"STANDARD" LENSES AND FOCAL LENGTH EQUIVALENTS
The classic (prime) "standard" lens has a focal length more or less equal to the negative diagonal. This dates from the days of contact prints and is based on the far from indisputable assumption that people look at bigger prints from further away and smaller prints from closer.
Let's start out with an 8x10 inch contact print shot with a 300mm lens.. The diagonal is near enough 300mm or 12 inches, which is why this is the "standard" focal length on 8x10. Look at the print from about 12 inches away, and everything in the print will subtend the same angle at your eye as it did at the camera lens, and perspective will appear natural.
Courtyard. This is not actually a contact print: it's a direct (laterally flopped) positive using Ilford's Direct Positive Paper. I used a De Vere monorail fitted (as far as I recall) with a 300/9 Nikkor, the 'equivalent' (on the diagonal) of a 43.3mm lens on 35mm.
Now go down to a 5x7 inch contact print. The diagonal is near enough 8 inches (203mm) and the standard lens is therefore 8 inches or 210mm. Odds are, you'll look at it from closer: maybe 8 inches. Again, the perspective appears natural.
In the event of invasion, church bells will be rung. I found the map in the attic of my last house; the gun is a replica, a present from a friend; the (empty) ammunition box came from my late father-in-law in the United States; and I remembered reading that in the event of a German invasion of the UK, church bells would be rung as a warning (they are supposed to be church keys). Of course the picture here is the same height (700 pixels) as the 8x10 inch picture above, which makes something of a mockery of contact print sizes. I used a 210/5.6 Rodenstock Sironar-N on (I think) a Linhof Technika V, shooting on 5x7 inch Ilford FP4 Plus. Print on Ilford Multigrade Warmtone.
Go down to 35mm, though, and few people make contact prints for display. This is just as well, for two reasons. First, the prints are too small to see much, and second, it is hard to examine them from 43.3mm away, the negative diagonal
No problem. Let's make a 5x enlargement, for a 120x180mm image: near enough 5x7 inch. Now we can look at it from 5x as far away. This is 216mm or rather over 8 inches: much the same as if we'd started out with a 5x7 inch contact print. There are however three complications, as discussed below.
Weston-super-Mare, c. 1974. In those days you could still take a camera on the beach in England without being lynched. I shot a lot in Weston, over two or three summers, always with 50mm lenses, mostly on Ilford HP4 (this was before HP5). I don't remember, but this may have been shot on a fixed-lens Leica from about 1929. Print on Ilford graded paper.
COMPLICATION 1: IMAGE SHAPE
(and why you can't compare focal lengths on different formats)
Unless the print is square, the vertical angle of view and the horizontal angle of view will be different. With an all-in print from a 35mm negative, the long side will show you a 50% greater angle than the short side.
Thus, a 45mm lens (the closest you normally find to 43.3 mm) gives a 50 degree angle of view on the diagonal of a 24x36mm negative, so the 24mm side covers about 27 degrees and the 36mm side covers about 41 degrees.
For comparison, on a so-called 6x6cm negative, which is normally 56x56mm, an 80mm 'standard' lens covers 53 degrees on the 79mm diagonal. This translates to an angular coverage of about 38 degrees both horizontally and vertically: not quite as wide as 41 degrees but a lot wider than 27 degrees.
Discarded film hangers. Maco Cube 400 was an excellent film with superb tonality but like many Maco products it was relatively short lived. I used a KowaSIX with its standard 85/2.8 lens for this shot; Frances printed it on Ilford Multigrade IV.
Use the same 80mm focal length on the Alpa 44x66mm format and you get 29 degrees on the short side and 44 degrees on the long side. Fairly obviously, you can compare all the 2:3 (1:1.5) formats directly: the 24x36mm of 35mm, the 44x66mm of the Alpa and the 56x84mm of the so-called 6x9cm format. A 21mm lens on 35mm gives almost identical coverage, vertically and horizontally, to 38mm on 44x66mm and 49mm on 56x84mm.
Equally obviously, you can't compare these 2:3 formats with the square 56x56mm (diagonal 79mm) format, or with panoramic ones such as 56x120mm (diagonal 132mm), because the horizontal and vertical coverages are so different from what they are on 2:3. You can't compare them to cut-film formats such as 4x5 inch or 8x10 inch either, for the same reasons.
The end of the road. This was shot on a home-brewed 6x17cm camera made by an ingenious friend from two chopped-up 6x9cm cameras grafted together. The diagonal is approximately 180mm, so the format is easily covered by a 90/8 Schneider Super Angulon (image circle 216mm at f/22). Think of it as a very slightly long standard lens on the short dimension and an ultrawide on the long dimension.
