Discussion in 'Large Format' started by shell|1, Sep 26, 2006.
What is focal length of a normal lens with a 4in by 5in film formal?
Thank you Randy. I am new at this so forgive me. Can you help do the formula for that just to help me understand. And how to convert it to inches. Thank you Randy
Image diagonal is sqrt(5^2 + 4^2) =~ 6.4in =~ 160mm. That just about defines what's
"normal" in the format.
In practice, a range of focal-lengths are normal: probably 130 to 210mm (pushing it).
You can also consider the aspect-ratio: 5:4 is not 4:3 nor 3:2 nor 1:1, therefore the
horizontal and vertical field of view will not be the same, but it's the diagonal measurement
that determines what coverage the lens requires.
Thank you Tim. You guys are really smart. Sorry, I am really bad at this math stuff. I am kinda confused. Randy said 150mm and you say 160 mm.The focal length in inches is approximately what?
The usable image area of a 4x5 film is about 95 x 120 mm. Using the Pythagorean theorem, the diagonal is 153 mm. (If you are not familiar with the Pythagorean theorem, I suggest searching the web.) The conventional definition of "normal" lens is a focal length of about the diagonal, so for 4x5, about 150 mm, which is also 5.9 inches. The conversion is 25.4 mm = 1 inch. Using lenses up to 210 mm for normal has been popular for 4x5, probably because of the increased coverage.
The majority of 4x5 cameras that have been made are press cameras, where a 127mm is the most common lens. This is the "normal" lens for a 4x5 press camera; with many millions made.
Divide the lens length 150 mm by 25.4mm/in = 5.904 in rounded off to 6 in.
The term "normal lens" is a technical term, and has a technical definition: a "normal lens" is defined as a lens of a focal length roughly equal to the diagonal measurement of the film. It is defined thus because such a lens will have an angle of view around 50 to 55 degrees, which is what someone, somewhere, once said was normal human vision. Hence, such lenses are also "normal." The concept is useful for choosing appropriate lenses for various film formats, but "normal" may be an unfortunate choice of words.
It's just a word that someone used back in the dark ages of photography, and which stuck. Focal lengths that are much longer or shorter than the film's diagonal measurement might be common lenses for that format, but they aren't normal. Lenses that are shorter than the diagonal are "wide angle" and lenses longer than the diagonal are "long lenses."
For 4x5 film, anything from about 135mm up to 165mm can probably qualify as a normal lens. That's roughly plus/minus 10% from the diagonal, but of course that's just an arbitrary number...
One justification for choosing the diagonal of the format as the normal focal length is the following. When viewing a print, people tend to get at about the diagonal of the print from it. This corresponds to the point made above that the comfortable angle of view for the human visual system is 50-55 degrees. If you think of the print as a scaled up version of the film format and the print viewing distance as a scaled up version of the lens to film distance, then the latter should be about the diagonal of the film format. For the great majority of pictures, the lens to film distance is either the focal length of slightly greater than the focal length. For close-ups, the lens to fim distance can be greater, even up to twice the focal length. So for close-up photography, you cold argue that the focal length should be chosen somewhat shorter than the diagonal of the format, so that the lens to film distance ends up roughly equal to the diagonal. Thus, for example, you could argue that for 1:1 macro photography, the focal length should be one half the diagonal. Be that as it may, I suspect that for macro photography, there are other considerations which play a more important role. Perhaps someon experienced in that type of photography can comment.
If you view a print made with a focal length other than the normal focal length, your eye will typically not be placed for appropriate viewing. This leads to what are sometimes called "perspective distortions", although that is technically a misnomer. But often you want to have such distortions for a deisred creative effect. If the ratio of the focal length used to the normal focal length is not too far from one, the resulting pictures won't depart from normal enough for most people to notice.
The diagonal of the 24 x 36 format is about 43 mm, but in 35 mm photography, 50-55 mm is usually considered normal. This results from the hisotry of such cameras, but since most of us have seen many pcitures taken with such lenses on 35 mm ameras, we may tend to be biased towards considering slightly long lenses as normal.
