For an elementary understanding of a digital camera we should take a look onto the human eyes. An interesting fact is, that the Iris, in contrast to a fingerprint, is unchangeable and every human has a unique one. That is the reason why an Iris scanner is much more reliable than a fingerprint scanner - additional the iris is different from right eye to left eye. But that is just a trivial fact.
For our comparison we want to take a look at the basic principles of the eye. Its special characteristics like the blind spot and the different focus zones are not relevant for us.
As a simplification a sketch of the eye - in it we have 2 for our comparison relevant elements:
When we observe the eye, the pupil stands out (i.e. the in general black opening). With this opening, we can regulate the amount of light which falls through our lens onto the retina. On sunny days the pupil narrows down, because otherwise the amount of light would not be editable. The less light there is, the further the pupil opens to catch the right amount of light.
Through the lens we are focusing onto the retina. The lens is steered by muscles, who stretch or thicken it. That way we can focus onto observed objects. It is only possible in a given frame - items, who are approximately 30 cm or closer cannot be focused upon. Also, in general not everything is sharp - but through cooperation between eye and brain and the extremely fast ability to focus other, further away or closer, objects many situations seem sharper in reality than on a photo. Our brain flutters ourselves and often we are unaware of it! Also we do not see "continuous" - even a blink interrupts our "constant vision", which gets blinded out by our brain.
The lens in the eye is comparable with the lens system in a camera. Still, there are differences - the lens in a photo camera does not change its form, instead it changes its position; For focusing they are moved back and forth.
Object lenses on the other hand have a specification about their focal length (More about it in the chapter about objectives and lenses) - our eye has only one focal length. We cannot "zoom" with our eyes. If we want to magnify objects, we actually have to move towards them. When we want to make a comparison, we should make it with lenses who have a fixed focal length. The contrast of a fixed focal length are zoom lenses, with which we can zoom in and out.
A fixed focal length for a good comparison are 50 mm lenses - that is the reason why it is called the normal focal length. 50 mm lenses are similar to the viewing habits of our eyes. Please take a look at this 50 mm lens with a full format sensor (format 35 mm camera).
Set your camera on 50 mm - first take a look through the viewfinder and then without the camera. The perspective should stay the same. Please note, that your eyes do not move (what is normal and expands our field of view). Please note: When you have a camera with a crop factor (take a look into the chapter about the image sensor of a digital camera), then the focal length is different which is a normal focal length - so, with a common crop factor of 1,6 the normal focal length is 32 mm.
The eye steers the incoming amount of light with the iris. A camera has a comparable mechanism - which works with lamellas, which open and close accordingly.
The aperture of a camera (normally, the aperture is inside the objective) is operated through a mechanical moving of the lamellas, which increases or decreases the size of the opening.
The important statement of this comparison is, that the eye "takes the pictures", but the human brain "sees" it - in other words the brain calculates and evaluates the information which the eyes deliver as a picture. The perceived picture is already heavily filtered.
One of the reasons why sharpness or blurring in a picture attracts our attention is the contrast to reality, in which we do not directly notice it. The information is gathered with our "mechanism" eye, but the brain does not process it, because we focus our attention on another point and that one is sharp. When we want to examine the background, then the brain refocuses onto it.
The sharpness of the background on a photo does not change anymore, even when you focus it (that means, when the brain wants to contemplate it closer). Here a slight irritation appears in our brains - that is the reason why playing with sharpness and blurriness is very interesting. We can increase this irritation when we deprive the brain of desired information. In general, we want to look people into the eyes - when the complete face is blurred, then a big irritation appears inside the brain of the observer.
Let us take a look at how "sharpness" happens when the eye focusses onto a specific point. As sketched in the following example with a red piece.
The in red drawn beam hits the image plane directly (with the eye it's the retina). The light beam of the green figure has its sharpest point behind the eye. It gets depicted onto the image plane not as a point, but as a small circle. The depicted circle is bigger onto the image plane of the blue figure
Till a certain size this depicted circle is perceived as a sharp "point". But, at some point when the seize of the circle increases the brain will recognize it as a blurred spot (as a circle).
Please note - when the brain decides it wants to see the blue figure, then the blue figure gets focused for the fraction of a second. Now, because the red figure is not important anymore for the brain, the brain does not recognize the emerging blur of it!
We will get more in-depth with sharpness in the chapter about depth of field.
Our eyes do have a resolving power (smallest recognizable gap between two points) of 0,5° and 1° - the resolving power is measure in angular distance. To image it better: 1° is equal to 1 mm on 3 - 6 meters.
Author: Axel Pratzner
Translator: Felix Pratzner