Saturday, November 29, 2008

Perceiving Light-The Difference Between The Human Eye And Camera Sensors

A very important concept to understand in photography is that the way you see something exposed is not the same way a camera does.

You may see a beautiful morning scene, trees illuminated softly with sun's light and a sky full of colorful clouds.

You want to take a picture of such moment, and if you don't understand this concept, you will find yourself in one of these scenarios:

  • Your shot comes out underexposed since you didn't use a slow shutter speed, you thought light was enough for 1/40 and it turns out you needed a 5 seconds exposure.
  • The trees come out properly exposed but the sky is blow out.
  • The sky came out properly exposed but the trees are underexposed.

By the time you get it right, sun has come out completely and the clouds are white now or gone.

The crucial difference

The subject of why what your eye sees is not what your camera sees is not exactly a simple one, there is a lot of science in between, however, I will give you the facts you need to know to understand it.

In a simple statement, the main difference between the human eye and a camera sensor is that the human eye can perceive a wider range of light than a camera sensor does.

Difference explained

So what does that mean?

Your eyes can perceive light in a way that you can see details even when your subject is illuminated from behind, or in scenarios where both the front and back planes are illuminated, your eyes can see both planes properly.

A camera sensor can not do this. If you took a picture of someone while being back lit, you would need to add light to the picture so the camera shows the person's face, in this case you would need to use a flash.

In the example of the first section, either you have to pick between the trees or the sky, but you can't get both.

Another good example would be while you're indoors. Usually you can see your way around inside a building pretty well, but a camera can't, in practical terms, it would be blind.

If you took a picture from inside a house at noon, you either properly expose the inside and have a blown out, practically white outside OR you get a properly exposed outside while the indoors look like a cave.


For explaining purposes, lets imagine a scale from 0 to 100. 0 would be really REALLY dark and 100 would be really REALLY bright.


In that scale, a human eye could perceive from around 20 to 80/85, while a camera sensor could perceive from 40 to 70/75.

It's not exactly a scientific scale but it gives you a notion of how much difference there is.

Mr. Gary L. Friedman explains this concept in his books for the Alpha DSLRs in stops. A stop would mean a factor of 2 in light intensity.

He uses the following examples:

Photographic paper: 6.5 stops

Digital sensors: 8 stops in JPEG, 10 in RAW

Color negative film: 12 stops

Human eye: 30 stops

Source: The Complete Guide To Sony's Alpha 700 Digital SLR Camera And The Alpha Mount System by Gary L. Friedman

This explains it in photographic terms, the human eye can sense a lot more light and shadows than a camera can. When a camera sensor goes beyond its range either for shadows or light, the data is lost, you can't recover it with post processing. In a histogram, this would show up as clipped lines on either sides.

So why do I need to know this?

Knowing and understanding the fact that a camera sensor doesn't see a scene as your eyes do is a critical thing for any photographer to know.

If know about it, you can know how much your sensor can really see and what things (shutter speed, aperture, ISO, etc.) you will need to set so the photo comes out as you want.

If you control this fact, you can create artistic pictures or unique looking photos, but if you don't, you will be facing constant frustration as you will be exceeding the sensor's range to perceive light, and your shots will be coming either underexposed or totally overexposed.

Now you understand why you need to add flash to portraits even when shooting outdoors with sunlight? Or why you need slow shutter speeds/high ISOs or really wide apertures to get shots indoors? Why if you get a properly exposed plane, the other one may be dark or over exposed? Why if you go out at night you can see properly but your camera can't?

Your eyes may see those situations properly exposed with no problem (unless its REALLY dark or REALLY bright) but your camera can't. That's because the range in which they can see light is completely different

Advices and closure

Here are some advices to overcome the limited brightness range of your sensor:

  • Buy a light meter
  • Use your camera's metering function
  • Use wide aperture lenses (In the Alpha line, the fastest lens available are the 50mm f 1.4 and the 35mm f 1.4 G)
  • Use high ISOs
  • Practice shooting in low light or in full light (like at outdoors at noon) to understand what settings you need to use in order to get a properly exposed shot
  • Always remember that what YOU see is not what your camera sees
  • Learn to see as your camera does

Now that you know the main difference between the human eye and a camera sensor, you should understand why such things as ISO, shutter speed, aperture work with each other to create a properly exposed picture.

