NikonGear'23
Gear Talk => Lens Talk => Topic started by: The_Traveler on March 22, 2016, 22:41:43
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If diffraction is an important issue at small apertures, how are macro lenses made to minimize that?
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It's all about optic physics; there is not much you can do about it. But there is a way to go around the problem: focus stacking.
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Yes and no. Diffraction cannot be avoided. However, some lenses are diffraction-limited in their performance and might deliver good results even well into the diffraction danger zone. The Coastal Optics 60 mm f/4 APO lens is a prime example. It can be stopped down to f/45 and the results, although clearly softer, are still highly acceptable.
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If diffraction is an important issue at small apertures, how are macro lenses made to minimize that?
Any lens having a diaphragm will show diffraction if you stop down, macro lenses included.
However, if I need to stop down, I do stop down, as a lack of DOF is more annoying than a hypothetical diffraction effect, that being of course my own opinion.
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Diffraction is an edge effect, so a circular aperture opening will minimize diffraction since it has the shortest edge length. The same f-stop with a non-circular opening will have a longer edge length, and proportionally more diffraction.
Most macro lenses these days have rounded apertures, although I suspect that's more to improve the bokeh at wider apertures, not to reduce diffraction. By the time you stop well down the opening usually become polygonal so less than ideal, but with 7 or more aperture blades the difference is minor.
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If diffraction is an important issue at small apertures, how are macro lenses made to minimize that?
Short answer: They aren't.
Long answer: They aren't, but some lenses seem to produce slightly more diffraction induced softness than other similarly spec'd lenses, and used under similar conditions.
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i am basically a bug shooter so diffraction is something that i have to deal with a lot of time. :o :o :o
here is my study of the 105mm VR (my main bug lens) and you can see for yourself how much this lens can handle before the images become real ugly (when pixel peeped).
http://richardhaw.com/2016/01/24/study-diffraction-on-the-105mm-vr/
apologies for the images. I wrote this when the blog was new so i didnt know how to embed stuff. not only that, the images were too big to be uploaded as-as but just read the commentaries and take my word for it.
macro lenses are just like any other lenses with the exception that it has a really long helicoid and an optical design to match. like what was mentioned, diffraction is connected to the iris and i am not aware of any macro lenses that have special irises.
another thing to consider is that as you get closer, the effects of diffraction increases because of the magnification (dont quote me on this yet, im investigating but this seems to be the case). a lens in a bellows unit set to full will exhibit that. the cause might be due to the iris being too far from the film plane as well. i am not an optical engineer or expert so i am curious as well.
dont worry too much about it unless you absolutely need to show a bug's compound eye or the like in clear detail.
one more thing that puzzles me is that FX cameras get at least 1 stop more before diffraction starts to be obvious. i tested this on a D7000 and a D800 a long time ago so the pixel pitch is roughly the same.
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If diffraction is an important issue at small apertures, how are macro lenses made to minimize that?
They aren't and they can't be.
If you are interested in the science there is a series of excellent tutorials on diffraction (and on many other things relevant to photography) at http://micro.magnet.fsu.edu/primer/lightandcolor/diffractionhome.html (There is a fair bit of science involved, but anyone with some tertiary physics or good secondary school physics and a willingness to work at it should be able to follow).
The skinny is that resolution increases as the aperture gets bigger: every decrease in aperture reduces resolution. This is a result of the nature of light, and it applies to reflecting as well as refracting imaging systems (which is why astronomers always want bigger telescopes). The catch is that lens aberrations also reduce resolution, and some aberrations improve as the aperture gets smaller (especially spherical aberration - related to the cube of aperture - but also coma, astigmatism and field curvature). So as the aperture gets smaller it is a race between more diffraction making the image worse and less aberration making the image better. A "diffraction limited" lens is one with no aberrations, and for practical purposes they do not exist, but folk talk about lenses that are "diffraction limited at f/5.6" and so on, meaning that at f/5.6 the diffraction effect is worse than the aberrations. The bigger the aperture at which the diffraction effect is worse than the aberrations the better corrected the lens.
Close focus means shallow depth of field, so you often need a small aperture to get adequate depth of field, but that is always bad for resolution. Macro photography is a struggle with optical limitations.
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Macro photography is a struggle with optical limitations.
