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1. ## equivalent lens

I am considering which lenses to use with my RB67 Pro S and was wondering what the 35mm equivalent lenses would be for the 50mm, 90mm, 127mm, and 180 mm lenses just to give me the right prospective. Thanks.

2. 50 = 28
90 = 50
127 = 70
180 = 100

The math is not exact, but on a practical level, I think you'll find that's pretty close.

3. 50 mm = 25 mm on 35 mm format.
90 mm = 45 mm on ditto,
127 mm = 64 mm,
180mm = 90mm on 35 mm format.

This math is exact, for the horizontal angle of view.

You can extend this table easily, once you notice that 6x7 is twice as wide as 35 mm format, i.e. 6x7 lenses are twice as long as 35 mm for the same horizontal angle of view.

4. ## many thanks

Thanks for your help. The math makes sense. Thats great.

5. 35mm to 6x7 is an easy AOV equivalent to figure out if using horizontal AOV instead of diagonal, as many (perhaps most) people use. This is because most 35mm frames are 36mm across (or close to it), and most 6x7 frames are about (and some are exactly) double that. Thus, divide your RB focal lengths by two, and that is your 35mm equivalent. However, because 6x7 is a "fatter" frame, you will have a wider vertical AOV for a given horizontal AOV. This can seem to make the RB lenses a bit wider than their 35mm equivalents, though you are not actually seeing more on the wide dimension – only on the narrow one.

Based on human vision giving about 50 degrees AOV of focused (i.e. non peripheral) vision, a 35mm lens on small format and a 75mm lens for the RB would give the most "normal" view based on horizontal AOV, while a 50 and a 90, respectively, would do this based on diagonal AOV. Obviously the camera makers have come to use the diagonal to make their FL choices.

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