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Thread: Optics: resolution versus contrast ?

  1. #51
    Abuser of God's Sunlight
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    Re: Optics: resolution versus contrast ?

    Quote Originally Posted by Struan Gray View Post
    Paul: you probably know this, but the effect you are describing is what makes the Einstein/Monroe hybrid image work. I've done similar double portraits of our twins. I don't know of anyone using the effect for art purposes, but I'm sure it's a factor (beyond the mere revelation of more detail) in many large scale abstract and figurative works.
    I don't know it ... I don't even know of it (had to check if there was another paul here ...)

    Thanks for the link ...

  2. #52
    Abuser of God's Sunlight
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    Re: Optics: resolution versus contrast ?

    Quote Originally Posted by Ken Lee View Post
    The MTF charts I've seen show that (in general) as the target gets smaller (demanding greater resolution from the lens), the contrast of the image delivered by the lens decreases.

    That's the reciprocal relationship to which I referred.

    I'm an amateur and appreciate all the help available here.
    This is a non-technical answer, but I think the simplest explanation is that all the natural forces that limit resolution do so by reducing contrast. optical aberrations, focus errors, and diffraction all introduce blur functions. The higher the spatial frequency of the detail, the more it's degraded.

  3. #53

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    Re: Optics: resolution versus contrast ?

    Fun indeed, microscopes are really a gateway into another world in similar ways to telescopes or other technology devices that extend human vision. It is an adventure waiting to be discovered.

    Alexanderson Alternator, these were the first attempts at generating wireless information transmission at high-power. These were basically alternators with lots of poles on the rotor and run a high speeds to generate high frequency energy around the early 1900's. An analogy would be a single tone (frequency) bell that is constantly struck with a rotary hammer at the same tone (frequency) rate as the bell with the sound of the bell as it's output.

    No small task to keep one of these devices operating. Still it is very much worth the effort as these devices are bits of wireless history.

    The rest of those images from the Point Reyes radio station can be found at the link below. These images are all of the film used that day.
    http://www.meetup.com/SFBayAreaLFers...bumId=20282972

    Curious thing did happen during that visit. The curators and volunteers of that historic facility figured out I knew something about the technology and contents of this facility. They began to share many, many interesting stories about the goings on with the transmitters and other technological bits and this particular facility. It became a tour within a tour and sharing places in this facility that are not often open to the general public. In turn, I shared the how and why this stuff does what it does and why the innards of these things were designed they way they were both good and bad. One of the last things that happened, they had a Fluke Frequency synthesizer on the bench being repaired. They were having difficulty with the phase detector. I spent a moment looking at the service manual schematic and explained to the guy working on it how this phase detector worked and why it could be mis-behaving, what to check for and other key items that must be in place for it to function properly which they greatly appreciated.

    -They really wanted me to volunteer and spend time helping them keep this facility up and running and with the restoration.

    Another interesting moment in the journey of life



    Bernice


    Quote Originally Posted by Struan Gray View Post
    Bernice, that would be a fun collection to have a play with.

    I saw your pictures in the 5x7 thread. I shared an office with one of the people responsible for renovating and restoring the Alexanderson Alternator at Grimeton. A mechanical radio wave generator - still used when the Swedish Navy are prepared to free up the antenna for frivolous transmissions.

  4. #54

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    Re: Optics: resolution versus contrast ?

    Arne, those microscope images are beautiful. Another wonderful example of the beauty found in nature.

    What happens to those who run the into the lower polarizer unit (OUCH)? Do they suffer the same fate as running the objective into the specimen or worst (REALLY OUCH)?

    There appears to be more Aristromet systems offered on ebay Germay than ebay US. The last time I noted one of these Aristromet systems appeared that appeared on ebay Germay, it was a very complete set up including the very desirable Variphot with 4x5, ergo head, four sets of objectives, full DIC and polarized light set up and more.. I kept pondering how to get the whole thing shipped to the US damage free and what the shipping cost might be.. Maybe one day...

    The Zeiss Ultraphot is a classic in many ways. It is extremely versatile and remained one of the great microscopes made to date. Zeiss produced these for a long time.
    For those who have never seen a Zeiss Ultraphot here is just one image of many on the web. Note the size and heft of this microscope. Worth noting, the Zeiss Luminars were part of the objective offerings for the Ultraphot.

    http://www.tianjixing.com/tbbs/Uploa...1292853765.jpg


    To Arne, who has impeccable taste in collectable microscopes..

    Berncie


    Quote Originally Posted by Arne Croell View Post
    Struan is right, of course, we are in full agreement, it is just fun to talk about and appreciate these wonderful instruments. I actually use the Aristomet and some of my scientific samples for my personal photography from time to time: http://www.arnecroell.com/p457195038

    But sorry Bernice, you can't have our Aristomet. We still use it regularly, besides having the Axio Imager sitting next to it. Actually, a few months ago there was a full Aristomet setup on the German ebay for €8000. It even had a 4x5" attachment for the Variophot photo unit. I was quite tempted at the price, but since I have access to the one here, I abstained. Btw, in that linked image the transmitted light system is missing a part of the condensor system, it is sitting in its box in the drawer as a cautionary measure. We use reflected light (DIC) 95% of the time and when the Ph. D. students have thicker samples and move the stage down, they sometimes forget that the condenser parts can hit the polarization filter unit below (Arrrgh...).

