DPR Forum

Welcome to the Friendly Aisles!
DPRF is a photography forum with people from all over the world freely sharing their knowledge and love of photography. Everybody is welcome, from beginners to the experienced professional. Whether it is Medium Format, fullframe, APS-C, MFT or smaller formats. Digital or film. DPRF is a forum for everybody and for every format.
Enjoy this modern, easy to use software. Look also at our Reviews & Gallery!

Does MTF describe a lenses accuratly

benwang

Well-Known Member
I went to photodo.com and they have tested a lot of lenses for their MTF and rate these lenses accordingly. However, in some case, although according to the MTF, some of the lenses are equivalent, but there is a different between them, the stand out lenses are usually the ziesis or leica. So, I want to ask everybody here, if the MTF can not describe the lenses properties 100%, what are the different between these lenses? What are the properties that is not objectively covered and how we can describe them or measure them accordingly?
 
I suggest that you look at some threads on this site about Bokeh, which is the characteristic of the out-of-focus areas(or try http://www.kenrockwell.com/tech/bokeh.htm). Zeiss glass generally has beautiful Bokeh. But I don't know of a way to measure this characteristic. BUt I sure do know it when I see it!
 
Robert is right on tack about the bokeh in Zeiss glass, at least those I've used. Nice, smooth transitions from sharp to out of focus, no geometric or chromatic distortions in the OOF area.
If you shoot color slides, there's a definite warmer cast to the images, not very important with scans, you can color correct to your hearts content there, but significant with projected images.
Another area that doesn't seem to be measured, or reported in photodo's tests is flare resistance. The Zeiss coatings seem to suppress flare very well, though I can't say whether Nikon or Canon are any different, not having done any side by side tests.
Perhaps It's related to flare, but ghosting seems to be less a problem with the CZ glass.
The sharpest lens isn't necessarily the best for your purpose. Doing portraits, are you sure you want to have every pore and tiny wrinkle rendered with maximum contrast and clarity? While high contrast is a plus in some situations, it can be less than useful in others. I don't know that contrast can be interpreted from the MTF curves. Perhaps if there were a graphical rendition of the output when plotted against a spectral sweep, there may be peaks or valleys which would explain the higher or lower contrast in a lens system. I haven't seen any such, nor do I know if it's possible, but thats really a bit off topic...
JW
 
re bokeh

at the risk of creating conflict i'll use nikon lenses which in the past wer over corrected (imho) for spherical abberation, this gave exceptional lpm at high contrast at the expense of bokeh and out of focus images where a windowpane dividers in the background would actually image a double image of the dividing strip compared to a cz or canon or leitz which would render a blurred divider. these images were available on the web years ago but i don't recall where. i have no experience with current nikon lenses and bokeh is a metter of personal taste.

re color/warmth again this is a matter of lens/coating design and then personal taste.

i suspect these nuances are what really drove folks to be a nikon/canon/leica/zeiss fanatic over the years.
 
I agree with you Robert that the Bokeh is great for the ziess and Leica, and IMO, this may beccause these lenses can capture more colour comparitively. in terms of MTF, although it offer similar, however the image appear sharper, this is because there are more colour. Any opinion?
 
Dale, MTF is a method to measure how sharp the camera lens is. What they do is they will take pictures with the lens testing against some very fine arranged lines (cotton thread?). They will take these picture with different distance and F-stop. So, the sharper the lens, the more accountable of these lines and hences you can objectively compare the lens side by side. And as long as you have follow the standard operating procedure, the test is objective!

"bokeh" is the first time that I heard this word, but going to the link Robert has provided, my understanding is basically the out-of-forcus area.

What I was thinking is that if it was the colour reproduction that Zeiss or Leica is differ from the other brand (canon, nikon e.t.c), isn't it that we could use the MTF as an ex&le, and construct an experiment similar to MTF and compare these lens side by side?

If we could establish a print with "true" colour, i.e.all the colour available, picture it with the comparing lens, and use the computer to count the colour and compare, will it work? is that objective?

Ben
 
Ben,

The test you describe is closer to an outright resolution test than an MTF test. The point of MTF testing is that it measures the contrast of a lens at a number of different spatial frequencies (spacings of the test pattern, IOW). If you look at the figure for 10 line-pairs-per-mm then you get a good idea of the contrast of the lens, while at 40lpmm a high figure indicates good resolving power for fine details. The old-style resolution test sets out to establish the finest spacing that the lens can pull any meaningful information out of, which is quite a different thing and isn't really as useful for assessing the real-world perforamance.

Your idea for comparing the colour rendition of different lenses is likely to run into trouble because there is no imaging system (based either on film or direct digital capture) that can faithfully portray the full range or gamut of colours that we perceive. Even for the colours that can be reproduced, there is always a bit of colour shift introduced by shortcomings of sensors and output devices. Colour profiling is supposed to compensate for these factors, but it's not perfect. Don't let that stop you from thinking about better ways of solving the problem though, you'll be rich and famous if you do, and a lot of photographers and reprographic people will warmly thank you! If you want to read up on the intricacies of colour theory, a good place to start is http://www.color.org, which is the website of the International Color Consortium who define the worldwide standards for colour processing and reproduction. Be warned though, it's a deep subject.

Regarding your original question, there are many other criteria beside bokeh and basic MTF measurements that can be used to characterise a lens. If you really wanted a full picture of a lens's performance, you'd need to know the following (others please feel free to add anything I've forgotten)...

* MTF at all f-stops and across various focusing distances from infinity to closest
* Contour map of the plane of focus at those distances and for each f-stop
* Measurement of flare characteristics of the lens under a broad selection of lighting conditions
* Light transmission properties of the lens
* Measurement of light fall-off effect at all f-stops
* Measurement of field distortion at all f-stops

In practice it's not easy to achieve this. The multiple MTF measurements are too costly to gather and produce an inordinate amount of raw data. There is no agreed way of quantifying the flare characteristics of a lens, and it's hard to see how this could be tackled well. Bokeh and colour rendition are hard to quantify objectively, for reasons already discussed. Note that because no lens has a perfectly regular plane of focus (and the true shape is not easy to measure), this can create anomalies in the MTF results. Most testers set focus for the centre of the field, then measure the whole field at that setting. If the plane of focus is strongly curved, then the edges will record a very soft image because they are not in correct focus. Similarly, some lenses experience a shift of focus as the aperture changes, which can lead to poor apparent performance at intermediate apertures. The Leica Noctilux f/1.0 50mm lens is notorious for this, for ex&le.

Ultimately there is no substitute for a real-world test of the lens you are using or thinking of buying. This rules out s&le variation and allows you to make some kind of assessment of some of the factors such as bokeh that cannot be easily measured on a bench. Over the years I have found myself choosing lenses based upon the recommendation of people I respect and thorough research of the body of opinion on the internet and in paper publications. I'd love to think that one day all the guesswork could be done away with, but given the complexity of the problem that doesn't seem likely anytime soon.

HTH

-= mike =
 
> "bokeh" is the first time that I heard this word, but going to the > link Robert has provided, my understanding is basically the > out-of-forcus area.

Bokeh describes the characteristics of the out of focus areas of the image. What your criteria is for good or bad bokeh is, are really up to you...but most people prefer smooth OOF area transitions, instead of abrupt, almost vibrating.

http://www.darkroom.com/MiscDocs/bokeh.pdf

Regards,

Austin
 
Back
Top