This is a potentially a pointless endeavour, but I'm looking into trying to assemble a mirrorless camera using a CCD sensor mounted to the back of an M42 lens using a series of converters.

Primarily, I'm doing this as a learning exercise in developing image capture software and to understand optics a bit more, however as an affectation, I would like to install the components in an old SLR body.

Due to the size of the sensor unit attached to the lens, it will extend back into the body where the mirror currently sits, so if I were to use the existing viewfinder, it would have to be converted to digital.

As it stands, my plan is to place a small LCD display where the focussing screen currently sits, and route the output from the CCD sensor to that LCD, effectively turning the viewfinder into a digital unit with a prism in it.

There will be no autofocus on the CCD, so all focussing will have to be done via the lens itself.

When the shutter release button is pressed, the LCD display will be turned off, and the CCD signal will be saved to a file, after which time, the LCD will be switched back on, essentially making a digital viewfinder.

Retaining the prism would allow me to use a larger (and cheaper) lcd than in commercial digital viewfinders, but at the expense of weight and size of the viewfinder assembly.

What I would like to find out is whether the split ring on the focussing screen will still work to aid focussing with an LCD attached to it directly. My gut feeling it that it wont, as the split ring will effectively be focussing on the LCD surface, rather than a real world view - is this assumption correct?

If this is indeed a dead end, if I were to remove the focussing screen completely and replace it with an LCD, assuming that I orient the image on the LCD correctly, would it be possible to view it through the viewfinder, or does the angle of the mirror when the shutter is closed play a part in the way the image is presented in the eyepiece?

Any help in this would be appreciated. I'm very new to photography, but have a background in software and electronics so the technical side should be feasible if it transpires that the optics will do the trick.

Many thanks

  • \$\begingroup\$ DSLR stands for "digital single lens reflex". One of the properties of reflex cameras is that a mirror provides a live view through a lens (the photographic lens for 'single lens reflex' and a separate lens for 'double lens reflex'. By definition, a mirrorless design is not like a DSLR. \$\endgroup\$
    – user50888
    Jul 25, 2017 at 20:45
  • \$\begingroup\$ This is why I referred to it as a mirrorless DSLR style camera rather than a DLSR. I have changed this for clarity. \$\endgroup\$
    – Alex
    Jul 25, 2017 at 20:57

1 Answer 1


Seems like an arduous project: The viewing screen of the SLR is ground glass. This is liken to etched glass (roughened surface). Its job is to provide a toothed translucent surface that captures the aerial image produced by the camera lens. The mirror is just a device to bend the image forming rays upward so they are square with the viewing screen. The etched viewing screen will be a poor cover glass for your LCD screen. The split image viewfinder and likely some micro-prisms are a feature of the viewing screen. They present a split view and or a super blurred view of a snippet of the image projected by lens. The image superimposes or snaps sharp depending on the angle of the arriving image forming rays from the lens. There is no chance they will function close haul to an LCD viewing screen. Additionally above the ground glass viewing screen is a Fresnel lens. Its job is to mitigate a vignette that is likely, fashioned by the camera lens.

Above the viewing screen and Fresnel lens is a “roof” prism. This produces multiple reflections of the image grabbed by the viewing screen. This image is upside down and inverted left to right. The “roof” prism corrects this view. Next is a eyepiece lens. This is simply an achromatic eyepiece (magnifier) that allows the human eye to focus on a screen only a dozen millimeters away. Seems to me, converting an SLR for your task, complicates the job. However you can do it, just strip away the reflex mirror and the view screen. Lots of extermination will be required.

  • \$\begingroup\$ Thank you, that is exactly the information I needed to find out. Essentially, building it the way I was thinking of would render all the lovely optical work useless, and would be a waste of a camera which would be better used for its purpose. I had pondered putting a CCD in place of the film, but the cost of the sensor and the time needed to write the driver software renders the whole thing too involved. I'm looking at fitting a simple digital conversion to an old rangefinder body at the moment, mainly just as a project for amusement. \$\endgroup\$
    – Alex
    Aug 1, 2017 at 15:20

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