pedro_casales

Here is an article on a way to reduce stray light in catadioptrics and reflecting telescopes, which improves contrast markedly. https://retrofocal.com/articles/improving-mirror-lens-image-quality/ The synopsis is that mirror lenses and reflecting telescopes show the sensor a reflection of itself, which hazes the image, but the spot that forms that reflection doesn’t participate in making the image, so it can be blackened.

Update: some newer copies of the Petri 7S have a revised shutter design with, among other things, a normal screw holding the lever onto the cocking axle instead of a pinned design. You still have to remove this to split the halves of the shutter and remove the shutter or aperture blades for cleaning, but no special tools are necessary. I haven't serviced enough copies to be sure, but I suspect that copies without an M-X sync switch on the side of the lens barrel are all of this design.

Here's an image of the tool and the direction the pin should be removed.

I know I'm responding to a bit of a stale thread, but I just wanted to confirm that the SPT service notes article is correct, there is NO WAY to split the halves of this shutter and gain access to the blades without pressing that pin out with a special tool. It's a frustrating design but it's what it is. I made my tool by buying a BeadSmith brand parallel pliers with one flat and one thin round jaw. They come in all kinds of combinations of jaw shapes. I used a Dremel with a cutting blade to cut a slot in the wide flat jaw, and cut down the pin-shaped jaw slightly (it was about 3mm at its tip, so it didn't take a lot of work). I could have modified a different pair of pliers but this felt like a head-start. For what it's worth, I have seen at least one other place in a book or article where a pliers like this will come in handy. I can't remember where I saw that, but I know there will be other places pliers like this are the right tool for the job. While you're buying BeadSmith parallel pliers, consider looking for a pair that has a set of thick blocks bolted sideways onto the nose, with an arc cut through them so the gripping surface is a long smooth curve. I have one, and it's been invaluable for straightening and bending things. Think dented filter threads, but also on the Petri 7S, the cocking ring has a tongue that mates with a cocking rod in the base of the camera, and the tongue is often distorted so the shutter can't be cocked fully; straightening that tongue is possible with these arc pliers. I appreciate the sentiment of fixing up these cameras. I too have a lot of them. Not one is without issues, and from inspecting and disassembling, these weren't all that robustly built and are prone to a variety of issues from damage and wear/age. Still, they feel worthwhile to me.

This is an old thread but it appears prominently in Google, so I think it is worth adding some info on how to set focus. First of all, advice to anyone disassembling the lens of cameras such as this one and the Olympus Trip 35, expanding upon some messages earlier in this thread: the ring around the inner portion of the lens, which rotates as you rotate the focus grip, is two-part. The shiny outer part with lens specifications engraved in it is a collar with a tab that you can't see protruding backwards to engage with the focus ring grip, and the inner blackened cone is a separate part; they are fastened together via three tiny set screws in the outside rim of the collar, and their position relative to each other determines focus calibration. Before you loosen these screws, make a way to ensure you can exactly realign the two parts when reassembling. I like to use a dental tool to make a very very small scratch in the paint of the black cone, aligned with the "f=" marking. The only way this works, by the way, is if you also mark the lens's orientation where the helicoid threads disengage when you unscrew it, because there are three threads and you have to get the right ones back together or you'll be at the wrong distance when rotated to the right orientation. Now, as to calibrating focus when you have lost it... based on my measurements, when focused to infinity, the engraved collar is recessed 0.5mm deeper than the outermost filter threads. In other words, if you lay a straightedge across the filter threads and focus to infinity, there will be a 0.5mm gap to the front of that ring. I don't think this is a super accurate way to calibrate focus, but it'll be better than nothing. I did measure this with a high quality Vernier caliper for what it's worth. A more accurate way would be to hold the shutter and aperture wide open and use a collimator with a focus screen against the film gate. I'm not aware of how to lock the shutter and aperture open on this particular camera. There's probably a way, since they likely did that at the factory, but it's not documented AFAIK. In the absence of that, likely the most reliable way for most people to calibrate without special equipment will be to take a series of shots in dim light to open the aperture wide for shallower depth of field, noting the focus distance to which the focus ring is set and including a big sheet of paper in the photo with that information written large in thick marker. You can then develop the film and examine where focus truly lies. For example if you can see in the negative that focus is at 10m but the sign in the photo says you had the focus ring rotated to 5m, now you can mark the rotation positions on the lens and shift the alignment between the engraved collar and the inner black cone by that much. Taking a series of shots at different distances will increase the chance that you have at least one negative with a clear point of focus to reference.