View Full Version : Measureing Focal Lenght

seawolf66

17-Jul-2008, 21:01

I Get darn Frustrated in this matter can never remember when Dealing with a Brass lens! From what point on the barrel lens does one measure to determine the focal point of the lens

In the foto I have attached I shined the light straight down thru the lens on to a piece of paper and have ruler set up there : When the lens board is at 3.25 inches, I get a sharpe focus of the light bulb bright spot! on the paper:

I know on a shutter lens one uses the center of the shutter as center point to the film plane , I Believe that to be correct !:

aduncanson

17-Jul-2008, 22:00

You measure from the rear nodal point of the lens, but since you can't know where that is (it depends strongly on the lens design), that is very unhelpful information.

I was trying to do exactly the same thing a few weeks ago and ended up measuring the size of the image of a window when projected onto the opposite wall of the room. That and the actual size of the window established the image magnification. Then the object distance is the distance from the lens to the window. (Here your question of the point to measure from comes up again, but is not important since the uncertainty is small compared to the total object distance.) The object distance, S(o), divided by the magnification, gives the image distance, S(i). Focal length can be calculated with 1/F = 1/S(o) + 1/S(i).

This procedure would seem to be highly sensitive to any error in measuring the magnification, so if you need a precise value for focal length you might want to do this with a camera and film where you can measure the image size with some precision on the negative itself.

Hopefully somebody else can provide a more accurate, less cumbersome procedure.

The following site gives two procedures. The first has the same requirement to measure image magnification, but solves the problem of glossing over the nodal point location. (This seems like the procedure I was taught in high school 35 years ago. If I had understood it then, I would probably have remembered it when I needed it.) The second is very clever, but may not be practical for your purposes.

http://www.bobatkins.com/photography/technical/measuring_focal_length.html

Good Luck

Ole Tjugen

17-Jul-2008, 22:21

... I was trying to do exactly the same thing a few weeks ago and ended up measuring the size of the image of a window when projected onto the opposite wall of the room. That and the actual size of the window established the image magnification.

...

This procedure would seem to be highly sensitive to any error in measuring the magnification, so if you need a precise value for focal length you might want to do this with a camera and film where you can measure the image size with some precision on the negative itself.

Hopefully somebody else can provide a more accurate, less cumbersome procedure.

...

That's what I do too - but a variation of the method. I use a piece of white cardboard taped to the wall for projecting the image; and that is already marked with the width of the image of the window when projected with every lens I own or have owned. That gives me a "calbrated FL scale". I have found a casket set with focal lengths from 150 to 750mm (single cells) to be an exellent standard scale. :)

Sergey Botvin

17-Jul-2008, 23:57

I have a book "Photographic facts & formulas" by Wall and Jordan (1940 edition). It describes 12 methods of finding the focal length of a lens. Please let me know if you're still interested. I can scan 5 relevant pages and e-mail them to you.

A question I am asking myself. I have a trousse Berthiot comprising 2 elements of

305mm and 575mm. Using a distant object and getting a rough measurement of the distance between lens center I got about 250mm. Ignoring the 1/df1f2 term cited above, I get 120.5...mm which is about 1/2 of what I thought. That indicates that teh d factor is important. Or does it? So looking for further references here, I fell on 3 different versions of the above formula!!!

1/f = 1/f1 + 1/f2 + 1/df1f2 least cited

1/f = 1/f1 + 1/f2 + d/f1f2

1/f = 1/f1 + 1/f2 - d/f1f2 most cited

I could not see any questions posed by other thread contribtors to the veracity of the chosen formula. It may be the third term is negligable, but my experience would suggest not.

So here goes::: which if any of the above is correct?

I did see the following with a technical reference :

According to the Puyo-de Pulligny book FL is determined as

F=(f1*f2)/D

where F - FL of the entire lens

f1 - FL of the front element

f2 - FL of the rear element

D - distance between nodal points of the elements

Ole Tjugen

18-Jul-2008, 01:38

Using F=1/(1/305 + 1/575), I get 199.3mm from your figures - not quite as far from the measured Fl as your 120.5mm...

Which formula is "correct" depends on how you define "d". Calling the three "d's" d1, d2 and d3 we get d1=1/d2, and d3=-d2. The only real difference between the last two is in which direction you measure the distance.

A classic example of the importance of "d" is the Unofokal lens. It consists (in its original form) of four lens elements; two positive and two negative. All the elements have the same focal length, except for the sign. Without the "d" the focal length of the system would be infinite...

erie patsellis

18-Jul-2008, 10:17

The method I use is simple, focus at infinity, measure film plane to whatever point on the lens you like, focus to 1:1, ( 2 rulers work wonders) measure to same point, subtract inf. from 1:1 and the focal length is found, with whatever precision used to make the original measurements.

Since the extension of a lens at 1:1 will be 2xf.l., the difference between infinity and 1:1 is the f.l. of the lens.

erie

Dan Fromm

18-Jul-2008, 11:53

Hey, Mike, in a recent discussion I gave a formula lifted from S. F. Ray's Applied Photographic Optics. The explanation is on p. 301 and references equation 8.3 on p. 47. Go read the book. Expensive, worth it just as a prop for a short table leg.

