Home · Register · Search · View Winners · Software · Hosting · Software · Join Upload & Sell

Moderated by: Fred Miranda
Username   Password

  New fredmiranda.com Mobile Site
  New Feature: SMS Notification alert
  New Feature: Buy & Sell Watchlist
  

FM Forums | Alternative Gear & Lenses | Join Upload & Sell

1       2       3      
4
       5       6       end
  

Archive 2010 · 100% Silver Mirror
  
 
U.C.
Online
• • •
Upload & Sell: Off
p.4 #1 · 100% Silver Mirror


Bobster2 wrote:
So?

I didn't say aluminum oxide is shiny. I said aluminum is shiny when it's polished. And the shine is protected by a 4 nm layer of aluminum oxide that keeps it from tarnishing.


Ok, so a layer of 4nm Aluminium oxide is transparent?
I can't find an index of reflection coefficients, so I don't know about the reflection difference between silver, aluminium and chrome.



Dec 03, 2010 at 10:02 PM
jotdeh
Offline
• • •
Upload & Sell: Off
p.4 #2 · 100% Silver Mirror


cogitech wrote:
I can't agree with this. There is no device in the light path that can "throttle" the input in the way you described. If the focus screen or pentaprism had some sort of photochromic action, then yes. But they don't.



The aperture Ed is referring to is a result of the roughness of the focussing screen. Essentially, higher light throughput demands bigger "grains" on the surface, and as a result resolution of such a bright screen goes down. Use smaller grains to increase resolution (as in the S-screens) but then the "aperture" of the screen will be reduced and the picture in the viewfinder will be accordingly darker.



Dec 03, 2010 at 10:19 PM
cogitech
Offline
• • • • • •
Upload & Sell: Off
p.4 #3 · 100% Silver Mirror


jotdeh wrote:
The aperture Ed is referring to is a result of the roughness of the focussing screen. Essentially, higher light throughput demands bigger "grains" on the surface, and as a result resolution of such a bright screen goes down. Use smaller grains to increase resolution (as in the S-screens) but then the "aperture" of the screen will be reduced and the picture in the viewfinder will be accordingly darker.


But it will not reduce bright light more than dim light. It is impossible.

Therefore, any increase in input light (whether a brighter scene, a brighter lens, or a more reflective mirror) will result in a brighter viewfinder, regardless of what focusing screen is used.

Look through your camera viewfinder in a dimly lit room. Turn on another light in the room. The image in the viewfinder will become brighter. This is universally true regardless of the lens, the aperture, or the focusing screen.

Increasing the reflectivity of the mirror is no different than turning that extra light on in the room.



Dec 03, 2010 at 10:44 PM
jotdeh
Offline
• • •
Upload & Sell: Off
p.4 #4 · 100% Silver Mirror


Oh sorry yes, matte screen roughness, lens speed and mirror reflectance the variables that determine the brightness of the picture in the viewfinder. I will pay more attention next time so as not to confuse people (and myself) even more


Dec 03, 2010 at 10:59 PM
RustyBug
Online
• • • • • •
Upload & Sell: On
p.4 #5 · 100% Silver Mirror


Ed Sawyer wrote:

Agreed


+1 on this (above).

Edmund Optics offers custom mirrors in any size, with any of a variety of coatings. Silver or aluminum would be the obvious choices.

If I was going this route, that's where I'd start. They (Edmund) have an amazing catalog, one can learn a lot by just reading it. And there's other providers too.
-Ed



That's where I started my Google search ... scientific mirrors. One such company was around 90% refelctance, so 100% probably is not actually achievable, but 90+% should be.



Dec 03, 2010 at 11:38 PM
theSuede
Offline
• • • •
Upload & Sell: Off
p.4 #6 · 100% Silver Mirror


Increased reflectivity in the main mirror has a 100% improvement translation into the viewfinder exit pupil. I.e, if you increase reflectivity by 20%, the VF picture also gets exactly 20% brighter. Relative light loss (in percent) is the same for all reflectivities.

The matte screen does not have an effective aperture in itself, it's just a lossy translation (diffusion) medium connecting the VF aperture and the lens aperture. The resulting projected image brightness is a complex angle interaction, an integration of space angle loss sums. Higher dispersion matte screens can project light from the rim of larger lens apertures into the VF aperture as more matte means "larger diffusion spread angle". But it also increases losses, as light from the center of the aperture is spread outside the VF aperture. The average light-loss for the entire (large) lens aperture throughput is fairly constant, but the aperture-edge light-loss goes DOWN. You get better DoF-conservation, but a slightly less bright image in some cases.

