[DreTAX]

No. The light entering the prism is not perpendicular to the surface. The light entering from the screen is scattered and a higher refractive index will make more use of the light scattered from the screen as more of it will be refracted sufficiently to be reflected by the internal surfaces in the prism. The ocular lenses are focused on the screen, receiving that light. The higher refractive index also makes the optical path length from the screen to the eye higher en then you need to ajust that part as well.

Think of the differences between the cheap pentaprisms that made small and dark finders, compared with the glass prisms of more expensive models.

Cheap pentamirror vs pentaprism

[DagT]

Cheap pentamirror vs pentaprism

Thanks. Now corrected.

[kwb]

A new prism made form different materials is actually quite a challenge. The change in refractive index of the glass means the whole shape of the prism had to change too. Directing light is never easy when you have to pass it through a substance.

Hi gaweidert, the shape and the size of a pentaprism is not dictated by the refractive index.

You don't want to change relative angles between surfaces of a roof pentaprism regardless of refractive index, because these define the function of the prism. Ridge of the roof is 22.5 degrees tilted relative to the input surface, roof panels are orthogonal to each other, internal bisector of the roof panels is orthogonal to the input as well as the third reflection surface, the third reflection surface is tilted 45 degrees relative to the roof ridge, and the output surface is orthogonal to the input surface.
You can make the overall size bigger or smaller, or make some surfaces closer to or farther away from other surfaces. This is bound by physical constraints like the size of the focusing screen (which is ultimately determined by the sensor size) and the position of the ocular lens (which is ultimately determined by the position of human eye), and by practical considerations like the size of the prism as well as the ocular lens system.

You can shape a roof pentaprism exactly like the one in K-3 using higher refractive index material, put that in K-3, and the new pentaprism still works in that it relays the light from the focusing screen to your eye without problem (actually better than the old one in a sense as I'll explain in my reply to ogl later). Of course the focusing screen "looks closer" viewed from the ocular lens due to larger refractive index, just like fish looks closer in water than it physically is. Existing diopter adjustment might not be enough to compensate for that, and you'll want to to change the ocular lens parameters. But the shape of the prism doesn't have to change just because of different refractive index.

BTW, roof pentaprisms are reflective prisms (reflective prism - Wikipedia), not dispersive ones (dispersive prism - Wikipedia).
By design, exit angle of light on the output surface is ALWAYS the same as the angle of incidence on the input surface regardless of color for reflective prisms. Among other things, this means that refraction effect of reflective prisms is equivalent of that of a glass plate with parallel surfaces (while that of dispersive prisms is equivalent of a wedge-shaped glass plate). Unlike dispersive prisms, reflective prisms don't separate co-propagating rays of light into different directions depending on color.

That's not to say that there's no dispersion ( Dispersion (optics) - Wikipedia ) effect in reflective prisms, but since that does NOT cause angular difference depending on colors, it's not that big of a deal for focusing systems like viewfinders, just like using an ND filter or UV filter, which are glass plates, won't give you color fringes for you camera, or looking through a glass window won't give you a rainbow-colored world for you to enjoy. The effect is there but is to small to matter.

[Parallax]

^^^^^^^^^^^^^ That ^^^^^^^^^^^^^^^^^^^^ was a great explanation.
Thank you.

[gaweidert]

Hi gaweidert, the shape and the size of a pentaprism is not dictated by the refractive index.

You don't want to change relative angles between surfaces of a roof pentaprism regardless of refractive index, because these define the function of the prism. Ridge of the roof is 22.5 degrees tilted relative to the input surface, roof panels are orthogonal to each other, internal bisector of the roof panels is orthogonal to the input as well as the third reflection surface, the third reflection surface is tilted 45 degrees relative to the roof ridge, and the output surface is orthogonal to the input surface.
You can make the overall size bigger or smaller, or make some surfaces closer to or farther away from other surfaces. This is bound by physical constraints like the size of the focusing screen (which is ultimately determined by the sensor size) and the position of the ocular lens (which is ultimately determined by the position of human eye), and by practical considerations like the size of the prism as well as the ocular lens system.

You can shape a roof pentaprism exactly like the one in K-3 using higher refractive index material, put that in K-3, and the new pentaprism still works in that it relays the light from the focusing screen to your eye without problem (actually better than the old one in a sense as I'll explain in my reply to ogl later). Of course the focusing screen "looks closer" viewed from the ocular lens due to larger refractive index, just like fish looks closer in water than it physically is. Existing diopter adjustment might not be enough to compensate for that, and you'll want to to change the ocular lens parameters. But the shape of the prism doesn't have to change just because of different refractive index.

