100% Silver Mirror
/forum/topic/960922/0

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RustyBug
Registered: Feb 02, 2009
Total Posts: 13189
Country: United States

If I were to replace the OEM mirror on either my 5D or my 1D MK II with a 100% silver mirror ... what issues would I have and what would be my work arounds.

I anticipate that I would lose AF & possibly metering ... thoughts ??



AhamB
Registered: Jul 11, 2008
Total Posts: 5030
Country: United States

You'd also need to match the exact thickness of the original mirror.



mirkoc
Registered: Jan 26, 2008
Total Posts: 631
Country: Croatia

What about weight and damping difference?



cogitech
Registered: Apr 20, 2005
Total Posts: 11365
Country: Canada

AhamB wrote:
You'd also need to match the exact thickness of the original mirror.


Not necessarily. If the new mirror was slightly thicker or thinner, then the focusing screen would simply need to be re-calibrated. Since it is easier to add more shims, I would say that a slightly thinner mirror would be a better choice than a slightly thicker one. If the mirror is too thick, then removing all shim(s) might not be enough to make up the difference. The only recourse at that point would be to reduce the edges of the focus screen so that it is inset into the pentaprism slightly. I had to do that once with an E-330. Not pretty.

Either way, the focusing screen would have to be re-calibrated after such an operation, so the mirror thickness is not critical (but must be within reason, of course).

Kent,

You would lose AF for sure, since the sub-mirror and AF sensor would be permanently in the dark, but you would not lose metering because it is in up behind the focusing screen.

Many have suggested that metering would be way off, due to much more light being reflected off the mirror, and would not be correctable with the 5D's meager +/-2 stops of EC. However, I am pretty sure it would be easy to deal with.

This is my logic:

1) When people replace the stock screen with an Ee-S screen, the 5D needs to have a custom function set to let the camera know that it has an Ee-S installed. This forces the camera to change its metering algorithm, which is necessary because the metering system is receiving less light through the Ee-S screen than with the stock screen (the viewfinder is "darker" for the same reason).

2) If a 100% silvered mirror is installed in that same 5D with an Ee-S and the custom function set to Ee-S, the meter will be receiving much more light than it should, resulting in under-exposure which may be beyond the 5D's +/-2 EC.

3) To compensate for the extra light reflecting from a 100% silvered mirror going through an Ee-S and hitting the meter, simply set the custom function back to Ee-A or whatever it is (not Ee-S).

In other words:

60% silvered mirror + Ee-S + C.Func(Ee-S) ≈ 100% silvered mirror + Ee-S + C.Func(Ee-A)

In this case I hypothesize that the ≈ could easily be compensated for by the 5D's +/-2 EC.



cogitech
Registered: Apr 20, 2005
Total Posts: 11365
Country: Canada

On another note; It is only a matter of time before this theory will be tested. As 5Ds get cheaper on the used market, people like us will surely begin to try things such as this. Things we would have never attempted on a $3000 camera.

I plan to do this myself once I can pick up a 5D for $500, and the camera will be worth more to me at that point than whatever photo-gizmos are available at the time. I have a Yashica FX-3 that I got for free with the purchase of a Contax 50/1.7 recently (both for $90!). The FX-3 will likely be the mirror donor.



Maximilian
Registered: May 06, 2006
Total Posts: 78
Country: Germany

Excuse me, what does "OEM" mean?



cogitech
Registered: Apr 20, 2005
Total Posts: 11365
Country: Canada

Original Equipment Manufacturer.



alexandre
Registered: Jun 30, 2005
Total Posts: 2624
Country: Brazil

cogitech wrote:
I have a Yashica FX-3 that I got for free with the purchase of a Contax 50/1.7 recently (both for $90!).


Hate ya.



U.C.
Registered: May 25, 2008
Total Posts: 632
Country: Netherlands

cogitech wrote:
Many have suggested that metering would be way off, due to much more light being reflected off the mirror, and would not be correctable with the 5D's meager +/-2 stops of EC. However, I am pretty sure it would be easy to deal with.

This is my logic:

1) When people replace the stock screen with an Ee-S screen, the 5D needs to have a custom function set to let the camera know that it has an Ee-S installed. This forces the camera to change its metering algorithm, which is necessary because the metering system is receiving less light through the Ee-S screen than with the stock screen (the viewfinder is "darker" for the same reason).

2) If a 100% silvered mirror is installed in that same 5D with an Ee-S and the custom function set to Ee-S, the meter will be receiving much more light than it should, resulting in under-exposure which may be beyond the 5D's +/-2 EC.

3) To compensate for the extra light reflecting from a 100% silvered mirror going through an Ee-S and hitting the meter, simply set the custom function back to Ee-A or whatever it is (not Ee-S).

In other words:

60% silvered mirror + Ee-S + C.Func(Ee-S) ≈ 100% silvered mirror + Ee-S + C.Func(Ee-A)

In this case I hypothesize that the ≈ could easily be compensated for by the 5D's +/-2 EC.

I thought that the AF only needs 30% of the incoming light and about 70% goes to the pentaprism. So you'll have about 1/2 stop extra light in the viewfinder/metering system.
Correct me if I'm wrong about the 70-30 ratio.



RustyBug
Registered: Feb 02, 2009
Total Posts: 13189
Country: United States

I used to use "wrong" ISO settings for exposure compensation with film. How would the ISO setting come into play here in digital ?

Any recommendations on source for replacement mirror?

