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How to Measure the Focal Length of a Lens
As some of you know I have been working on a means to construct my own lens housings with my 3D Printer, but before I get into all that, I thought I would show you how I have been measuring the focal lengths of the glass elements I intend to use and I thought I would take a picture showing the layout.
Essentially, the focal point is the distance from a lens to the media where your projected image is in focus.
Probably the easiest ways to measure focal length is to use a bright light in front of a pure white background, and you simply move the lens from the white background towards the light until the projected image is in focus as you can see on the paper here.
Using my calipers, I estimated this particular lens was about 24mm in focal length, with the diameter of lens being 33mm. The candle looked pretty good, so my reasoning was this should work for a good wide aperture test lens, with a potential f stop well below f1.0, reserving judgement on this until I actually got a good result.
One major problem here, the distance from the sensor to the lens mount on Canon cameras is about 45mm, there would be no way to get a lens with a 24mm focal length inside with the mirror locked up, unless there was no mirror . Which thankfully I do have a Canon Mirrorless M Camera. The distance between the sensor on the M and the mount is closer to 17mm.
This should trigger a few questions:
If you can't move a lens closer than 45mm in a Canon DSLR, how in the world do we have all these wide angle lenses, even as short as 8mm? The short answer is that most professional camera lenses are made of many, many lens elements which do things such as correct aberration & distortion, but also redirect the light rays themselves. The shape and angle of curvature of the lenses are such that they can "project" through that lens mount 45mm away from the sensor to render and equivalent focal length on the sensor.
Hopefully if I get a chance Ill diagram how I understand this to work and show you different kinds of lenses that do different things.
I spent pretty much all day yesterday on designing & making a 3D printed lens housing for the Canon M, and I was able to figure it out, lens mounted and all. I've done some test shots, and Ill be honest, there are some problems, which Ill show you soon, but it also was a great lesson about optics themselves.
The strange thing is…there is not a lot of good information out there about how to construct a quality photographic lens from scratch, so I have started just breaking lenses apart and studying the components. Its quite frustrating, but Im starting to realize now why there are limits on the functionalities of lenses and what I should be able to make on my own