MTF again

[bob_salomon3]

The statement that prompted the response from Rodenstock:

 

<p>

 

" 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."

 

<p>

 

 

The response from Rodenstock:

 

<p>

 

"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. "

<p>

<b>Editor: This is the followup to

<a href =

"http://hv.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=004ZDQ">

http://hv.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=004ZDQ</a>

</b>

[dmin-99]

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.

[kerry_thalmann2]

Bob,

 

<p>

 

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.

 

<p>

 

From the Miriam-Webster online dictionary:

 

<p>

 

Main Entry: cal·cu·late

1 a : to determine by mathematical processes

b : to reckon by exercise of practical judgment : ESTIMATE

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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).

 

<p>

 

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.

 

<p>

 

Thanks for clearing this up once and for all.

 

<p>

 

Kerry

[michael_mutmansky1]

Kerry,

 

<p>

 

I think that we're splitting hairs here, however, I contend that the

original statement that the curves are theoretic is absolutely

correct.

 

<p>

 

The American Heratige Dictionary (dictionary.com) lists the definition

of theoretic as:

 

<p>

 

1.Of, relating to, or based on theory.

2.Restricted to theory; not practical: theoretical physics.

3.Given to theorizing; speculative.

 

<p>

 

Note that the first definition is 'based on theory'. They list

the applicable definition of theory as:

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

 

---Michael

[dmin-99]

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.

 

<p>

 

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.

 

<p>

 

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".

[steve_singleton2]

Wayne, who you callin' a runt? Come over here and say that to me and

my curly-haired sister!

[dmin-99]

Steve

 

<p>

 

Send your sister over and I'll apologize to her, and send her back

with an apology note for you (eventually).

[andreas_carl]

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).

 

<p>

 

Still it would be nice if Rodenstock could clarify what their

calculations are based on!

 

<p>

 

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!?

 

<p>

 

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%???

[neil_poulsen1]

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.

 

<p>

 

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.

[jonathan brewer]

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.

[pete_andrews]

I too, would like a clarification of what the "-10%" figure refers to.

Which axis of the MTF curve is involved?<br>If it's a drop of 10%

contrast at a given spatial frequency, then that's significant. If

it's a 10% variation <i>of</i> the contrast then it's not very

significant.<br>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.<p>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.<br>If the MTF figures really are measured

results, then the sample size and statistical data should be readily

available.

[michael_mutmansky1]

Andreas,

 

<p>

 

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.

 

<p>

 

The German use of the word is not at debate here, since you are

clearly correct that they meant 'theoretic' in the 'unachievable'

sense.

 

<p>

 

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. ;-)

 

<p>

 

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.

 

<p>

 

 

---Michael

[glenn_kroeger]

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.

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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.

[kerry_thalmann2]

Michael,

 

<p>

 

I agree we are splitting hairs here, and arguing semantics more than

science.

 

<p>

 

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.

 

<p>

 

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:

 

<p>

 

"The MTF curves are not theoretic but they are calcuted (sic)."

 

<p>

 

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.

 

<p>

 

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).

 

<p>

 

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).

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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).

 

<p>

 

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.)

 

<p>

 

NAH, I'd rather just go out and take some pictures.

 

<p>

 

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.

 

<p>

 

Kerry

[dave_bulmer1]

If I wanted to photograph these "split hairs" which lens

manufacturer's product would produce the most hairs split per mm.

[bruce_lee4]

How much wood would a wood-chuck chuck if a wood-chuck could chuck

wood?

[geoffrey_chen]

Dave,

 

<p>

 

Any lens will do, just fit your lens with a Tiffen filter and all

hairs will be gone. Cheers,

[john_doe33]

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.

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

I know of no other manufacturer who physically tests lenses for MTF

in this way. As Bob stated, other manufacturers calculate the MTF

curves.

 

<p>

 

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.

 

<p>

 

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.

[ani]

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?