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Author Topic: shallow DOF limits of current MF systems  (Read 19070 times)
Gary Ferguson
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« Reply #40 on: October 28, 2006, 08:23:14 AM »
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If one chooses the same focal length and aperture, ones get the same DOF with any format.

Kornelius Fleischer, Zeiss's Marketing Manager, has said that two differently designed lenses of the same focal length may have different DOF even with the same format and the same aperture. He gives the example of the Hasselblad 250mm superachromat and the regular Hasselblad 250mm, at the same aperture the Superachromat has a shallower DOF.

It's just one of the "DOF myths" that he debunks, another example is that the theoretical DOF tables are computed only for the centre of the image, off axis different DOF rules apply but these are ignored by DOF tables.
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howiesmith
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« Reply #41 on: October 28, 2006, 08:31:05 AM »
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Kornelius Fleischer, Zeiss's Marketing Manager, has said that two differently designed lenses of the same focal length may have different DOF even with the same format and the same aperture. He gives the example of the Hasselblad 250mm superachromat and the regular Hasselblad 250mm, at the same aperture the Superachromat has a shallower DOF.

It's just one of the "DOF myths" that he debunks, another example is that the theoretical DOF tables are computed only for the centre of the image, off axis different DOF rules apply but these are ignored by DOF tables.
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You have kept focal length the same but changed designs - "two differently designed lenses of the same focal length".  I think the effect is essentially changing CoC.

The superachromat lens has a smaller circle of confusion at the focal plane because it focuses all wave lengths of light better.  The standard lens does not focus all wave lengths the same and produces a circle of confusion on the focal plane for the out of focus wave lengths.

A fuzzy image will have greater DoF than a sharp one, all other things being equal.

True, you are comparing the same focal length, but you have changed designs.  I claim you are actually comparing the designs, not the focal length.  The variables changed (compared) are the design, not the focal length.
« Last Edit: October 28, 2006, 09:07:50 AM by howiesmith » Logged
Ray
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« Reply #42 on: October 28, 2006, 12:20:39 PM »
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But I think it is the whole story.

My proof of this is if you stick a 645 lens on a 35mm body you get the same image - just less of it
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Hey! Morgan, I think I might like to employ you. I could offer you a good salary, then later when I cut it in half and you complain, I could say, 'but you've still got the same salary - just less of it.' I'm sure you would understand   .
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BJL
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« Reply #43 on: November 01, 2006, 02:19:02 PM »
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I'm not so sure of 1, in the sense that I'm not so sure that two lenses of different design set to the same aperture would have the same depth of field even when mounted on the same camera and focused to the same distance.
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That is of course only an approximation, but a widely used and trusted one. Like all the standard formulas for DOF and the size of the circle of confusion on the focal plane, it is computed with the geometric optics approximation and for an on-axis subject, so chromatic aberrations and other lens aberrations make it inexact except for an imaginary aberration-free lens. (Gary and Howard have mentioned parts of this.)

However, such imprecisions are probably irrelevant to my original comment and questions.

For one thing, variations between lens designs are unlikely to be enough to offset the substantial differences in these approximations for minimum DOF between f/1.2 or f/1.4 portrait lenses in 35mm format and the fastest "portrait" lenses in the various current MF systems.

For another, it seems unlikely that there is significant systematic design induced DOF difference between 35mm and MF. Indeed, since the smaller format has to go to lower aperture ratios to get low DOF, and thus has to struggle more with aberrations, it would make sense if anything for 35mm format portrait lenses to be designed to get the lowest DOF that they can out of a given minimum f-stop.
« Last Edit: November 01, 2006, 02:21:43 PM by BJL » Logged
rljones
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« Reply #44 on: November 01, 2006, 10:32:43 PM »
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Kornelius Fleischer also mentioned (probably in the same thread I read, which Gary is quoting) that APO designs inherently have shallower DOFs than the non-APO equivalent. He went on to say this is why many Zeiss lenses are not APO designs---especially those that Zeiss designs for military use.
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eronald
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« Reply #45 on: November 02, 2006, 01:27:47 AM »
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Look here, and ant the two photos of the lady in front of the blue background (search for "sagittal") http://www.imx.nl/photosite/zeiss/test85/t004.html
Those two pictures look separated by 3 stops, and I don't think that such an experienced reviewer as Erwin Puts would have compared them if they were 3 stops different.

