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02-19-2016, 03:13 AM   #1
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A crazy idea

We all know that camera sensors are not capable of handling high contrast scenes. Our eyes though are very complex sensors that are capable of adjusting automatically and simultaneously even if there are extreme differences in light intensity in a single “frame”.

Here is my proposal: We use photochromic filters between the lens and the sensor.

Photochromic glass is very commonly used in prescription eye glasses. A popular brand is called Transitions. I wear them. In direct sunlight photochromic lenses turn dark and automatically return to their perfectly transparent state under the shade.

We can probably use this technology in photography. If the contrast is very high, areas of the frame that are bright are automatically darkened proportional to the intensity of light. Areas that are dark remain fully transparent.

This photochromic filter should be optional because there are times when you do want a high contrast shot such as in silhouettes. I would propose that there will be increasing levels of dymanic range control. Maybe have several layers of this thin filters that can be lowered just like mirrors in SLR cameras. I don’t know yet how to implement this.

02-19-2016, 04:36 AM   #2
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QuoteOriginally posted by dtmateojr Quote
Photochromic glass is very commonly used in prescription eye glasses. A popular brand is called Transitions. I wear them. In direct sunlight photochromic lenses turn dark and automatically return to their perfectly transparent state under the shade.
The problem is that photochromic lenses aren't colour neutral and have less than uniform transmission spectra.
02-19-2016, 05:28 AM   #3
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QuoteOriginally posted by Digitalis Quote
The problem is that photochromic lenses aren't colour neutral and have less than uniform transmission spectra.

Possibly because they weren't intended for this kind of application. I'm not familiar with their construction. Lots of filters aren't neutral. Cokin filters are a good example. Since typical sensors are only sensitive to three colours maybe the filter can be optimised for those colours as well?
02-19-2016, 05:43 AM   #4
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what I'm saying is that the areas with density will have a different colour from the areas without density, it would be a nightmare to colour correct the shadows and highlights without the mid-tones getting messed up real bad.

02-19-2016, 06:03 AM   #5
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QuoteOriginally posted by Digitalis Quote
what I'm saying is that the areas with density will have a different colour from the areas without density, it would be a nightmare to colour correct the shadows and highlights without the mid-tones getting messed up real bad.

How is this different from GND filters or variable ND filters?
02-19-2016, 06:21 AM   #6
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Eventually we'll see LCD gradiant glass which can dim itself. Whether that would be applicable to photography is an open question. That glass may not be optically neutral enough for lens applications. What may be acceptable for a window and our eyes may not be so for a sensor.
02-19-2016, 08:09 AM   #7
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QuoteOriginally posted by dtmateojr Quote
How is this different from GND filters or variable ND filters?
Their name implies the difference. Neutral Density filters are designed to be just that. At or around the magic 16% gray, there is no, or minimal, color shift. Higher quality full frame coverage ND and gradient ND filters essentially do nothing but change the exposure value of the scene by x stops and don't effect the WB.

I'm not an optical engineer, but from what I can tell about those photochromatic glasses, they need to be "activated" and they respond to the level of UV light hitting them. More UV, the darker they get. This could be very problematic for high angled light, lens hoods, etc, since not all of the exposed glass would be equally activated. That's assuming it was a front element. If a deeper element, I would think stray light bouncing around in the lens itself, might hyperactivate some parts of the photochromatic glass. ND filters avoid this by having a controlled amount of film applied evenly across the surface of the glass.

However, as mentioned, not all ND filters are created equal, I've experienced first hand some "bad" ND filters that had a blue cast to them, since they were actually GND, it destroyed the images. They probably wouldn't be as bad for sunsets/sunrises, where the heavy reds of the highlights and the blues of the shadows would have balanced a little, but one particular scene had a lot of whites and grays, and the blue cast was impossible to remove.
02-19-2016, 10:19 AM   #8
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QuoteOriginally posted by nomadkng Quote
Their name implies the difference. Neutral Density filters are designed to be just that. At or around the magic 16% gray, there is no, or minimal, color shift. Higher quality full frame coverage ND and gradient ND filters essentially do nothing but change the exposure value of the scene by x stops and don't effect the WB.



I'm not an optical engineer, but from what I can tell about those photochromatic glasses, they need to be "activated" and they respond to the level of UV light hitting them. More UV, the darker they get. This could be very problematic for high angled light, lens hoods, etc, since not all of the exposed glass would be equally activated. That's assuming it was a front element. If a deeper element, I would think stray light bouncing around in the lens itself, might hyperactivate some parts of the photochromatic glass. ND filters avoid this by having a controlled amount of film applied evenly across the surface of the glass.



However, as mentioned, not all ND filters are created equal, I've experienced first hand some "bad" ND filters that had a blue cast to them, since they were actually GND, it destroyed the images. They probably wouldn't be as bad for sunsets/sunrises, where the heavy reds of the highlights and the blues of the shadows would have balanced a little, but one particular scene had a lot of whites and grays, and the blue cast was impossible to remove.

The filter is very close to the sensor. Think of where the AA filter is. It will see what the lens sees. It means that if you are shooting at a wall with a window, the portion of the image with the very bright window will be the only part that darkens.

There are now more advanced photochromic lenses that respond to the visible spectrum not just UV.

I have used cheap GND filters before and they have a magenta colour cast. It is a known and common problem. I'm hoping that they could combine modern designs with old designs to achieve colour neutrality. The older photochromic materials used silver crystals just like in film except that they are reversable.

Anyway, just a wild idea that might sound good in theory but impractical to implement.

