## Question for lens design Guru's; Aperture Calcs?!greenspun.com : LUSENET : Large format photography : One Thread |

Michael Mutmansky asked a question that I thought I knew the answer to and found out I didn't.(ie. which ap scale is right etc?)I thought apertures were simply the mathematical inverse relation of the focal length. Simply F10 for a 210mm lens would be 1/10th of 210 or 21mm.

I started checking "knowns" on lenses that I knew were in their manufacturers original configuration and found that it "just ain't so." I did find that a very general rule of thumb that would probably get you into the ballpark is Focal Length minus 25% and then do the inverses. For example the inverse relation works well on a 120mm Symmar if I reduce that 120 by 25% to 90 and then calculate 1/5.6, 1/8, 1/11 of the 90, not 120.

I surmise that light transmission, number of air surfaces, modern coatings vss. single coatings vss. no coatings probably all play into the formula.

Someone who knows, help me out here. If I mount a barrell lense in a shutter is it impossible for me to correctly calculate an aperture scale myself? Is it black magic that only the original designers can do correctly? How does Wisner do it with say a casket of plasmats that are mix and match in a hundred different ways?

-- Jim Galli (jimgalli@sierra.net), January 31, 2001

Hi Jim, ideally, wouldn't it be done with a light meter. I mean, I know you would have to have a reference point, like calculating f16, but from there on, isn't one stop double or 1/2 the amount of light as the previous stop. I wonder if the stops on your 120mm are so spaced to half or double the light? Best, David

-- david clark (doclark@yorku.ca), January 31, 2001.

Your thinking is correct, except: Where do you measure the aperture? If you measure the diaphram, remember that there is some lens between the diaphram and the film, which magnifies the diaphram somewhat. The f number refers to the cone of light that the film receives, thus the size of the diaphram as it appears from the film. Another way is to put a scale accross the front of the lens, and view it from a distance with a telescope, with the lens elements in place. You need the distance to avoid parallax. For most lenses, once you have one diaphram diameter, you can measure the diaphram itself for the other stops: it should change diameter by square root of 2 for each stop change. If you include the transmission, using a light meter, then you get what is termed T stops. I think the measurements are difficult to do.

-- Jim Galvin (jegalvin@lbl.gov), January 31, 2001.

The physical size of the iris diaphram in the centre of a complex lens will almost never match the theoretical aperture diameter of F/f-number. The reason for this, is that the amount of light gathered by the lens is governed by its entry pupil diameter, which nearly always differs from the iris diameter.

You can see this entry pupil diameter, very approximately, by looking into the front of the lens from arms length, and measuring the size of the apparent 'hole' that you can see. This apparent diameter will be a much closer fit to the theoretical aperture.

So, the entry pupil size is governed not only by the diameter of the iris diaphragm, but also by the optical magnification or reduction of the front component group.

For a normal or telephoto lens, the front lens group will be positive, and will have the effect of magnifying the diaphram diameter; therefore the iris will be smaller than the theoretical aperture.

Conversely, for a wide-angle or retrofocus lens with a negative front group, the physical iris size will be larger than the calculated aperture.I hope we can forget the relationship between the exit pupil, back focus, and cos^4 fall-off this time round!

With the ready availability of little laser pointers, there's a simple way to measure the aperture of an LF lens quite accurately.

Set the lens to infinity focus on the camera, and illuminate the centre of the ground glass screen with a laser spot. A parallel beam of light should now be shining from the lens.

Project this beam onto a sheet of white paper, and directly measure, or mark off, its diameter. This is the true aperture diameter of the lens.

Provided that infinity focus is properly set, this method is accurate to within the spot size of the laser, and I've used it to measure the aperture diameter of a variety of small format lenses in the past. (To settle an argument about 'T' stops actually, but that's another story.)

-- Pete Andrews (p.l.andrews@bham.ac.uk), January 31, 2001.

Sorry about echoing some of what Jim Galvin said, I was composing a response while Jim posted.

-- Pete Andrews (p.l.andrews@bham.ac.uk), January 31, 2001.

Thanks very much for some great help! Can't wait to try the laser pointer trick. And yes, for my purposes, once I found the one known the other points would be set off of that, either with a light meter which I haven't tried yet or just measuring with a caliper. Jim

-- Jim Galli (jimgalli@sierra.net), January 31, 2001.

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