Also, I never wondered about portrait lenses, I wondered about extreme wides - eg a 28 mm gaussian and a 28mm retrofocus used with an MF digital back. In digital, as we all know, lane of focus effects are considerably magnified with respect to film !

Edmund



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That is of course only an approximation, but a widely used and trusted one. Like all the standard formulas for DOF and the size of the circle of confusion on the focal plane, it is computed with the geometric optics approximation and for an on-axis subject, so chromatic aberrations and other lens aberrations make it inexact except for an imaginary aberration-free lens. (Gary and Howard have mentioned parts of this.)

However, such imprecisions are probably irrelevant to my original comment and questions.

For one thing, variations between lens designs are unlikely to be enough to offset the substantial differences in these approximations for minimum DOF between f/1.2 or f/1.4 portrait lenses in 35mm format and the fastest "portrait" lenses in the various current MF systems.

For another, it seems unlikely that there is significant systematic design induced DOF difference between 35mm and MF. Indeed, since the smaller format has to go to lower aperture ratios to get low DOF, and thus has to struggle more with aberrations, it would make sense if anything for 35mm format portrait lenses to be designed to get the lowest DOF that they can out of a given minimum f-stop.
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Edmund Ronald, Ph.D. 
Ray
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« Reply #46 on: November 02, 2006, 02:46:52 AM »
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It's surprising how really bad most fast lenses are at full aperture. It seems to me that this really poor performance is masked by the extremely shallow DoF which, nevertheless, is not as shallow as it should be due to a lack of 'real' sharpness at the plane of focus.

An exception would be the Canon 200/1.8 which just happens to be the finest lens that Canon make (do they still make it?) and very expensive of course. At f1.8 and 40 lp/mm, contrast is still in the range of 60-70% right to the edges. Compare this with the Hasselblad Planar CF 80/2.8 which at f2.8 and 40 lp/mm has a best MTF response at the centre of just 40% falling off to a miserable 10% at the edges. If you came across a lens that had this sort of performance at f8, it would be fair to call it absolute crap.
« Last Edit: November 02, 2006, 02:51:32 AM by Ray » Logged
BJL
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« Reply #47 on: November 02, 2006, 09:18:25 AM »
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Look here, and ant the two photos of the lady in front of the blue background (search for "sagittal") http://www.imx.nl/photosite/zeiss/test85/t004.html
Those two pictures look separated by 3 stops, and I don't think that such an experienced reviewer as Erwin Puts would have compared them if they were 3 stops different.

Also, I never wondered about portrait lenses, I wondered about extreme wides - eg a 28 mm gaussian and a 28mm retrofocus used with an MF digital back. In digital, as we all know, lane of focus effects are considerably magnified with respect to film !

Edmund
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My original post specifically mentioned portrait lenses, and the much discussed need for large apertures for the sake of extremes of background blur in portraiture. So I will stay with  that topics, as in your Sonnar 85/Summicron 90 "woman in blue" examples.

In those photos, the most likely explanation is the combination of substantially greater aberration blurring in the Sonnar (as Ray has suggested?) and the different perspective: the Sonnar is clearly closer to the subject, as the woman's image is larger despite the slightly shorter focal length. And the Sonnar image seems less sharp overall, on the woman and dress as well as the background, which is more evidence for its having significantly greater aberrations (or incorrect focus). Reducing focus distance will increase background OOF effects.

There is no doubt that under-corrected aberrations can soften backgrounds (and even main subjects) such as in "soft-focus" lens designs, in which under-correction of some aberrations is a deliberate design feature. Such design choices are irrelevant to my initial curiosity of comparing MF to 35mm format options, since soft-focus lenses exist for 35mm format too. Positioning of the aperture diaphragm can also affect the appearance of background blur, making the large "disks of confusion" harder or softer at the edges: I believe that macro and portrait lens designs often take opposite approaches on diaphragm placement, favoring maximum sharpness versus "nice bokeh" (softer backgrounds) respectively.

