Maxwell's House 3

Maxwell's House Episode 3

It's All In Your Head: Part 1

Ray demonstrates a slide that shows that a camera sees as the same color but a human eye does not. It is impossible to build a camera that reacts to color the way the human eye does. The human eye reacts in an extremely non-linear way. It is affected by surrounding color, how much sleep one has had, the surrounding lighting, etc. The eye tends to find the lightest thing in the scene and decides that the lightest thing it is seeing is white. It then performs a white balance to make everything else relative to the lightest thing it sees.

Guests

There were no guests on this episode.

Topics

Local Colour Contrast

Ray demonstrated an image previously shown on The Lab With Leo that shows that human eyes don't always see things. The image shows a dark blue arc on the left and a light blue arc on the right but the RGB values of those two colors are absolutely identical. This local color contrast occurs because the arc on the left has black next to it and the arc on the right has yellow next to it. A digital camera does not see this effect.

The reason the eye perceives the colour as darker has to do with how the cones (the colour sensors) in your eyes encode the colour and how they transmit it to your brain. They do it as a difference signal which has to do with the colours that are adjacent to the colors.

Human Ear Reaction to Sound

All the human senses are non-linear. Hearing: if you have a very low sound, a whisper, and you double the intensity of the sound, it doesn't sound twice as loud. Your ear can hear a whisper and understand it and if a jumbo jet takes off beside you, your hearing does not quit on you. It can withstand a 120 decibel sound or so as well as a 2 decibel sound whisper. The ear reacts by turning down the gain on your hearing the stronger the sound. There are muscles in your ear that that de-couple the bones as the sounds come in. This gives humans this logarithmic hearing that is not nearly as sensitive to loud sounds as it is to quiet sounds.

Eyes Reaction to Brightness

The eye is much more sensitive in very low light levels. As the light levels increase, it becomes less and less sensitive. This is a very non-linear reaction. Film reacts exactly the same way caused by the photochemical reactions of silver halides in film. When you get enough light to start the exposure, the density that is produced when you develop the film is not linear with regard to energy. It takes more and more energy to make the film darker and darker. A d-log-e-curve plotted against a log of exposure looks linear but a log scale on both axes means it is not linear.

Camera Adjustments

f stops/ stopping down: halves the amount of light coming into a camera. the higher the f stop number relates to the opening going down in size and halving of the amount of light that comes in. Turning down the shutter from 1/60th of a second to 1/125th of a second halves the amount of light. every 0.3 of a density unit we move in film equals one stop. Density is measured with a densitometer.

Film

Film and the eye react in a similar fashion. when you over-expose film and approach the maximum density that the film can produce, you get to a point where it won't get any blacker (negative film). As it approaches this maximum blackness (maximum density) it smoothly rolls off the shoulder of the curve on the graph.

CMOS, CCD Pickup

Film and the eye react do not react in a linear way but CMOS and CCDs do react in a linear way to energy. Ray demonstrates a slide titled 'Gamma Corrected Distribution' that demonstrates CCD/ CMOS array reaction. an example 12 bit analog to digital converter inside a digital camera would give 4096 levels, 2048 of them are used going from the brightest thing picked up to one stop down (half the light). This is why the scale of the linear distribution of the CMOS/CCD array is so much on the bright side. The CMOS responds better to bright light than it does to less light. This is linear with regard to energy unlike film which is non-linear. The Gamma corrected distribution appears visually linear but is not energy linear. This means the numbers you get in a RAW file are proportional to energy and not proportional to the brightness the human eye would see. Adobe camra RAW for instance redistributes into what is visually linear grayscale. This conversion occurs on the data that comes out of a CMOS or CCD to transform it to something that is pleasant for the human eye. This results in the rule of thumb in photography: instead of exposing until there is no detail in the dark and then moving up, you expose up until there is no detail in the light and then bump it down as desired. Max up to the point where you max out your brights.

Histogram

The histogram on the back of your camera is made from the visually linear file created by the digital image processor remapping of the linear RAW data from the CCD/CMOS chip. The visually linear histogram is not made from the RAW file as it would be jammed up on one end. What is important is the histogram is affected by digital image processor controls (sharpness, contrast, saturation, color tone, color space, white balance,etc) that are in addition to the only three controls that affect the RAW data (aperture, shutter speed, ISO setting). If you want an accurate histogram that shows the full dynamic range of your camera, shoot RAW and turn the contrast to its lowest level. This is because the contrast adjustment affects how the histogram is displayed. This adjustment will not affect the RAW file but will affect the sidecar JPEGs in modern cameras.

Astrophotography

The fact that these effects are linear has revolutionized astrophotography (photography through a telescope). In order to image the very dim images of galaxies you have to do incredibly long exposures. Film is not only non-linear but also has reciprocity failure. Exposures above about a second or so results in failure; it takes a certain number of photos coming in to even get the electrochemical reaction started in the silver halides in a film.

In astrophotography the chip in digital cameras is supercooled to about -25 degrees C lowering thermionic noise to allow long exposures without any build up of noise. This is not to be confused with the dark correction that digital cameras use to get rid of the fix pattern noise in the picture. This correction causes a delay during which the camera takes a second picture with the shutter closed and measures the dark current of every pixel which is subtracted off of the picture.

Amateur astronomers can now take pictures through their small 8 to 16 inch telescopes which was previously impossible. There are some amateurs capturing deep space imagery superior to the ones done with the big telescopes.

Websites

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Notable Comments

• ray@twit.tv for questions to Ray or suggestions for show topics

• Future suggested topics
  • Battery technology
  • How to build a Laser/How Lasers work: Ray proposed that he could borrow from his friend Dan Gelbert an exact replica of the very first laser that actually worked which was given to him by Dr.Theodore Maiman, the inventor. Ray met Dr. Maiman when he gave a talk in Vancouver and saw and put his hands on the very first Laser. All the pieces were loaned to Dan Gelbart who made the exact replica.
• It's All In Your Head: Part 2 will be about how the brain creates color.

Production Notes

• Recorded Date: November 6, 2008
• Release Date: November 6, 2008
• Duration: 37:57
• Log line:
• Edited by:
• Notes: