Life is a series of moments that leaves us with memories, both splintered and whole.
If you are just joining us, the PRELUDE & SYLLABUS section is the logical starting point for the series.
Welcome to Digital Photography #101
by Virtual Studio Photography (VSPHO)
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The analogies outlined here are insight from my IT and Photography background at AT&T. No proprietary information is disclosed from research performed by Bell Labs.
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In 35mm FILM versus DIGITAL IMAGING Part 1 and Part 2, we laid the foundation for this section: ISO Sensitivity.
The Digital ISO Sensitivity is possibly the single biggest advantage in Digital Photography, along with the unlimited possibilities of the Digital Darkroom.
Let’s make this section a little easier to navigate for future reference:
PART 1: ISO Defined
PART 2: Quantifying Light
PART 3: ISO Efficiency
PART 4: ISO Detail
PART 5: ISO Manipulation
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PART 1: ISO Defined
What is ISO Sensitivity?
From a practical standpoint, the ISO Speed is a standardized sensitivity to the amount of light required to expose film or a digital imaging sensor at any given setting of Aperture and Shutter Speed. (If needed, please see the PRELUDE section for a comprehensive tutorial outline.)
When we get to the LENSES section we will see that the EXPOSURE based on the Aperture and Shutter Speed correlated to the ISO Sensitivity can vary slightly due to the quality of the LENS.
We previously covered in Part 1 that 35mm film evolved to include an ISO Sensitivity range of ISO 25 through ISO 6400.
In comparison, Digital ISO Sensitivity on top end DSLR Cameras include ISO 100 through ISO 12800. Note: With a little Digital trickery, the virtual ISO Sensitivity range can be expanded from ISO 33 to ISO 25600.
As discussed with film in a comparison (Part 1 and Part 2), the ISO speed was determined by the film. Along with the task of changing the film, every ISO increase, decreased the image resolution by about 40%.
The undisputed advantage to Digital Photography is that the ISO Sensitivity can be changed between every shot if needed. On most DSLR Cameras, the ISO can also be incorporated into the computer controlled EXPOSURE variables to change automatically (EXPOSURE next section).
First we will reflect on the initial information presented in the PRELUDE section. We now need to refer to light as a quantity of photons defined as “Quantification” to fully understand the advantages and ramifications of Digital ISO Sensitivity.
When looking at 35mm film’s Dye Clouds absorption of photons, it is like little cotton balls absorbing water. The smaller the cotton (Dye Cloud), the finer the resolution (smaller absorption pieces) but the less water (photons) each piece can absorb.
***** With Digital Imaging, the Pixel Resolution remains the same at any ISO, but the Spatial Resolution (Tonal Definition) begins to deteriorate with very high ISO Sensitivity (pixel averaging explained below).
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PART 2: Quantifying Light
Research is quantifying the quantum limits of the amount of light needed to expose a digital image. Let’s use some historical data as a reference point for comparison.
In 1969, Bell Labs invented the CCD chip (Charged Coupled Device) used in many Digital Cameras. Note: The CMOS chip (Complementary Metal Oxide Semiconductor) is now more commonly used and will be discussed in a section called CCD versus CMOS.
In a nut shell, in the 1980s when Bell Labs was testing the Quantum Limits of Photons for Fiber Optics, they found the magic number for current technology. As I recall it was 8 photons to give a positive reading within an acceptable error margin.
When they submitted 8 photons in a highly controlled testing environment, the sensor was able to detect a burst of 8 photons with certainty. When they submitted 7 photons or less in a burst, Gammy Radiation (which is bombarding the Earth 24 hours a day) would interfere with the sensor, creating an unacceptable rate of false positives. In other words, Gamma Ray looked like additional photons when 7 or less photons were actually sent.
On the other end of the spectrum of quantifying photons, light is measured in moles (1 mole = approximately 6 x 10 to the 23rd power of photons). A sample of direct Sun Light for One Second over an area of One Square Meter may have around 600,000,000,000,000,000,000,000 photons.
