Calculating lens focal length

pete oakley · Aug 3, 2007

I have aquired a small lens which I believe to be a Petzval design. I need to make some Waterhouse stops for it but I don't know how to calculate the focal length. Do I measure from the Waterhouse stop slot, tge front element or the rear element?
Thanks in anticipation.
Pete.

Ole · Aug 3, 2007

I think I've described my lens focal length measuring method before, but here it is again:

I have a long corridor with a window at one end and a door at the other. On the door I stick a piece of paper with one vertical line on it.

I then focus the image of the widow through the lens onto that piece of paper, and line it up so that one side of the window aligns with the mark. I then mark the other side of the image of the window.

Once that is done with a couple of lenses with known focal length, interpolating to other unknown focal lengths is easy.

This is far more exact than guessing where on a lens barrel to measure from. The measure point is often close to the aperture, but can be quite a long way from it.

Ed Sukach · Aug 3, 2007

pete oakley said:
I have aquired a small lens which I believe to be a Petzval design. I need to make some Waterhouse stops for it but I don't know how to calculate the focal length. Do I measure from the Waterhouse stop slot, tge front element or the rear element?
Thanks in anticipation.
Pete.

from the Waterhouse stop position. The f/stop is calculated as the distance from the diaphragm - in this case called the "Waterhouse Stop" - to the film plane, and is actually independent of "focal length at infinity"... which is normally corrupted by omission; "focal length", in lens descriptions.

This is most commonly evident in close-up photography, were the lens is used extended far out from the film plane. If the actual focused distance, diaphragm to the film plane is doubled, e.g., 100mm instead of 50mm, the f/stop must be recalculated; f/d - 100/diaphragm diameter, instead of 50/dia.

Focus the lens on something far away, and measure; that will result in f/stop at infinity.

Ole · Aug 3, 2007

Ed, I'm afraid you're wrong.

The f in f-stop refers to the focal length; that is why older (especially German) lenses use the form F/x. The f-stop is defined as the diameter of (a circle with the same area as) the entrance pupil, divided by the focal length of the lens.

The aperture is not necessarily located one focal length from the film plane - look at telephoto and retrofocus lenses.

When you focus closer than infinity a correction factor must be applied. In most cases, but far from all, the correction you gave is correct.

Ed Sukach · Aug 3, 2007

Careful ... do NOT confuse "F" with "f". One example lies in the calculation of "f/stop" in a lensless system - see "Pinhole Design Calculator". There is no focal length (squint when you read that - there are other limiting factors ... but for the present...) when there is no lens. The major use for an f/stop is in regulating exposure; NOT to say that exposure is the only and exclusive use.

Lens design - Optics - is an involved and wonderful science. I know "very" little about it, damn near *nothing* - when compared to a half-decent Optical Engineer. I guess I know something about definitions, etc. (I've never had my fingers broken in communicating with the Engineers) - enough to realize the gravity of "centering", alignment, different glass choices, coatings... more.

I miss the "day when..."

BTW - All my lenses, German and otherwise, do not even use "f/ .." They are inscribed without it ..., e.g, "2.8/80". I never noticed that before. I can only say that there seems to be a close correlation in the data sheets for the lenses between the "/80" value and "focal length at infinity".

Ole · Aug 3, 2007

Ed, to regulate exposure you should really use T-stop, not f-stop. f-stop does not take light loss (from absorption or reflection) into account; it's a purely geometrically derived number. Its primary use is in determining DoF and CoC, not exposure.

A pinhole, when used instead of a lens for taking photographs, does have an effective focal length: The distance from film plane to pinhole.

I can assure you the the F in focal length and the "f" in f-stop are one and the same.

Ed Sukach · Aug 3, 2007

Ole said:
Ed, to regulate exposure you should really use T-stop, not f-stop. f-stop does not take light loss (from absorption or reflection) into account; it's a purely geometrically derived number. Its primary use is in determining DoF and CoC, not exposure.

