WindSoul
 lvl.3
Canada
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You can buy filters based on what a guy or another claims based purely on their own experimenting or even worse-based on nothing. Real present-day knowledge has certain attributes: is seeded in curiosity, is built on logic and amazingly, proves ultimately wrong, or false, as is getting replaced by new knowledge. There is a fundamental value in knowledge: no further progress can be achieved unless based on something the future proved fundamentally wrong. Next is the knowledge I have in regard to filters, my understanding on how photo sensor works and why buying filters is the same with buying Coca-Cola: foolish in any possible way.
Long story then:
1. Sensor photoelements
The 4k sensor of the camera has a bit over 8 million photoelements.
Each photoelement is composed of three photodiodes (or phototranzistors, but is not relevant here).
Each photodiode is able to detect light in the visible range, of one particular colour: either red, green or blue.
The photodiode outputs a current which is digitized by an analog do digital converter (ADC), into a number say between 0 and 256, corresponding to a 8-bit
2. Characteristics of a photodiode:
If the light of a certain intensity falls on the photodiode, then the analog output keeps on increasing, so the ADC output will end up being 256. Therefore the exposure time needs adjusted as to obtain a reading relevant for the real colour.
2.1. Note on sensitivity: the sensitivity of a diode is the speed at which the diode output translates into the max ADC value and can be adjusted in the same way as the exposure time. Since sensitivity and exposure are two variables, i will exclude sensitivity in order to maintain the optimum logic of a mono-variable problem.
3. Exposure time on a photodiode:
If the exposure (time) is too short, the colour is more likely to be rendered a dark one. If on the contrary, the exposure is too long, the colour rendered from the photodiode will become the purest color visible by the eye. Please note the main difference between short exposure (rendering black) and long exposure (rendering not white but the purest colour).
Exposure time is the same for all photoelements of the 4k frame. The ideal exposure of a photoelement is the time needed such as one component (either red, green or blue) reaches saturation. Imagine a sensor where each photoelement is sampled after ideal exposure. The ultimate HDR.
Exposure range of a photodiode:
1. under-exposed: the ADC outputs a value under 32: the eye can hardly see anything but black. This is the range of black
2. correctly exposed: the ADC outputs a value between 33 to 224: the eye can easily see all different nuances. This is the visible range
3. over-exposed: the ADC outputs a value between 224 and 256: the eye can see a pure colour, but can hardly detect nuances. This is the saturation range.
How a photoelement gets the colour depends on each photodiode. A photoelement is correctly exposed when at most one photodiode reached saturation. Now turning from photodiodes to how photoelements work together to generate a unique exposure for all.
4. Metering
There is a process of adjusting exposure time for the entire 4k frame, called metering. It functions like this:
1. There are a number of photoelements on the frame which can be read independently. These photoelements are involved in exposure metering at first, but are also part of the entire 4k frame sampling which follows the metering.
2. These special photoelements are scattered around on the frame in a way to make the exposure relevant for the entire frame.
3. The stages of metering:
3.1. The shutter opens
3.2. The metering process makes the first sampling (of the metering photoelements)
* at the end of the sampling, a ratio is generated: how many elements are dark to how many are bright (saturated). After first sampling, that ratio is usually well above unity
3.3. Second sampling
* since the photoelements have the additive property while the shutter stays open, then the dark/bright ratio decreased towards 1, meaning more photoelements which before rendered dark colours now render correctly, but also that more photoelements now render bright colours.
3.4. Further sampling
* the sampling process continues untill the number of correctly exposed photoelements is maximized. This metering process is irreversible (happens while the shutter stays open) and requires that the ratio dark/bright doesnt fall well bellow 1, because if the scene becomes over-exposed, the only way to reduce brightness is close the shutter and restart the process from 3.1.
5. Sampling the entire 4k frame
* after consecutive samplings and measurements while the shuttter stayed open, the metering has reached an optimum of maximum correctly exposed photoelements.
* next, the entire 4k frame is sampled and the image is stored. Note: since the shutter speed can be up to 200 faster than frame rate (1/4000 to 1/24), then consecutive shutter actuations can be employed in order to further correct the optimal shutter speed, before the shot is taken and sent to video compressor.
6. The role and limitations of the ND filter
6.1. About colour refection from material surfaces:
* any material has a light absorbtion characteristic. Any surface has in addition, a light reflection characteristic. For example the leaves are green because they absorb red and blue light entirely, and whatever they reflect back is the green component of the incident light;
* in certain illumination, surfaces reflect a certain amount of color. If the incidental white light increases, then the surface is able to reflect more of the specific surface colour. This ability is limited and if white light increases further, then the surface will start reflecting along with the max amount of coloured light it can, also the extra white light. This renders the surface colour white-washed.
6.2. The role of the filter is to remove the white-wash from the light reflected, therefore rendering the colour at its strongest, without any extra white in it.
6.3. The limitation of a filter is that it removes white light from all photoelements of a sensor. While the subject of a shot may appear with the best colouring possible, the rest of the scene will appear dark or even black.
6.4. Filter use:
* in cinematography, to render colours in close setups-requires the lights on the stage;
* in still photo-protraiture, for exactly the same purpose as in cinematography;
* in still landscape photo, where the motion-induced blur can generate artistic effects, while long exposures take advantage of additive effect of photo sensor in order to render the still subject with clarity.
7. In conclusion:
* there is nothing a filter can do which can not be obtained by the variable shutter speed;
* sensor sensitivity only gives extra leverage by offsetting light detection levels up or down;
* the camera on the P4 has no shortcomings. the P4Pro camera is hardly an improvement at all.
Epilogue: how HDR works
HDR is a techique of combining different exposures.
The main goal is to render as many photoelements at their ideal light output, where at most one light component is at saturation.
The usual method is to render the same scene at different sensor sensitivities, each sensitivity bringing a different region of the scene at ideal colour output.
For clarifications on sensitivity and exposure, read note 2.1.
I believe that is only a matter of time until each photoelement will be sampled at ideal exposure. Is technologically complicated in present day to sample 8 million photoelements each at its own exposure and all at the same time, but I am confident that a means to measure the saturation levels in real time for each photoelement via a dedicated micro-circuitry is only a matter of microelectronics. What we see with artificial coloring of deep space shots taken beyound visible range along with pictures of the sun, i expect to be unified as a whole picture during our lifetime.
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