MTF charts for lensesgreenspun.com : LUSENET : Large format photography : One Thread
It seems that there are two different ways of depicting MTF charts for lenses even though this shouldn't be. Ernst Wildi explains in his book on the Hassablad camera that the horizontal axis is image height with the leftmost border as the lens center. The fourth spacing to the right is then 40 degrees off axis. More conventionally, the horizontal axis shows the modulation frequency of the reproduced image. Why is there a difference between the two? I also wanted to know if Tessars have better contrast than symmetrical lenses. Thanks in advance.
-- John Dorio (email@example.com), May 05, 1999
hi John, i'm also interested in which lenses have what kind of contrast, although, i've been calling it tonality. i hope we are on the same page, as they say. i've asked Congo and Schnieder; here's what they said. Congo makes a lens which, as the representative stated, gives more abundunt tonal gradiation especially with subjects in low illumination; this is when you compare their Congo (and he said the Commercial Congo is similar to a Commerical Ektar which i think is a tessar) with lenses with 6 or more elements. Schnieder Optics said that the Xenar ( and as far as i know they are tessar design) was originally designed for portraiture, and the Schnieder representative says they have very good tonal quality. evidently, the 6 or more element design gives a sharper image, and the sharpness in image makes an image that some people think is too contrasty. so if you wanted high contrast, it's my guess you would look for a lens with 6 or more elements, and according to Schnieder, optimal performance by design is f 16 1/2. i guess you would achive good contrast with 6 or more elements at f 16 in sun light. so, i don't know if tessars have more contrast that symmetrical lenses, but evidently they have much less than what is considered state of the art now. i hope this comes close to being informative. and if anyone out there knows anything about this topic and the names of lenses for 4X5 and 8X10 which are known for smooth tonal gradiation vs. contrast please let me know.
-- david clark (firstname.lastname@example.org), May 06, 1999.
MTF is essentially a description of how pairs of black/white bars are reproduced in the image plane. As such, there are a number of independant variables:
- object contrast - object distance - bar frequency - distance from axis - angle from the radius - lens aperture - colour of the light - probably a number of others
Any of these could be used as the x-axis.
I don't think you can generalise and say whether Tessars or symmetrical lenses have the highest contrast. The larger number of elements will tend to increase flare (and thus reduce contrast), but this is usually controlled with surface coatings.
If you really want a generalisation, I would suggest 'modern lenses have higher contrast than older ones', essentially because of the coatings.
-- Alan Gibson (Alan.Gibson@technologist.com), May 06, 1999.