Camera and the Human Eye

When I created this site a few months ago, I had the goal of sharing interesting knowledge I learned in photography in a manner that anyone can easily understand. So this article start a series that each aims to cover a particular subject, today, I want to discuss the similarity between a camera and the human eye.

Why do we need eyes?

The world is full of light, it give us crucial information about things around us, whether it be a potential prey — if we were in the stone age — or an incoming car — if we were on a crosswalk. But lights do not always work with us, sometimes there’s too much light than we need, sometimes there’s too little. mid day in a clear summer outdoor sky contains plenty of light whereas a bedroom at night contains very little. A useful and ideal image often requires a sweet spot in between.

Every single photography class will start by explaining the exposure triangle, namely, Aperture, Shutter Speed and ISO, but few ever point out that our eye work with these properties too on the daily.

What is aperture?

Aperture describes the opening of a hole through which light reaches an sensing area. The hole that light travels through is called Entrance Pupil. The bigger the hole the more light that goes through and vice versa. The obvious naming here gives clue to its origin in the biological counterpart.

Aperture in the eyes

Here I took some pictures of my left eye in varying light levels. And you can see how my pupil slowly enlarged as I turn off the light in my room to compensate for the lack of light available.

Aperture in camera

On a camera, instead of having muscles in our eyes to control the aperture, we use aperture blades, a sophisticated mechanism that evenly expands and closes the “iris” just like an eye.

There are two ways to measure the degree of this opening. First is by just measuring the diameter of the hole when it’s at its largest, e.g. 42mm (human eye opens to around 2-8mm). The second, and the most common way in photography, is by measuring the diameter in relation to the focal length of the lens. This is because when comparing two lenses, the one with larger diameter iris doesn’t always let in more light, we have to consider the length light needs to travel through as well. A larger focal length for the same amount of diameter will reduce the effective amount of light that goes through. We can write this relation in a very simple formula

Aperture = Focal Length / Diameter of the entrance pupil

The resulting aperture will be written in the unit of “f”, and written like f/number. The smaller the focal length and the bigger the diameter the more aperture we have, though the f number will be smaller. So f/1.2 has larger aperture than f/8.

As an interesting comparison, human eye usually has an f/2.4 aperture, where as an owl’s is f/1.3 and a cat’s f/0.9! It’s no wonder why cat can be so comfortable navigating in the dark with its enormous aperture.

What is ISO?

ISO is a photography jargon that actually doesn’t have a meaning itself. It’s used more as a unit of how sensitive of the light sensing unit is. The higher the number the more sensitive and hence more light that appears in the final image.

Whereas a camera denotes ISO in nice numbers like 100, 1600, etc, Human eyes’ sensitivity is much harder to measure, and often takes time to reach its most sensitive state in the dark. But a camera’s sensor and an eye’s retina’s primary mechanism behaves much the same. A camera sensor has millions of pixels that each captures a primary color – red, green, blue, and arranged in such a way that for every red and blue pixel there are two green pixels. This is actually done to mimic our photosensitive cells in the retina, which also consists of red, green, blue photo receptors called cones. But human eyes have one more trick up the sleeves, we have another type of receptors called rods, which, although cannot give us color, are very good at sensing in very low light. This allows us to see so much more in the dark given an equivalent shutter speed and aperture compared to a camera.

Shutter speed, a messier subject

Now we are onto the last part of the triangle, shutter speed. Instead of using individual photos as examples, a continuous stream of photos, or video, is most similar to the human eye.

In photography, each image is exposed in a controlled time, it can be as short as thousands of a second or as long as several minutes. Human eyes do not have this freedom as survival in the wild requires attention every second. But through simple experiments we figured out that when more than 24 images are presented in a second, we can see a life-like imagery, a proof that human eyes have a minimum shutter speed of 1/48 of a second. This is also why video is shot around that shutter speed to give us the most natural looking movements to our eyes.

Now we have covered the exposure triangle, don’t you feel that camera is very much like our eyes? Don’t forget that the next time you pull open the curtain in a dark room that it is your eyes working hard to balance the exposure so you could see the wonderful world out there.