I too, would like a clarification of what the "-10%" figure refers to. Which axis of the MTF curve is involved?If it's a drop of 10% contrast at a given spatial frequency, then that's significant. If it's a 10% variation of the contrast then it's not very significant.OTOH, if the -10% refers to the spatial frequency at which a certain low value of contrast is reached, again, that's an important variation. A difference between 'resolving' 80 cycles per millimetre and 72 cycles per millimetre can effectively mean the difference between a high-end lens, and an average one.
If Rodenstock really want to lay this one to rest, they should test a statistically meaningful sample of lenses (>30), and issue the results in the unequivocal form of Mean, Standard deviation, together with the best and worst case figures.If the MTF figures really are measured results, then the sample size and statistical data should be readily available.
You're not looking at the correct quote in Bob's original message. My statement was only in reference to the first quote and the use of the word 'theoretic'. If you look at thta statement, you will see that the definition is exactly as I state, the performance based on 'expected' behavior of the lenses.
The German use of the word is not at debate here, since you are clearly correct that they meant 'theoretic' in the 'unachievable' sense.
I just wanted to clear that up for you, because I have never been wrong about anything important, and if I were, it clearly wasn't as important as I thought. ;-)
Ultimately, it would be great to get a statement from Rodenstock, (and Schneider), that clearly annunciates how they derive their curves, and what the curves represent as a performance expectation, so that the task of comparing them will be much easier. This, I suspect is exactly the reason they do it they way they do; so it's harder to compare the two brands directly. Simply the marketing departments working their magic.
I am sure we would all like custom MTF curves for the lens we buy... but lets be real. We now know what the curves are, and we have some clue as to the variation (although it would be nice to know whether the 10% is relative or absolute) in production lenses.
Since there is no rash of people reporting on Rodenstock lemons, and in fact most users regard their Rodenstock lenses as superb, it is clear that Rodenstock exercises appropriate quality control measures to ensure that lenses leaving the manufacturer meet their quality goals.
I still run chart and field tests on new lenses. My latest Rodenstock 55mm Apo-Grandagon resolved nearly 80 lp/mm. Thats pretty impressive, and the images back it up.
I have always assumed that real lenses vary downward from published curves by some amount. The curves are mostly useful for defining the possibilities and priorities of the lens design.
Finally, except for Zeiss, Rodenstock has now provided us with more information than any other manufacturer. Schneider publishes calculated curves, but not information on chromatic aberration or production tolerance. Nikon and Fuji provide nothing.
I agree we are splitting hairs here, and arguing semantics more than science.
A couple of points. I don't believe the first unattributed quote in Bob's post came from anyone at Rodenstock. The first quote is a blanket statement and makes no specific mention of Rodentock (or any other manufacturer). It appears to be generic in that respect. Who made that quoted statement, I have no idea, but I believe it is Bob's contention that the statement is false and hence he asked Rodenstock for a clarification on the derivation of THEIR MTF curves.
If you look only at the second statement, that Bob attributes to some unknown being at Rodenstock, it is very clear that the source of that quote considers "theoretic" and "calculated" to be two different things. The first sentence in that statement states:
"The MTF curves are not theoretic but they are calcuted (sic)."
Even if we disagree on the precise meaning of "theoretic" in this context, I think we both agree that the Rodenstock MTF curves are mathematically derived, NOT based on measured data.
That seems to be the exact opposite of what Bob was trying to prove in the previous thread on this topic. However, with the complete first sentence provided here, it is very clear and unambiguos that the Rodenstock MTF curves are based on calculations, NOT measurements (there is no mention of measuring anything in that first sentence).
Now, the question becomes, how accurate are those calculated MTF curves? There seems to be a lot of confusion surrounding the "tolerance of -10 %" claim. Not necessarily the accuracy of the -10% figure, but the meaning of -10% in the context of MTF curves (absolute or relative).
Based on the first sentence in the Rodenstock quote, I do believe lossy models were used by the Rodenstock engineers in generating these curves. And, therefore, the curves represent something less than the theoretical ideal, or perfect, lens. The accuracy of the curves then becomes a function of the accuracy of the models used. We have no idea how accurate those models are, but can safely assume the engineers at Rodenstock have a pretty good handle on it.
Simulation results can be VERY accurate and correlate VERY closely to real world performance. Again, it is a function of accurate models and a comprehensive set of simulation scenarios. In my former career, I did a LOT of simulations, and the models were constantly refined based on measured data. Also, the simulations were generally run using "best case", "typical" and "worst case" scenarios for all variables involved. This results in a set of curves that bound the performance of the system. Measured data is then used to verify that the real world performance does indeed fall within these bounds.
WRT to the Rodenstock MTF curves, what we don't know is if they represent "best case", "typical" or "worst case" conditions (I won't speculate on which). It would be nice to have a complete set of curves, on the same graph, showing best case, typical and worst case performance, but I doubt if we'll ever see it. Even if it was provided, it's probably overkill for our applications (and besides, we have no way of verifying it - at least I know I don't have my own personal MTF test machine). Unlike semiconductors, where the system designers NEED to know upper and lower performance boundaries for each component to guarantee a functional system, our LF cameras will not completely cease to function if your lens is slightly out of spec (either slightly better, or slightly worse).
Sure, we want the best system performance possible, but in most cases (with modern lenses), it will not be the taking lens limiting that performance. Perhaps we need to run some simulations on the complete image producing system that take into account the best case, typical and worst case scenarios for all variables involved (taking variables - lens performance, film performance, focusing errors, film plane location, film flatness, camera movement, subject movement; developing variables - time, temperature, agitation, developer strength; printing variables - enlarger lens performance, enlarger vibration, paper performance, paper flatness, focusing accuracy, etc.)
NAH, I'd rather just go out and take some pictures.
Seriously, we've beat this to death, and without more data, or at least a clarification from Rodenstock, we can't really take it any further without a lot of assumptions and speculation (not that that has stopped us so far). At least, by providing the entire first sentence of the quote from Rodenstock, Bob has has clarified, beyond any doubt, that the Rodenstock MTF curves are based on calculations, and not measure data. So, we did learn something new. Thanks Bob.
MTF curves for Zeiss lenses for Hasselblad (medium format) are generated by taking ACTUAL measurements of the lens in the appropriate laboratory testing device for determination of MTF and using the appropriate software (mathematical equations) to convert the measurments to MTF data.
They do not test every lens manufactured. They test a statistically sufficient number of lens of a specific lens type (e.g. 180/4 Sonnar) and with the manufacturing tolerances allowed for the final finished lens as a product, they determine the MTF which is published in graphical form as a technical specification for that particular lens.
Zeiss manufacturing tolerances are so tight, that any lens so tested on an appropriate MTF laboratory system should "fall" on the line of this published graph - the error in reading this "rough" published graph should encompass any tested lens.
I know of no other manufacturer who physically tests lenses for MTF in this way. As Bob stated, other manufacturers calculate the MTF curves.
Your discussion of what is meant by "calculate" and "theoretic" is an area I have not investigated nor discussed with other lens manufacturers. It would be very interesting to know the exact procedure (measurements and mathematics) to arrive at the "calculated" MTF.
At the website www.photodo.com, MTF values and curves for 35mm amd medium format lenses are tested on the exact MTF equipment used by Hasselblad and the tests are performed by the Hasselblad technician who is qualified to determine MTF measurements. There is no information on the number of lenses tested for a specific lens type nor any (if any) statistical methodology to determine the "error" in the stated MTF and published MTF curves.
Do uou think that the MTF equipement for optical imaging have received his theoretical limites. Do you think that there are new areas left for future research?
Reply With Quote
Bookmarks