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Thread: MTF again

  1. #1

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    MTF again

    The statement that prompted the response from Rodenstock:

    " MTF curves are theoretic only (at least as published by the manufacturers) and represent the 'expected' output of their designs. I don't know of a correlation between these simulations and reality."

    The response from Rodenstock:

    "The MTF curves are not theoretic but they are calcuted. The reality is when you measure MTF curves on the MTF machine with a lens it could have a tolerance of -10 % at the most. In other words there is a difference when you measure the MTF together with the lens. "

    Editor: This is the followup to http://hv.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=004ZDQ

  2. #2

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    MTF again

    Once again you've done nothing except shoot yourself in the foot. "Calculated" refers to mathematical equations - it does not infer actual measurements of production lenses being taken. Read the statement as "The MTF curves are not theoretic, BUT they ARE calculated. And once again they speak of the "reality" of actually "measuring" a lens and the "-10%" tolerance. The published graphs are not from measurements derived from production lenses. At least you didn't repeat the snotty, condescending tone of your first post. But the Germans reading comprehension skills, even in a foreign tongue, still surpass yours.

  3. #3

    MTF again

    Bob,

    Thanks for supplying the rest of that first sentence. It definitely clears things up and puts this to bed as far as I'm concerned.

    From the Miriam-Webster online dictionary:

    Main Entry: cal?cu?late 1 a : to determine by mathematical processes b : to reckon by exercise of practical judgment : ESTIMATE

    So, either the MTF curves are arrived at by mathematical processes or they are estimated approximations. And when actual lenses are measured, they are within -10% of these calculated curves.

    Based on my background in the semiconductor industry, this makes perfect sense. We did all of our design work using complex computer models and simulations representative of real world operating conditions. When the actual silicon arrived, we took extensive measurements in the lab to correlate the simulation results with the actual measurements.

    Theoretic values are considered to represent ideal conditions not attainable in a real world environment. Although our simulated results were still arrived at via mathematical calculations, they also took into account the real world variables that would cause deviation from the theoretical ideal. Our simulation results, by including real world losses, offered much better correlation with the actual measured results (real world performance).

    This is EXACTLY what is said in the first sentence in the complete quote provided above. To expand slightly: The MTF curves are not theoretic (not ideal, pie-in-the-sky, unattainable in the real world) but they are calculated (arrived at through complex computer models that accurately simulate real world conditions). Even if you ignore my parenthetical comments, and just read the complete original sentence, there can be no other way to interpret what is being said.

    Thanks for clearing this up once and for all.

    Kerry

  4. #4

    Join Date
    Dec 1998
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    MTF again

    Kerry,

    I think that we're splitting hairs here, however, I contend that the original statement that the curves are theoretic is absolutely correct.

    The American Heratige Dictionary (dictionary.com) lists the definition of theoretic as:

    1.Of, relating to, or based on theory. 2.Restricted to theory; not practical: theoretical physics. 3.Given to theorizing; speculative.

    Note that the first definition is 'based on theory'. They list the applicable definition of theory as:

    1. a.Systematically organized knowledge applicable in a relatively wide variety of circumstances, especially a system of assumptions, accepted principles, and rules of procedure devised to analyze, predict, or otherwise explain the nature or behavior of a specified set of phenomena. b.Such knowledge or such a system.

    Now, note that the original unattributed quote from Bob says "MTF curves are theoretic only... and represent the 'expected' output of their designs..." That's exactly the definition that the original quote is referring to. I don't believe that the word 'theoretic'is referring to the 'restricted to theory' definition that you refer to at all.

    The last part of the original unattributed quote from Bob says, "I don't know of a correlation between these simulations and reality." Well, that's what Bob attempted to answer, and the information about -10% can tell you what is reasonable to expect in a real lens sample.

    However, this raises a question in my mind about the legitimacy of data, and the ability of a person or organization to manipulate the numbers to achieve a desired outcome. I contend that these curves are in fact the very best possible performance for the lens design, even if they do include discounts for imperfect application of an ideal optical design.

    And, I contend that the only way that these curves would be of real applicable value is if you reduce the performance by approximately 5%, so that the curve falls in the middle of the expected performance range, and not at the very top. What I'm getting at here is that I don't believe that these curves are a fair representation of the performance of a typical lens, because the curve ultimately represents the _upper limit_ of the potential performance, not the _reasonable expectation_ of a typical lens.

    If this were a reasonable expectation of a typical lens, the error would be + or - 5% or so, not -10%. I believe a sampling of a few lenses and a statistical analysis of the performance curves would reveal a much more useful set of curves, and one that could then have an error based on the statistical sample.

