Foto: (c) Dré de Man 2010

My lens test system differs from the system(s) other internet sites or magazines use. I neither use vulgar MTF testing (meaning: testing by means of a system that suggests an MTF-test whereas in reality it only looks for the resolution at which a certain minimum contrast value can be reached), nor real MTF testing (nowadays only used by the German magazine Color Photo and manufacturers), nor just simple resolution testing.
My system combines a standard resolution test with two tests that evaluate the rendition of photographic details. The system has been developed with the criteria experienced photographers use for evaluating lenses. A lens that scores high on my test at a given aperture will do much more than render test patterns well, it will make excellent pictures that show all details you want to see in a picture - at that aperture of course. On the other hand: a lens that doesn’t score well, will most likely disappoint you, if not now, then at the moment you get more experienced.
The test consists of three parts: RES, IR 1 and IR 2. The result are being displayed for the centre (white bar), margin (light grey bar), and corners (black bar). In all cases only the lowest resolution in any direcion is given, I don't show sagittal and meridional results in the graphs. One reason for this is that you don't know in advance which details wil be important in your picture, least what angle they will make. Another reason is that astigmatism is a very ugly aberration, and in this way it's weighed in the test score.

RES tells you the resolution of lens. Resolution is simply said the ability to resolve details, where details are mostly (as here) black and white line pairs. Because the contrast of the object is relatively high, the contrast of the lens doesn’t  influence the score much. Resolution is much less important than most people think. Even if a lens is able to resolve the maximum amount of line pairs, the picture can look as if it is not sharp. On the other hand: if a lens doesn’t show the maximum resolution (at a very large aperture) image quality can still be very good. The popularity of resolution tests dates from the film era, where special very high resolution (and contrast) films or projection methods were used to get as close as possible to the theoretical resolution of the lens.
Even then it was not always a good predictor of the photographic qualities of a lens. In the digital era quality lenses are strongly limited by the resolution of the sensor (+ OLPF). Only with very large or small apertures (or low quality lenses) you will get scores that will be clearly lower than the maximum the sensor plus OLPF will allow. A simple resolution test is a very good test though to check for lenses that are decentered, due to production problems or falls. The differences in resolution are more important and more interesting, depending on the resolution of the sensor. With a D70 (6MP) a resolution test is almost useless, but even with a D3x it doesn’t tell us that much. Why do I do it then? Because it’s the ideal test to start with. If a lens fails this test, there is already something wrong with it.

IR 1 & IR 2
IR 1 and IR 2 show the rendition of details, again as a percentage, but now in a different, more reality bound way than other tests do. Both give a combination of sharpness, contrast and other factors. IR 2 is the most difficult test for lenses. It is important tough, especially for landscape pictures, but in the centre of the image even for photojournalism. A good lens should have excellent scores for both RES, IR 1 and IR 2 at all places in the image that are important to you, at the apertures you intend to use it with.

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voorlopige versie: 0.85

19 mei 2010 






How I test lenses (1)