So somewhere between 8 photons and a WHOLE BUNCH of photons is the sensitivity level of 35mm film and censor chips in Digital Cameras.
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PART 3: ISO Efficiency
Next , let’s look at the efficiency of film versus digital sensor chips. Because of the multiple colored layers (covered in Part 1) on 35mm film to filter out different light frequencies, color film only has a quantum efficiency of about 2%. Meaning 98% of the light trying to get past the color filters does NOT make it to expose the film.
Conversely, the Digital Sensor has a quantum efficiency of about 70%. 70% of the light that gets through the Lens (which also has a loss of light covered in the upcoming LENSES section) is able to penetrate the sensor’s color filters and expose the digital sensor.
So either with film or digital sensors, we are still talking about a lot of photons for exposure, but the Digital Camera clearly has an overwhelming advantage in light efficiency.
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PART 4: ISO Detail
This takes us to the composition of Digital Cameras. Every Digital Camera is a computer unto itself. You have Hardware (the physical camera and peripherals), the Firmware (computer code (BIOS) that runs the camera) and the Software (the computer code that produces the image data files). The Firmware in this case is also the Software as it permanently resides in the camera.
The ISO Sensitivity is directly correlated between the Hardware and the Software of the Digital Camera.
To delineate what was presented in the Previous Section:
There are TWO primary parameters to a Digital Image.
1. The Resolution (how many PIXELS)
2. The Bit Depth within the DynamicRange (how many Gradient Levels)
Each of the two have a whole sub-set of parameters which can be manipulated in almost infinite combinations.
We will only look at the implications of ISO Sensitivity at the top level.
All photographic images degrade with faster ISO settings, I’ll explain why.
We outlined previously that 35mm film has a range of ISO 25 up to an ISO 6400. The Degradation of the Image Quality of the higher ISO values starting as low as ISO 400 was the result of the Dye Cloud size and the chemical compound variations used in the upsized Dye Clouds.
With the Digital Imaging Chip there is also trade offs with higher ISO Sensitivity.
Film or Digital, a HIGHER ISO Level means LESS light is needed to create an Image. The QUALITY of the Image is what we are analyzing.
Number 1, The Resolution Potential is directly correlated to the Hardware. To reiterate from the Previous Section, the Imaging Sensor Chip is composed of PIXELS. The number of PIXELS on the chip defines the Resolution in Pixels Per Inch or Dots Per Inch when printed.
Number 2, The Bit Depth within the DynamicRange is a combination of both:
The Hardware’s ability to sense Voltage Variations of tiny Capacitors with Electricity converted from Photons;
And the Software’s intelligence (algorithms) to convert the Difference in Voltages to meaningful information.
Don’t worry, we are going to explain in detail.
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PART 5: ISO Manipulation
Each PIXEL has a Light Sensor (Photodiode) and a tiny Capacitor (a capacitor stores electricity). Yes, basically on a 36 Mega-pixel imaging chip there are 36 million Photodiodes and 36 million Capacitors. The Capacitors are obviously microscopic, they are limited to a very small amount of electrical voltage measured in units called Microfarads or even Nanofarads.
So back to the water analogy to explain Bit Depth, Resolution and ISO Sensitivity.
In our water analogy, we are going to quantify the difference of water in ounces. With our real life capacitors, it would be the difference in the voltage of each capacitor.
Most top DSLR Cameras start with an ISO Sensitivity of ISO 100 or ISO 200 (Sensor Sensitivity explained in a future section). Therefore, I will be referring to the Lowest ISO Setting as ISO 100/200. The next ISO Setting will be referred to as ISO 200/400 and so on…
Both ISO 100/200 are very accurate with light and produce the same visual results as the Lowest ISO Setting. We will also explain the compensation of ISO Settings in the next section, EXPOSURE.
In this first illustration we use a 2 Bit Depth for simplicity (2 to the 2nd power equal 4 gradient levels). The four glasses represent just ONE CAPACITOR with four possible voltages, four possible gradient levels. The actual voltage in the capacitor would have been rounded to the nearest value.
Hold the CNTL key and hit + or – to control the image size when open.
