The 'auto' thing

I want to explore the history and working of some of the most common automated features in film and digital cameras.

In 'An incomplete history of cameras' I mentioned how before the digital cameras were born on planet earth, a lot of electronics started appearing in film cameras. Most important in my opinion were the introduction of in-built light metering, auto focussing and auto exposure. All three helped in making cameras popular and easy to use. 

Through the lens (TTL) metering & Auto Exposure:

Many of you may have seen professional photographers with their funky looking light meters pointing and poking the subject. Not poking maybe. What they are doing is measuring the light reflected off the subject. It can give very accurate exposure and these meters are still used by many professional photographers.

During the 60s, SLR film cameras fitted with TTL metering started appearing in the market. The basic concept includes a light sensor array located within the camera body capable of measuring the light entering through the lens and providing that data to the photographer or to the camera circuit to adjust shutter speed or aperture to get correct exposure. Initially the TTL metering was limited to average metering but soon SLR cameras started sporting more metering options. Modern cameras have mainly 3 types of metering options:

Spot,

Center-weighted and

Multi-zone.

Spot metering is useful for close up/ macro shots or when the scene to be photographed has strong contracting elements- say a person standing in front of a bright window. Using spot metering, you can measure the correct exposure for the subject while ignoring the bright background. You can do the reverse by correctly exposing the window and get a silhouette of the subject.

Center weighted is similar to spot but here the metering is averaged for the entire frame with added emphasis to the center of the frame. It's use is similar to spot metering but preferred when the subject fills a larger portion of the frame; for example a portrait against a bright or dark background.

Multi zone metering: As the name implies, in multi-zone metering, the camera takes data from multiple points distributed throughout the frame. This setting is pretty reliable for average scenes except as mentioned above for spot and center metering. All cameras have a visual indicator to show the light intensity. I remember using a manual SLR camera that had a tiny needle on a semi-circular dial visible through the viewfinder. The needle would show if the scene would be over exposed or underexposed and you could adjust the aperture and shutter speed till the needle comes to the center of the dial.

Modern cameras have a digital display  normally visible though the viewfinder or a separate LCD screen. Fully automatic cameras also do not require you to change aperture or shutter speed; they would do it automatically for correct exposure.

So how does the camera know when the scene is correctly exposed? 

The metering system starts with an assumption that a typical scene reflects about the same amount of light as 18% gray. The assumption is not random, in fact under most circumstances, this assumption works pretty good. However, things are not always this simple in the real world. The 18% gray assumption does not work well if you photograph a scene with mostly dark colors- say a forest or a black wall. The way most cameras are calibrated, you'll most likely get an over exposed photograph. Similarly for a scene filled with light/ reflective colors like a beach or snow will result in an underexposed photograph. That's where your camera's exposure compensation setting comes handy. Another way to deal with tricky scenes is to get the exposure reading from a standard 18% gray sheet lock the exposure and click. You can find one from a photo shop. There's a very good book on photography from National Geographic which has the inside of the back cover in 18% gray.

Most SLRs have an exposure bracketing feature that allows you to take 3 successive shots with one correctly exposed per camera's settings and the other two under-exposed and over-exposed by a predetermined amount. Many photographers do this to increase the odds of getting a correctly exposed shot. With digital cameras, bracketing also allows you to combine the three photographs to get a High Dynamic Range (HDR) Image.

Auto Focus:

Auto focus has been around for some time and I'm sure many of you reading this may have never used a manual focus lens. Though all autofocus SLR lenses have an option to use manual focus which is handy when you are trying to focus on a flying kite and your camera is trying to focus on a cloud.

Let's look at the basic concepts of auto focus.

There are two major Types - Active AF and Passive AF

Active AF:

Active AF uses ultrasonic sound wave or infrared beam to measure distance between the camera and the subject and adjust the focus accordingly. Active AF was popularized by polaroid and was normally used in compact film cameras. I haven't seen Active AF in any of the modern cameras.

Passive AF:

Passive AF analyzes the light entering through the camera lens to achieve focus. It's done in two ways:

Phase Detection: Phase detection is much faster and the default focussing method in most SLRs (except in live view option when it shifts to contrast detection). Phase detection works by splitting the incoming light into two separate beams and calculating the phase difference by analyzing the intensity. It allows the camera to know if the object is in front focus or back focus and it can instantly move the lens in that direction.

Contrast detectionContrast detection is used in most compact digital and mirrorless interchangeable lens cameras. It is very precise but slower that phase detection. In principle it works on a simple assumption that an image has the highest contrast between pixels when it is in focus. In this method, the camera sensor is used to autofocus and there is no way for the camera to know if the image is in front focus or back focus and it has to move the lens back and forth a few times to achieve correct focus.

Compact digital cameras also do not have shutters since the sensor has to be used for focussing (The sound that you hear on clicking is just an electronic sound byte for effect and can be turned off in many cameras). Digital SLRs shift to contrast detection in live view mode and if you have one you can easily notice the extra time the camera takes to focus when you use the LCD instead of the optical viewfinder.

The next time you take your camera out of the bag, think of all the things that work together to make that picture.

 

Vishal Charles

June 30, 2011

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