medium format backs

[John Fass]

I am using a horseman 6X7 roll film back on my old but immaculate sinar norma for a job soon and have a question about focal lengths. When shooting 5X4 i commonly use a 90mm grandagon and find it the perfect lens for what i do which is mostly architecture exteriors. The 72mm XL i find far too distorting but wonderfully sharp and bright. What would be the equivalent focal lengths of these 2 lenses for the 6X7 and 6X9 backs respectively?

Thank you in advance for your advice.

John

[steve simmons]

Probably something in the 58- 65mm range.. 75/72 might be a touch long. When I used to shoot 6x9 I used the 65 a lot for interiors.

steve simmons
viewcamera magazine

[Emmanuel BIGLER]

John 4"x5" and 6x7 being very similar in aspect ratio (1.27-1.28) the conversion factor is easy, simply apply
a conversion factor equal to the ratio of diagonals. 6x7 cm is actually 56x72mm (at least for Linhof) and the corresponding diagonal is 91 mm. 4"x5" is actually 94x120 mm in image size and the diagonal about 155 mm. So the conversion factor is 1.7 or 1/1.7 ~= 0.59
90 *0.59 yields 53 mm.

6x9 is actually 56x82 or 56x84 with a longer aspect ratio closer to 1.5 so it is less easy to define equivalent an focal length vs. 4"x5". But you may consider the diagonal of 100 mm for 6x9 as an approximate starting point, the conversion ratio being 1.55 or 1/55 ~= 0.65 vs. 4"x5"
90 * 0.65 = 58 mm.

[Kirk Gittings]

I agree completely with Steve. I predominantly use 6x9 now for architecture after twenty years of 4x5. For some reason the 6x7 format just seems too small. I use a 65mm predominantly which is almost identical to a 28PC on 35mm (and would be in the ballpark of a 90mm on 4x5) I also use a 47mm which is closer to a 24mm on 35mm. The other lenses are 90mm, 150mm, 210mm and 305mm.