The Human Eye Rule

This week I want to talk about something I figured out about photography and human vision. I see people asking online “what would the specs of the human eye be if it was a lens?”. Today, I want to go through the specifics and answer that question.

I remember realizing something a while back while I was using a 50mm fixed lens on a Canon 80d with its cropped sensor (making it an 80mm lens). I realized that the subjects in photos shot with an 80mm lens on a full frame camera appear to be the same size as the human eye perceives it. I concluded this by having both eyes open while shooting with a 50mm lens on my 80d, with one looking through the viewfinder, and the other looking a the subject with the naked eye. One eye saw the subject like the human eye sees anything else, and the one looking through the viewfinder saw that the subject appears to be the same size and “distance” away as the other eye saw. As if one eye was completely unobstructed and the other had to look through a rectangular peephole, but saw the same image. This wouldn’t happen with an 18mm focal length lens or any lens with a focal length significantly more or less than 80mm on a full frame sensor camera body. This means that using an 80mm lens on a body with a full frame sensor (or a 50mm on a body with cropped sensor with a crop factor of 1.6x) creates an image with the subject having the same perceived size and shape as the photographer saw that object with their naked eye. This is one of the reasons why I love using a 50mm lens on my 80d, because the lens is just like my eye and I can use my eyes as a way to determine where the best place to take a picture is.

Valle Redondo, Mexico, January 2017

While the human eye has the same “zoom” as an 80mm lens on a full frame sensor, the human eye doesn’t have the same angle of view, or how wide you can see. The human eye can see more than what’s in the confines of the rectangular frame of an image taken using an 80mm lens, but it sees what’s in the frame the same way.

There are two parts of the image perceived by the human eye, peripheral vision, and the cone of attention. With peripheral vision, the human eye can see around 160 degrees, so lets make it interesting and just go with the cone of attention. The cone of attention is the primary source of information from your sense of sight. It’s what you pay the most attention to and see the most detail with. With peripheral vision, it is very difficult to see details, but you can make out generic shapes and react to things happening in your peripheral. Peripheral vision is very important while driving, for example.  The cone of attention is 55 degrees, it’s what you’re looking through in order to read this. Try reading the next paragraph while focusing on this sentence. See? You can’t. This is because the next paragraph is in your peripheral vision.
A lens with the equivalent field of view of the human eye’s cone of attention is a 43mm lens. So the human eye is unlike any camera lens. Compared to when using a full frame sensor, the human eye sees objects in the perspective and relative size and distortion of an 80mm lens, but has the field of view of a 43 mm lens. I call this the “human eye rule”, and I think about it a lot when I’m shooting. There are three kinds of focal lengths according to this rule:

 

1. Focal lengths that create images with objects that appear to be in equal proportion and distance to how the human eye perceived them (=80mm on full frame).

Photo taken at 80mm (1.6x crop factor, taken on fixed 50mm)


2. Focal lengths that create images with objects that appear to be further away from and smaller than how the human eye perceived them (<80mm on full frame).

Photo taken at 28.8mm (1.6x crop factor)

3. Focal lengths that create images with objects that appear to be closer and bigger than how the human eye perceived them. (>80mm on full frame).

Photo taken at 300mm (full frame)

I think about this when shooting because I want to know if I’m creating an image that attempts to recreate the world as we see it (=80mm), or if I’m creating an image that will show the world in a new lens. Whether I’m showing more than what you can see in that moment with your eyes by expanding your field of view, or by shrinking it down to focus on a detail you didn’t notice. The main difference between photographs and images seen by the human eye is this. The human eye can see 160 degrees, but we can only focus on a small 55 degree portion of that image. A full frame sensor camera with a 10mm lens on it can take a picture with a 130 degree diagonal field of view and condense the entire image to a 35mm plate.

Photo taken at 43mm on a full frame from 2 ft away, the focal length equivalent of the human eye’s cone of attention.

The only way to create an image that mimics the perspective of the human eye is to take a full frame sensor camera with an 80mm lens, put it on a tripod, and take a picture while rotating the camera 160 degrees. If it’s printed to the right size and the viewer stands the right distance away from it, the photograph will fill the viewers vision and they will be able to look at the image with peripheral and cone of attention view.

This is the reason why, even today with the newest high quality cameras and lenses, every picture still looks a bit unrealistic compared to how we see the world. Once the image quality reached that of the human eye, we still wondered why photos still don’t look like real life. The issue wasn’t entirely resolution, perspective was a factor as well. In the digital age, an image can be resized however big or small, but most of the time, the image is small enough to fit in the 55 degree cone of attention of the human eye. This is a beautiful part about photography. In a way, it enhances our own vision and perspective of the world.

 

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