The Human Eye and Digital Camera (2)
Now let’s see how it compares with a digital camera from a quantitative standpoint.
Before, we relate the different parts of both: the goal is the combination of the lens and the cornea (which in turn serves as a UV filter as well). The diaphragm is constituted by the combination iris / pupil (which expands or contracts to change the opening), and the sensor is the retina. Finally, the image processor is the brain.
The Human Eye in Figures
There is some controversy regarding the focal length of the human eye: some authors speak of 16 -18 mm although it seems that consensus is another figure: 22-24 mm , whose value depends on several factors such as distance from the eye to the subject, age and health of the individual.
The Diaphragm
The maximum diameter of the pupil depends on the age (this is well know amateur Astronomy and Astro !) in young people come to dilate the pupil to 7 mm, a figure that is reduced from 5-6 mm of 40-45 years. In contrast, the minimum diameter of the pupil is 1.5 mm. With these figures we can calculate the focal ratio (f) of the human eye, from f3.3 (of 4.6 for adults over 45 years) to f16 at its opening minimal, ie, a relatively short range compared with a normal lens ( for example 50 mm f1.8 – f22).
The ISO of the Human Eye
ISO What the human eye? In fact, the human eye has a sensitivity native or base such as digital cameras, but surprisingly adapts to ambient light levels very extreme. In this way it is calculated that the human eye in a bright environment has an effective ISO of about 1 , and change in conditions of very low luminosodad has an equivalent ISO of 800 . However, the contrast ratio detected by the human eye than 10,000:1 , far from what has been achieved by any digital camera (or analog).
Shutter Speed
This parameter is interesting: in fact, the eye has a vision system constantly sends continuous information to the brain for processing. The shutter speed is equivalent aproximadamanete one hundredth of a second (the time interval that the eye is able to distinguish), but under certain conditions is capable of detecting light emissions of only two hundredths of a second long.
The Sensor
The retina has a diameter of approximately 40 mm (if extended to form a flat surface). That is, almost twice the area that a sensor full-frame 35 mm format (which is 24×36 mm) and more similar to the sensors of digital backs. It has about a hundred million light-sensitive receptors, of which 7% are cones, the cells with substances called opsins , separately detect red, green and blue and are insensitive to light (photons require at least 1,000 to activate.) 93% are Poles, cells by rhodopsin , a substance that is most sensitive to green light, are responsible for monochromatic vision and are very sensitive to light intensity (a single photon can he activate). The diameter of the cones is about 6 microns and 2 microns sticks (the photosites of the SLR CCDs have a diameter of between 5 and 8 microns, somewhat higher than the rods and cones similar to).
Quantum Efficiency
A parameter on the CCDs little known but important in this context is the quantum efficiency (CE) : measures the efficiency of a CCD and represents the percentage of photons incident on the CCD that are able to release an electron. For example, a CD of 50% means that for every 100 photons that affect the photosensitive surface of the CCD, it generates 50 electrons. Importantly, the EC varies by wavelength of light (usually maximum toward 550 nm, which corresponds to the green).
By way of comparison, the film has an EC of between 5% and 10%, while CCDs have an EC of between 40% and 90% (green), the human eye has an equivalent EC 15% (green), a value closer to the film to the CCD.
Random Posts
Posted in Photography Tips