I agree. One manufacturer, I think it was ... "Realistic" (???? ... could that be correct? I remember the ads ... "Joan Crawford" uses ... whoever they were) attempted to use "T" stops, a far more accurate and realistic way to determine exposure, precisely for the reasons you referred to. Unfortunately, there was GREAT competition for "the fastest lens", at the time. Having a low "f/ stop" or "T/stop" was a critical concern in the purchase of a lens. "T/stop" lenses were, of necessity, considered to be slower - solely, and unreasonably, because of the number itself.

A pinhole, when used instead of a lens for taking photographs, does have an effective focal length: The distance from film plane to pinhole.

Yes. Exactly what I wrote.

I can assure you the the F in focal length and the "f" in f-stop are one and the same.

I disagree - but I cannot argue this. I have no immediate reference at hand, and I will not try to argue this on the strength of my memory alone.

Ole · Aug 3, 2007

Ed,

I have several optics books from 1890 to 1920 close at hand.

Ole · Aug 3, 2007

BTW - back to the original question: A small lens is unlikely to be a Petzval. The length of a petzval lens is about 3/4 of the focal length, and the diameter about 1/3 of the focal length, so a 150mm / 6in Petzval is at least 2" wide and 4.5" long - most people wouldn't consider that "small". I have a 3" petzval, and even that is quite a big chunk of glass and brass!

pete oakley · Aug 3, 2007

Thanks

many thanks for all your help, I'll give it a try. It is small to be a Petzval and I did wonder (I have a 16" Vitax and that is REALLY big). Thanks again,
Pete.

nicolai · Aug 3, 2007

As a side-note, cine lenses often use t-stops rather than f-stops.

jimgalli · Aug 3, 2007

Another aside. Smallish Petzvals almost never get used at infinity. I take the approach of making my stops to round numbers in millimeters. 10mm 20mm 30mm 50mm 80mm etc. Then the focal length is irrelevant. Measure from the GG to the center of the barrel roughly half way between the elements and divide the stop used into the length for your f stop.

ie. I have a little petzval that focuses at infinity about 150mm. For a nice shot of a coke can sized object I've got the bellows drawn out to about 260mm. I use my 20mm stop. 20 into 260 is 13. Im at an effective f13

epatsellis · Aug 3, 2007

Easiest way I know is to focus on infinity, make a mark on the bed or rail. dig out 2 rulers, without moving the front standard, focus to 1:1, the f.l. is simply the difference between the two positions. The f# becomes f.l. divided by the apparent aperture (I measure with a caliper, lens held at arms length) an easy way to do it with waterhouse stops is to glue a piece of 10 square graph paper to a blank stop (or if you have a hard time seeing, a huge stop, as large as you can manage), measure a convenient # of squares, and determine the correction factor, if 5 squares measure .6 inches, then the correction is 1.2. (.5" x 1.2=.6") Calculate the desired f# aperture, multiply by correction factor and drill away. Easier in practice than to explain, I think.

erie

seawolf66 · Aug 6, 2007

Do you realize's that some threads here can drive one nuts: your talking about F-stops // or are you talking about Focal lenth's that perverial F2.8 vs 1-2.8 which was the first way for write-ing the Iris opening for the lens:
Back in the early days of 1890 aand to what unknowen date the symbol was 1-4.5 and the F 15cc which worked out to a Iris opening of 4.5-?? on a 150mm lens : Why in gods name can we not simply call a dog a dog and a cat a cat! to me the Iris is the thing that allows a certain amount of light thru the lens in a constrained manner and the bloody F is for focal lenth:

I know I am about to get my fantail shot off:

freygr · Aug 7, 2007

All the tempest in a teapot.... Quick and dirty. Mount lens in a view camera. Focus on the horizon. Take a tape measure and measure the distance form the aperture ring or water house stops to the face of the ground glass. That measurement is the focal length. The F stop is the Focal length divided by the diameter of the Stop. example the measurement for the focal length was 12 3/4 inch and the hole in the stop was 1/4 inch so the f stop is 12.75/.25 = F51

To get standard F stop like F 11, formula is focal length divided by f-stop, example focal length - 12.75 (12 3/4) inch F-stop = 11, so the hole to cut is 12.75/11= 1.16 inch

It doesn't mater it you use metric or inch the size of the hole will not change

Ole · Aug 7, 2007

freygr said:
... Quick and dirty. ...

