro.

I would predict your 120mm macro with the 1.4 TC would be marginally sharper than the 200mm until you got to f/8-11 on the 200.
06-12-2018, 04:21 AM - 1 Like   #3
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QuoteOriginally posted by Alex645 Quote
Rick, Are these shot on film or digital?



On film, my 75mm is the sharpest, followed by the 35mm (not including the outer edges), 120, 150, and then my 200mm. For portraiture, I find my 200mm too long and soft and prefer the 150mm. Sorry, I don't have a 90 to comment, although I suspect it would be sharper than the 120 macro.



I would predict your 120mm macro with the 1.4 TC would be marginally sharper than the 200mm until you got to f/8-11 on the 200.

I’m using a 645z.

I agree with your prediction.

My only 75 is the leaf shutter lens, which I suspect will be very sharp in the conventional sense (it’s an old design, like the P67 105, but in the sweet spot for the format). I’ll make a ranked center sharpness list when I’m done (why not?), maybe as a follow up to my “what lenses should I take to Alaska?” thread. My impression of this lens might have been inflated by the softness I encountered with the two Ukrainian lenses.

Rick “still has the 75 to go, and the 80-160” Denney

Last edited by rdenney; 06-12-2018 at 02:24 PM.
06-13-2018, 01:35 PM   #4
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The 120 macro has a unique cross section, at least to my eye. I have never seen a lens design that resembled it. Your thoughts?

06-13-2018, 08:50 PM   #5
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QuoteOriginally posted by desertscape Quote
The 120 macro has a unique cross section, at least to my eye. I have never seen a lens design that resembled it. Your thoughts?


It’s sort of a Plasmat design, with additional correctors front and rear.



Compare to the very similar (and newer) Canon EF 50mm Compact Macro (also a superbly sharp lens):



The front group (behind the front element) is a strong meniscus but fairly neutral. In a Plasmat, this would be the front element and it would be (but isn’t always) a cemented doublet. You can see the rear Plasmat element in both lenses. Neither has the usual Plasmat inner element in the front group, and both have three corrector elements behind the rear Plasmat group.

A perfectly symmetrical Plasmat can perform beautifully at 1:1, but they are usually slightly asymmetrical to optimize for distance over ten times the focal length.

Both lenses use floating elements to maintain optimal correction over the wide focus range.

Rick “wondering if the 9-element Mamiya and Schneider 120/4 macro lenses are similar, but can’t find any diagrams” Denney
06-13-2018, 10:06 PM   #6
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The Canon looks like a 7 element Double Gauss similar to a Leitz Elcan but with an achromat at the rear. The 120 macro looks very different to me. I just can't categorize it. It may be unique.
06-13-2018, 10:23 PM   #7
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Grand Lens Test 9: Pentax 645 A 120mm f/4 Macro

QuoteOriginally posted by desertscape Quote
The Canon looks like a 7 element Double Gauss similar to a Leitz Elcan but with an achromat at the rear. The 120 macro looks very different to me. I just can't categorize it. It may be unique.


Take elements 2 and 3 of the Canon lens and make them a single similar meniscus, and you have the same order of positive and negative elements as the Pentax, and the same general plan.

A double Gauss has cemented doublets in the middle and positive meniscus elements outside. A Plasmat has doublets on the outside and single elements in the middle. These are a little of both.

But the 645 diagram is a sketch, and not a correct one. The 120/4 Macro has nine elements in seven groups. We are missing a cemented doublet somewhere in that diagram.

Rick “unlike the earlier 67 lenses, designed in a later age of computers, when the old formulas became blurred” Denney

Last edited by rdenney; 06-13-2018 at 10:31 PM.
06-16-2018, 03:07 PM   #8
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QuoteOriginally posted by rdenney Quote
The 120/4 Macro has nine elements in seven groups. We are missing a cemented doublet somewhere in that diagram.
I'm betting that whoever did the sketch, left out the line in the front meniscus. It is probably a cemented achromat, not a single element.

06-16-2018, 06:16 PM   #9
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QuoteOriginally posted by desertscape Quote
I'm betting that whoever did the sketch, left out the line in the front meniscus. It is probably a cemented achromat, not a single element.
My thoughts exactly.

Rick "turning my attention to the A* 300/4" Denney
06-17-2018, 11:02 AM   #10
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120 Design

Okay, I think I found the heritage design that the 120 was based on. It is a Makro-Plasmat, a hybrid between a Double Gauss and Plasmat. Double Gauss in front, Plasmat in the rear. The 120 has a corrector in the middle which is a bit different.
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06-17-2018, 12:09 PM   #11
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QuoteOriginally posted by desertscape Quote
Okay, I think I found the heritage design that the 120 was based on. It is a Makro-Plasmat, a hybrid between a Double Gauss and Plasmat. Double Gauss in front, Plasmat in the rear. The 120 has a corrector in the middle which is a bit different.

Good find!

Rick “wondering which elements float” Denney
06-17-2018, 08:30 PM   #12
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QuoteOriginally posted by AstroDave Quote
I did go back and look now. For sure, as you stop down, things get less sharp (~ my "more blurry"), but I think (without doing some calculations) that this is happening faster than diffraction would be kicking in. This may be due to other effects such as commented upon by kaseki.
In the discussion of the test of the 300mm A*, @AstroDave brought up a good point, one that I initially refuted but now I think he's right.