But in the "woman in blue" examples, the Sonnar 85 disks are clearly far larger, not just softer.

If such design differences could adequately soften distracting backgrounds and reduce DOF, why would photographers bother with f/1.2 or f/1.4 lenses for that purpose (as opposed to the main large aperture purpose of higher speed); they could just use soft focus lenses, some of which allow turning the "softness" on or off, or at least increasing and decreasing it. Probably there is more to the desire for "background softening" than that.
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BJL
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« Reply #48 on: November 02, 2006, 09:22:31 AM »
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An exception would be the Canon 200/1.8 which just happens to be the finest lens that Canon make (do they still make it?)
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No, the 200/1.8 was discontinued several years ago. Do you not remember the debate here as to whether Canon would soon replace it by something even better, or at least something as fast? Instead, technological changes like increased usable ISO and stabilization keep driving a trend away from the most extreme large aperture lenses.
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Ray
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« Reply #49 on: November 03, 2006, 05:32:12 AM »
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Do you not remember the debate here as to whether Canon would soon replace it by something even better, or at least something as fast? [a href=\"index.php?act=findpost&pid=83380\"][{POST_SNAPBACK}][/a]

Only vaguely, which is why I asked. My memory is not always at its best on issues like this when I am never likely to buy such a lens whether it's available or not  .
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damien
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« Reply #50 on: November 06, 2006, 03:25:04 PM »
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It is sometimes an advantage to have a lens that produces pleasing pictures with the out of focus parts of the scene rendered in a beautiful way. Absolute resolution, contrast at the image edges, or circles of confusion factors are second to the ability of a lens to make good pictures in my opinion. Rarely do I see a 'fast' lens make pleasing pictures wide open. My 210 f4 HC is not particularly fast but is wonderful wide open at f4. I'm glad not to be carrying around the extra glass for a 2.8 version only to shoot it at f4.

Damien.
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« Reply #51 on: December 25, 2006, 02:34:57 PM »
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This is a very interesting thread.  But....

Has anyone directly compared (e.g. portrait) a 35mm lens like the canon 85mm 1.2 with a MF setup with say an 80-120mm f2ish lens. I would love to see some direct comparisons at full or near full aperture.
« Last Edit: December 25, 2006, 02:35:29 PM by MarkKay » Logged
eronald
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« Reply #52 on: December 25, 2006, 03:45:25 PM »
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Only vaguely, which is why I asked. My memory is not always at its best on issues like this when I am never likely to buy such a lens whether it's available or not  .
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I believe the replacement is ready, at least ready designed. They probably have simply not been able to find production capacity.

Edmund
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Edmund Ronald, Ph.D. 
Ray
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« Reply #53 on: December 26, 2006, 12:25:19 AM »
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I believe the replacement is ready, at least ready designed. They probably have simply not been able to find production capacity.

Edmund
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It'll be interesting to see how it compares with the discontinued model, but unless there's a significant reduction in price, there's not much chance of my ever buying one. I suspect such lenses are so expensive, not only because it costs more to produce such high quality lenses, but because sales are not high and profit margins have to be greater. I guess manufacturing this new lens is not a priority for Canon. There are other lenses that they know will generate more profit, such as the 70-200L f4 IS.
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Ray
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« Reply #54 on: December 26, 2006, 01:05:57 AM »
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Negative refractive index???!

How does (speed of light in a vacuum)/(speed of light in the material) ever become negative?  Or even become less than unity?
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Steve,
I'm reminded of your question because, not long ago, at the end of a day's trek in Nepal, during the evening's conversation when everyone huddles around the wood fire to get warm, the subject of digital camera's and the rapid development of technology came up.

One of the trekkers claimed to be a physicist, so I sounded him out on this interesting research on the concept of metamaterials with a negative refractive index.

He too immediately reacted as you did. How can the speed of light in any material exceed the speed of light in a vacuum?

