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There are two caveats to this article:
- It’s almost impossible to correctly map the human eye to a camera sensor. The eye is a miraculous system, and not all its secrets are understood. We’re doing this just for fun, from a cinematography perspective.
- I’m going to be using 35mm full frame equivalents for my focal length. If you don’t know what that is, I suggest you read this first:
What is the focal length of the human eye, considering the entire field of view?
Let’s do this logically, step by step.
The Total Area of the Retina
The human retina, a thin layer at the back of the eyeball, functions essentially as the sensor in a camera. As an approximation, you could think of it as a spherical dome; it’s not flat.
The retina’s complex structure, populated with rods and cones, is responsible for converting light into neural signals, enabling vision.
The approximate area of the retina is 1,094 square millimeters.
Field of View of the Human Eye
The field of view (FOV) of the human eye is remarkably panoramic, with an average horizontal FOV of about 200-220 degrees and a vertical FOV of around 135-150 degrees.
This extensive range allows for a broad perception of the environment, crucial for both survival and complex tasks like driving or sports. We don’t see equally in both directions of course, due to the nose, chin, blind spot, etc. However, our perception is one of a uniform field in daily use.
The aspect ratio of the human eye is 200/135 (or 220/150) = 1.48:1 approximately.
Designing a Rectangular Camera Sensor That Approximates the Human Eye
There are many ways to slice a spherical dome to arrive at a rectangle, which is the shape of a common camera sensor. I could crop the sphere or make a rectangle that covers the sphere but with extra sides sticking out.
However, in both these scenarios, you’re either wasting rods and cones, or adding more. I think a better representation is to use the area and aspect ratios of the retina to arrive at a rectangle that incorporates the same receptor (pixel) density.
If the aspect ratio is 1.48 and the area is 1,094 sq. mm., using basic geometry the sensor size works out to be 40mm x 27mm. This is just slightly bigger than a full frame 35mm sensor. The aspect ratio and size of our retina is very similar to full frame 35mm.
The equivalent camera sensor size of the human eye is 40mm x 27mm.
Focal Length of the Human Eye and its 35mm Equivalent
The effective focal length of the human eye varies approximately between 16.67mm and 22mm, assuming a simplified model. The lens in our eye doesn’t zoom in or out, but “accommodates”, i.e., changes shape.
The 35mm equivalents focal lengths of the human eye, considering its entire field of vision (FOV), is between 15mm to 22mm.
If you look through a 15mm or 22 mm lens on 35mm full frame, do you see the world as you see it with your eyes?
Maybe. Maybe not.
It’s definitely one way to look at it. There are other ways.
Other ways to define the field of view
Binocular Vision and Field of View
Binocular vision in humans, which involves the overlapping fields of view from both eyes, provides a combined horizontal FOV of about 120 degrees horizontally and 60-degrees vertically. This overlap is crucial for depth perception.
The area outside of this range is monocular, since only one eye can see those areas. The aspect ratio of this range is 2:1.
The equivalent sensor size is 24mm x 12mm. This is very similar to Super 35mm.
The 35mm equivalents focal lengths of the human eye, considering the binocular field of view (FOV), is between 25mm to 37mm.
25mm to 37mm is a pretty popular focal range. A lot of people consider 28mm, 32mm, 35mm and 40mm as the “normal” lens, and you can see how this is with good reason.
However, there are yet other ways to see.
The Near Peripheral Region
The retina’s rods and cones have different distributions. Cones, which are densest in the fovea, handle color vision and fine details. Rods, more dispersed and absent in the fovea, are sensitive to low light.
The concentration of cones drastically reduces as one moves away from the fovea, with rods becoming more prevalent. This drop-off significantly impacts peripheral vision quality and sensitivity.
The first drop happens at about 120-degrees, or the binocular vision.
The second happens at about 50-60 degrees. Typically people take this as 60-degrees.
This is the near peripheral region. This is where the maximum cone activity takes place. Sharpness, color, most of the critical functions of the eye during normal lighting conditions are carried out in this zone. In humans color vision and shapes are concentrated in the center of the visual field.
If we consider this range, the sensor size is 12mm x 6mm. Guess which format this is awfully close to? Super 16mm.
The 35mm equivalents focal lengths of the human eye, considering the binocular field of view (FOV), is between 49mm to 73mm.
Personally I find this range the most “normal” for my particular way of seeing the world.
Macular Region
We go further deep, into the macular region. This has a diameter of about 5.5mm.
This is where most humans focus their attention when they have to. This is circular so the aspect ratio is about 1:1.
If you consider the Macular area only, you get a focal length of about 160mm.
You go even try the fovea, but the fovea only has a diameter of about 1.5mm.
If you consider the Fovea only, you get a focal length of about 600mm.
Is there one answer to the question?
Yes, the focal length of the human eye is pretty much fixed between 16.67mm and 22mm. The equivalent is 15-22mm. But, our idea of the focal length of the eye as it relates to cinematography varies between 15mm to 160mm.
This is probably why we accept lenses in different focal lengths in a movie. We, too, selectively view the world, even if it is presented to us in a wide field of view at all times. We only pay attention to what we want to pay attention to.
If we’re reading or playing games our focus is at the telephoto end of this range.
In a day to day situation, it can vary between 25-75mm, which is also typically the normal range of a zoom lens.
We find this range “normal” because it satisfies most of us. Very few people would equate the widest angle 15mm to how they see the world, even if the information (field of view) is similar.
Don’t get me wrong, we do experience the peripheries when we’re looking at landscapes or the sky. When humans or objects enter the frame it takes our attention away from the peripheries. The closer the person gets, the tighter our attention. Rarely do we comprehend a human face as a whole at intimate distances. We’re always paying attention to some aspect of the face.
The intricate mechanics of the human eye, when translated into cinematographic terms, provide fascinating insights into how our natural vision compares to modern camera technology. It’s complex.
Just for fun, though, if you want one single answer, then let’s do it.
What is the focal length of the human eye for cinematography and photography?
First of all, don’t take this section seriously. There’s a lot of room for error, even in a camera lens. A 50mm lens by one manufacturer might not have the same angle of view as a 50mm from another manufacturer. And, the eye isn’t the same exactly for everyone.
If you average out all the focal lengths mentioned earlier, the focal length you arrive at is 55mm. If you ignore the macular focal length you get 37mm.
If you ignore the widest and the macular (the extremes) you get 42mm. Keeping the variability in mind, you could argue for between 40-45mm.
However, from a democratic perspective, to hold the peace in cinematography land, 42mm is as democratic as you can get.
The focal length of the human eye for cinematography is 42mm, in 35mm equivalent terms.
What do you think?
In photography and cinematography, a normal lens is a lens that reproduces a field of view that appears “natural” to a human observer.
-“it is possible to see a scene made with any lens in normal perspective if one adjusts the viewing distance”-
I’ve always heard that the “normal” focal length for a 24mmx36mm size negative (or sensor) should be the length of the diagonal/hypotenuse, or approximately 43mm, resulting in a natural-looking perspective. So your calculated number of 42mm is very close. With a APS-C sensor, approximately the same size as Super35, the normal FL would be around 28mm. It seems to me that many classic movies (i.e. Casablanca) employed those natural-looking FLs.
42mm is just an average that won’t work for a majority of people. Like me!
Well considered analysis! … covering all the aspects. I like your conclusion of 42MM, which represents the conscious visible field well I think. There seems to be a semi-conscious “visible” field that is wider still… designed to detect encroaching saber tooth tigers and such. I like the Zeiss Batis 40mm as a walking around lens… very normal, and with close focus.
Thank you!