Can I calculate focal length from aperture size?

[DanK]

I have an unmarked focal length 11x14 lens (11x14 Sterling Improved)

Does someone know the focal length of this lens?

If not...

Can I calculate the focal length of this lens from the lens diameter and aperture diameter?

Has three stops...

I have yet to build a board for it, and I'm not sure I even have the bellows length to check infinity focus from camera....

Thanks in advance,
Dan

[Bob Salomon]

Point the lens out the window. Hold a piece of large white cardboard behind the lens. Move the cardboard torwards or away from the lens until the image projected on the cardboard is sharp. Measure the distance from the lens rear to the cardboard and that is the appx. focal length.

[DanK]

Thanks Bob...I appreciate the response....I will try your method tomorrow AM....


I am still curious as to whether the focal length can be calculated from the lens aperture and lens diameter?

Or is the design of the lens too large a variable?

Thanks again,
Dan

[Peter K]

I am still curious as to whether the focal length can be calculated from the lens aperture and lens diameter?
No, a wide-angle lens with the same focal length like a "normal" lens can have nearly the same diaphragm diameter but the front-lens diameter is as double as big.

[sun of sand]

out a window is better obviously but you can get an estimate with a lightbulb ..or streetlamp or moon

ill estimate to be a normal lens for that size film

[Flauvius]

If you follow these directions, you can determine the focal length of any lens.

(1) Mount your camera with your mounted lens on a tripod,

(2) Tape a sheet of news paper to a "straight" wall,

(3) Measure in millimeters the length of a sheet of newspaper, or anything else that you can easily measure, and be precise.

(4) Focus on the newspaper, and use a lupe to focus as best you can.

(5) Measure the length of the newspaper on the ground glass. For best results, use a seamtress' cloth tape measure.

(6) Divide the length of the newspaper as it appears on your ground glass into the length of the newspaper that you have taped to a wall.

By way of example, the newspaper on the wall is 800mm, the newspaper on the ground glass is 40. So the result would be 20. (Stay with me, as I use this "20" result as part on the next step.)

(7) Add "1" to whatever is the result of your having divided the size of the newspaper on the ground glass into the size of the newspaper that you have taped to a wall.

So, now you have "21", the result of adding steps (6) and (7).

(8) Measure the distance, and be as precise as you can, from the wall to the nodel point of you lens.

(9) Now, divide the result of steps (6) and (7) into the distance from the wall to your lens' nodle point.

(10) Congratulations ! You now have the focual length of your lens.

*** This procedure only applies to lenses that are not tele fotos, or any unconventional lens type.

---------------

*** For the mathematically minded, the forumula for focual length determination is:

f/ = 1/
1/ + (1)
1/m


Robert J. Triffin

[Bob Salomon]

If you follow these directions, you can determine the focal length of any lens.

(1) Mount your camera with your mounted lens on a tripod,

(2) Tape a sheet of news paper to a "straight" wall,

(3) Measure in millimeters the length of a sheet of newspaper, or anything else that you can easily measure, and be precise.

(4) Focus on the newspaper, and use a lupe to focus as best you can.



This assumes that the camera has enough bellows to focus in that range.

[Peter K]

This procedure only applies to lenses that are not tele fotos, or any unconventional lens type.
If one only uses the distances in the subject space together with the image scale, magnification or reduction - e. g. distance between subject and front nodal point - the formula is valuable for all lenses.

focal lenght = subject distance / (image scale +1)

image scale = subject size / image size

With telephoto lenses or reversed telephoto lenses compared with "normal" lenses of the same focal-lenght the distances differs only in the image space.

Have fun
Peter

[goamules]

If you follow these directions, you can determine the focal length of any lens.

(1) Mount your camera with your mounted lens on a tripod,....[9 more steps]



To the OP, or...after step 1, measure from the center of the lens to the ground glass. Unless you like challenging, convoluted ways to determine something.

If so:

I have a lightbulb, is there a way to calculate the wattage without reading what is marked on the bulb?

[Mark Sawyer]

As others have said, the focal length is unrelated to the aperture, but you can figure the f/stop by dividing the focal length by the aperture after measuring both.

[Peter K]

As others have said, the focal length is unrelated to the aperture, but you can figure the f/stop by dividing the focal length by the aperture after measuring both.
As mentioned here very often, the f-stop is the diameter of the image of the mechanical aperture in the entrance pupil. Not the diameter of the mechanical aperture. :rolleyes:

Peter

[Peter K]

To the OP, or...after step 1, measure from the center of the lens to the ground glass. Unless you like challenging, convoluted ways to determine something.
What about a telephoto lens or reversed telephoto lens? In this cases the distance between the center of the lens to the groundglass is either shorter as the focal-lengt, telephoto lens, or longer, with a reversed telephoto lens.

[ic-racer]

Can I calculate the focal length of this lens from the lens diameter and aperture diameter?

Has three stops...



Of course you can. If the stops are marked. It will give you a ballpark figure. Do it with all 3 and get the average.

[DanK]

Thanks again...

I anticipated not having the bellows length for an 11x14 lens (on a 4x5)....and tried Bob's suggestion.....was a little skeptical, but it actually worked well and was in focus at approx. 255mm

Thanks Bob.

Later I mounted the lens and focused on the GG, lens focuses at 250mm

It is a wide angle 11x14 lens.

Mathematically, the closest I could get was from using front lens diameter divided by aperture at f64, then multiplied x 64 - showing 229mm - larger apertures with the same procedure weren't close...

IC, should I be squaring the measurements? Or might you elaborate on the formula?

Thanks,
Dan

[Peter K]

Mathematically, the closest I could get was from using front lens diameter divided by aperture at f64, then multiplied x 64 - showing 229mm - larger apertures with the same procedure weren't close...