100%
i never got this until i started shooting greater than 1:1 :o :o :o
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The dedicated photomacrographic lenses should always be used "wide" open as they are designed for that use.
The technique of focus stacking has allowed wonderful solutions to the limited depth of field issues troubling us before. However no tree grows into the sky and stacking can only be of so much help, plus introduces separate issues.
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Long answer: They aren't, but some lenses seem to produce slightly more diffraction induced softness than other similarly spec'd lenses, and used under similar conditions.
That is what I have found out testing my lenses (mostly AIS Nikon lenses) at small apertures: there are some difference between lenses, they are very small (I would say less than 1 stop, I mean that the best lenses at f/22 are not better than the worse ones at f/16) but still visible.
So what can make a lens behave better than another in the diffraction field?
I think that at small apertures (f/11 and smaller) the different optical quality of different lenses is very small compared to the deterioration of IQ due to diffraction. So in my opinion differences of IQ at small apertures is mainly due by different level of diffraction.
As diffraction is produced by an angular deviation, could the position of the iris diaphragm have impact on diffraction effect? Consider two lenses of the same FL at the same aperture, lens A has the iris diaphragm closer to the sensor than lens B, given that the light rays are bended by the same angle, rays of lens A should arrive at the sensor with a smaller displacement.
As far as I know aperure is determined by the diameter of the entrance pupil and not by the diameter of the physical aperture of the iris diaphragm. So at the same f/stop lens A could have a bigger physical aperture of the iris diaphragm than lens B and be less affected by diffraction. Could this be another reason for little differences between lenses in the diffraction territory?
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one more thing that puzzles me is that FX cameras get at least 1 stop more before diffraction starts to be obvious. i tested this on a D7000 and a D800 a long time ago so the pixel pitch is roughly the same.
Might that be because the DX image must be magnified 1.5x to make the same size image be it print or Web display? Then again the DX image has more DoF so for DX you might not stop down as far as you would with FX.
Dave
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f-numbers are relative .... f/N really where f=focal length, N=aperture number. That'll take care of the absolute size.
The physical aperture opening is usually placed at the internal lens position where the ray bundle is near-parallel.
FYI, it is the exit pupil that acts as the film/sensor illuminating source on behalf of the optical system.
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f-numbers are relative .... f/N really where f=focal length, N=aperture number. That'll take care of the absolute size.
The physical aperture opening is usually placed at the internal lens position where the ray bundle is near-parallel.
FYI, it is the exit pupil that acts as the film/sensor illuminating source on behalf of the optical system.
Thank you Bjørn for the replay.
I was referring to two lenses of the same FL at the same aperture, having different position of the iris or/and physical aperture size.
So there is no much room for lens designer to move the iris closer to the sensor? But could it help?
Thanks for the correction about exit/entrance pupil.
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"So there is no much room for lens designer to move the iris closer to the sensor? But could it help?"
Affirmative on the first question, NO on the other.
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You can place the aperture in various positions within the optics, before the front element, after the rear element, or in between any of the groups. Diffraction is an edge effect, placing the aperture at a point before the incoming light is narrowed down and collimated allows a larger diameter to be used.
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Of course an aperture can be placed anywhere, if you don't care for shading or vignetting due to the cutting off of peripheral rays. However, it is usual to put the aperture where it does the least harm, namely in the internodal space at the position where the ray bundle is (near) parallel.
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The dedicated photomacrographic lenses should always be used "wide" open as they are designed for that use.
ehemm, not in general. Quite a few of them work much better when stopped down a bit, ZEISS Luminars for instance are best 1 stop down.
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OK, my statement was too broad. I stand corrected. "More or less wide open" would be more apt.
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One thing I have wondered about, is diffraction affected by focal length? If for example we frame the same subject identically (eg a brick wall) using two lenses - a 1200mm lens at f/16 and a 28mm at f/16, will the diffraction be the same?
With the 1200mm lens, the physical opening is much larger (greater area : edge ratio) so diffraction is less. On the other hand the path to the image plane is much longer so any spreading of light due to diffraction is much greater. So is the overall effect the same between the two lenses?
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This is not easy to answer yes/no. Detail magnification will be vastly different and because things are rendered bigger with the long lens, more softening can often be tolerated. Traditionally long lenses offered much smaller apertures such as f/45 or f/64.
