Hello: I am interested in knowing how you in this forum read or interpret MTFs a nd what you know about MTF (summary). If you please, Your own official and expe rt versions and even the very private unofficial versions, including those you w ere afraid to ask about. What do you look for in MTFs? What to you is OK and no t?
Are you asking how to properly read an MTF chart?
Or are you asking how someone thinks it should be read?
It is a scientific chart and there is no guess work involved.
if you are in the US and want a detailed and correct explanation of lens curves that Rodenstock published then I would be happy to mail you one. Providing you are in the US.
Read them with a grain of Salt if they're from the lens maker or importer.
Importers/distributor do not do MTF, distortion or color testing.
Lens and camera manufacturers do.
Now why would you be so gilded to not trust MTF curves?
In the case of Linhof some curves are also done by Hasselblad for the Arc camera lenses from Rodenstock so it is very easy to compare the ones from zRodenstock to the ones from Hasselblad.
As for the accuracy Rodenstock has ISO 9001 status. this assures government and industrial purchasing agents of a given level of assured quality. This encompasses the performance characteristics as stated by the MTF cuves, distortion curves and color curves.
Now while few photographers know or understand ISO 9000/9001/9002 reqierments and fail to comprehend its meaning and people in organizations who don't opt to spend the large amount of money required to qualify or just ignore the benefits of ISO status that does not mean that specs are automatically suspect.
In the case of Rodenstock it represents the basic quality level you should receive.
My question pertains to how people in the forum interpret MTF charts and what information they derive therefrom. It is your own personal views that I am interested in, whatever they are. ALso I would be interested in knowing what effort it takes to gain a solid technical understanding of what the MTF data conveys and what effor the respondent has made to that effect. To all, thanks.
As in my original response we would be happy to mail you a brochure on how to read the curves used by lens manufacturers.
[pre] Bob, Julio lives in CANADA. So please stop saying you would send him anything. The US postal services is so expensive especially with the weak American dollar that you can't mail up there. Julio, I do not understand MTFs. I look at the specs and user experiences. While its not very scientific, looking at the images is key for me. I try to keep in mind I am making images not MTFs graphs. I am not trying to be dogmatic, just sometimes we can take the technical side too far. For example my Rodenstock Sironar S 150mm is suppose to be optimal at 1:5 to infinity. However macro shots look fine. [/pre]
Bob: thank you for your reply. I am asking how the people in this forum read MTF charts, the information they get from them and also how various people think these should be read. And you are right, it is a scientific chart and there should be no guess work involved. However, since the information available on how to read these charts is not detailed enough, there are holes left that leave room for personal interpretations, which I believe there are many and varied. That is what I am interested in finding out and perhaps my own have been wrong all along? Bob, thank you for your kind offer to provide the Rodesntock lens data, I do have it for all their lenses. If however Rodenstock has a white paper on MTF testing and interpretation, other than what is said about it in their lens brochures, I would appreciate having such and if so, I will contact you directly. Your statements on ISO certification are a valid point, besides, Rodenstock's reputation transcends ISO.
Julio:
I look at MTF charts all the time. I am a seismologist and my work involves many facets of signal processing, which, like MTFs are based on Fourier analysis of signals. In the case of MTF charts, the signal is the input image, and the Fourier decomposition is done as a function of spatial frequency. MTFs are like spatial spectra, except that they omit phase information (essentially they are spatial power spectra). If you need pointers to technical explanations of such analysis, I can provide them, but as I read your question, you are more interested in how I use them in my decision making, so I will comment on that a bit.
First, you need to know that many MTF charts provided by manufacturers (Schneider and Rodenstock in particular) are theoretical not measured. They are based on computer models of the lens design and glass characteristics, and they assume a default imput color spectrum of light. Thus, they cannot address manufacturing tolerances or variations. They are mainly useful in determining the priorities that the lens designers used in designing the lens.
Next, you need to decide what format film you are interested in. I use 6x9 roll film, so I am interested in getting the highest performance at the highest spatial frequencies possible. Most LF lens manufacturers only show MTF curves up to 20 lp/mm. For MF work, I would love to have curves at 40 lp/mm, and I have some for some Rodenstock lenses off of the Hasselblad web site for the ArcBody lenses. Since I use 6x9, I am most interested in lens performance out to about 50mm off-axis.
If I were shooting 4x5, I wouldn't care about 40 lp/mm, or even 20 lp/mm but would focus on 10 lp/mm, and would be far more interested in performance farther off-axis. Most LF lenses are optimized for better performance farther off-axis and less emphasis on higher spatial frequencies since their primary audience is 4x5 shooters. This is why I look at the MTFs, to find lenses that work best for the MF situation which is not always the design goal.
Specifically, I try to determine at what aperture, the MTF values in the center of the field reach theoretical limits. Since I shoot MF, I like to use apertures like f/11 and f/16. Many LF lenses are not optimized to work well until f/22 or smaller.
Finally, I like to see how the performance falls off as a function of off-axis angle. Some lenses have flat curves that break sharply and fall off rapidly at some point. Other lenses, notably the Super Symmar XL lenses have MTFs that roll off more gently with off-axis angle. Since I shoot mostly landscape, I don't use alot of movements, so I am happy with flat curves and abrupt fall off so long as I have a reasonable usable image circle. If I were doing architecture, I might prefer a more gentle roll-off since I might need extreme movements in certain situations.
If I were shooting 4x5, I don't think I would worry much about MTF curves. Most LF lenses are optimized for 4x5, most do great for lower spatial frequencies, and I suspect most are at or near the limits of what is optically possible to produce. But since I am "swimming up river" so to speak, in using LF lenses on MF images, I use them to pick lenses that work well for that particular application.
As for numbers, I worry when modulation at 20 lp/mm drops below about 40%. That means 40 lp/mm is probably going below 30%. Given that most color transparency films are at about 50% modulation at 40 lp/mm, that means that the combined modulation is nearing 15% which is probably about the value corresponding to "resolution" of that frequency.
I am one of "the few" photographers to understand ISO9000. I have been working in the electronics industry for 20+ years and dealing with ISO9000 for the last 9. ISO9000 has nothing to do with quality. It was instituted by the European Union in order to discourage American competition in their home markets. It's only a paper trail, nothing more. If you make crap, and state that the purpose of your business is to make and market crap, you too can become ISO9000 certified. When I worked on the west coast the company that I worked for had "one-man" garage shop subcontractors that were ISO9000 certified. Let's not inject mumbo-jumbo into the debate.
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