Let's look again at the f/11 center crop:


It's not as sharp as at f/4. But here's the thing: Diffraction is the same across all lenses--it's not a defect. Diffraction is caused by the image being slightly polluted by light that bends around the edge of the aperture. At wide apertures, the pollution is hardly visible--it's too much diffused into too much good stuff. At small apertures, the polluted light rays represent a higher percentage of what's hitting the sensor, and the effect is more visible. But f/11 for one lens shouldn't be worse than f/11 for another lens just because of diffraction. So, I went back to compare, despite that comparisons are difficult simply because my test subject is different sizes with different lenses, making it hard to know what is simply the result of greater magnification. Even so, here's a quick exploration.

The 45=85 is the longest lens I tested that is shorter than the 120. When magnified so that the subject is the same size as with the 120, it should be less sharp, if both are limited only by diffraction. So, I blew up the f/11 photo from the 45-85 at 85mm to 400%, and took a screen capture. Here it is:



It's hard to know for sure--they are still different sizes and I don't want to move these to Photoshop where I can scale them to be the same. But it seems to me the 85 resolves a little more detail at f/11 than the 120..

Let's look at the f/11 shot from the FA 200/4. Here's the center crop:


Okay, I see much more detail in this image. We would expect a little more resolved detail simply because the 200 pre-magnifies the subject for us. But I don't think the difference in visible detail between this lens and the 120 can be explained solely by the difference in focal length.

So, something is causing the 120 to lose sharpness as it is stopped down. Here's a couple of ideas from Kaseki:

QuoteOriginally posted by kaseki Quote
...In addition, although this should be minimal when changing among the smallest apertures, there might be chromatic aberrations balanced over the lens aperture that change in value with f/#. These might be mistaken as a diffraction effect as the iris is closed. Similarly, the point of best focus (compromise) could change with aperture.
It is certainly true that some lenses suffer a slight focus shift as they are stopped down. And, if the aperture isn't where it should be, the shift can be more severe. It was always necessary to refocus at taking aperture when using an old Schneider Symmar Convertible--a symmetrical plasmat that provided an acceptable triplet with the front cell removed. But that left the aperture in front of the remaining lens elements, which is what caused the focus shift.

But in studying the whole f/11 image at 400%, I can't see that it gets sharper in front of or behind what was the focus plane at f/4. So, I don't think it's focus shift.

The chromatic aberration issue is intriguing, but I don't know enough about what Kaseki is saying to know how to evaluate it in the images.

In any case, diffraction may not be the whole story here.

Rick "it's still going to Alaska" Denney
06-18-2018, 09:01 AM   #13
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QuoteOriginally posted by rdenney Quote
...

The chromatic aberration issue is intriguing, but I don't know enough about what Kaseki is saying to know how to evaluate it in the images.

In any case, diffraction may not be the whole story here.

Rick "it's still going to Alaska" Denney
Well, there is the possibility that I don't know what I'm saying either.

Actually, my occupation never involved me designing the lens assemblies our systems required. My yob, inter alia, was to specify what the lens performance had to be for the system it worked in to meet its requirements. Optical designers would construct and tune lens designs and then statistical ray tracing would be used to determine how well the design worked. And this had to be done over a range of tolerance values. From this experience and the complaints of the designers that certain combinations of requirements were nearly impossible (given other constraints), I gained a feeling for many of the "second order" effects that can degrade an image relative to the detector's pixel size.

One measure of flatness of field, for example, was a plot of focus error over the focal plane. This was usually not perfect even if designed for a narrow range of wavelengths. If the lens assembly had to work over a wide range of wavelengths, then the flatness of field function could be different for different colors. My systems did not normally have irises, so I can't say I saw explicit cases where trying to get all of the colors within some tolerance of field flatness had to deal with changing f/#. However, tolerancing lenses for best focus at a given f/# had to take into account that the flatness of field shapes were different. From that I conclude that when a lens is optimized for some f/#, changing that f/# could cause chromatic differences in focus, just as would result from a single wavelength's best focus over a focal plane changing with f/#. These are normally subtle effects, and only in a highly magnified image are they likely to be an issue.

Bottom line: thin lens approximations may lead one astray when the number of lenses is large (and not thin) and performance is important over a large field angle, and not just at the optical axis.

---------- Post added 18th Jun 2018 at 12:34 ----------

Addendum:

When focusing my 645N, whether manually or with autofocus, the action is normally taken with the iris wide open for best light in the viewfinder. While I might use the lever to force the aperture closed for examining depth of field, I wouldn't normally (or at least previously) check to see if the focus is still optimal at the higher f/#. Even if I were to do so, in less than maximum lighting it might be difficult to determine best focus at high f/nos. Without an optical lab to support focus experiments, evaluating this might be possible by having a set of resolution targets set up at modestly different ranges near the line of sight to the "lamppost" to see if the point of best focus was observably different over a range of auto-aperture photos relative to the wide open focus condition. This should be more practical with a digital camera than a film camera.

Last edited by kaseki; 06-18-2018 at 09:20 AM.
06-18-2018, 11:34 AM   #14
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QuoteOriginally posted by kaseki Quote
However, tolerancing lenses for best focus at a given f/# had to take into account that the flatness of field shapes were different. From that I conclude that when a lens is optimized for some f/#, changing that f/# could cause chromatic differences in focus,
Spherical aberration by color has been a challenge for lens designers for years. Most photographic lenses tend to have as many colors as possible focus at the focal plane that emanate from the area 40% out from the center of the optic. They know that the diaphragm can be used to truncate the poorly corrected marginal rays, so the serious spherochromatism at the margin is somewhat ignored. Conventional glass telephotos were afflicted with massive fringing wide open due to spherochromatism but the newer, low dispersion long lenses do much better, mostly because the glass used just does not separate the colors very much in the first place.

Last edited by desertscape; 06-18-2018 at 12:49 PM.
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