A low-matte screen makes sure that more of the lens-aperture central rays (chief rays) gets through unhindered to the VF (isn't scattered outside the VF entry pupil), but it also loses a lot of the aperture-rim light (peripheral rays) since it doesn't allow for enough diffusion - random angle bending of the rays. Longer DoF in the VF, less accurate focusing, but a bright image even with smaller apertures.

Unless you introduce some shaping optics into the equation, the maximum brightness for any combination of lens aperture > screen > VF aperture is fairly constant (!) - you just choose the lens F-no where the system is at "maximum light transfer" by choosing the amount of (and type of) matte. Generally you can say that the system has an efficiency pivot, where light throughput starts to decline. High matte screens start falling off in brightness at lower F-no's than low matte screens - but they can also start from a slightly higher point if they're correctly designed for the system.

The mirror is totally separate from all this, it's a linear loss medium.



Dec 03, 2010 at 11:55 PM
Bobster2
Offline
• • • •
Upload & Sell: Off
p.4 #7 · 100% Silver Mirror


Note that Canon does not specify the reflectance of the 5D mirror. All they give is the ratio of transmission to reflection which is NOT the same thing because they are not specifying how much of the light is lost (neither transmitted nor reflected). So all we can do is guess. My guess for reflectance is only 50% but it would be nice to have actual data.



Dec 04, 2010 at 12:05 AM
wickerprints
Offline
• • • •
Upload & Sell: Off
p.4 #8 · 100% Silver Mirror


Isn't it the case that the stock reflex mirror is only semi-transmissive in the central portion, directly in front of the secondary mirror? If this is the case, then wouldn't replacing the primary reflex mirror also change (to some extent) how evaluative and center-weighted average metering work, because in these metering modes, the camera is not expecting the mirror to be fully reflective in the center? That is to say, it is not merely a matter of increased brightness to the VF, but a change in the distribution of light intensity as a function of image height.

Granted, I imagine such differences are likely to be very small; but certainly if my understanding of the way these mirrors are designed in relation to the AF system is correct, changing the mirror could theoretically result in changes to metering behavior in some modes that cannot be easily predicted.



Dec 04, 2010 at 12:18 AM
cogitech
Offline
• • • • • •
Upload & Sell: Off
p.4 #9 · 100% Silver Mirror


wickerprints wrote:
Isn't it the case that the stock reflex mirror is only semi-transmissive in the central portion, directly in front of the secondary mirror?


I don't believe so.

Can anyone confirm this?



Dec 04, 2010 at 01:40 AM
Bobster2
Offline
• • • •
Upload & Sell: Off
p.4 #10 · 100% Silver Mirror


I believe that is correct. The mirror is only semi-transparent in the area in front of the secondary mirror.

Therefore, using a fully reflective mirror will not make a huge difference in viewfinder brightness because much of the mirror is already fully reflective. I'm not sure about this yet.

By shining a flashlight into my 20D, I can see that the mirror is more complicated than I realized. It does have a big piece in the middle that is different, and small circles near the edges that are different.


Edited on Dec 04, 2010 at 02:29 AM · View previous versions



Dec 04, 2010 at 02:17 AM
 

Search in Used Dept. 



Daniel Heineck
Offline
• • •
Upload & Sell: Off
p.4 #11 · 100% Silver Mirror


Aluminizing would be the wise idea. Edmund optics probably puts a CVD Si3N4 coating on top of that (silicon nitride--tough tough stuff)

If you're looking for the esoterica of mirror materials, then a dielectric mirror is what your really want. Open wallet.

IMHO, this sounds like WAY more of a PITA than the benefit, but that metric is pretty personal.



Dec 04, 2010 at 02:28 AM
theSuede
Offline
• • • •
Upload & Sell: Off
p.4 #12 · 100% Silver Mirror


According to the information I have, mirrors are uniformly reflective over the whole front surface. What you see as a "darkening spot" in the middle is just an effect of the translucency being non-diffused in the portion where the sub-mirror sits.
Over the rest of the surface the translucency is diffused, which makes it seem brighter on direct inspection. But bounce a spotlight off a lose mirror if you have access to one, and you will see that the reflected spot is very uniform.

So both yes and no.
I guess this (the diffusion of the rest of the mirror) removes or minimizes the risk of excessive stray light forming bright-spots and bouncing around behind the mirror and maybe upsetting the AE metering, but that's just a guess.