After a bit of Googling, I get the impression that there's a ways to go before these new materials will be able to transmit light. The researchers are currently working on frequencies in the 1 to 3 micron wavelength. The wavelength of red light begins around 0.7 microns. However, I believe the same principle applies. Below is an extract from [a href=\"http://www.aph.uni-karlsruhe.de/ag/wegener/meta/meta.html]http://www.aph.uni-karlsruhe.de/ag/wegener/meta/meta.html[/url] .

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Metamaterials and negative refractive index
The basic physics of "magnetic atoms" is quite simple: In order to obtain a magnetic response from a metal nanostructure the incident light field has to excite local currents circulating in loops (solenoidal currents). These currents in turn give rise to a magnetic dipole-moment. By properly designing the metal nanostructure, one can obtain a resonant enhancement of the local currents leading to a strong magnetic response and potentially a negative magnetic permeability.
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godtfred
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« Reply #55 on: December 26, 2006, 01:52:24 PM »
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This is a very interesting thread.  But....

Has anyone directly compared (e.g. portrait) a 35mm lens like the canon 85mm 1.2 with a MF setup with say an 80-120mm f2ish lens. I would love to see some direct comparisons at full or near full aperture.
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There is a comparison of the Canon 5D with the 85 f1.2 mk II against the Hasselblad H2 with 39' back and 100 f2.2 in the latest edition of swedish magazine proffsfoto. They come out pretty similar, but the MF system wins on most aspects.

-axel.
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Axel Bauer
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MarkKay
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« Reply #56 on: December 26, 2006, 02:04:34 PM »
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I would love to see the comparisons.  Let me ask another question.  If you were using a Hasselblad H system and could select the best medium focal length (80 to 150mm) lens for DOF, OOF, and Bokeh, which one would you select.  I have tried to read comparisons on the web after doing a google search but nothing really substantial comes out of it. mark

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There is a comparison of the Canon 5D with the 85 f1.2 mk II against the Hasselblad H2 with 39' back and 100 f2.2 in the latest edition of swedish magazine proffsfoto. They come out pretty similar, but the MF system wins on most aspects.

-axel.
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ericstaud
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« Reply #57 on: December 26, 2006, 03:55:41 PM »
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I would love to see the comparisons.  Let me ask another question.  If you were using a Hasselblad H system and could select the best medium focal length (80 to 150mm) lens for DOF, OOF, and Bokeh, which one would you select.  I have tried to read comparisons on the web after doing a google search but nothing really substantial comes out of it. mark
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It would be great to throw the 50-110 lens in this comparison group.  It seems so many shooters use this as a starter lens for the H1 because of it's versatility.   With the Canon and Nikon lenses there are many fixed lenses that provide so little improvement over zoom lenses these days that it is often not worth the effort to switch back and forth.
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MarkKay
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« Reply #58 on: December 26, 2006, 05:06:58 PM »
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I can state as a general purpose lens, this zoom is absolutely outstanding. It is the best zoom i have used period.  It is tack sharp even at full aperture.  the bokeh is nothing special but I suspect this may be the fuji vs zeiss factor.  I went hiking in the colorado mountains and with one exception,  this lens remained on the camera the whole time.  It is big and heavy though and I almost never use it other than on a tripod.  So i am trying to find one lens for  those narrow DOF shots or I am going to go back to a canon 85 1.2. Mark


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It would be great to throw the 50-110 lens in this comparison group.  It seems so many shooters use this as a starter lens for the H1 because of it's versatility.   With the Canon and Nikon lenses there are many fixed lenses that provide so little improvement over zoom lenses these days that it is often not worth the effort to switch back and forth.
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BJL
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« Reply #59 on: December 28, 2006, 02:53:16 PM »
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I believe the replacement [for the discontinued Canon 200/1.8--BJL] is ready, at least ready designed. They probably have simply not been able to find production capacity.
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That seem very unusual: discontinuing a product several years before a replacement is ready. Can you reveal your source for that belief? Forgive my skepticism, but a lot of people confidently but incorrectly predicted the imminent arrival of a replacement back when the 200/1.8 was discontinued: hopes got transformed into claims of fact back then, as often happens.
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