Robert, Excellent, useful info. Thanks very much.
Gene Robert Mueller wrote: > > The method you first describe should be fine but you should be sure the > lens is focused for infinity: in that case the light exiting from the lens > is a parallel beam and the distance from lens to screen is unimportant. > > A pretty much equivalent method is to place a ruler in front of the lens > (ideally a transparent one) and measure the apparent diameter of the > opening when looking from as far away as possible while still being able to > read the ruler markings (you could use binoculars, but that is a bit of > overkill.) > > With the light meter you do have the problem of establishing a > calibration. If you know the F# at the largest opening you can just scale > the values for the other openings according to the light meter > readings. One way to get a reference is to use a calibrated aperture, like > a hole of known diameter and placed at a known distance from the > meter. This system has an F# equal to the usual value, (focal length) / > (diameter). I would tend to use a small source for this task, a bright > light bulb, far, far away, for example. Then place your lens to be > calibrated in place of the hole but with the spacing from lens to meter or > screen equal to what you need to focus objects at infinity (You want to > avoid that bellows correction otherwise required when doing close-ups.) > > An easy way is to select a shutter time on the meter so the F# of your > reference hole is what is shown by the meter for the selected speed. Then > just read off the F# of your lens. > > You must take precautions so only light coming through the lens or standard > hole illuminates the meter! > > You are correct about the F# for an iris in front of the lens. This is > hardly an ideal arrangement, but if there is no alternative, go ahead and > do it! > Bob > At 10:10 14.12.01 -0800, you wrote: > >Hello Cameramakers annd Richard Knoppow, > > > >I have a 2 questions regarding establishing F-stops on a lens. First, > >I'm looking for suggestions on how to determine the stop settings on a > >lens of known focal length, having an iris with no markings. This came > >up a while back and Richard, I think I remember what you said which is > >scary for me, but here goes. I think I recall that the "compression" > >effect of having the iris between the groups could be worked out by > >putting a point source well behind the lens, and a groundglass as close > >as possible to the front of the lens, then measuring the dimeter of the > >illuminated circle on the groundglass. Did I get that right? In other > >words, on a 160mm lens, a circle diameter of 10mm would indicate the f16 > >setting and so on. I also remember less well some suggestions using > >light meters. I would appreciate comments on this problem. > > > >The second problem is a lens that has no iris at all, and no practical > >way to install one. I have a big iris that will fit on either the back > >or the front of the lens. I am pretty certain installing the iris this > >way will cost me a lot of coverage, but on this lens I may have it to > >spare. Would the stops in that case be just the aperture divided by the > >focal length? > > > >Gene Johnson > >_______________________________________________ > >Cameramakers mailing list > >[EMAIL PROTECTED] > >http://rmp.opusis.com/mailman/listinfo/cameramakers > > _______________________________________________ > Cameramakers mailing list > [EMAIL PROTECTED] > http://rmp.opusis.com/mailman/listinfo/cameramakers _______________________________________________ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
