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Common pitfalls with interpretation of MTF light fall of vignetting and distortion


Reading Zeiss data sheets

As you can see in the download section. Zeiss is providing for each lens data for MTF, vignetting and distortion. This is outstanding in the industry, because the data is measured in the normal production line, not as a proto-type or computer model.

Zeiss is the only company worldwide, which is doing this. So far so good.

A different story is the interpretation of these curves. At first sight it seems to be obvious. For the MTF and light fall off - the higher the line, the better – for distortion the lower the number, the better.

You might be surprised to hear, that this is wrong !

Every part has its own rules and has to be interpreted differently. So lets start with the MTF curves:

1. MTF-charts

Some manufactures publish 4 line-pair per mm (lp/mm), some just 3 lp/mm. This has nothing to do with quality differences. If you see 4 lp/mm, these are 5lp/mm, 10 lp/mm, 20 lp/mm and 40 lp/mm. If you see just 3 like in the Zeiss data sheets these start with the 10 lp/mm, 20 lp/mm and 40 lp/mm.

If you see the distances between the other 3, you can conclude easily in which ballpark the 5 lp/mm is located.

I do not want to go to much into details here. For that you can read the original article of Zeiss. As a quick overview you, just have to know that there a certain “improvement-steps” needed, before you can see with your eye (not with a microscope) any difference in the picture.

Talking in numbers, that means, the line of the 10 lp/mm have to increase by ca. 5%, before you even can see any difference in the picture with your eyes, the threshold for the 20 lp/mm is 10% and for the 40 lp/mm is even 25%.

In other words, if i.e. the 40 lp/mm line is going up from 60 to 70, you would not see with your eye any improvement in the picture, because the increase is less then 25%!

2. Light fall off (Vignetting or Relative Illuminance)

Everybody knows them, the darker corners of the picture, compared to the center. But the intensity is not only influenced by the lens.

Other factors influence also this phenomen. If you i.e. shoot against even dark blue sky, you will see the light fall of stronger, then in a “normal” mixed colour picture.

Also the film is an consideration. You will see Light fall off with a Fuji Velvia stronger then with Fuji Sensia.

The Chart always start at 100 at the centre of the image. This is because the benchmark (=100%) is always the middle of the picture. If you go along the horizontal axis, you see that the line drops, the more you are going to the outer zone of the film.

This means that the more you are going to the corner of the picture the more is the difference to the centre. If the line drops by half, i.e. 50% this means 1 f-stop light fall off at that point.

3. Distortion

A general rule of thumb is, that you can not see without a very fine fixing point the distortion up to 1%. Between 1%-2%, you have to look very carefully i.e. architecture etc. and over 2% it is easier to see.

But this is not all of the story. Here is the surprise: If you look i.e. at the 15mm and 21mm Zeiss data sheet, you will see that the 21mm has a distortion of 2% and the 15mm of 4%. By our rule of thumb, the 21mm must be better. Wrong!

The 15mm is actually better. This has something to do with our ability to see with the human eye and the fact that at the border lines are always more curved than in the centre. If you imagine a gun-target with several circles around the centre, you realize, that the outer ones are more curved, then the inner circle. A stronger distortion in the outer circle is less obvious/recognizable than in the inner-circle.

These 3 points make it easier to understand why Zeiss is always considering whether it is worth it or not to improve an older lens-design (see the article of Zeiss).

Our human eye is one of the limiting factors. Another factor is the quality control in the normal production. If you can not get reasonable improvements in the final picture without extraordinary costs and a controllable production process, it does not make sense to do it.

You might be able to design something better, but you have also to make sure, that what you are calculating is what is coming out of the production line with every lens (!) you produce.

But the final image quality is a result of a “chain”. Other limiting factors are the aperture and the film your using and whether you shoot handheld vs. using a tripod. But I put this in another article.

To sum it up, it is nice to have these data sheets, but think twice, before you give to fast judgements about the image quality of a lens.