Probably impractical idea I came up with to improve low-light focusing

Kit Lens Jockey

EOS 7D MK II
Nov 12, 2016
677
407
I'm a fiend for low light photos. It's really striking to me how much light is wasted in a DSLR by bouncing it off the primary mirror into the viewfinder. I mean, obviously, you need to see what's going on, but I'd happily trade off something like a 50% dimmer viewfinder for 50% more ability to focus in low light. I wonder if there would be any possible way to take the mirror box out of a DSLR, use either some kind of chemical or mechanical process to strip off some of the reflective coating, and then put it back into the camera, resulting in a modified camera that has amazing low light focusing abilities. Or, perhaps just replace the primary mirror with a custom made piece of glass that has a lower reflectivity.

I guess the obvious drawback is now you have a camera that has a dimmer viewfinder but is modified to perform in the exact kind of conditions when you would WANT a bright viewfinder. I should probably just trust that Canon has already figured out the perfect reflectivity of the primary mirror to get both adequate viewfinder brightness but to leave enough light to get adequate autofocus.

I was also thinking about some kind of hypothetical super low light focusing mode where there's no modification to the mirror, but when autofocus on the camera begins, it flips only the primary mirror up, allowing all of the available light to reach the secondary mirror and focusing sensor. Then, as soon as focus is achieved, the focus point still blinks red, albeit on a totally black viewfinder, and you can snap the photo. This is probably way too much viewfinder blackout time for most people, especially in those crucial few seconds leading up to the moment a photo is taken. But honestly sometimes I'd take this sort of situation over having a viewfinder that I can see the entire time, but the camera just hunts and hunts for focus... It would be kind of a last-ditch effort autofocus mode for very low light situations.
 

Talys

Canon 6DII
Feb 16, 2017
2,062
341
Vancouver, BC
Kit Lens Jockey said:
I'm a fiend for low light photos. It's really striking to me how much light is wasted in a DSLR by bouncing it off the primary mirror into the viewfinder. I mean, obviously, you need to see what's going on, but I'd happily trade off something like a 50% dimmer viewfinder for 50% more ability to focus in low light. I wonder if there would be any possible way to take the mirror box out of a DSLR, use either some kind of chemical or mechanical process to strip off some of the reflective coating, and then put it back into the camera, resulting in a modified camera that has amazing low light focusing abilities. Or, perhaps just replace the primary mirror with a custom made piece of glass that has a lower reflectivity.

I guess the obvious drawback is now you have a camera that has a dimmer viewfinder but is modified to perform in the exact kind of conditions when you would WANT a bright viewfinder. I should probably just trust that Canon has already figured out the perfect reflectivity of the primary mirror to get both adequate viewfinder brightness but to leave enough light to get adequate autofocus.

I was also thinking about some kind of hypothetical super low light focusing mode where there's no modification to the mirror, but when autofocus on the camera begins, it flips only the primary mirror up, allowing all of the available light to reach the secondary mirror and focusing sensor. Then, as soon as focus is achieved, the focus point still blinks red, albeit on a totally black viewfinder, and you can snap the photo. This is probably way too much viewfinder blackout time for most people, especially in those crucial few seconds leading up to the moment a photo is taken. But honestly sometimes I'd take this sort of situation over having a viewfinder that I can see the entire time, but the camera just hunts and hunts for focus... It would be kind of a last-ditch effort autofocus mode for very low light situations.
It's not 50% of the light, though. Much more goes to the viewfinder than that (otherwise, your image would look much darker). I believe only a little bit of light passes through the viewfinder mirror to reflect down to the AF sensor.
 

Kit Lens Jockey

EOS 7D MK II
Nov 12, 2016
677
407
Right, but I'm just saying however much light goes up to the viewfinder, cut that by 50%. So say the viewfinder gets 80% of the light now, cut that down to 40%. I guess ultimately what gets in the way of this is the feasibility of it. I can't imagine that type of glass that's translucently reflective is easy to make, and I can't imagine the existing mirror would be easy to modify or disassemble to replace it outright.

Maybe some mirror glass from one of those DSLTs could be sized for a moving mirror mechanism and then swapped in? Remember, the cameras that had a mirror that was much more transparent but didn't move? Again, I know this would be very difficult to do, require a lot of specialized tooling. Just interesting to think about.

I'd still be curious about a mode in which only the primary mirror gets flipped up and then the camera focuses. The blackout would be unacceptable for most people, granted, but for subjects that are fairly stationary and people who are steady enough to hold composition for a second or two while the mirror is flipped up and focusing is happening...
 

LDS

EOR R
Sep 14, 2012
1,623
176
It may be impractical to focus things you can't see because the viewfinder becomes too dark :) Or the AF system is blinded by too much light in other situations. You have to strike a balance between the two needs.