    As an engineer by training, I understand what having only a negative error means, and that tells me that the curves have a certain amount of 'spin' to them, to pad the performance. This is marketing, and I expect that every optical manufacturer (and just about every other company that sells performance products around the world) does to improve the overall appearence of the performance specs.

    Until I see a set of measured performance curves from a reliable, independant source, the MTF curves will always be a little bit suspect. They're not unuseable, but you have to take into consideration the source of the data to make an intellegent analysis of the data.

    ---Michael

  5. #5

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    MTF again

    Sigh, time to add my two cents. I would rather be presented with "theoretical" MTF numbers identified as such rather than "real world" numbers. Since Kerry and Michael are both knowledgeable in this they will understand where I'm coming from. Knowing that the numbers are "theroretical" ideals, and that they do not take into account the variability in materials and manufacturing, does give you a starting point at least. But, if you are presented with "measured" MTF numbers, I believe that you are more likely to read information into them that is not there.

    Suppose your favorite lensmaker has their technicians test a "representative" (whatever that is) sample of 20 lenses (still not statistically valid except for very small production runs). Suppose that the resulting averaged graph is presented to the head of marketing or engineering who says "the numbers aren't good enough". What do you think they will do? They will either pretest and hand pick their next "sample" or they'll make sure that Hans and Fritz (their most skilled assemblers) assemble the next batch with elements tested and handpicked by Bertha (their most capable line inspector), using mechanical parts machined by Helmut (their best machinest), and then "cherrypick" the resulting lenses. Just stating that the tests are "representative" of their product doesn't give you any assurance that your sample will fall within the projected distribution curve.

    We all have someone in our family who doesn't fit. Whether it's the 5'6" runt brother in a family whose mother is 6', or the curly-haired blonde daughter whose siblings all have straight brown hair. All manufacturers have "off-days", "bad-runs", and "outliers", it doesn't matter what the graphs look like when you get a dog. I just believe that if you know going in that the numbers are theoretical (or "calculated") you will not be subject to the "sleigh-of-hand" that can be used to massage "real numbers". The variables are still present but they are not falsely "accounted for".

  6. #6

    MTF again

    Wayne, who you callin' a runt? Come over here and say that to me and my curly-haired sister!

  7. #7

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    MTF again

    Steve

    Send your sister over and I'll apologize to her, and send her back with an apology note for you (eventually).

  8. #8

    Join Date
    Jul 1999
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    84

    MTF again

    As a native German speaker I wish to add my two cents: In German, the word "theoretic" does not mean "based on a theory" but it has a very strong connotation of "not achievable in the real world". I therefore believe that Kerry's (and not Michael's) interpretation is the correct one. (Note, Rodenstock states that the curves are NOT theoretic, implying that they are achievable in real lenses).

    Still it would be nice if Rodenstock could clarify what their calculations are based on!

    Also, what does -10% tolerance mean??? How can they be sure that no lens is below 10%, unless they measure each and every one and discard them in case they are below!?

    Another burning question: Lets say the curve shows 40% MTF at a certain frequency. Does -10% tolerance imply that no lens is below 36% or does it imply no lens is below 30%???

  9. #9

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    MTF again

    I would propose that the situation is someplace in the middle. I would be very surprised if lens design is accomplished by the trial and error making of lenses! The alternative is to design lenses on the basis of a model. All models are based, at least in part, on a theory; they can't be based entirely on emperical observation. (It's theorectically not possible!) However, I am sure that, before commiting a design to glass, lens designers apply models that, to the extent possible, represent real life. Would we not expect these models to calculate MTF curves as an important standard against which lenses are compared? I would hope so. So, there we go, MTF curves are "calculated" from models that are based, at least in part, on a "theory". See, right smack dab in the middle. How about that.

    With all of this said, why would MTF curves NOT be based on data collected from the lenses themselves? Wouldn't this be the most representative form of MTF curves? It's not as if the test were destructive! Sounds mighty suspicious to me.

  10. #10

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    MTF again

    I cannot resist jumping in with my 25 cents. There is system resolution and gate resolution. There's not only the Modulation transfer Function and lenses, but also the issue of the cumlative error of the whole system. The lens/camera set-up, the film plane, the film, the films MTF, the enlarger lens & set-up and so on. The greatest lens ever made cannot strut its stuff if its performance is compromised by other variables that are off the mark.
    Jonathan Brewer

    www.imageandartifact.bz

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