    In terms of first rate collectable microscopes of the 1950's- 1980's, Reichert is also worth mentioning. My personal dream microscope for collecting would be a Zeiss Ultraphot III though...

  5. #55

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    Re: Optics: resolution versus contrast ?

    Quote Originally Posted by Bernice Loui View Post
    The rest of those images from the Point Reyes radio station can be found at the link below. These images are all of the film used that day.
    http://www.meetup.com/SFBayAreaLFers...bumId=20282972
    What a great place for a LF excursion. I'm not surprised that array of globe-like valves worked like catnip :-)


    Quote Originally Posted by Bernice Loui View Post
    -They really wanted me to volunteer and spend time helping them keep this facility up and running and with the restoration.
    Nice to be appreciated. A bit like with attempts to resurrect Autochrome, there's a difference between understanding the principles of operation, and having the deep practical knowledge needed to actually do something.

  6. #56
    Drew Wiley
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    Re: Optics: resolution versus contrast ?

    Yes... this certainly has been a pleasant thread, even if a slight detour from the original topic. And it's nice to become aware of the range of micro expertise available
    on this forum, just in case I try something nutty like modifying a scope for 8x10 film.

  7. #57
    the Docter is in Arne Croell's Avatar
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    Re: Optics: resolution versus contrast ?

    Quote Originally Posted by Drew Wiley View Post
    Yes... this certainly has been a pleasant thread, even if a slight detour from the original topic. And it's nice to become aware of the range of micro expertise available
    on this forum, just in case I try something nutty like modifying a scope for 8x10 film.
    Drew, the only reason to use 8x10" on a microscope would be if you want to get the "je ne sais quois" of contact prints, and that is certainly a valid reason.

    Otherwise, a larger film format does not buy you anything on a microscope due to diffraction (with a conventional microscope - not Emmanuel's near-field microscope which makes an end run around diffraction, or Struan's STM/AFM instruments, but those images use scanning and can only be viewed on a screen). The influence of diffraction is in my opinion overrated in regular photography, but in microscopy its an everday fact of life, easily visible. There is a reason why there were no 8x10 options available on microscopes, at least in the last 80 years.

    Here is a little off the cuff calculation: The 20x objective on our Aristomet has a numerical aperture of NA=0.45, quite typical, and would show an object field of approximately 0.75mmx0.9375mm for a 4x5 aspect ratio of the image. Provided you could find a photo eyepiece that gives you a good 8x10" image right away, that is an overall magnification on the negative of about 250x. The normal criterion for a "sharp" contact print is 5lp/mm, i.e. 200µm for the line pair width. Divide by 250, and the resolution in the object plane needs a width of 800nm for the line pair, or 400nm for the line. The microscope resolution based on the (slightly simplified) Rayleigh criterion is R=lambda/2NA (if the condenser aperture and objective aperture are the same, which is the case for reflected light), with lambda the wavelength of light. If we assume lambda=550nm (green) and the aperture above, you end up with 610nm, already 50% above your allowable line width. One can get a bit better by using blue light and/or an oil immersion objective (larger effective numerical aperture), but the basic gist is that an 8x10 print is usually the end of the line, if not already too much. My personal microscope image prints linked in the earlier post are usually around 5x7" for that reason. Using objectives with smaller magnifications helps a bit, but not as much as one thinks, because the numerical aperture typically goes down with decreasing power. Using a 10x objective would only be a 125x enlargement, of course, so your allowable line width would be 800nm instead of 400nm, but the NA of our 10x lens is 0.25, so the resolution is then 1100nm.

    When we got the Leitz Aristomet new, it came with a 35mm camera attachment for the Variophot unit as well as a Polaroid pack film attachment. In the early 1990's I did a comparison of prints from 35mm fine grained film such as Agfa APX 25 vs. Polaroid 665 P/N (essentially type 55 in pack format), and there was no visible resolution or other advantage of the Polaroid negatives vs. the 35mm ones in prints of about 5x7" size - and larger prints are just not sharp due to diffraction. Again, the tonality of contact printing is another story, in this comparison both negatives were enlarged.

    And we have not yet talked about film flatness issues - most camera attachments on microscopes look straight down...
    Last edited by Arne Croell; 23-May-2014 at 07:51.

  8. #58

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    Re: Optics: resolution versus contrast ?

    I was looking for a chance to drop my 1920s Zeiss camera lucida into the conversation :-)

    Allows reproductions onto large paper sizes (limited by arm length - your arm length) and works well with watercolour and other handmade art papers.

    Simply bursting with indexality.

  9. #59
    the Docter is in Arne Croell's Avatar
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    Re: Optics: resolution versus contrast ?

    Quote Originally Posted by Struan Gray View Post
    I was looking for a chance to drop my 1920s Zeiss camera lucida into the conversation :-)

    Allows reproductions onto large paper sizes (limited by arm length - your arm length) and works well with watercolour and other handmade art papers.

    Simply bursting with indexality.
    Yes, of course! ;-) I actually had to do drawings of mineral thin sections when I had my first microscopy courses - one eye looking into the (single) eyepiece, one looking at the paper. I wasn't very good at it...

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