Anyway I guess replacing the mirror without re-tuning the rest of the system may yield issues, it could also impact metering.

Mirrorless does solve this issues, so I don't think there much will to invest a lot in the mirror technology.
 

old-pr-pix

EOS 7D MK II
Dec 26, 2011
402
52
LDS said:
Mirrorless does solve this issues ...
down to about EV -3 which is an 15" exposure at f1.4 ISO 100. Most metering systems also won't work at any lower EV value. How low do you want to go?
 

Talys

Canon 6DII
Feb 16, 2017
2,062
341
Vancouver, BC
Mirrorless doesn't solve anything. The problems start in low-light situations, where mirrorless cameras don't do very well.
 

lexaclarke

EOS M50
Mar 10, 2018
48
0
26
Southampton, England
Mirrorless slows down in low light more than SLR but they stay just as accurate as they do in good light. I find my SLR stays faster in low light than my mirrorless but it loses accuracy instead. I'd rather have accuracy than speed. No point focusing quickly if you're not actually in focus.

But OP's suggest sounds familiar, isn't that basically what the Sony A99 does? It has a semi-transparent mirror so less light is bounced up and more passes through to the AF system, right?
 

3kramd5

EOS 5D MK IV
Mar 2, 2012
3,084
405
lexaclarke said:
But OP's suggest sounds familiar, isn't that basically what the Sony A99 does? It has a semi-transparent mirror so less light is bounced up and more passes through to the AF system, right?
Or the canon Pellix, for that matter (both are different implementations of what the OP suggests, however).
 

Valvebounce

EOS 5D SR
Apr 3, 2013
4,328
243
53
Isle of Wight
Hi Kit Lens Jockey.
See this video for the reason you can’t flip the primary Mirror and leave the secondary mirror in place, at least without a very significant redesign, the secondary mirror is fixed to the primary mirror probably to ease the problem of synchronising both mirrors to get out of the way in time for the shutter to open! :) Also, hanging the secondary mirror part way down the primary mirror enables it to be significantly smaller reducing the moment of inertia of the mirror.

Cheers, Graham.

Kit Lens Jockey said:
.
I'd still be curious about a mode in which only the primary mirror gets flipped up and then the camera focuses. The blackout would be unacceptable for most people, granted, but for subjects that are fairly stationary and people who are steady enough to hold composition for a second or two while the mirror is flipped up and focusing is happening...
 

9VIII

EOR R
Feb 8, 2013
1,843
0
I've always wondered what would happen if I drilled a hole in the middle of my mirror.

Given the spread of line detecting elements on the AF sensor I'm betting it would actually have to be a series of perforated lines at very specific angles across most of the mirror, or maybe it would work best if the mirror were more of a mesh that reflected just enough image up to the viewfinder to be able to tell what you're pointing at?

The idea probably would work, and no-one would want to use it.
 

9VIII

EOR R
Feb 8, 2013
1,843
0
Alternatively (and if I remember correctly this is something Canon actually has patents for), you could use an SLR design with an EVF, put the AF sensor where the prism is, and then you only need to let a little light onto the sensor for it to amplify that into a usable image. It could even use contrast detect as a redundant AF system for better accuracy.

The biggest problem with Sony's old SLT design was when they decided to take out the "Reflex" part.
 

Talys

Canon 6DII
Feb 16, 2017
2,062
341
Vancouver, BC
9VIII said:
I've always wondered what would happen if I drilled a hole in the middle of my mirror.

Given the spread of line detecting elements on the AF sensor I'm betting it would actually have to be a series of perforated lines at very specific angles across most of the mirror, or maybe it would work best if the mirror were more of a mesh that reflected just enough image up to the viewfinder to be able to tell what you're pointing at?

The idea probably would work, and no-one would want to use it.

This picture actually says it all :) Light passes straight throught he mirror, hits another mirror, and is reflected downwards on an angle to the sensor.

 

9VIII

EOR R
Feb 8, 2013
1,843
0
Talys said:
9VIII said:
I've always wondered what would happen if I drilled a hole in the middle of my mirror.

Given the spread of line detecting elements on the AF sensor I'm betting it would actually have to be a series of perforated lines at very specific angles across most of the mirror, or maybe it would work best if the mirror were more of a mesh that reflected just enough image up to the viewfinder to be able to tell what you're pointing at?

The idea probably would work, and no-one would want to use it.

This picture actually says it all :) Light passes straight throught he mirror, hits another mirror, and is reflected downwards on an angle to the sensor.

Which we can be pretty sure will accomplish nothing once you look at the layout of an AF sensor: https://www.the-digital-picture.com/Photography-Tips/canon-eos-dslr-autofocus-explained.aspx