SLR Mirror Size and Rambles

Hi,

I received a Spotmatic F with a detached and broken mirror. Looking into the screwmount opening, the right lower corner is gone - about 20% of the mirror's surface. (The little piece is missing - the big piece was rattling around the camera's interior loose. Seller obviously knew if this situation - anyway that's a different story).

So, I decided to cement the remaining piece roughly in place - used paper cement on a cotton swab.

When dry, and after a few tentative dry firings, I put on a 28mm, f3.5 SMC Wide Angle lens and looked through the viewfinder expecting to see an 80% image. What I saw was 100%. I then removed the lens and looked again - still 100%.

Here's what I figure - ready for this all you scientists out there in Pentax land? The mirror, viewing surface and prism are all sized the same as the rectangular opening to the film plane.
But the mirror in the 45 degree viewing position is at a point where the image lines are at the narrow point of the X, and thus only the center portion of the mirror is necessary to project the image to the complete viewing screen. The mirror could be much smaller, but it is used to completely block any light from entering through the viewing lens while the exposure is made, and therefore must be the same size as the viewing screen.

I may be in the unique position to experiment with just how small the mirror can get by breaking it down further - or has this already been worked-out mathamatically?

I plan to run a roll through - just for fun - to see just how bad - good this poor specimen performs. I'll have to block the viewer eyepiece during exposure, as the mirror doesn't seal properly when "up".

One more observation - question. With Super Multi Coated and SMC lenses with the open metering hardware, there is a little tiny pin which, when you hold the lens in your hands, allows you to switch between auto and manual if depressed. When the lens is screwd on to the "F", I assume the little pin is depressed, but the lens refuses to leave Auto. If screwed on while in Manual it willing goes to Auto but locks in.
Is this normal? (I can see why Auto is preferable - just don't understand why the little pin isn't fully depressed - like its owner).

Oh, Wait, I figured it out! F's have a little channel in the mounting ring and this little pin isn't depressed at all as it would be on non open-metering models.
How very clever!

George, if I have this much trouble with hardware, imagine me with digital software!

Have a grand day, All!

Hi Mac,
The X as you put it will also depend on aperture and focal length. The full mirror may only be needed for a 1.4 Tak. Most light diagrams are shown as an X for simplicity but the real picture is a bit more complicated.

If the only problem with the F is the mirror, I wouldn't mess too much in case you damage other parts. I should have a spare mirror or 2 at home. They are mainly from other Spots but they should be the same size. I will need to check and get back to you.

Kim

Kim,

I just ran over and put on my 50mm Super Tak 1.4 (which is happily baking in a south-facing window - almost 100% clear - but you never know when enough is enough do I?). Still don't see any dark corners wide open, so I may be getting away with something here. Of course, my pasted-on fragment isn't resting properly under the clips and therefore only developed pictures will indicate how far off my focusing is at max openings.

If you do find a mirror you can part with, please email me at [email protected] and we'll work out details of the transaction. Thanks, Kim to the rescue again!

I just ran a light meter test with a grey card and my hand-held Sekonic spot meter, and I am blown away with the range and accuracy of the F's TTL meter. (Combinations from 1/1000sec to 1sec, and f1.4 to f16 all within 1/4 stop of the hand-held). I'm presently using a Wein 625 cell - haven't started modifying Silver Oxides yet.

The only other complete fatality on this poor old wreck is the self timer which seems to have totally jumped the track. (As I mentioned earlier, I'm sure wee children played with it - and there's nothing like the 10 second whir/buzz of fine clockwork to amuze little ears). Ah well, self portraits are not my strong suit, the only thing I'll miss is mirror lock-up on longer exposures. Having two other fully functioning Spots will take care of those situations.

I tested the hot shoe and X terminal - both work fine. Had a moment of doubt when pushing my flash unit into the hot shoe - there isn't a black plastic bumper along the front of the shoe - allowing the flash's foot to slide too far forward (beyond proper contact). The only thing that prevents the flash foot from sliding right through is the prism housing. Is this normal on an F? When looking at the black triangular shim upon which the shoe is mounted, it seems to be much thinner than earlier models, precluding the integration of the aformentioned front stopper. (Howszat fer a buncha big words? Hooweee now mah haid hurts!

Cheers

Gluing on a mirror fragment is a no-no. I think the only way - if one cannot use the proper clips to hold the mirror to the door, would be a spray adhesive which leaves virtually no build-up.

It is amazing how out-of-focus the F became with just a smushed dab of glue stick goop twixt mirror fragment and metal door! With a 55mm, infinity looked like 50 feet.

Kim C. if you do have a spare full mirror you could part with, I'd love to buy it from you. I think I've figured out how the spring clip works, and I've cleaned off all the goop.

Thank's, [email protected]

Cheers