Calculating f-stop sizes

[Anton Lukoszevieze]

OK, I know this question has been posted before, but I still could not find a simple, informative answer.
I have a 9 inch petzval lens, which takes waterhouse stops, none with it. The front lens is 55mm in diameter, the lens is f4 (or maybe 3.6?). What size would the holes be in descending order for about 6 stops? How can I calculate them?
It is a great lens for portraits, but I want to be able to have some different possibilities for depth of field. My maths is crap, so please don't blind me with science
Harry

[Nick Zentena]

9" is 225mm [not exactly] F/stop is the focal length divided by the apparent size of the stop when looking at the front of the lens. So if you wanted F/8 you need a hole that would look just under 30mm from the front of the lens.

[rob champagne]

as Nick says but in reality you say your lens is 9 inch and is F4.

9 inch = 228.6mm and F4 diameter would be 228.6/4 = 57.15mm.

However, you say the front diameter is 55mm so either your lens is not 9 inch or it is not F4 or it is not both.

To work it out accurately you need to know the exact focal length of the lens.

Or if you sure that the front collecting area is 55mm diameter, then you can work backwards giving exact 1 stop difference by halving the area of each aperture.

So area of 55mm is pi*r² = (3.142 * (55/2)²) = 2376mm²

next stop area would be 2376 / 2 = 1188mm² and the diameter required for that would be (√(1188/3.142))*2 = 38.89 diameter (F5.6)

next stop area would be 1188 / 2 = 594mm² and the diameter required for that would be (√(594/3.142))*2 = 27.5mm diameter (F8)

etc

or you can assume it is f4 and that the focal length must therefore be:

4 x 55 = 220mm and then work out stops as :

f5.6 = 220/5.6 = 39.29mm diameter
f8 = 220/8 = 27.5mm diameter
f11 = 220/11 = 20mm diameter
etc

but the common f stop numbers are rounded numbers so need to be calculated first for more accurate calculation.

however for an old lens like yours, I guess being accurate to hundredths of a millimeter is not critical so any of the aproximations supplied should be OK and none of them take exit pupil diameter into account anyway.

[Anton Lukoszevieze]

thanks guys,
i know that it is a 9 inch lens because it has a 9 written on the radial drive. f4 is the estimated speed that came from the seller. My 55mm could be out a couple of mm.
Harry

[GeorgesGiralt]

Hi !
I had the same question a few month back.
I was givent this answer :
Set the lens at infinity; and when i'ts done, put a piece of enlarging paper into the front lens cap.
Light the lens fromm back, develop the paper and measure the diameter of the spot. This is the exact measure of the opening as it should be.
As your lens use waterhouse stop, you've to make some guess to draw the correct curve (one wide open, one medium, one small and devise the curve from here...)
Hope this helps

[phfitz]

Harry,

a down & dirty, sweet & simple way is to drill the stops in standard sizes: 2", 1.5", 1", .5", ect..., then measure from the stop plate to the GG and divide. 9" / 2" = f4.5, 10" / .25" = f40. Eliminates all the guess work, magnification factors, bellows extension factors, ect...

Try it, it works well