"About" Equivalent

I've seen references that have comments like "a 50mm lens in the 6x6 format

is 'about' equivalent to a 35mm lens in the 35mm format." Why is it 'about'

equivalent? Is there a formula to calculate it exactly?

William, the comparison valid when the different formats have the same proportions; for instance, 35mm and 6x9 have very nearly the same length/width proportions, so comparisons are meaningful. And digital "APS-C" sensors are also about 1:1.5 proportion. Just calculate the diagonal of each format and compare them in a ratio. The 35mm format diagonal is 43.27mm, the APS-C size sensor in Pentax dSLRs is 28.26mm for a 1.53x comparison "factor". The 6x9 (56x83mm) diagonal is 100.12mm...

 

When the proportions are radically different, such as 56x56mm versus 24x36mm, then how does one make a valid comparison? Just go with the ratio of the full diagonals as usual, or consider the 6x6 cropped to 4x6, or consider the 35mm cropped to 24x24, or what? Meaningful comparison is elusive...

See http://www.photo.net/equipment/medium-format/focal-length-conversion

Most of the time, the film diagonal length corresponds to this comparison.<br>

Of course, as Doug says, it depends on the "shape" of the film format. A 6x6 camera (56mmx56mm) has a 80mm diagonal : square root of ((56^2)+(56^2)) = 79.2 mm.<br>

If you need exact correspondance, you must take into account not the "theoretical" format, but the exact film exposure window. It depends sometimes of the manufacturer. A 6x9 format can be 56x82mm to 56x88mm depending of the back.<p>

 

You can also use the angle of view of a lens as a comparison data, because it is independent of the "shape" of the film : <br>

- A Mamiya Sekor 110mm for 6x7 format has an angle of view of 44 deg. and a 90mm lens has an angle of 53 deg.<br>

- A Bronica GS1 (also 6x7) 100mm has an angle of 48 degrees.<br>

- A Bronica 6x6 80mm lens has an angle of view of 50.7 deg.

All these lenses can be considered as "nearly equivalent" to a 50mm in 24x36 which has an angle of view of 46 deg. Thus, all are considered as "normal" lenses.<br>

But these are only approximations.<br>

Doug's point about the different ascpect ratios is a very good one. You can't compare diagonal angles of view without correcting for aspect ratio.<br>Since we tend to compose such that things fit in vertically (sometimes), or horizontal (most of the time), comparing the horizontal angles of view would be more useful.<br>But, alas, most data available specify the diagonal angle of view only.

To find the horizontal equivalences (which is what most people are interested in) from 35mm to Hassy just divide 36mm (length of 35mm neg.) into 56mm (length of Hassy neg) which will give you 1.55. Then multiple 1.55 by by your 35mm lens to give you the equivalent focal length (for horizontal) in MF. So to duplicate the horizontal view of a 35mm SLR 28mm lens for a Hassy it would be 1.55x28=43.4mm. Conversely, to figure out what a 50mm Hassy lens is in 35mm divide 56 into 36 which will give you .64. So .64x50=32mm. Hope this is clear.