Paul, I'm surrprised you haven't pulled the trigger on it already.

I'm not overly concerned about the metering, since (not in order):

1) You've always got the histogram
2) I've always got another body
3) Bracketing
4) Handheld meter
5) PP lattitude
6) Sunny 16 Rule
7) Spot meter off a darker subject area



cogitech
Registered: Apr 20, 2005
Total Posts: 11365
Country: Canada

RustyBug wrote:
I used to use "wrong" ISO settings for exposure compensation with film. How would the ISO setting come into play here in digital ?


Irrelevant, because when you change the ISO on a digital camera you aren't tricking the camera in any way.

In any case, I am confident that my method would work.


U.C., I'm 99% positive that it is 60/40, but I'll dig up the official info...



jotdeh
Registered: Apr 07, 2009
Total Posts: 880
Country: Belgium

RustyBug wrote:
I used to use "wrong" ISO settings for exposure compensation with film. How would the ISO setting come into play here in digital ?

It has no effect in terms of exposure compensation. The camera meters and calculates appropriate aperture / shutter speed for the ISO selected. And that's the ISO the camera uses to record the image.



U.C.
Registered: May 25, 2008
Total Posts: 632
Country: Netherlands

cogitech wrote:
RustyBug wrote:
I used to use "wrong" ISO settings for exposure compensation with film. How would the ISO setting come into play here in digital ?


Irrelevant, because when you change the ISO on a digital camera you aren't tricking the camera in any way.

In any case, I am confident that my method would work.


U.C., I'm 99% positive that it is 60/40, but I'll dig up the official info...

Ok, but even 60/40 does increase the amount of light in the viewfinder/lightmeter by only 0.73 stop. Which easily can be compensated.

edit: checked the official ratio and you are correct about the 60/40 reflection/transmission-ratio.



cogitech
Registered: Apr 20, 2005
Total Posts: 11365
Country: Canada

... from Canon's site:

"Quick-return half mirror (Transmission: reflection ratio of 40:60, no mirror cut-off with EF 600mm f/4 or shorter lens)"



cogitech
Registered: Apr 20, 2005
Total Posts: 11365
Country: Canada

U.C. wrote:

Ok, but even 60/40 does increase the amount of light in the viewfinder/lightmeter by only 0.73 stop. Which easily can be compensated.


Yes, it can.

However, take into consideration that a very good amount of EC is often required to get good exposure with manual focus lenses to begin with. The extra light from a 100% silvered mirror will just exacerbate the issue, so it makes sense to try to compensate for it in the way that I have described. No?

For instance, my 5D tends to overexpose by about 1 stop with my Rokkor 58/1.2 when wide open, so I consistently have my EC set to -1 when using the lens wide open. Throw .73 stops more error into the equation and I would then be consistently stuck at -1.66 EC, which gives me very little room on the low end of the EC scale to compensate for other exposure factors.

I'd just rather mitigate that .73 stop of error if I could, to maintain maximum exposure flexibility, and I think the C.Func tweak would go a long way to mitigating the error.



RustyBug
Registered: Feb 02, 2009
Total Posts: 13189
Country: United States

Screen setting sounds reasonable ... even if not perfect, should move it in the direction you need it to go.

I'm thinking that mentally ajusting my metering habits of finding a 'middle gray' subject area to a more shadowed area might be viable as well ... meter from shadows, expose for highlights, or some such modified approach. Who says an old dog can't learn a new trick.



U.C.
Registered: May 25, 2008
Total Posts: 632
Country: Netherlands

cogitech wrote:
U.C. wrote:

Ok, but even 60/40 does increase the amount of light in the viewfinder/lightmeter by only 0.73 stop. Which easily can be compensated.


Yes, it can.

However, take into consideration that a very good amount of EC is often required to get good exposure with manual focus lenses to begin with. The extra light from a 100% silvered mirror will just exacerbate the issue, so it makes sense to try to compensate for it in the way that I have described. No?

I think by selecting the normal focussing screen, you'll quite compensate the extra amount of light. So you're right about that too.



cogitech
Registered: Apr 20, 2005
Total Posts: 11365
Country: Canada

RustyBug wrote:
Screen setting sounds reasonable ... even if not perfect, should move it in the direction you need it to go.

I'm thinking that mentally ajusting my metering habits of finding a 'middle gray' subject area to a more shadowed area might be viable as well ... meter from shadows, expose for highlights, or some such modified approach. Who says an old dog can't learn a new trick.


Sure. Or just chimp the histo, re-dial EC, re-shoot. Once it is set, it'll be mostly consistent for that lens/aperture (as you probably know).

BTW, is this a pipe dream or are you actually going to try it? For me it is still a pipe dream, so I'll be very excited to hear about your results if you go for it!



TweakMDS
Registered: Aug 12, 2008
Total Posts: 240
Country: Netherlands

You could always hack some sort of very mind ND filter over the metering sensor to compensate this extra light...



RustyBug
Registered: Feb 02, 2009
Total Posts: 13189
Country: United States

If you can help lead me in the right direction ... I'll do it.

I never use my 5D with AF glass (except for my Canon 100/2.0) and I've got a pair of 1D MK II/N which has much better AF/metering/screen/shutter lag/etc. anyway. The FF is really the main attraction I have for the 5D over the 1D MKIIN and that only matters mostly in the wider end of things.

If it wasn't for the fact that I can get my 20mm to do 20mm on the FF, I'd probably stick with my 1D MK II's.



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