Quick and dirty indeed. In most cases this will be fairly close to correct. In some cases it can be very far from correct, though; and it will never be exact.

epatsellis · Aug 7, 2007

The error will be greatest with retrofocus and tele lenses, and it can be significant, hence the method I use of measuring the difference in extension at infinity and 1:1. It will always give you the exact fl.

freygr · Aug 7, 2007

Ole said:
Quick and dirty indeed. In most cases this will be fairly close to correct. In some cases it can be very far from correct, though; and it will never be exact.

epatsellis said:
The error will be greatest with retrofocus and tele lenses, and it can be significant, hence the method I use of measuring the difference in extension at infinity and 1:1. It will always give you the exact fl.

Both of you are correct but to use the lens quickly, most LF lenses have there nodel point at the iris. I don't think the errors will cause much of exposure error .
Example a 135mm F4.5 lens is measure at 5 inches (12.7 cm) the f stop is 4.233 the error will not effect the exposure. The longer the lens is the smaller the error will be. At least you will be using the lens.

Ray Heath · Aug 8, 2007

g'day all
i've always considered that f = focal length divided by diameter of aperture

but to think about it another way; focal length is actually a measure of how far the light needs to travel from the lens to the film, given that the shortest distance is when the lens is focused at infinity

so, wouldn't it be more accurate to consider f = the distance from lens to film divided by the diameter of the aperture regardless of focus distance or lens focal length?

epatsellis · Aug 8, 2007

Ray, a few things to consider, first f.l. and physical distance from the focal plane need not be the same, i.e. retrofocus and telephoto lenses have either longer or shorter distances.
Given the following:

In an optical system, a lens focused at infinity will have the principal point (or node) at F (focal length)
In an optical system, a lens focused at 1:1 will have it's principal point at 2F

Thus, one can accurately determine the f.l of a lens to the accuracy you are capable of measuring the distance between 2 the standards by simply focusing at infinity and 1:1 mag. and subtracting the infinity measurement from the 1:1 measurement. Using a tape measure, yardstick, grains of wheat, it doesn't matter.

The f-stop of an optical system is determined by the effective aperture when viewed from the front. Hence Ole and my comment in regard to holding the lens at arm's length and measuring with a caliper.
(note that the f-stop of a lens is theoretical, the T-stop actually measures light transmission, and accounts for light loss through the elements.)

erie

HerrBremerhaven · Aug 9, 2007

I had a quick look through Kinglake's tome about lenses, and it seems the original Petzval design was f3.6. Many of the copies of his design were f3.6, though a few were f3.4. Given that published data, and measuring the front element diameter, would that give an accurate focal length guess (without going through measuring with the lens mounted to a camera)?

The other reason is that the few Petzval design lens diagrams I have seen place the nodal point closer to the rear group than the front. So the comments about measuring to the middle of the lens would not seem to work. Maybe I am confusing something else by viewing these diagrams?

The other reason I ask is that I now have an ancient Holmes, Booth & Haydens lens. Using a few tricks to mount it onto my 4x5, I managed to fire off two Polaroids. I did one expecting exposure using an f3.4 aperture, and the other expecting an f3.6 aperture. The exposure with f3.6 aperture guess was properly exposed, while the other was a little darker. It seems reasonable, though maybe this gets into light transmission?

Ciao!

Gordon Moat
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