Hi Shelly. I can't do the math either. Basically "normal" is defined by how you see and what you shoot. If you primarily shoot tabletop, a 210mm may work best. If you shoot landscape, a 135mm may work best. As has been mentioned, "normal" for 4x5 runs from 135-210mm. Many people start with a 210mm lens and then work wider. I've found over the years that I see slightly wide (40mm for 35mm, 80mm for 6x7), so I use a 135mm as a normal lens for 4x5. My current lens set for 4x5 is 90mm, 135mm, and 210mm.
Since it comes up so often in large format photography, every large format photographer should understand the Pythagorean theorem. That says that if you have a rectangle, the diagonal, i.e., the distance between opposite corners, is found as follows. Square each side of the rectangle, and then add. Finally take the square root of that sum. This can be done easily on almost any calculator which has both a square key and a square root key. As Michael pointed out, the usable area of a 4 x5 frame has dimensions 95 x 120 mm. Using my computer's built in calculator, I find 95 squared = 9025, 120 squared = 14400. The sum is 23425. the square root of that is 153.05227865, but we would normally ignore anything past the first three digits and simply take the answer as 153 mm, which is close enough.
I don't know how many of our fellow ciitizens can do such arithmetic, but I would be willing to bet it is tens of millions, if not more. There is no reason to be afraid of it. Try it for another example such as the 35 mm format 24 x 36 mm, for example, to see what you get.
But Leonard, everyone knows that the normal focal length for 24x36 is 50 mm. Or 52 mm. Or 55 mm. Or 57 mm. Or even 58 mm.
If you don't believe me, ask the camera manufacturers who sold interchangeable lens 35 mm still cameras with normal lenses.
Thank you everyone for your help. I appreciate it and better understand how it works
What is considered a "normal" lens varies with the type of camera, also whether still or movie. With a 4x5 press camera, the normal lens has been between 127 and 135mm since the 1930's. In movie cameras the focal length of a normal lens is alot longer than the old tired silly diagonal stuff. With a regular 8mm camera the lens is a 13mm lens, ie about 1/2 inch. With a 16mm camera is a 25mm lens, ie about 1 inch. The vast majority of lenses made for 4x5 cameras have been for press cameras, 4x5 view cameras are a small dinky subset of 4x5 cameras ever made. The 127mm and 135mm lenses for 4x5 cameras is like a 16oz claw hammer, a 7 1/4" circular saw, a 50mm lens on a 35mm still camera, the most common and normal tool used. Those who get upset about 127mm being a normal lens for 4x5 cameras tend to be view camera chaps, who use a small dinky subset of all the 4x5 cameras made. It might be better to mention that a view 4x5 camera tends to use a longer lens as a normal lens, than erase 80 years worth of history with a press camera. In press cameras a 90mm is a wide lens, a 127 or 135 as the normal, a 150 as a mild long lens, a 210mm as a long lens. With view camera adjectives jsut move up one grunt.<BR><BR>Its nothing really to get in a tizzy about, its like worrying about what a small, med, or large coke is in LA, Tokyo. Auckland, versus London. What matters is angular coverage, not which lens is a normal, or what some books says it is. for a completely different camera.
Ah, "normal" is in the eye of the beholder, and often also defined by the planned use of the camera. I've heard a number of individuals state they perfer 135mm for their normal 4x5 lens, and others the 150mm. IN the end, it is a good idea to try out a few lenses on a camera while photographing what you normally do.
IF you've use a 645 format camera with a number of lenses, then look up their conversion to a 4x5 lens for an idea of what will work best for you. I think Calumet still has the tables for doing that, but don't try it with 35mm since that is actually very close to a 5x7 format (~2x3 in dimension).
Go to http://www.google.com<br>
Paste this into the search window: sqrt(5^2+4^2)*24.5<br>
There is your answer in millimeters<p>
For any other format just replace the 5 and 4 inch film dimensions
If you are entering millimeter dimensions, leave off the *24.5<p>
NOW, that said please keep in mind that the actual exposed area of 4x5 film is a bit smaller than 4x5" due to the film holder.<p>
AND when comparing lens/perspectives for films of different aspect ratios, it is best to use the long side of each in the formula.
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