Lens Coatings and Colour Response

[johnamazement]

Using black and white film—specifically HP5—my uncoated Summar seems to render foliage and grass in darker tones compared with my coated Canon 50/1.8. In a side-by-side comparison the Canon looks as if it had a light yellow filter on it.

Is this a characteristic of the lens coating or just an effect produced by the lower contrast? Or am I imaginging the whole thing?

 

[Sonnar Brian]

A few things come to mind.

How are you determining the exposure? The Canon transmits more light than the Summar, so with both lenses set to F2, with the same shutter speed, the Canon will deliver more light to the film.

The other- The Balsam used in the lenses can yellow, and produce the same effect as a light yellow filter. Look through the lens at a sheet of white paper. also, shine a light through the two lenses. Internal haze can cause a lot of contrast loss, and will make the foliage look lighter.

 

[Ranchu]

Hold the two lenses up to your monitor on a white page, that will allow you to compare tints.

 

[sparrow6224]

Why does the Canon at f2 transmit more light than the Summar at f2?

 

[Ranchu]

Ooh, I know this! It's because the aperture is only a ratio of the aperture size to the focal length, it doesn't account for the transparency of the glass. They have a thing called t stops which is apparently more accurate... http://en.wikipedia.org/wiki/T_stops#T-stops

 

[pgeobc]

Don't forget that uncoated lenses lower image contrast, a desirable thing for a lot of B&W work. That, alone, might account for what you see.