Also, "standard" lenses on cut film and on all but the smallest roll-film formats are often nearer to the negative diagonal than lenses for 35mm. Diagonals and "standard" lenses for some of the roll-film formats are:
8x10 inch: diagonal 12 inches = 300mm, 'standard' lens 300mm
5x7 inch: diagonal: diagonal 8.6 inches = 218mm, 'standard' lens 210mm
4x5 inch: diagonal 6 inches = 150mm, 'standard' lens 150mm
"6x9" cm format (56x78-84mm): diagonal 96-101mm, 'standard' lens 100-105mm
"6x7" cm format (56x68-72mm): diagonal 88-91mm, 'standard' lens 80-105mm
"6x6" cm format (56x56 mm): diagonal 79mm, 'standard' lens 75-80mm
Preston-next-Wingham, Kent. It is east to forget that peel-apart Polaroids are contact prints, but it is easier to remember when you see a picture like this, made from a Type 55 P/N negative (which has in this case been enlarged). Camera: probably a Toho with a 120/6.8 Angulon. This print on Ilford Multigrade Warmtone.
To sum up, "standard" lenses on cut film and roll-film often are somewhat wider than "standard" lenses on 35mm, and with the square "6x6" format, many people find that they look wider still.
COMPLICATION 2: LENS DESIGN
Although you might expect 45mm (close enough to 43.3mm) to be the "standard" for 35mm, even the Leica started out with a nominal 50mm lens: "nominal" because Leica lenses are often a little longer than 50mm, up to about 52mm, though they are very rarely shorter. Since then, many "standard" lenses have been 50mm, 55mm or even 58mm.
"Nullserie" pre-production Leica. The lens is a 50/3.5 Leitz Anastigmat: remember that Leitz were microscope manufacturers, well able to make their own lenses. The Anastigmat was a 5-glass derivative of the Cook Triplet.
There are at least three reasons for "long standard" lenses. One is that there were already 50mm or 2 inch (50.8mm) lenses available, because it was a popular length for movie cameras. This would have been important to small manufacturers who had to buy in lenses. Movie lenses were designed for the 18x24mm format but some, especially with focal lengths of 50mm and above, would cover 24x36mm with varying degrees of success.
Another is the question of print sizes and viewing distances. These are further discussed below. When the Leica first came out, many people wanted only postcard sized prints: 3.5 x 5.5 inches, 9x14 cm. This is about a 3x enlargement after allowing for the border, implying a viewing distance of 150mm or 6 inches, as close as many people can comfortably focus. With a 43.3mm lens the viewing distance would be only 130mm, a bit over 5 inches.
If postcard sized prints sound hopelessly unambitious, remember that in the 1920s and indeed for a couple of decades afterwards many people were perfectly happy with 6x9cm (nominal) contact prints. In any case, the films of the day were hard put to sustain a 6x enlargement or 6x9 inches. Sharpness and grain were simply not up to it.
Bristol, c. 1978. It wasn't just film grain and sharpness that limited early enlargements. This was taken with a pre-WW2 Plaubel Makina with a 6x9cm back and a 100/2.9 Plaubel Anticomar. I have heard of sharp Antiocomars but I have never encountered one. Enlargements are OK at 3x (168x240mm, rather bigger than whole-plate, which is six and a half by eight and a half inches, 168x216mm) but after that the quality declines noticeably. In the 1930s, remember, whole plate was the normal size to submit for newspaper reproduction.
Third, longer lenses make it easier to make evenly illuminated negatives that are reasonably sharp in the corners as well as in the middle. Even a comparatively tiny increase of 15% (50mm instead of 43.3mm) makes the lens designer's job much easier.
When SLRs came in, the problem was exacerbated because there had to be enough room behind the lens for the flipping mirror to flip up and down. With a completely symmetrical lens, the nodal point of the lens will be 50mm from the film or sensor at infinity. If the lens is 30mm thick, this means that the rear element will be 50-15=35mm from the film or sensor. The mirror normally requires more than 35mm between the back of the lens and the film if it is to flip up and down. A focal plane shutter further eats into the space available. Again, the easiest way around this is to increase the focal length of the lens.
A fast lens can be a lot thicker than 30mm, too, which explains why the first f/1.4 "standard" lens for the Nikon F was 58mm, which is much sought after today: I wish I hadn't sold mine. It's a wonderful length for portraits. I still have have a 58/2 Biotar in Exakta fit, though, and there have been other "long standard" lenses. Certainly, 55mm was popular: I have a 55/1.4 Super-Takumar (Pentax) and my first ever serious standard lens was a 55/1.8 Super-Takumar.
