Converting for Focal Length - 6x9cm Out of 4x5 in

[Scott Fleming]

Please. Simple math only. I don't understand some of the computer symbols for some math functions anyway.

Can I just figure the area of the relative formats and multiply the 'crop factor' by the focal length? Is there an easier way? Is there a chart on the web somewhere?

Thanks.

[Oren Grad]

Scott -

There are simple formulas, but there really isn't a simple answer. Because the shapes of 4x5" and 6x9cm are so different - one is very square, the other rather elongated - which focal lengths will "feel" most similar to you across different formats will depend on how you see.

For many years, I used to instinctively key on the vertical dimension of the frame. By that measure, the 6x9 "equivalent" of say, a 150mm "normal" lens for 4x5, would be (56/96) x 150 = 88mm. (I'm just multiplying by the proportion of the respective vertical dimensions of the two formats.)

More recently, as I've experimented extensively with different sheet-film formats, I've found myself becoming more attuned to the format diagonal as the standard for measuring how a given focal length "feels". By that standard, the 6x9 equivalent 0f a 150mm lens would be (99/154) x 150 = 96mm. (This time I'm using the proportion of the format diagonals.)

Finally, although this has never worked for me, if you are especially sensitive to the horizontal "sweep" of your pictures, you might be most comfortable scaling with the horizontal dimension. For the 150mm lens for 4x5, this would give you a 6x9 counterpart at (82/120) x 150 = 103mm. (Same principle of proportionality.)

In each case, however, the proportionality is in terms of a linear measure, not an area measure.

So my advice is, think about how you normally frame a scene. If you're especially sensitive to the vertical or horizontal, use that proportion. If you don't have a strong feeling that you key on either dimension in particular, the diagonal would be a reasonable basis for the calculation.

A concrete example from my own experience: the focal length that feels "normal" for me tends to be about 7/8 of the format diagonal. For 35mm that's about 38mm, so I reach for a 35 or a 40. For 8x10, it's about 273mm, which is one reason why I've always had a hard time with the format - the only lenses commonly available in that focal length to cover 8x10 are the 270 G-Claron and the 10 3/4" Dagor, and I don't especially care for either (though I do have the G-Claron and use it occasionally), while my favorite Apo-Sironar-S is available in 240 and 300 but not 270. In 4x5 that rule of thumb comes out to 134mm, and I'm very happy with a 135 as my normal. For 6x9cm it's about 87mm, so a 90 seems about right for me.

[Scott Fleming]

Oren, that was REALLY helpful. Thank you very much.

[Oren Grad]

PS - You can find different focal-length conversion charts floating around the web. This one...

www.schneideroptics.com/software/Lens_Conversion_Chart.xls

...was calculated by scaling in proportion to the diagonal.