How to calculate "Mamicon" wide angle lens fov?

[lowep]

OK so I have an aging Mamiya 7 with an 80mm lens gathering dust in my cabinet. Right below it I have a Canon WD-58 0.7 x 58 wide-converter leftover from my dead video camera. Today I find out both have a filter diameter of 58mm so the Canon 0.7 wide-converter fits perfectly on the Mamiya 80mm lens.

Before I did not have a wide angle lens for the Mamiya 7. Now I do

The only problem is that since the Mamiya 7 is a rangefinder I have no idea how wide the field of view is when the 0.7 Canon converter is mounted on the Mamiya 80mm lens. This is not a serious problem as I figure it will give a surprise bonus every time I develop whatever I shoot this way ie I can be sure whatever I see in the yellow frame of the rangefinder will be in the photo but will not know for sure what else I get at the edges of the frame until I see the negative - a bit like shooting blindfolded.

But being a calculating geek, before I shoot a test roll I am now trying to calculate about how wide the resulting field of view (fov?) will be when the 0.7 wide converter is mounted on the 80mm lens.

Two questions:

Could it be something like: 80mm lens x 0.70 converter = 56mm fov?

Would 56mm fov on the Mamiya 7 (6x7 format) be roughly equivalent to (about 2x) 28mm on a 35mm DSLR?

If this is the case maybe I can count on whatever ends up in the rangefinder will be in view (not just whatever is in the yellow reference frame inside the rangefinder) :loco:

 

[Vivek]

lowep,

You may have even more serious of a problem than not knowing the angle of coverage. The wide converter on that lens may not let the combo focus correctly.

You can demystify the RF by doing a simple thing. Add a tracing paper or a ground glass plate at the film gate, open the shutter on the cam/lens and check if the infinity focus is correct.

Oh, in that process, you will also be able to find the angle of coverage.

 

[lowep]

Hey Vivek, thanks for this good suggestion about tracing paper. I will check. Maybe what I lose in focusing accuracy I gain in depth of field?

 

[lowep]

according to the specs this is a zoom-through (afocal*) adapter -- like the lens elements in the front part of this telephoto lens diagram -- so if i am very lucky the focus will not be effected.

* "...A "zoom through" wide-angle*adapter is what's technically known as an "Afocal" attachment. “Afocal” means that there is no focal point produced by the adapter, but rather a resizing of the image...."

Will try with tracing paper tomorrow (maybe more fun than actually using this thing to take photos :-/)

 

[Vivek]

Well, since I have tried many of these "afocal" adapters on various lenses, I said that you may not get the focus right with the combo.

Try it and let us know.

 

[lowep]

Well, since I have tried many of these "afocal" adapters on various lenses, I said that you may not get the focus right with the combo.

Try it and let us know.

OK I tried it with a piece of semi-transparent baking paper from the kitchen, some sticky tape, a magnifier, tripod and a reading lamp.

So guess who has for sale a pristine Canon WD-58 paperweight?

The 80mm lens focussed at 1.5 meters on the lamp whereas the superdooper-zoom through-afocal Canon WD-58 mounted on the 80mm lens focussed at 1.75 meters, the rest as they say is history - but it was fun to try and I do appreciate very much this suggestion that saved me a roll of film and a trip to the lab.