Tim Parkin - Landscape Photographer

The subject of lens equivalents has come up on Large Format Photography mailing list recently and I thought it a good time to come up with the definitive answer to the 35mm to large format (4×5) ratio.

The first thing was to make sure we really know the size of a large format and a 35mm photography. Well I thought this would be easy but there was a surprise in store. The 35mm size was not 35mm (but I think I knew this). The size was actually 36mm x 24m (35mm was the size of the width of the film strip).

You would expect large format film to be 4″ x 5″ but actually it’s not this, it’s 3.74″x4.72″ (or 95mm x 120mm). This means the aspect ratio is actually 1:1.26 and you lose over 10% of the film area you thought you had

So… the ratio between LF and 35mm can be calculated based on three things. The first is comparing by short dimension. This is the method I use as I used to crop my 35mm pictures to 4×5 and I now use a mask. This preserves the short dimension of the 35mm picture size and doing this gives a ratio of 3.96 to 1.

The second way to calculate the ratio is presuming that if you like the amount of landscape you can include with your 35mm camera, you should use the long dimension as as the ratio. i.e. if you decide you want to take a landscape orientation picture and you stand in front of Buachaille Etive Mor and want to get the whole mountain in, then you’ll want to get the whole mountain in with 4×5 too.. So you’ll need the longer side of the aspect ratio to convert properly. This gives a ratio of 3.33 to 1.

The ratio that most people use however is the diagonal. I haven’t seen a single justification for this apart from “It uses bits of the short and bits of the long dimension”. Anyway, this gives, 3.54 to 1, which is probably the ratio a lot of people recognise.

In summary: –

short dimension comparison gives ~ 4 : 1
long dimension comparison gives ~ 3.33 : 1
diagonal dimension comparison gives ~ 3.5 : 1

Whilst I was doing this I also used google docs spreadsheet program to calculate a few more things, like what lens distribution would you get it you use 40% spacing of focal length and worked from a 150mm lens? What ratios do Leica and Nikon recommend (based on a posting at LFP.info)? What happens if you use 40% difference in angles instead?

Visit this google spreadsheet to see my calculations.. If you want a copy of the spreadsheet just ask or I think you may be able to copy it from the google site.

I should say that the common consensus seems to be to choose your lenses so that the spacing is approximately 40% by focal length. If you want to carry less lenses, use 50%.

As an example from my spreadsheet. If we start with 150 and 210 as a couple of focal lengths that have a lot of common lenses in, we get the 40% ratio and the extrapolated focal lengths in this range would be 77, 107, 150, 210, 294, 412 or if we map to real lenses this would be 75, 110, 150, 210, 300, 400.

I chose to use slightly longer spacing at the top end and slightly shorter spacing at the bottom end. Thinking about this since, I think it might be more useful to have closer spacing at the top end. The reason is that most uses of longer lenses are to pick out details at a distance and it’s harder to ‘use your feet’ when you are working in a 3D environment, for instance if you are on the side of a hill looking down a valley, moving 30% closer to the other side of the valley changes your viewpoint somewhat. My lenses are 80, 110, 150, 240, 360, 500 which give ratios of 40%, 40%, 60%, 50%, 40%. The gap between 150 and 240 is a bit long, I could do with a 190 or 200 to plug the gap but my pack is heavy enough as it is

Joe Cornish uses (from the first light book) 58, 72, 90, 120, 150, 210, 300 (although I doubt he uses all of them at once!) which gives 25%, 25%, 33%, 25%, 40%, 43%. The steps are longer at the top end which is probably because these lengths are used a lot less frequently by Joe (although by his own admission he rarely takes out the 58).