BTW, roof pentaprisms are reflective prisms (reflective prism - Wikipedia), not dispersive ones (dispersive prism - Wikipedia).
By design, exit angle of light on the output surface is ALWAYS the same as the angle of incidence on the input surface regardless of color for reflective prisms. Among other things, this means that refraction effect of reflective prisms is equivalent of that of a glass plate with parallel surfaces (while that of dispersive prisms is equivalent of a wedge-shaped glass plate). Unlike dispersive prisms, reflective prisms don't separate co-propagating rays of light into different directions depending on color.

That's not to say that there's no dispersion ( Dispersion (optics) - Wikipedia ) effect in reflective prisms, but since that does NOT cause angular difference depending on colors, it's not that big of a deal for focusing systems like viewfinders, just like using an ND filter or UV filter, which are glass plates, won't give you color fringes for you camera, or looking through a glass window won't give you a rainbow-colored world for you to enjoy. The effect is there but is to small to matter.

Not what I learned in college. If you change the material of the glass the prism is made of there may be a slight change in the refractive index of the glass. This would mean some slight redesign may be necessary. Fortunately today all those calculations are done by a computer and not teams of people laboriously calculation the design by hand. A very tedious and time consuming activity. Within the glass there is no refractive index coming into play. It is only when moving from one medium, say air, into the prism that the beam path changes a bit. This is the refractive index I am referring to.


A prism is a very efficient means of changing the path of a beam of light. In the olden days, some less expensive SLRs used penta mirrors instead of prisms. These were lighter and less expensive, but also darker than a prism.

[kwb]

I don't understand how new prism could make OVF 10% brighter...

Good question. There could be two things, I'll start with the one that probably has a bigger effect.

Reason 1.

Through a higher refractive index material, the focusing screen looks closer to the ocular lens. This is not just an optical illusion in that the higher the refractive index, the larger solid angle (Solid angle - Wikipedia) of light originating from any given point on the focusing screen reaches the ocular lens.

Forget about the prism shape, it's just a hunk of glass with one input surface and one output surface, reflections don't have anything to do with refraction. Instead, let's think about a point on the focusing screen, an empty space, and an ocular lens (Figure A). The point P emits light in a wide angle, and only a fraction of that reaches the lens. In this figure, I used the apex angle of cone of light that reaches the lens instead of the solid angle to represent how wide the cone is.

Next, without changing the distance between the focusing screen and the lens, fill the space with a glass plate with parallel surfaces (Figure B). Because of refraction, the apex angle of cone of light at P that reaches the lens is larger than before. From the lens, the focusing screen appears to be at a new location P', and the apex angle viewed from the lens is the same with the apex angle at P. The larger the refractive index, the larger the apex angle, thus the larger fraction of light from P reaches the lens to be relayed to the eye (thus brighter).

Suppose that you have two glass plates of the exact same thickness, one made of typical glasses like BK7 (n=1.516) and the other made of Ohara S-LAH66 (n=1.7725) that is one of the high refractive index materials mentioned in the patent. How much more solid angle do you gain by using the latter? Let's calculate the ratio of the solid angle!


It turns out that you don't need to know the exact dimensions for the calculations. As far as most of the space is filled with the glass (i.e. distance between the focusing screen and the pentaprism as well as between the pentaprism and the ocular lens are much smaller than the apparent path length of light), and as far as the ocular lens diameter is smaller than the apparent path length of light, the ratio is pretty much the same.
And that ratio is,

• solidAngle(S-LAH66)/solidAngle(BK7) ~ 1.37

(for example if the entire space between the focusing screen and the lens is occupied by the glass, the ratio is ~1.369 when arctan(R/d)~5 degrees, 1.376 if arctan(R/d)~10 degrees).
That is, assuming that the reflection at the input and output surfaces are negligible in both S-LAH66 and BK7 because of good anti-reflection coating, 30-something percent more light reaches the ocular lens originating from P.

Note that this is an optimistic estimate and should be considered an upper limit. In reality, though the light is indeed emitted with a large angle from P, the light intensity is not distributed evenly to all directions (e.g. the light is stronger at the center of the cone) and therefore the ratio of the light energy (number of photons) is somewhat smaller than the ratio of the solid angle. There could be other gotchas. 37 percent could easily be 30 or even 20 percent. But this is NOT a few percent effect.

But the view is larger in the new VF, thus increase in the photon number is distributed over a larger area for your view. This will counteract to make your view dimmer. Using the square of the magnification ratio, this effect is

• mag(K-NEW)^2/mag(K-3)^2 = (1.01/0.95)^2 = 1.13.