IC, should I be squaring the measurements? Or might you elaborate on the formula?
As mentioned before the front lens diameter says nothing about the focal-lenght. Also the diameter of the physical aperture. :rolleyes:

If you know the f/stop measure the diameter of the image of the mechanical aperture in the entrance pupil. This is always smaller as the front-lens!

Peter

[GPS]

...

IC, should I be squaring the measurements? Or might you elaborate on the formula?

Thanks,
Dan

There isn't any mathematical formula to calculate the IC from a lens focal length of an unknown design.
As for the rest, if I were you, I would listen better to the good advice Peter K has given you...;)

[ic-racer]

IC, should I be squaring the measurements? Or might you elaborate on the formula?



Just multiply your estimation of the pupil size (by looking from the front and usng a ruler) by the indicated number on the scale for each stop.

Example: Measured pupil size = 4mm, f = 64 then FL = 4 x 64 = 256mm
Example: Measured pupil size = 8mm, f = 32 then FL = 8 x 32 = 256mm etc.

[GPS]

Just multiply your estimation of the pupil size (by looking from the front and usng a ruler) by the indicated number on the scale for each stop.

Example: Measured pupil size = 4mm, f = 64 then FL = 4 x 64 = 256mm
Example: Measured pupil size = 8mm, f = 32 then FL = 8 x 32 = 256mm etc.

And while doing so, don't forget - unlike someone ;) - that FL is not IC...:)

[Tracy Storer]

Also remember, for Ic-Racers procedure, that the lens may not be marked in "F-Stops" but in US stops or some other system. In US stops, 16=f/16...

[DanK]

This lens is a bit odd to me....hence the curiosity...apparently designed to cover 11x14...but it wouldn't seem so looking at the lens itself...but I've heard of its use on 11x14 with movements...

It is a small lens - 23mm across the front element and 25mm front element to rear element

Has three marked stops - 16, 32, 64

Aperture diameter is (+ -) 15mm at 16, 10mm at 32, and 7mm at 64

All in all, I was just curious....I didn't figure an 11x14 lens would be this short (250mm)...and it made me wonder if there was a way to calculate focal length from the lens measurements, rather than through it....and as yet, it does not appear to be possible....

Thanks all for your input....

Dan

[DanK]

And while doing so, don't forget - unlike someone ;) - that FL is not IC...:)

IC = ic-racer (in the above post) sorry for the confusion (I'm not concerned with image circle at the moment, I'm sure an 11x14 lens has plenty of coverage for 4x5, and 5x7) :)

Thanks,
Dan

[aduncanson]

The inconsistency of the calculated focal length across the three apertures might be because the apertures are not marked in the familiar f-stops as suggested above. Or you might not be measuring the aperture accurately. You don't say what procedure you used.

There is no accurate procedure for measuring focal length that is as simple as the task would seem to be. The complication arises because for "thick lenses" (that includes all well corrected photographic lenses) the focal length is the distance from the rear nodal point to the image plane, when focused on a object an infinite distance away. (Surprisingly perhaps, the "infinite distance" part turns out to be the easiest part of the process.) Unfortunately the location of the rear nodal point is usually unknown. It may be either ahead of, at, or behind the aperture.

Emmanuel Bigler has made a comprehensive post (http://www.largeformatphotography.info/forum/showpost.php?p=584863&postcount=2) containing three ways to accurately measure focal length. For good measure, his post concludes with a procedure for measure a lenses aperture which also may be of interest to you.

[Ole Tjugen]

<snip>
It is a small lens - 23mm across the front element and 25mm front element to rear element

Has three marked stops - 16, 32, 64

Aperture diameter is (+ -) 15mm at 16, 10mm at 32, and 7mm at 64
<Snip>


That looks a lot like US stops.
US 16 = f/16
US 32 = f/22
US 64 = f/32

So if f/16 = 15mm, then f=240mm.
And in f/22 = 10mm, then f=220mm.
Also if f/32 = 7mm, then f=224mm.

Close enough. Assume a slight magnification effect from the front cell and a little measurement uncertainty. Then a focal length of 256mm is not unreasonable, or 10" for the metrically impaired.

A 10" w/a rectilinear would be a nice little lens, about as described here. It should easily cover 11x14"; image circle at small apertures should be around 17".

[DanK]

Thanks Ole....I appreciate your response....

If I might ask......Why is there a difference in the US stops vs. f/stops? I'm unfamiliar with older lenses and getting a crash course....

More specifically, when I use this lens at the marked stop of 64, I should be calculating exposure for f/32 rather than 64...or f/64...

Thanks again,
Dan

[Ole Tjugen]

The US, or "Uniform System", is one of several historic ways of stating an aperture.

It just so happens that US 16 is equivalent to f/16, but whereas the f-stops progress by the square root of two for each step (16, 22, 32,45, 64, 90 and so on), the US progresses by doubling for each stop: 16, 32, 64, 128 and so on.

So to use a lens with US stops, at US 64, treat it as f/32.

Another common system is what I like to call AU - Arbitrary units: 1,2,3,4,5. Each step can be assumed to correspond more or less to one stop, but it's up to you to determine what "1" means. An offshoot of this is "Time" - 1,2,4,8,16 where each step shows the exposure time factor relative to "1" - again, you need to determine what "1" means!

[Mark Sawyer]

A quick question on the Uniform System... Most of us know how to convert between the US and f/stop systems, but was there a formula used to calculate each US stop's number? Something similar to f/stop = focal length / aperture?

I'd guess something to do with exposure values since that's what doubles or halves with each stop, as the US numbers do, but how did they determine where US/16 was at?

Never really thought about it before, and it's hard to think much about it now, fighting off a bad cold...