Calculating Petzval lens f stops

[Ian Grant]

Dallmeyer Petzval f stop calculation is not the simple f stop = focal length divided by the diameter of the aperture.

I have a Dallmeyer 2B Quick Acting Portrait Petzval f4 back focus 6", and the serial number is in the Dallmeyer records confirming it is a 2B made in 1864. Dallmeyer adverts list the lens diameter as 2¾", and 8¼" equiv focal length (210mm), it came with one Waterhouse stop.

The front element is approx 65mm diameter, so on that basis the lens would be f3.2, the 1866 Patent Portrait Petzvals were f3 but not manufactured until 1866. So knowing the maximum f stop is f4 and the pupil diameter we get a figure closer to 10" for focal length. This is where the lone Waterhouse stop comes in the aperture diameter is 32mm, the tab which would have been marked has broken off and lost at some point, this gives a vital clue it's relationship the entrance pupil, it's the f8 Waterhouse stop. On that basis a nominal FL figure of 256mm is used to calculate the aperture diameters for other f stops,

Last year I made an f16 Waterhouse stop out of thin Aluminium, both stops have given accurate exposures using studio flash, I use a large Gitzo leaf studio shutter with flash sync.

So why the anomaly, it has to do with J.H. Dallmeyer's redesigning the Petzval in 1860 to correct astigmatism, the results are nothing like uncorrected early swirly Petzval lenses. I will post a BJP Almanac entry on Dallmeyer's The Anastigmatic Corrector, it's hard to OCR and needs correction.

Ian

 

[bernard_L]

The front element is approx 65mm diameter, so on that basis the lens would be f3.2, the 1866 Patent Portrait Petzvals were f3 but not manufactured until 1866. So knowing the maximum f stop is f4 and the pupil diameter

The pupil diameter need not equal the diameter of the front element. Look into the lens from the front, with some even backlighting. The circle of light that you see is delimited by the pupil; it is likely to be located somewhere inside the lens. Its diameter is the effective aperture. A little difficult to measure because it is out of reach. The front element is larger to minimize (if not avoid entirely) vignetting over a finite field of view.
Hope this helps and resolves your apparent contradiction.

 

[Ian Grant]

The pupil diameter need not equal the diameter of the front element. Look into the lens from the front, with some even backlighting. The circle of light that you see is delimited by the pupil; it is likely to be located somewhere inside the lens. Its diameter is the effective aperture. A little difficult to measure because it is out of reach. The front element is larger to minimize (if not avoid entirely) vignetting over a finite field of view.
Hope this helps and resolves your apparent contradiction.

Effectively in this case that is almost the same, like all lenses taking Waterhouse stops there are two brass guides (essentially washers) for the stops. and the internal diameter is 64mm, quite easy to measure.

Ian

 

[ic-racer]

If you think you have facilities to detect a 1/3 stop change in exposure, make an f4 waterhouse stop and compare exposures with and without the stop. That should give you your answer.