Ratio of brightness per area on your retina is therefore

• brightness(K-NEW)/brightness(K-3)~ 1.37/1.13 ~ 1.21.

Wow, 20% brighter, maybe this is too much but as an optimistic estimate that should be considered an upper limit, I claim success.

Reason 2.
The higher the refractive index, the smaller the critical angle (Total internal reflection - Wikipedia), therefore smaller transmission on roof surfaces for angle of incidence (AOI) smaller than the critical angle. (FYI if AOI is larger than the critical angle, light is 100% reflected without transmission.) This is something that is very briefly mentioned in the patent. There should be such an effect, but it's hard to imagine that this is a huge effect that can possibly explain 10% brighter VF.
The reason why I don't believe this to be the dominant effect is because the AOI on the roof panels is already larger than the critical angle even for BK7 for the vast majority of rays. Critical angle for BK7 and S-LAH66 are 41.3 degrees and 34.3 degrees respectively, while the AOI on roof panels for the light perpendicular to the input surface is 49. 2 degrees.

It is explained in the patent. Have you read and understand it? No? Then, you have no choice than to trust the professionals who tells you how it is.

Rather than, say, pretend you know better and they're liars.

Wow, ogl didn't call people liars as far as I see. And no, actually it's not explained in the patent, the patent is not about the brightness of the VF per se.
The patent is about the recipe for multi layer dielectric coating on the roof panels of the roof pentaprism and other prisms with a roof. Without coating, the view quality (contrast and sharpness) suffer, regardless of the refractive index and brightness, according to them. Prior art exists for regular glass materials like BK7, but not for higher refractive index materials, thus this patent.

Sorry for long post!

[Zygonyx]

Ouch, that is top level optics kwb

[rawr]

Ouch, that is top level optics kwb

And that's just about the pentaprism and viewfinder. Plus the AF has it's own optics and mirrors.

[mysterick]

Um.....I think I'll go use my camera. ; - )

[texandrews]

Yeah...these are not the sort of posts you see on painting and drawing forums (conservators excepted).

[DagT]

I don't understand how new prism could make OVF 10% brighter...

the loss of light on the prism is insignificant compared
to the loss in the lens, on a translucent mirror and frosted glass,
no matter how much the transparency of the pentaprism is increased,
the overall increase in brightness in the viewfinder will be just as insignificant.

I think this patent is more relevant to the prism, with an explanation of the influence of refractive index etc.
Espacenet - Original document

[gaweidert]

Good question. There could be two things, I'll start with the one that probably has a bigger effect.

Reason 1.

Through a higher refractive index material, the focusing screen looks closer to the ocular lens. This is not just an optical illusion in that the higher the refractive index, the larger solid angle (Solid angle - Wikipedia) of light originating from any given point on the focusing screen reaches the ocular lens.

Forget about the prism shape, it's just a hunk of glass with one input surface and one output surface, reflections don't have anything to do with refraction. Instead, let's think about a point on the focusing screen, an empty space, and an ocular lens (Figure A). The point P emits light in a wide angle, and only a fraction of that reaches the lens. In this figure, I used the apex angle of cone of light that reaches the lens instead of the solid angle to represent how wide the cone is.

Next, without changing the distance between the focusing screen and the lens, fill the space with a glass plate with parallel surfaces (Figure B). Because of refraction, the apex angle of cone of light at P that reaches the lens is larger than before. From the lens, the focusing screen appears to be at a new location P', and the apex angle viewed from the lens is the same with the apex angle at P. The larger the refractive index, the larger the apex angle, thus the larger fraction of light from P reaches the lens to be relayed to the eye (thus brighter).

Suppose that you have two glass plates of the exact same thickness, one made of typical glasses like BK7 (n=1.516) and the other made of Ohara S-LAH66 (n=1.7725) that is one of the high refractive index materials mentioned in the patent. How much more solid angle do you gain by using the latter? Let's calculate the ratio of the solid angle!


It turns out that you don't need to know the exact dimensions for the calculations. As far as most of the space is filled with the glass (i.e. distance between the focusing screen and the pentaprism as well as between the pentaprism and the ocular lens are much smaller than the apparent path length of light), and as far as the ocular lens diameter is smaller than the apparent path length of light, the ratio is pretty much the same.
And that ratio is,

• solidAngle(S-LAH66)/solidAngle(BK7) ~ 1.37

(for example if the entire space between the focusing screen and the lens is occupied by the glass, the ratio is ~1.369 when arctan(R/d)~5 degrees, 1.376 if arctan(R/d)~10 degrees).
That is, assuming that the reflection at the input and output surfaces are negligible in both S-LAH66 and BK7 because of good anti-reflection coating, 30-something percent more light reaches the ocular lens originating from P.

