90mm on 4x5 field with 6x7 120 film back?

[rustyair]

90mm is 90mm - it does not matter what size the film is. The image is the same regardless of how big or small the film is! The only difference is that the smaller film only "sees" less of the projected image.

put the 90mm lens on a 6x7 camera, take a photo on 120 roll film.

put a 90mm lens on a 4x5 take the same photo from the same camera position...now, cut out the 6x7 image from the 4x5 negative...it is exactly the same as the one made with the 6x7 camera.

Now it makes perfect sense! Thanks. To use a horseman 6x7 roll film back on my 4x5 field, Does that mean I need an optical viewfinder for 6x7 film? Wouldn't it be really hard to composing without one?

[Kevin Crisp]

Your photo will look the same only if the negative size stays the same. The smaller piece of film "seeing" less of the projected image makes a big difference, though strictly speaking the image hasn't changed in the sense that it remains the same size. But since you are shooting 6X7 negatives either way, then one 90mm lens is going to record the same area as the other and you don't need to worry about it.

To answer your new question, an optical viewfinder is typically not needed. You use reference lines on your ground glass to mark off the area of the 6X7 negative on the roll film back. Focus on the ground glass, slide in the roll film back (or take off the ground glass with a graflok style back) and everything is all set so long as you don't bump the camera.

[Kuzano]

Taking a different tack on the answer... look at the lenses considered normal on Medium Format Camera's

The typical lens on a 6X6 is 75-80mm. My Perkeo 6X6 is 80mm.
The typical lens considered normal on a 6X7 is usually 90-105 (Pentax 6X7)
Considered Normal focal lengths on Fuji 6X7 and 6X9.... 90-100mm

So your 90 falls into normal focal length for the longer Medium Format negatives.

Actually, that seems appropriate, considering on 4X5.. the 90 is wide angle, and the normal for 4X5 is often regarded as about 180mm.

[BrianShaw]

I love this question... every time. There are so many different ways to say the same thing.

Now let's talk about "normal" on 4x5... "about 180"?