The Pentax MX prism

[David Lyga]

Through the years I have oftentimes changed prisms on SLRs due to dirt or fungus that I could not remove with solvents. One such SLR that I attempted a while ago was the Pentax MX. I got the top cover off but, to my rather amazement, that prism's bottom was slightly curved!

I had never before seen an SLR prism whose bottom was curved. Thus, I could not replace that prism. My questions are these: Why is it curved? Is there an optical advantage? If so, why don't far more expensive SLRs have such a curve? - David Lyga

 

[flavio81]

Through the years I have oftentimes changed prisms on SLRs due to dirt or fungus that I could not remove with solvents. One such SLR that I attempted a while ago was the Pentax MX. I got the top cover off but, to my rather amazement, that prism's bottom was slightly curved!

I had never before seen an SLR prism whose bottom was curved. Thus, I could not replace that prism. My questions are these: Why is it curved? Is there an optical advantage? If so, why don't far more expensive SLRs have such a curve? - David Lyga

Hi David,

Cameras often have a curved lens below the prism. The MX integrates it into the prism to keep it compact.

 

[AgX]

Prism (or porro) SLRs cameras need a collective lens between groundglass and prism to get most of the light transmitted through the groundglass into the eyepiece. And I do not know of model which does not have one, but am willing to learn.
Such collective lens can be arranged in different ways. One way is incorporating it into the prism.

 

[flavio81]

Prism (or porro) SLRs cameras need a collective lens between groundglass and prism to get most of the light transmitted through the groundglass into the eyepiece. And I do not know of model which does not have one, but am willing to learn.
Such collective lens can be arranged in different ways. One way is incorporating it into the prism.

I think some don't have a lens but use the fresnel printed on the focusing screen. I think the Nikon EM does it that way, for example. I took apart one, and there's no glass lens. And the prism has a flat seat.

I might be wrong of course...

 

[AgX]

The fresnel lens is just a flat version of the field-lens (as a lens of that function is called). Often a fresnel field-lens, as integral part of the ground-screen, was added by a classic field lens,

 

[Ron (Netherlands)]

Perhaps a prism of a cheap donor MV or ME could be used?

 

[flavio81]

Perhaps a prism of a cheap donor MV or ME could be used?

I've taken apart an MG and an ME and the focusing screen dimensions are different. I'd suspect the prism sizes are different too. The MG and MV should have identical screen and prism sizes.

Now, I'm not sure the MX would have a big parts interchangeabilty with the ME. They were introduced simultaneously but they're very different cameras. Different body castings, body dimensions, different shutter (vertical vs horizontal), etc.

 

[David Lyga]

Perhaps a prism of a cheap donor MV or ME could be used?

Those have FLAT prisms. I had already tried. - David Lyga

 

[flavio81]

Those have FLAT prisms. I had already tried. - David Lyga

Great info David!!

 

[Ron (Netherlands)]

Those have FLAT prisms. I had already tried. - David Lyga

Pity, thanks for the info! (have currently an OM1 with deteriorated prism.....)

 

[David Lyga]

Pity, thanks for the info! (have currently an OM1 with deteriorated prism.....)

Well, the OM-1 should be flat if I remember correctly, Thus, easier to find a replacement. There were not many SLRs which had prisms which were not flat. However, even flat prisms do vary in overall size. Still, I have used slightly different sizes and have made do. It has worked. I value pragmatism over "exact". As long as I can get precise focus (depends largely upon precise mirror angle) I am content. - David Lyga

 

[Ron (Netherlands)]

I value pragmatism over "exact". As long as I can get precise focus (depends largely upon precise mirror angle) I am content. - David Lyga

Fully agree - since I have many pentaxes laying around, I'll try a Pentax flat on the OM-1