Silves, Portugal. Even today, limiting enlargement ratios can have its rewards. A 3x enlargement off the Linhof 56x72mm format is almost identical in size to whole plate at 168x216mm and with the right film it can be indistinguishable from a contact print. Ilford XP2; 'baby' Linhof Technika IV; 105/3.5 Linhof-selected Schneider Xenar; print on Ilford Multigrade paper.
COMPLICATION 3: VIEWING DISTANCE AND PRINT SIZES
It's hard to control the distance from which people look at prints.In fact, it's all but impossible. Old-timers spoke disparagingly of "sniffing the print" because they held it as an article of faith that no-one should look at big prints from close up. They were on to a loser, because people will insist on looking at prints of any size from close up despite the disapproval of self-appointed "experts". They look at them from far away, too. You have almost certainly had the experience yourself of walking into a gallery and looking around from a distance, then looking at one or more prints closely.
Also, print sizes vary widely. At my last exhibition in Arles, The Secret Life of Chairs, the prints were about 90x135mm, rather under 4x6 inches. At the one before, Recycled Religion, they were about 300x450mm (12x16 inches). These were the sizes I felt suited the images and themes. I wanted people to look at the small pictures quickly and from fairly close up, and the big ones more slowly and from further away; but I couldn't control it either way. There's a lot of fashion in print sizes, too: cynics often say of today's exhibition prints, "If you can't make it good, then at least make it big."
Reculver church. "Natural perspective" is a bit meaningless in the context of fish-eye lenses, and this one is especially unusual. The same ingenious friend who made the 6x17cm camera adapted a Russian full frame fish-eye made for 6x6cm to use as a circular fish-eye on a specially made, shuttered 4x5 inch camera. The irregularities at the edges of the circle are where he sawed off the built-in "petal" hood. He described this as a nerve-wracking procedure. Ilford FP4, printed on Ilford Multigrade IV.
To go back to the original premise about contact prints and enlargements, if the perspective in a print looks 'natural' at (say) a 10x enlargement when viewed from 10x the focal length of the lens, then enlargements of a constant size will need different viewing distances according to the focal length used. If the perspective in a picture taken with a 50mm lens looks right at 40cm (8x50mm) or 16 inches, then one that was taken with a 35mm should look right at 28cm or 11 inches and one taken with a 100mm should look right at 80cm or 32 inches. This ain't likely to happen. Alternatively, if a picture taken with a 50mm lens looks natural when viewed from 40 cm, then one taken with a 25mm lens should be blown up to twice the size for the same viewing distance. Again, how likely is this?
An interesting aside here is that ciné lenses are almost invariably longer than "standard" because viewing distances are usually much longer than for prints.
Also, of course, you don't always want the perspective to look 'natural'. There are times when you want the compressed perspective of a long telephoto or the stretched perspective of an ultra-wide. Don't listen to anyone who tells you that perspective depends only on viewpoint. It's nonsense, and common experience shows us it's nonsense. Perspective in a print (or on a screen) is affected by focal length, enlargement size and viewing distance. Anyone who says "Ah, but that's only apparent perspective, not real perspective," can usually be shut up very smartly by asking them what "real" perspective is. As there is no such thing, there is no answer. Besides, there's a lot more to perspective than just linear or vanishing-point perspective.
IT'S ALL PRETTY APPROXIMATE ANYWAY
A common error in photography is looking for more accuracy than exists in the system. As long as focal length, format, print size and viewing distance are all reasonably close to the parameters given above, everything looks about right.
Also, most people have grown used to different perspectives. In old photo books you often read references to the 'violent' perspective of 28mm lenses, something which would pass unnoticed today. In fact, many people don't even find the perspective from a 21mm "violent", though a lot depends on the composition. Sooner or later, though, perspective starts to look compressed or stretched, and you lose the "magic window" effect.
Graffiti, Arles. Frances used a 28/1.9 Vivitar Series 1 on either a Nikon F or a Nikkormat for this picture. It's clearly a wide-angle shot, but does the perspective strike you as "violent"? Ilford XP2, print on Ilford Multigrade IV.
As with most things in photography, there's little sense in getting obsessive about any of this, but equally, if something is not quite working and you can't see why, some of what I have written above may come in handy. At the very least, you'll understand why you can't really talk about "equivalent" focal lengths on formats of different shapes.
Go to Photography
Go to Index
Go to Home Page
Words and pictures copyright (c) Roger Hicks 2016