The power F of a compound lens is given by 1/f (that's the reciprocal of the effective focal length) = 1/fa + 1/fb - d/fafb where fa is the focal length of one element, fb is the focal length of the other, and d is the distance between the elements. This can be rewritten as

f = fafb/(fa + fb -d)

You can see that when fa and fb are large and d is small that d's effect will be small. I have no idea whether there are real lenses that satisfy this condition.

None of this has much to do with actually taking pictures.

Will each of your two Berthiot cells focus to infinity on your camera? Can you guess the distance between them -- I'm not clear on where to measure from, suggest front and rear vertices -- when they're in the barrel? A small error won't make much difference.

Good luck, have fun,

Dan

aduncanson

18-Jul-2008, 22:13

The method I use is simple, focus at infinity, measure film plane to whatever point on the lens you like, focus to 1:1, ( 2 rulers work wonders) measure to same point, subtract inf. from 1:1 and the focal length is found, with whatever precision used to make the original measurements.

Since the extension of a lens at 1:1 will be 2xf.l., the difference between infinity and 1:1 is the f.l. of the lens.

erie

That does seem simple and free from both sloppy approximations and error prone measurements.

I found a site the describes how to determine focal length. Using this method, I arrive at a value that is 1mm longer than I expected (I expected 1250mm, and measured 1251. Since I measured from the edge of the mirror, not the centre spot, then I need to add the depth of the parabola, which is about 3mm,

Using the method outlined in the link actually gives double the focal length, and I consistently measured (5 trials) 2502mm, which means that the measured focal length is 1251mm. Add in the 3mm depth of the parabola .

So, the measured mirror diameter is 249.5, and the measured focal length is 1254mm, so the calculated f/ ratio is 5.02

I think that that result is close enough to the advertised f/5 that I won't quibble.

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dolly

Bjorn Nilsson

5-Aug-2008, 04:13

Eh Dolly, you forgot to give us the link...

//Björn

quick and dirty. i hold the lens up to my camera focus at infinity and measure to the water house stops (aperture). granted this may not be the nodal point.....but it should be close enough for government work/brass lenses.

oh! and no "real" math to do.

eddie

roger12

19-Sep-2008, 22:59

Focal Length- For a thin double convex lens, all parallel rays will be focused to a point referred to as the principal focal point. The distance from the lens to that point is the principal focal length f of the lens. For a double concave lens where the rays are diverged, the principal focal length is the distance at which the back-projected rays would come together and it is given a negative sign. in this way we can easily find out the focal length.

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roger

ANCHOR LINK : http://www.globoinfotech.com (globoinfotech software solutions)

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I tried the same thing for determining focal length.but the value what i get is 1253mm and i did not measure from edge of mirror rather i measured from centre of spot.i dont know why ther is so much difference in the results.

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dorson

[url="http://www.globoinfotech.com"] globoinfotech(software_solution)[/url

scamy06

20-Sep-2008, 00:01

Since i am Computer student, I am not able to understand the calculation and science words in this topic "Focal length". Could anyone tell me in detail what the focal length is all about.

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scamy

GloboInfotech Software Solutions

http://www.globoinfotech.com

Anchor text : GloboInfotech

Jenny13

20-Sep-2008, 00:21

I am not not understanding about the focal length calculation. I have a camera and i want to know how to use the lenses in an efficient way.

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Jenny

Globoinfotech Software Solutions

Globoinfotech ( http://www.globoinfotech.com)

Peter K

20-Sep-2008, 06:42

I am not not understanding about the focal length calculation. I have a camera and i want to know how to use the lenses in an efficient way.

But it helps to choose the right tool like a lens with an adequate focal lenght.

Peter K

BTW, this link Globoinfotech ( http://www.globoinfotech.com) isn't working.

Emmanuel BIGLER

22-Sep-2008, 02:01

For a long focal length, there is a strange method that will allow you to estimate quickly the value. Without knowlegde of where the principal planes are located. So the method is valid even for a telephoto.

A precise measurement would need a precise method of measuring angles.

You aim at distant object (at infinity) with the lens mounted on a view camer, mounted on a panoramic head or any tripod head that can rotate and where you have readable graduations for angles.

You determine the angle that is required for the object to move on the ground glass (GG) by a certain distance. For example 4" or 10cm.

The focal length is roughly equal to the ratio of the amount of image travel on the GG divided by the corresponding rotation angle expressed in radians, 1 radian is about 60° (actually 1 radian = 180/pi degrees = 57.3°)

For a lens of 1250mm of focal length, the angle required to move the image by 100m on the GG is 100/1250 = 0.08 radian = 4.6°

Well, this angle is small and not easy to measure on a regular tripod head with graduations every 5 degrees !

However if you can fix an horizontal rod to the rotating tripod head, the amount of travel of the rod's end will be equal to the amount of travel of the image on the GG when the rod's length is equal to the focal length !

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