• Welcome to BirdForum, the internet's largest birding community with thousands of members from all over the world. The forums are dedicated to wild birds, birding, binoculars and equipment and all that goes with it.

    Please register for an account to take part in the discussions in the forum, post your pictures in the gallery and more.
ZEISS DTI thermal imaging cameras. For more discoveries at night, and during the day.

Straight or Angled (1 Viewer)

Kimmo,

I was surprised to find that Zeiss use a Schmidt-Pechan prism in the straight Diascopes; certainly a very odd choice since it's more expensive and has lower light transmission than a porro.

Henry
 
Henry,

I stand corrected. Now that I think about it, I may have read about the straight-through Zeiss Schmidt-Pechan, but since I rarely if ever encounter straight-through scopes I must have filed that information into the "no retrieval needed" part of my brain.

I wonder if Zeiss just wanted to have the eyepiece lined with the objective better, to avoid the "step" in most porro scopes.

Kimmo
 
henry link said:
I was surprised to find that Zeiss use a Schmidt-Pechan prism in the straight Diascopes; certainly a very odd choice since it's more expensive and has lower light transmission than a porro.

Henry,

You are quite right. Before purchasing my straight Diascope last year I contacted Zeiss to ask if the prisms had a silver or dielectric coating. They confirmed that it was silver and that the only Zeiss sports optics using dielectrically coated Schmidt-Pechan prisms were the 32mm Victory FL binoculars.
The price and performance were, however, convincing and, even with a potential 3% light loss I should still be getting more light throughput than the 60mm and 62mm competition.

John
 
henry link said:
Kimmo,

very odd choice since it's more expensive and has lower light transmission than a porro.

Henry

Henry.

Your statement in general is true, however, could it be possible that advances in the coatings, especially the mirror surface of the SP, shows that some SP prism's do give a higher transmission figure than some Porro's.

This is not to say that all SP prisms achieve the same transmission, as it is right to say that not all Porro's achieve the same transmission figures.
 
John,

Thanks for that information. Since dielectric coating is the only one worth bragging about now, I suppose it's safe to assume that when mirror coating material is not specified it is probably silver at best. I think it is roughly accurate to say that a 1% change in light transmission is about the equivalent of a 0.5% change in aperture, so a 3% drop in light transmission would be like subtracting about 1.3mm from the aperture of an 85mm scope, not much.


Mak,

If we assume equal quality glass and anti-reflection coating I don't see how a Schmidt-Pechan prism (just the prism, not the entire scope) could ever be made to have transmission quite as high as a Porro or an Abbe-Koenig, even with dielectric coating. The difference might become undetectable to the eye (as it appears to me to be in the 32mm FL), but not unmeasureable.

To really split hairs I would expect a Porro II or a cemented Porro I with only two glass to air surfaces to have the very highest possible transmission. Slightly below those would be air spaced Porro or AK. SP with dielectric coating would be a little lower and SP with silver coating would be lower still, perhaps 5-7% lower than Porro or AK.

Henry
 
Henry,

And Schmidt 45 degree prisms such as those in Swarovski ATS and Zeiss Diascope angled models, as well as the Nikon Fieldscope's modified Schmidt all have a single chunk of glass, two glass-to-air surfaces and no need for mirror-coated surfaces, so they should be in the same class with a Porro II or a cemented Porro I?

Kimmo
 
Warning! This thread is more than 18 years ago old.
It's likely that no further discussion is required, in which case we recommend starting a new thread. If however you feel your response is required you can still do so.

Users who are viewing this thread

Back
Top