If one could arrange so magnification differences are eliminated, perspectives would be completely different, and again direct comparison would not be easy.
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Maybe I'm misunderstanding ... If the subject is framed identically with the two lenses (the 1200mm proportionally further from the subject than the 28mm lens) then the detail magnification is the same. Is the amount of diffraction also the same?
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Maybe I'm misunderstanding ... If the subject is framed identically with the two lenses (the 1200mm proportionally further from the subject than the 28mm lens) then the detail magnification is the same. Is the amount of diffraction also the same?
Diffraction is the same assuming both lenses behave similarly to f stop vs diffraction.
Unless I'm wrong what will make the difference is that the 1200mm lens will have a more abrupt sharp to soft transition zone compared to the wide angle. This might increase perceived sharpness.
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Maybe I'm misunderstanding ... If the subject is framed identically with the two lenses (the 1200mm proportionally further from the subject than the 28mm lens) then the detail magnification is the same. Is the amount of diffraction also the same?
I have already discussed this, see above.
The massive difference in perspective will render a direct comparison very difficult. That is, unless the target depicted was perfectly flat with no depth to it at all. In the latter, hypothetical case, there should be no difference between the two lenses if the angle of incidence was perfectly normal.
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Of course an aperture can be placed anywhere, if you don't care for shading or vignetting due to the cutting off of peripheral rays. However, it is usual to put the aperture where it does the least harm, namely in the internodal space at the position where the ray bundle is (near) parallel.
One would want to place the aperture in a position where the light has diverged in order to make the physical opening as wide as possible to implement the F-Stop. Effects of diffraction caused by the aperture mechanism of the lens the optical engineer can take that into account of the lens design. Looking at the retro-focus design of the "art" lenses, aperture placement could be done to reduce effects of diffraction.
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You cannot play two horses at once and expect both to win.
When the aperture stop is put in the non-optimal position, lens characteristics such as vignetting will suffer badly.
Besides, you cannot "enlarge" a given aperture number. This is a fixed relationship, combining focal length and entrance pupil size.
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You can expand the wavefront and then place the Iris at that location.
Physical size for a simple, single-element lens of fixed diameter is given by focal-length/F-Stop. Complex lenses allow a degree of freedom in placement and size of the Iris. The Leica Elmar vs Zeiss Tessar is a good example of placing the iris in different positions: both lenses are the same optically, the Elmar places the Iris after the front element, the Tessar places it in front of the rear doublet. The Olympus RC and several other fixed-lens RF cameras place the Iris behind the lens, the MS-Optical 4/28 Perar places the Iris before the front element.
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one more thing that puzzles me is that FX cameras get at least 1 stop more before diffraction starts to be obvious. i tested this on a D7000 and a D800 a long time ago so the pixel pitch is roughly the same.
I'm sorry I'm repeating my self...
The answer might be the "Enlargement Factor." To make an 8x10 inch print from a 24x36mm (FX) camera you'll need an 8x enlargement but for 16x24mm (DX) camera you'll need a 12x enlargement. The greater the enlargement the easier it is to reveal diffraction: make an 8x10 inch print from an FX image shot at f/45 and it will be surely be soft. Make a contact print from an 8x10 inch negative shot at f/45 and you won't likely notice. I believe the diffraction for each image shot at f/45 will be the same before enlargement.
Dave
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I think the discussion is going in circles. I tend to assume the optical designers know what they are doing, and put the aperture stop where it does the least harm to other characteristics of the lens. Thus, the typical position is within the internodal space.
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I've had a number of Optical Engineers with me in the 80s, and work for me in the 90s. If you tell them what to optimize for, the design changes. If the size of the Iris is going to be the limiting factor in the design they will move it to a location to minimize that effect. Positioning the Iris where the cone is most constrained reduces the size of the mechanism, but maximizes diffraction. If light traveled as a cylinder through the lens, it would not matter. In most lenses, light travels as a cone that expands and contracts as it goes through the elements. This gives an opportunity to place the Iris in a position to minimize diffraction.
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We are stating the same concepts and ideas so obviously no disagreement exists. The aperture stop is put where it does the least harm to other lens characteristics.