Dec 04, 2010 at 02:37 AM
wickerprints
Offline
• • • •
Upload & Sell: Off
p.4 #13 · 100% Silver Mirror


That contradicts my own observation of the reflex mirror on my 5D2. Just now, I took a small flashlight and pointed it directly at the mirror. Through the small gap at the bottom of the mirror box, you can just barely make out the secondary mirror. When I position the beam in just the right manner, I can see in the secondary mirror reflection an image of the flashlight. When I moved the flashlight toward the edge of the darkened central region of the primary mirror, I could see the reflection of the flashlight become partially obscured in the same shape as the darkened central region. I was very careful to observe that this obscuring was not due to the reflection in the secondary mirror being cut off due to its smaller size. This strongly suggests that the primary mirror is not uniformly transmissive.

Upon further thought, whether this means the mirror is uniformly reflective, however, I did not determine. I suppose it is possible that the mirror is partially transparent in the center and opaque in the periphery, but the reflectance is somehow the same.

Edited on Dec 04, 2010 at 03:16 AM · View previous versions



Dec 04, 2010 at 03:02 AM
cogitech
Offline
• • • • • •
Upload & Sell: Off
p.4 #14 · 100% Silver Mirror


It seems to me that the primary mirror is uniformly 60% reflective but there is an opaque frame that covers all but the rectangular central opening behind which the sub-mirror sits, which is exactly what I see when I lift up the mirror slowly and look in at the back of it. It actually isn't a perfect rectangular opening, but whatever. You will see it for yourself if you look.

On another note: Think about the economics of either making a uniformly 60% reflective mirror vs. making a mirror that has different reflectivity in the middle than the edges. The uniformly silvered mirror is far, far cheaper to mass produce for too many reasons to list. Now consider the added costs of doing it the other way against the very small benefits of doing such a thing. No company in their right mind would do it, because it has zero potential to increase profit. Quite the opposite.

Edited on Dec 04, 2010 at 03:22 AM · View previous versions



Dec 04, 2010 at 03:15 AM
wickerprints
Offline
• • • •
Upload & Sell: Off
p.4 #15 · 100% Silver Mirror


So the question becomes, what are the potential consequences of a mirror design such as this with respect to metering? Surely the backing material cannot be perfectly absorptive, but on the basis of these investigations, I am now thinking that replacing the stock mirror with an opaque one would not pose any significant issues with respect to metering.


Dec 04, 2010 at 03:19 AM
Bobster2
Offline
• • • •
Upload & Sell: Off
p.4 #16 · 100% Silver Mirror


The SLR mirror is not silvered. It's made with many layers of dielectric material, to cause constructive and destructive interference of the light waves.

Or something like that. It's not simple.



Dec 04, 2010 at 03:31 AM
cogitech
Offline
• • • • • •
Upload & Sell: Off
p.4 #17 · 100% Silver Mirror


wickerprints wrote:
So the question becomes, what are the potential consequences of a mirror design such as this with respect to metering? Surely the backing material cannot be perfectly absorptive, but on the basis of these investigations, I am now thinking that replacing the stock mirror with an opaque one would not pose any significant issues with respect to metering.


We covered the metering issue earlier. If you increase the input to the meter (by increasing the reflectivity of the mirror) without adjusting something else, it will generally lead to exposure issues. (see theSuede's excellent explanation about the efficiency pivot, etc. above).

This is why cameras have a custom function to tell it what focusing screen you are using, because the different focusing screens affect how much light the meter will "see" at different apertures (just as they will affect how much light you will see).

As I proposed, this could be counteracted by setting the Cust. Func. to Ee-A (on 5D) while using an Ee-S because at that setting the camera is (in general) expecting to "see" more light. EC can take care of the rest.



Dec 04, 2010 at 03:34 AM
cogitech
Offline
• • • • • •
Upload & Sell: Off
p.4 #18 · 100% Silver Mirror


Bobster2 wrote:
The SLR mirror is not silvered. It's made with many layers of dielectric material, to cause constructive and destructive interference of the light waves.

Or something like that. It's not simple.


Isn't this only true for partially reflective/transmissive mirrors?

A regular SLR mirror (with no submirror) could be "just silvered", no?

If not, why?




Dec 04, 2010 at 03:38 AM
Bobster2
Offline
• • • •
Upload & Sell: Off
p.4 #19 · 100% Silver Mirror


I'm not sure. The dielectric is supposed to be more efficient and lasts longer than silver. But I guess you're right, it's only for beam splitters.



Dec 04, 2010 at 03:53 AM
LightShow
Offline
• • • •
Upload & Sell: Off
p.4 #20 · 100% Silver Mirror


The mirror would have to be uniformly reflective, or the center would be dim in your viewfinder.


Dec 04, 2010 at 05:47 AM
1       2       3      
4
       5       6       end




FM Forums | Alternative Gear & Lenses | Join Upload & Sell

1       2       3      
4
       5       6       end
    
 

You are not logged in. Login or Register

Username   Password    Retrive password