Click HERE for PICTURE: Then hit the “back arrow” at top left of page to return. If the picture comes up BLACK, just hit the screen REFRESH icon. Round Arrow upper left of screen.
The lowest bit depth in any consumer digital camera is an 8 Bit Depth: 2 to the 8th power = 256 gradient levels of grey. That is also an 8 bit color depth with three colors, RGB, that is 256 x 256 x 256 = over 16 millions color variations. An 8 Bit Depth is also the JPG industry standard for printing as in your 4×6 color prints.
Therefore, in all of these examples, imagine 256 glasses each with a slight variation of water levels with over 16 million color variations. Even better, imagine a 14 Bit Depth = 16,384 gradient levels of grey or 16,384 glasses of water with 4,398,000,000,000 color variations.
Note that the glass on the left represents NO light, but still has a small amount of water (representing photons). Our four levels are 1, 2, 3 and 4. There is no ZERO allowed, so even if the voltage is ZERO, the computer code (algorithms) always adds a small numerical amount to eliminate the ZERO called DITHERING. You will commonly hear the term Dithering in Digital Photography and Audio Reproduction for the same reasoning.
Like our glasses of water, our Digital Capacitors can be full, represented as a level 4, down to no light represented as a level 1 with 2 levels in between.
In our next picture we have the equivalent of doubling our ISO Sensitivity from ISO 100/200 to ISO 200/400.
We now have the sensors evaluating exactly half of the light. The difference in the voltage is now only half as well defined, but still highly accurate.
Hold the CNTL key and hit + or – to control the image size when open.
Click HERE for PICTURE: Then hit the “back arrow” at top left of page to return. If the picture comes up BLACK, just hit the screen REFRESH icon. Round Arrow upper left of screen.
At ISO 400/800, the Sensors can still do a very adequate job, but errors will start to accumulate. (next picture)
Hold the CNTL key and hit + or – to control the image size when open.
Click HERE for PICTURE: Then hit the “back arrow” at top left of page to return. If the picture comes up BLACK, just hit the screen REFRESH icon. Round Arrow upper left of screen.
As the voltages continue to diminish in variation, multiple problems arise in the digital processing. (next picture)
Hold the CNTL key and hit + or – to control the image size when open.
Click HERE for PICTURE: Then hit the “back arrow” at top left of page to return. If the picture comes up BLACK, just hit the screen REFRESH icon. Round Arrow upper left of screen.
At high ISO Settings, there are Two Parameters the computer programmers can play with in the Software Algorithms.
First, on top DSLR Cameras with a high Bit Depth, they can reduce the effective Bit Depth to say 8 to Decrease the error rate. At an extended ISO rating, the picture will still have the same tonal and color quality as the industry standard of JPG’s 8 Bit Depth.
Also at all ISO settings, we can employ another common technical trick of Pixel Averaging called Interpolation. There are various implementations of Interpolation by the software’s design. But basically if you take two or more Pixels in an area and weigh their averages, then you can assign the Pixel in question an appropriate value based on the averaging.
NOTE: Another definition of Pixel is that after the measured voltage is converted (Analog to Digital Conversion) to a Digital Value (number), that Value officially becomes a Pixel.
Pixel Averaging effectively Decreases the overall Tonal Accuracy of the Bit Depth, but can still gives satisfactory results in high ISO Settings without decreasing the Pixel Resolution.
Currently, very high ISO values are still plagued with Digital anomalies like Posterization (loss of definition) and Artifacting (undesired image alterations). But at the same ISO levels compared to 35mm film, I consider Digital Imaging already superior.
Likewise, as technology advances, ISO Sensitivity will approach the Quantum Limits, Pixel averaging could be done at the Hardware level with the actual voltages with more precision.
With each new generation of Digital Cameras, the accuracy and constancy will improve as Digital Imaging will produce new artistic techniques for the 21st Century.
This added to supliment this section:
In the next section (EXPOSURE) we put all three variables (APERTURE, SHUTTER SPEED and ISO Sensitivity) together, please join us.
Virtual Studio Photography (VSPHO)