02-19-2016, 10:55 AM   #9
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QuoteOriginally posted by dtmateojr Quote
The filter is very close to the sensor. Think of where the AA filter is.
The problem is that borosilicate glass attenuates UV considerably.

QuoteOriginally posted by dtmateojr Quote
There are now more advanced photochromic lenses that respond to the visible spectrum not just UV.
But you still run into the issue of how much light the lens is transmitting to get the filter to respond, there is a threshold of light intensity required by photochromic lenses.


QuoteOriginally posted by dtmateojr Quote
The older photochromic materials used silver crystals just like in film except that they are reversable.
Hence the green cast. If they used something with a more neutral colour: like platinum, they would get better colour response - but the filters would be expensive as hell.
02-19-2016, 02:56 PM   #10
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QuoteOriginally posted by Digitalis Quote
The problem is that borosilicate glass attenuates UV considerably.
Why would you want non-visible light on your sensor?



QuoteQuote:
But you still run into the issue of how much light the lens is transmitting to get the filter to respond, there is a threshold of light intensity required by photochromic lenses.
But isn't that the point? Only the really intense light sources will get filtered.

QuoteQuote:
Hence the green cast. If they used something with a more neutral colour: like platinum, they would get better colour response - but the filters would be expensive as hell.

I can't comment on the material. I know nothing about them. I'm the architect, I will let the engineers figure it out LOL!
02-19-2016, 04:44 PM   #11
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QuoteOriginally posted by dtmateojr Quote
We all know that camera sensors are not capable of handling high contrast scenes. Our eyes though are very complex sensors that are capable of adjusting automatically and simultaneously even if there are extreme differences in light intensity in a single “frame”.

Here is my proposal: We use photochromic filters between the lens and the sensor.

Photochromic glass is very commonly used in prescription eye glasses. A popular brand is called Transitions. I wear them. In direct sunlight photochromic lenses turn dark and automatically return to their perfectly transparent state under the shade.

We can probably use this technology in photography. If the contrast is very high, areas of the frame that are bright are automatically darkened proportional to the intensity of light. Areas that are dark remain fully transparent.

This photochromic filter should be optional because there are times when you do want a high contrast shot such as in silhouettes. I would propose that there will be increasing levels of dymanic range control. Maybe have several layers of this thin filters that can be lowered just like mirrors in SLR cameras. I don’t know yet how to implement this.
There are a few flawed points here

I too wear transitions, they are not linear, they get darker in the cold, they work based upon UV exposure not actually visible light, and they darken the whole scene, so they don't reduce contrast just overall light.

The only way it can work would be if they could respond to the fvisible spectrum, because the uv would be filtered by the lens, and they would need to be at the focal plane, over the sensor, not in the lens or as a filter
02-19-2016, 05:05 PM   #12
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QuoteOriginally posted by dtmateojr Quote
I can't comment on the material. I know nothing about them.
IF you don't know anything about the characteristics of the material, how can you devise a practical way to deploy it in relation to photography.

QuoteOriginally posted by dtmateojr Quote
But isn't that the point? Only the really intense light sources will get filtered.
At f/8 light at the focal plane is considerably less intense than it is at f/1.8 - would the transition filter be sensitive enough to respond and offer any benefit? Probably not. Will the filter be able to respond uniformly to the entire visible spectrum? probably not.
02-19-2016, 05:07 PM   #13
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I think the OP's idea was to put independent pieces of filter in front of each pixel, so the amount of darkening would vary across the frame. In theory it could work.

My glasses have the same issues as others mentioned: not color neutral, darkening varies with temperature.

---------- Post added 02-19-16 at 07:10 PM ----------

Slow change is another issue. My glasses need around a minute to stabilize when light levels change. The camera would have to be used on a tripod and sit for a minute whenever composition changes.
02-19-2016, 05:23 PM   #14
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QuoteOriginally posted by DeadJohn Quote
I think the OP's idea was to put independent pieces of filter in front of each pixel, so the amount of darkening would vary across the frame. In theory it could work.
you would have issues with the transition in dynamic range at the edges of areas with extreme contrast.


QuoteOriginally posted by DeadJohn Quote
My glasses need around a minute to stabilize when light levels change. The camera would have to be used on a tripod and sit for a minute whenever composition changes.
And that would be too long if you work in rapidly changing lighting conditions like sunsets.
02-19-2016, 05:31 PM   #15
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QuoteOriginally posted by Digitalis Quote
IF you don't know anything about the characteristics of the material, how can you devise a practical way to deploy it in relation to photography.



At f/8 light at the focal plane is considerably less intense than it is at f/1.8 - would the transition filter be sensitive enough to respond and offer any benefit? Probably not. Will the filter be able to respond uniformly to the entire visible spectrum? probably not.

Cameras meter with the lens wide open. So the filter will react at f/1.8. Newer photochromic lenses now react to visible light. No sensor as of present is capable of capturing the entire visible spectrum. They nly interpolate from RGB.

---------- Post added 02-20-16 at 10:35 ----------

QuoteOriginally posted by Digitalis Quote
you would have issues with the transition in dynamic range at the edges of areas with extreme contrast.




And that would be too long if you work in rapidly changing lighting conditions like sunsets.

It will improve with time.
In the old days, film required minutes of exposure but now they react in milliseconds.
And besides, the ones that would greatly benefit from this are landscape photographers. If it takes a minute to darken that's fine. Since lenses concentrate light on the focal plane it should not take longer than that. It takes me way longer to compose a shot than waiting for my glasses to darken.
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