I have a Six-20 Brownie Special for which I’m working on determining its focal length and f-stop. I’ve measured the lens and figured out its inner and outer radius of curvature, for which I plan to use in the lensmaker’s equation on this page. I also have the refractive index of air, but I’m at a loss as to what to use for the lens’ refractive index. I don’t really know what kind of glass it’s made from. This camera is from 1938 - 1942. What is the most likely type of glass for an inexpensive camera’s lens from that time?
I think it would be easier to measure the focal length and use that info to calculate the index of refraction. Just set up the lens so that it forms an image, and measure the distance from object to lens, and lens to image.
I agree that would be easier, but I don’t have any optical equipment other than cameras.
>I don’t have any optical equipment other than cameras
Use the image scale the lens creates. Rays that pass through the center of the lens, whether on or off axis, go straight through it. When an image forms, a distant building appears with a physical size in the image (you could take a photo and measure the negative for instance). Triangulate out what the path of the central rays must do to create an image of the size you got - the central rays would have to pass through a pinhole some specific distance from the negative to give you your image - that specific distance is your focal length.
Note - you should use a distant object to fix the small error that results if the lens is retrofocus (in which case it has extra physical length that doesn’t appear as optical length) or telephoto (I mean in the technical optical sense, which is that it has missing physical length that appears in the optical length, rather than that it just has a long focal length). You want the distant object to be much further than the dimensions of the lens to make this error neglible.
Youtr typical garden-variety glass has an index of about 1.5. The most common glass used for simple models and for beginning optics students is BK-7, with a refractive index (at sodium D) of 1.517. I know this because the beginning optics classes drill it into you incessantly. If you have a singlet lens on that camera, you ought to be able to use that 1,.5 figure.
Most cameras try to correct for chromatic aberration at least (and other aberrations generally) by having at least two elements, and generally more (which is why professional photographers’ lenses are so big and expensive). Usually you’ll have at least a doublet, with two elements, called crown and flint glasses for historic reasons that don’t really apply any more. The classic mate to BK7 is SF6, with a refractive index of about 6 and a very different dispersion, so you can make the overall focal length relatively insensitive to color across the visible spectrum. If your camera has a doublet lens, or anything more complicated, your Lensmaker’s Formula won’t work. Welcome to the world of optics.