Note that this is an optimistic estimate and should be considered an upper limit. In reality, though the light is indeed emitted with a large angle from P, the light intensity is not distributed evenly to all directions (e.g. the light is stronger at the center of the cone) and therefore the ratio of the light energy (number of photons) is somewhat smaller than the ratio of the solid angle. There could be other gotchas. 37 percent could easily be 30 or even 20 percent. But this is NOT a few percent effect.

But the view is larger in the new VF, thus increase in the photon number is distributed over a larger area for your view. This will counteract to make your view dimmer. Using the square of the magnification ratio, this effect is

• mag(K-NEW)^2/mag(K-3)^2 = (1.01/0.95)^2 = 1.13.

Ratio of brightness per area on your retina is therefore

• brightness(K-NEW)/brightness(K-3)~ 1.37/1.13 ~ 1.21.

Wow, 20% brighter, maybe this is too much but as an optimistic estimate that should be considered an upper limit, I claim success.

Reason 2.
The higher the refractive index, the smaller the critical angle (Total internal reflection - Wikipedia), therefore smaller transmission on roof surfaces for angle of incidence (AOI) smaller than the critical angle. (FYI if AOI is larger than the critical angle, light is 100% reflected without transmission.) This is something that is very briefly mentioned in the patent. There should be such an effect, but it's hard to imagine that this is a huge effect that can possibly explain 10% brighter VF.
The reason why I don't believe this to be the dominant effect is because the AOI on the roof panels is already larger than the critical angle even for BK7 for the vast majority of rays. Critical angle for BK7 and S-LAH66 are 41.3 degrees and 34.3 degrees respectively, while the AOI on roof panels for the light perpendicular to the input surface is 49. 2 degrees.



Wow, ogl didn't call people liars as far as I see. And no, actually it's not explained in the patent, the patent is not about the brightness of the VF per se.
The patent is about the recipe for multi layer dielectric coating on the roof panels of the roof pentaprism and other prisms with a roof. Without coating, the view quality (contrast and sharpness) suffer, regardless of the refractive index and brightness, according to them. Prior art exists for regular glass materials like BK7, but not for higher refractive index materials, thus this patent.

Sorry for long post!

Thanks for setting me straight here. I see what you are talking about now. I wonder if there have been any changes made to the mirror to help boost brightness. Haven't had to engage my brain at this level in a long time. Been concentrating my efforts on other things since I retired back in 2016. In any event it is going to be fun to see exactly what they have done once the camera is out.

[DagT]

Thanks for setting me straight here. I see what you are talking about now. I wonder if there have been any changes made to the mirror to help boost brightness. Haven't had to engage my brain at this level in a long time. Been concentrating my efforts on other things since I retired back in 2016. In any event it is going to be fun to see exactly what they have done once the camera is out.

This is explain in the patent I referred to earlier today.

[MMVIII]

I think this patent is more relevant to the prism, with an explanation of the influence of refractive index etc.
Espacenet - Original document

Thank you for reposting this here, it is a very interesting read, even if I would claim to fully understand only a fraction of it.

Anyway, has anyone noticed that figure 3 shows an alternative embodiment to superimpose information on the image in the viewfinder? What I noticed is that the space for this system, that normally would be in front of the prism is noticably reduced in K-new. I would not be surprised if they went for this new solution. Whatever the consequence of this might be, we will see.

[Serkevan]

Thank you for reposting this here, it is a very interesting read, even if I would claim to fully understand only a fraction of it.

Anyway, has anyone noticed that figure 3 shows an alternative embodiment to superimpose information on the image in the viewfinder? What I noticed is that the space for this system, that normally would be in front of the prism is noticably reduced in K-new. I would not be surprised if they went for this new solution. Whatever the consequence of this might be, we will see.

Do you mean "Block 80" on Figure 2 (Block 90 on Fig. 3)? That looks like the metering sensor... might have to do with the absolute brightness of the prism since in the Fig.3 arrangement they don't need to account for transmission through one of the internal surfaces of the pentaprism, on top of reducing the space in front of the prism and providing the famous extra 3mm of eyepiece relief?

... and if they have increased the resolution of the exposure sensor (92) to do face recognition à la Canon 90D, the new location might also, like you say, allow for superimposing the AF area on the OVF, showing the detected faces.

Great catch! Exciting times ahead, indeed.