View on GitHub

Quorten Blog 1

First blog for all Quorten's blog-like writings

So, you’re wondering about optical viewfinders for a Raspberry Pi camera? I’ve give you the final verdict on this. Start out with the cardboard viewfinder trick.

First of all, some discussion on camera viewfinders. What does Wikipedia mean by “reverse telescope” when discussing camera viewfinders? Basically, they mean a refracting telescope that uses a convex objective lens and a concave eyepiece lens so that you see the image rightside up, also known as a “Galilean telescope.” By contrast, the much more popular Keplerian telescope uses a convex lens for the eyepiece, so the image appears inverted, i.e. upside down and left side on the right.

The cheapest of camera viewfinders, “direct optical viewfinders,” are constructed exactly as an “inverse telescope.” The main problem with using an inverse telescope for a camera viewfinder is that you do not see a sharp line at the edge of the camera’s viewable surface. Rather, a blurred line is seen due to the edge that defines the image boundary to be out-of-focus.

By contrast, DSLR camera viewfinders use a more complex design to achieve a sharp edge at the image boundary in the viewfinder. The camera optics project the image onto a matte surface. This image is upside down. Additional optics adjust the convergence of the light so that the user can focus their eyes on the image that is physically right in front of their eyes. These optics also flip the image upside down again so that it appears rightside up. With the image boundary in focus, in a DSLR camera, it is also very easy to insert data displays that the user can see just beyond the edges of the image boundary.

Of course, all this being said, we’re not interested in high-end optical viewfinders for a Raspberry Pi camera if our goal is to be saving as much money as possible. So, back to the cardboard viewfinder concept. What do I mean by “cardboard viewfinder”?

Basically, review the concept of a cheap optical viewfinder that uses telescope optics. The main caveat that I’ve pointed out with this system is that the edges of the viewing boundary appear blurred to the use. The cardboard viewfinder also shares this limitation. Now you know what I’m talking about: yes, you just cut out a frame with cardboard and you look through that to determine the contents that are visible within the image boundary. With this concept in hand, the remaining problem that needs to be addressed is the field of view. With a simple window, looking through it at different distances will give you different fields of view visible. How do you design the system so that you know you are getting the “right” field of view?

Easy, you just use two cardboard windows rather than just one. The only way you can be looking through the cardboard viewfinder at the right field of view is then if both windows visually appear to be the same size as each other. You move your eye close enough to just start seeing the bounds of the far window, then you move away until the bounds are no longer visible.

The main disadvantage with cardboard viewfinders is that they must be constructed to be larger than telescope optics viewfinders, due to the exclusion of optics. But, that being said, there’s another problem looming. How do you construct a correct cardboard viewfinder for ultra-wide optics? The problem we’re facing is that the far window is going to need to be huge compared to the near window. But, we have an modification that works somewhat well. Use a viewfinder for a narrow field of view simply for the sake of making sure the user places their eye at the correct distance. Then, around the either the near side, place a larger cardboard frame that indicates the wide-angle viewfinder boundaries. Now you have a reasonably compact wide-angle cardboard viewfinder.

Finally, another option that works well if you’re willing to use but a single plastic lens. Construct a through-the-lens viewfinder that projects an upside-down image on wax paper, simply by placing the lens in front of the wax paper and adjusting the image bounds over there.


Finally, if you are willing to cut open old cameras to harvest the viewfinder parts, that is also an option to get a viewfinder on the cheap. My recommendation in that case is to use the viewfinder from an old disposable camera, but if you want something higher-end, you can try taking the viewfinders out of other kinds of old cameras.

20181117/https://en.wikipedia.org/wiki/Viewfinder
20181117/https://en.wikipedia.org/wiki/Refracting_telescope
20181117/DuckDuckGo raspberry pi disposable camera viewfinder
20181117/DuckDuckGo disposable camera viewfinder
20181117/https://www.lomography.com/magazine/139546-viewfinder-from-a-disposable-film-camera
20181117/https://www.pentaxforums.com/forums/136-pentax-q/171113-make-your-own-optical-viewfinder.html
20181117/https://www.adorama.com/ipxqvf.html?utm_source=rflaid65093
20181117/https://www.35mmc.com/02/08/2016/diy-35mm-mini-viewfinder/