Dimish prism finder on Mamiya TLR

[fparnold]

Quick question: I added a prism to my TLR a while back, and while it's easier to focus quickly than with the WLF, it seems to be significantly dimmer. Is this due to the magnification, or is there something in that prism that should be checked out? I realize that a f3.5 lens on a cloudy day isn't the brightest, but the difference is pretty noticable.

I suppose the other question is whether there's any solution to safely removing haze from the ground-glass (or fresneled plastic?) on a C220.

Thanks for your time.

 

[mgb74]

Was it the prism finder or the "porrofinder"? See: Dead Link Removed

The porrofinder is inherently dimmer than the prism.

 

[grahamp]

The true prism is a little dimmer. The light has to pass through the focus screen, through the glass base of the prism, and bounce off at least two mirror surfaces to reach the eyepiece. The waist-level finder or chimney finder avoids the extra surfaces that lose light. The apparent magnification is smaller, too.

Most of the TLR screens are plastic, so the best thing to clean them with is distilled water, with maybe a _tiny_ drop of washing-up liquid to a litre or so of water. Solvents are not advised. De-mount a C220 screen at your own risk - there are usually some leveling washers underneath. You can clean the base of the screen through the lens throat, but turn the camera upside down to protect the mirror and use a barely damp Q-tip _after_ a blower. These screens are soft.

 

[jadphoto]

The Mamiya prism is, as stated before, a little bit dimmer than the wlf. The porrofinder, on the other hand, uses mirrors and will be a lot dimmer.

If the eyepiece is in the middle you have a prism. If the eyepiece is off-set you have a porrofinder.

Joe D.

 

[fparnold]

I've played with optics long enough, that by appearance and weight, this is definitely a prism. Probably some combination of viewing screen, 3.5 lens, and light-loss through the prism. Any comments on replacement viewing screens for a 220 which are still easy to focus? It's amazing how much quicker it is to focus now, so on the whole it's an improvement.

Is there any chance it needs resilvering, or did Mamiya sensible just use evaporated aluminum like the telescope-types?