This difference between your eye and the sensor regarding brightness range can be used for all sorts of purposes, but if you don't know how to handle it or know about it at all, it may affect your pictures in a bad way.

If you ever wondered why you couldnt get that shot of the trees and the sky properly illuminated in both planes, now you know why. In that case there is no other thing to do but to pick which plane you want to get...or you can try High Dynamic Range photography.

The limited range of a sensor may be a nuisance, since you can't capture photos the exact same way the human eye does, but it's overcoming such limitations like this one that make a great photographer and create great pictures.


  1. This is why I got online to research this topic "Learn to see as your camera does". This phase has been mentioned a lot lately and they all seem to describe the differences between camera and human but do not explain on how to go about doing it. I would like to know what steps I am missing to begin seeing like my camera. I'm sure it something simple that I'm overlooking or perhaps overthinking but right now I seem to be clueless on where to start. If you can help just say it straight it is getting old trying to read between the lines. Well that how it feels from everything that I have been finding and reading about. Just give a straight answer please.

  2. Hello Matt:

    To see like a camera does, first you got to understand the concepts in this post; your eyes and brain process the information in a different way than a camera does, that along with the extended dynamic range our eyes have, make a lot of people frustrated when taking pictures.

    "Seeing like your camera does" is a mental process. The thing you always got to keep in mind is that a camera will always need more light than your eyes and brain require.

    Take this as an example: During a full moon, if you're outdoors and the only light available is the moonlight, then your eyes will adjust to the very low light, but if you try to take shots with just that source of illumination, you will get underexposed shots if you don't a) use a very slow shutter speed and tripod, b) use a very wide aperture or c) raise your ISO to the top level your camera allows. While this will yield a better exposed shot, there will probably be a lot of dark zones where you can actually see the detail.

    The mantra (for lack of a better word)you got to keep in mind always is "If I can see the detail in a very dark or very bright situation, my camera will most likely won't, therefore, I need to add and/or block the light in order to balance it out to how I see it"

    The best example of this kind of mantra I can think of, is film lightning. If you watch a "behind the scenes" documentaries of movies, you will notice that in practically all of them they use A LOT of lights, be it daylight or night time. To our eyes it seems redundant and pointless, but because film doesn't have the whole dynamic range our eyes and brain have, they need to add or block light in order to make things seem "normal" to our eyes. We don't usually notice it because if our eyes don't have a problem looking at what's in them in terms of brightness, we don't look for another source to light things up. It's only when we have trouble seeing that we either add more light or reduce light (like wearing sunglasses).

    There isn't a recipe of how to see like your camera, it's a mental process. Keep in mind that your camera doesn't see a scenario like you do and you will have to add or block light, or even do both at the same time.

    As sensor technology progresses, the dynamic range of sensors improve. The latest middle format backs from Mamiya Leaf have sensors with up to 12.5 stops of dynamic range, that's quite a lot! But every good photographer knows that usually ambient lightning is not enough and you got to add more or block it if it's too much.

    I suggest you practice during dawn, twilight and night in order to get this concept. You will under and overexpose shots horribly at first, but eventually you'll figure out the limits of your camera regarding light and you will be able to control it, be it by the use of filters, exposure values or direct control of the light in the scene.

    But always remember that your camera doesn't always see detail where you can.

    I hope this helps.

  3. Thanks Diego, I appreciate the time taken for your reply. Your the second person to suggest shooting the light albeit low light the other person suggested shooting bright light. In any case I was having a hard time wrapping my head around this concept but I have a better understanding now.


  4. Hello again Matt:

    You're welcome. I'm glad the information I gave you made more sense now.

    Shooting bright light does help you understand the concept as well, but I find that modern sensors handle bright lights well most of the time and they can render the scene as you see it with your eyes and brain. It's on situations of stark differences, like during dawn or twilight, that you see the problem the camera has with dealing with a very bright and a very dark scene at the same time.

    Still, practicing in very bright light does help to understand the whole concept of seeing things as your camera, except that under bright light, you usually need to block or reduce the brightness of it with the use of filters, whereas on low light situations, you need to get creative with the lightning.

    Thanks for dropping by!