Lens Equivalents -- the missing table

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johnielvis

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Lens equivalents among the formats is often misstated as being scaled up based on the ratio of the format diagonals. The problem with this method is that it breaks down with larger formats because the magnifications become significant. Most people seem to want to know which lens to choose for shooting portraits based on their experience with the 35mm format. Invariably, the answer given to them is the scaled up focal length with a disclaimer that "shorter focal lengths seem to work better with the larger formats, so you can get away with a shorter lens." This is not really true. What happens is that when your magnifications start getting significant (say about M=0.1), the lens extensions become more significant. Your "effective focal length" becomes longer. For example, if you're shooting a 300mm lens at a magnification of 1/3, your effective focal length becomes 300*(1+1/3) = 400mm--this is now, effectively, a 400mm lens--you are getting the angle of view of a 400mm lens.

To make things easier, there are 2 tables attached. One is for infinity distance, where the lenses scale exactly with the format size. You'll see that the focal length for a 5 inch format length (4x5 inches "tall" length is 5 inches) is exactly half that of the 10 inch format length (10 inches is the "tall" or "portrait length" for the 8x10 inch format. The other table is calculated for a distance of 5 feet to the lens--this is the standard "go to" portrait length for pleasing perspective. Note that this measurement is TO THE LENS--to the entrance pupil--so the similar cameras with different format sizes will have the lenses positioned at the same position--the same perspective. You'll see that there is quite a difference at 5 feet as compared to at infinity for focal length requirements.

For example--you'll see that for a 135mm lens on 35mm camera focused on a distant object, you'd need a 1333mm lens for 11x14 to get the same distance on the 14 inch length as the 35mm camera gets on the 36mm length. However, up close at 5 feet, the 11x14 camera would need only a 746mm lens to get the same field of view, same angle of view, same perspective as the 135mm lens on the 35mm camera.

The tables are calculated for common 35mm lens sizes starting with 21mm up to 135mm.
 

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wiltw

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Sinar resolved the issue of lens FL equivalency across different aspect ratios by simply relating things to the vertical frame dimension (the short dimension)

13mm in 4/3 format is like
15mm in APS-C is like
24mm in 135 format is like
43mm in 645 is like
56mm in 6x6 and 6x7 and 6x9 is like
93mm in 4x5"
 

AgX

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Whether you relate to the diameter or to one side of the frame (and with panoramic formats that even gets more difficult), there remains the fact that with larger formats you more likely will end up in the macro range, with extraordinary extentions and cropping of field of view.

So, for those cases a modified relation to the common focal lenthts/angle of view would be appropriate. As proposed by the OP.

I'm not sure though, seen the possible variation of aspect ratio, whether such modification is just academic. I mean, do people want exact replication of angle of view?
 
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johnielvis

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Good question. People without extensive large format experience, it seems, want something to relate so something familiar. They had a table printed of these equivalents (at infinity) as relates to the diagonals in calumet photo's catalogs for years and it likely helped people make decisions to buy lenses without having them "try it first".
 

Renato Tonelli

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Good question. People without extensive large format experience, it seems, want something to relate so something familiar. They had a table printed of these equivalents (at infinity) as relates to the diagonals in calumet photo's catalogs for years and it likely helped people make decisions to buy lenses without having them "try it first".

I fall into this category. When I got started in large format I needed some guidance as to which focal length lenses would be somewhat equivalent to my 35mm arsenal. The Calumet table was helpful and I referred to it again when I got into 5x7 and 8x10. I find your tables very useful, especially for the shorter focusing distance.
 

Alan Gales

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Good question. People without extensive large format experience, it seems, want something to relate so something familiar. They had a table printed of these equivalents (at infinity) as relates to the diagonals in calumet photo's catalogs for years and it likely helped people make decisions to buy lenses without having them "try it first".

The good thing is that if you buy a used lens today at a fair price you can sell later and get most of your money back. I consider it as a cheap rental. I originally started trying to buy 35mm equivalents to what I shot on my Contax 139 and found out that I prefer different lenses for 4x5 and 8x10. For me I had to see for myself what I liked. Doing that years ago would have been very expensive!

Thanks for the interesting information.
 
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