This is a fundamental "problem" with cameras, that is, their apparent lack of dynamic range, especially when compared to the human eye. Most cameras seem to have an average dynamic range of around 11 stops of light.
What is a "stop" of light you ask, thought you would never ask!
What is a stop anyways?
A stop of light is a relative measure of light based on a number of factors, such as lens aperture (the amount of light the lens allows to enter the camera), the sensor sensitivity (ISO) and the amount of time the aperture is open (shutter speed). Kind of not an answer to the question isn't it?
But this is where it gets to be fun. So remember our 18% gray? Well, consider that to be 0 stops of light, properly exposed. If you double the amount of light, you have increased the number of stops by 1. If you take away half the amount of light, you have decreases the amount of light by 1 stop, or -1. The same geometric principles that applied to determining 18% gray applies to stops of light. Stops of light is really a relative scale, such as that below:
-2 -1 0 1 2
Half light Half light Properly double double
of -1 of 0 exposed light light
stop stop of "0" of "1"
The "Dynamic Range" of Your Camera Versus Your Eye
Perhaps the second great debate among photographers is the dynamic range of the human eye as compared to the camera. And this is like comparing the similarities between a Saturn V rocket which launched the Apollo moon missions and the model rockets I used to make that used "c" or "d" size cartridge solid fuel engines. They are both rockets, they both go up. They both are cylinders They both have mechanisms to produce thrust. And that is where the similarities end.
The human eye and a camera are like that, the human eye is the 'Saturn V rocket' and the DSLR is the 'model rocket' of light capturing ability. Unlike a camera which views an entire scene in its entirety, the human eye roams throughout the scene, in essence painting the image for our brains to view the scene. The eye will focus first on those areas which are the brightest or considered most important, such as eyes on a person's face, then will wander throughout the scene and 'paint' the picture.
In this sense, the human eye, wandering through a scene has a dynamic range of around 24 f-stops. This is due to the opening and closing of the pupil, which allows the eye to regulate the light coming into the eye. Our eyes are more similar to video camera technology than it is to still camera technology.
If we just stop and view a single object, then the dynamic range of the eye closely matches the range found in still cameras, which is around 8 to 11 stops (yes, some camera manufacturers claim upwards to 14 stops, but in real life the range doesn't quite match the laboratory).
Ansel Adams and the "Zone System." Remember, there is a test after this! (Just kidding, kinda!)
Ansel Adams, perhaps the most famous of landscape photographers, developed a system to understand light and the tones in a scene to help him determine proper exposure. What is nice about his system is that it fits well with the concept of measuring light using stops.
He developed a system called the 'Zone System' where there are 11 zones of gray, 9 zones really if you count black and white which are at either end of the zones. Hence zone 5 is middle gray, and each zone moving up or moving back correlate to the use of stops of light discussed above.
Zone 0: Pure black
Zone 1: Near black, with slight tonality but little to no texture captured
Zone 2: Textured black; the darkest part of the image in which slight detail is recorded
Zone 3: Average dark materials and low values showing adequate texture
Zone 4: Average dark foliage, dark stone, or landscape shadows
Zone 5: Middle gray: clear north sky; dark skin, average weathered wood, what your light meter determines to be 0 stops (see chart above for zone 5).
Zone 6: Average Caucasian skin; light stone; shadows on snow in sunlit landscapes
Zone 7: Very light skin; shadows in snow with acute side lighting
Zone 8: Lightest tone with texture: textured snow
Zone 9: Slight tone without texture; glaring snow
Zone 10: Pure white: light sources and specular reflections
In digital photography, the zone system is an important tool to understand proper exposure for the scene before you. Most cameras, especially DSLRs, all have histograms. Histograms are another important tool in your toolbox. And the histogram will normally cover from zone 3 up to zone 7. Anything below zone 3 will be seen as just black, and anything above zone 7 will be seen as just white.
In Ansel Adam's days, he shot using negative film, which has a fairly high dynamic range. He would often "exposure for the shadows and process for the highlights." In digital photography, we generally expose for the highlights and process for the shadows. Shadows being black is generally more acceptable than blown highlights - pure white - which is what you get when your exposure range is beyond zone 7.
Links to Explore
- Lee Varis has a great website devoted to understanding the zone system and applying it to digital photography. You can click here to reach his website, which is at https://www.varis.com/DigitalZoneSystem/Digital_Zone_System-Part_1.html
- Russell Cottrell also has an excellent article on how cameras expose and the zone system titled Beyond the Digital Zone System, which can be reached here, https://www.russellcottrell.com/photo/btdzs/index.asp
- Alan Ross, who worked with Ansel Adams, has a great article about some of the myths and applications of the zone system. His article can be reached here, https://www.alanrossphotography.com/category/tech/zonesystemandmetering/ He also has a bunch of great articles on the same page that go into more detail on what I have already talked about in the prior articles. All in all, this is a great resource to help you understand the zone system and it's application to photography.
Next blog article will discuss the application of the zone system as it is a great way to assess the dynamic range of a scene.
P.S. Please understand that this is not an attempt to write a technical article, so some specifics have been glossed over for the sake of explaining the concepts. This is a very complicated subject and one can quickly find themselves deep in the weeds on this subject. This is NOT a definitive account of how exposure works, but just a way to explain the concept so that the average photographer can use the concepts in the application of photography.