Rule number 1 - Bob can't be wrong. Rule number 2 - If Bob is wrong, see rule number 1.
Bob, all the posters raise very sensible points, instead of fighting everyone, why not just state the facts in clearly defined english - after you have confirmed it from the factory. Isn't it obvious you confused us all? Please, have a little respect for some of the extremely intelligent posters to this forum.
Well, I must be misguided and uneducated, but the quote does not speak to whether the published curves are measured or calculated, only that IF measured, real lenses have about a 10% variance. The latter is important information. Bob may be right that Rodenstock publishes a measured curve, or the average of many measured curves, but the evidence is yet to be presented.
Glen -
Sorry but I have to disagree with you also. And I have to thank you for pointing out the most obvious reason why in a statement that I overlooked. The statement "tolerance of -10 % at the most" indicates that the measurements are most definitely theoretical. If the data was from real measurements any tolerance would be stated as "+/-" not "-" only. Since you can only be worse than theoretical values, never better, the "+" cannot be used. If the measurements were of a sample they would state tolerances as "+/-" since the lens tested is subject to the same variation as the rest of the production, and could be worse than other samples. All indications point to the data being theoretical since I don't believe that the Germans would express themselves so poorly - even in a foreign language.
Wayne: excellent observation! I didn't catch the - sign.
Glen -
I just read the other answers and Kerry also noted the discrepancy in an earlier post. Since his response was so long (sorry Kerry) I skipped it the first time I read through the responses. Germans might struggle with english grammer, but math is math.
My interpretation is this: Rodenstock publish "calculated MTFs" but they know from experience that when production lenses are actually tested their tested these tests show a 10% variation from the calculated MTF. Rodesntock does not MTF-test every lens produced, although by all rules of quality assurance, spot checks are carried out on a regular basis. Did Rodenstock's words say that? No, but it this is what I believe they meant and only Rodenstock can validate affirm my interpretation. Bob just tried to clarify unclear statements thought in German and written in English, and in the process put his own spin on them. As to taking pictures to see if the lens is OK, the infallibility and allure of this simple approach is too obvious and its allure too much for some to refuse. However, this approach succeeds only when the results are positive. It is succeptible to too many false-negatives and hocus pocus to be a scientifically valid and universally reproducible approach. There are sound, scienfic methods for evaluating lenses today and I personally believe those to be more reliable than joe's next door. If not, it would be quite OK to design lenses according to Joe's test methods. Personally I rather buy Rodenstock or Schneider lenses than Joe's. On another issue, the Japanese do not publish MTFs but the microscope and binocular division of one of those camera lens manufacturers do use MTFs for evaluating their lenses and making claims as to their quality. Of the lens manufacturers, only Zeiss's MTFs are for production units. When all is said, it would be helpful for Bob to address this issues with Rodenstock and ask Rodenstock to issue a position paper. Like many in this forum, I guess, we would rather hear it from the horse's mouth.
That's the problem with insufficient data. If you want to reach any sort of conclusion, you have to make assumptions. Based on the original quote, Bob had to make some assumptions to reach his conclusions. He chastised me to "not read between the lines", but that's exactly what he did (unless he has additional data he is not sharing). The problem is, there isn't enough information "in the lines" to support any conclusion one way or the other. Based on the incomplete, out of context and confusing quote that initiated this discussion, Bob can neither prove his conclusions, nor can we conclusively disprove them. We need more (or at least better) data. Since it is pointless to speculate without additional data, I suspend further commentary (but will not be holding my breath) until that data is provided.
Kerry
I am a native German speaker and am afraid the statement quoted by Bob does not make any sense to me either - it certainly does not address the question as to whether the published curves are measured or calculated.
"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." How do they know that a lens could be within 10% tolerance at the most unless they measure every lens? There must be occasional "lemons"! Perhaps what they are saying is that Rodenstock measures every lens and if it is below 10% of the published curves, it is discarded??? (This is pure speculation on my part).
Another question is what exactly do they mean by by 10%? Since MTF is expressed in %, this becomes ambiguous. Lets say the curves specify 40% MTF at a given frequency. Does -10% tolerance imply not worse than 36% MTF or not worse than 30% MTF?
"In other words there is a difference when you measure the MTF together with the lens." A difference compared to what? I thought that MTFs are always measured with a lens? If so, it must refer to a difference with calculated data, which would suggest (but I am guessing here again) that the Rodenstock curves as published are calculated, not measured.
Bob, I too would be very glad if you could clarify these issues with Rodenstock and post on photo-net:
1.) Are the published curves calculated or measured?
2.) If they are measured, do they reflect the very best lens they came ever across in their production line, or an "average" lens.
3.) What exactly does Rodenstock mean by "a lens could have a tolerance of -10%" at the most?
The "10% at the most" deviation in the MTF for any random sample is also incomplete. As other posters mentioned, there is an occasional lemon. Of course there is! It's IMPOSSIBLE to claim that ALL lenses will deviate by at most some amount. Most manufacturers use a probability model to tell them the tolerances are within so much for some number of samples. It might be something like you get 1 lemon (MTF's are off by more than 10%) every 1000 units or 10,000 units.
Not to flog a dead cat with a radioactive lens, but there is another interpretation, (I think)- MTF data is calculated from theoretical design inputs for the lens - measured lenses fall within +90% of the calculated theoretical figure. ie, measured lens deviation is less than 10% typically. Did someone say that already?
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