Quote Originally Posted by Steve Smith View Post
So it's designed to focus correctly with a filter in place, so to compensate for not having a filter, a flat piece of glass needs to be there instead.

Was it supplied with a (non) filter?

Also, the depth of field would be huge. I assume it's only really an issue when used wide open.

Mamiya supplies the lens with a couple of filters yes - including a (clear) UV filter. One of them always needs to be in place. I'd also just like to address your last statement, which is a bit of a misconception - the field (in front of the lens, in subject space) is indeed very deep, but the focus (at the film plane) is extremely shallow. Wide angle lenses are much less tolerant of any inaccuracy of distance between the lens and the film. A slight problem behind a 37mm lens will mean a great deal more than the same problem behind a 200mm lens.

Think about it - focusing, say, a 21mm lens on a 35mm camera from infinity down to a 1 metre moves the lens assembly less than 3mm - imagine how sensitive this space is to inaccuracies! Compared to, say, a 200mm lens, that has to move a great deal (a couple of centimeters) to achieve the same focus distance.

Lenses with a deep depth of field (wide angle lenses) have shallow depth of focus at the film, and lenses with a shallow depth of field (e.g. telephoto lenses) have a deep depth of focus at the film. Mamiya 37mm lenses have awesome depth of awesomeness all 'round