Photographic Theory
Cameras don't take pictures. People do.
You can spend hundreds of dollars on 'how to' photography books. They are filled with beautiful color photographs. They talk about composition and lighting in great detail. But, until you know and understand the real nuts and bolts of photographic theory, you're only shooting in the dark, so to speak.
As digital photography becomes more and more popular, the basic concepts of photography become more and more obscured by the technology, but none the less important. The relationship between aperture, shutter speed and media sensitivity still are the basic building blocks of image capture. For the purposes of this discussion I will be using the term 'media' to describe the image recording material used in the photographic process, whether electronic (CCD) or photochemical (film). Under "normal" conditions any reasonably sophisticated camera will produce acceptable results with no intervention from the photographer. It's that 10% of situations where intelligent intervention is required that can turn an automated failure into a triumph of photographic skill.
This document covers the following elements of photographic theory:
- Shutter speed: The amount of time light is allowed to contact the media
- Aperture: The size of the opening through which light passes.
- Media sensitivity: The amount of light necessary to produce a recordable image
- "Stop": an adjustment up or down resulting in either twice as much or half as much light reaching the media.
- Light Meter: A device for measuring the amount of light reaching a subject
- Depth of field: The distance between the closest and farthest objects in acceptable focus
- Exposure: The carefully balanced relationship of aperture, shutter speed and media sensitivity optimized to capture a suitable image
- Flash: a means of delivering artificial light to illuminate a subject.
- Photomacrography: taking pictures of very small subjects
The Basics
The easiest to understand is shutter speed. This is the amount of time the media is exposed to light. The amount of light reaching the media in 2 seconds is twice as much as reaches the media in 1 second. Typically, shutter speed is measured in fractions of seconds. A typical camera will allow variable shutter speeds from 1 to 1/1000 of a second in 1 "stop" increments. The scenario might look like this: 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000. The little variances in the 'one half' rule is for convenience and does not significantly impact the overall exposure. Shorter shutter speeds allow less light to pass than longer shutter speeds.
Why do we care about what shutter speed we use? Fast moving action will cause blur with a long shutter speed. Waterfalls and ocean waves look really cool at very long exposures. Dark conditions may require longer shutter speeds to allow sufficient light to pass to the media.
What factors determine whether I should pay attention to the shutter speed? The amount of action in a scene and your vision for its effect will be an important factor. If motion is not a factor, look to other issues for clues to exposure priorities.
The aperture is easy to understand, but those numbers! 1.4, 1.8, 2.8, 5.7, 22, What's that all about? It's actually easy to understand, and a little math will help explain it in detail. Aperture is expressed as a ratio of the diameter of the light admitting opening to the focal length of the lens. A 50 mm lens with a light opening diameter of 50 mm would be rated at 1:1, and expressed as f1. This 50 mm opening would have an area of 1963 square millimeters (pi * r2 or 3.14 * 25 * 25). To reduce the amount of light passing through the opening by one half, we would reduce the area of the opening by half to 981. Divide by pi (= 312.2619983463), take the square root to determine the radius (= 17.67093654412), multiply by 2 to find the diameter (= 35.34187308824). Then, divide the focal length by the diameter and we discover a ratio of 1:1.4 or, f1.4. This process continues all the way DOWN the scale. Smaller apertures are referred to as stopping down, even though the number becomes larger. The sequence - 1, 1.4, 2, 2.8, 4, 5.7, 8, 11, 16, 22, 32... each allowing half as much light to pass through the opening over a given period of time.
Why do we care about aperture? Large apertures require careful focusing but let in lots of light. Very small apertures have a greater 'depth of field', which we will talk about in a little more detail later, but restrict the amount of light and require longer exposure or more sensitive media.
What factors determine whether I should pay attention to the aperture? Again, your subject makes all the difference. Are you more concerned with how much of your picture is in focus, or how action is rendered in the final image.
In a film camera, media sensitivity is indicated by the ISO rating of the film. Again, an exponential scale is used. Film with an ISO rating of 200 is twice as light sensitive as film with a rating of 100. 400 speed film is twice as sensitive as 200. Many digital cameras have selectable sensitivity control and rate that sensitivity on an ISO scale for convenience. I do not know the standard on which the ISO scale is based, but it can be understood in a purely theoretical sense. Whether the scale is a referential value or purely arbitrary is not important.
Why do we care about media sensitivity? With film, faster or more sensitive films are 'grainy', producing lower resolution. Slower film has a finer grain, thus higher resolution. Digital equipment is overcoming some of these issues, but even the current 'mega-pixel' cameras still fall short of the resolution and color rendering capabilities of even the fastest analogue films.
What factors determine the media sensitivity to use? With film, grain must be balanced against speed depending on the size of the enlargement, or sensitivity against the available light on the subject. With electronic media it is important to understand the tradeoffs between image quality and light sensitivity.
A "stop", as alluded to previously, is photo-technical jargon for an adjustment that allows twice or half as much light to reach the media. Manual camera equipment has mechanical detents, or stops, that allow the photographer to 'feel' the adjustments without needing to divert his gaze from the viewfinder. A skilled photographer could quickly adjust his equipment to meet the demands of his subject without the aid of built-in light meters and other 'helpful' automations. Once the amount of light falling on a subject is known, adjustments may be made to accomplish various photographic goals. Both the shutter speed and aperture are usually adjustable in one stop increments. Double check your equipment. Some cameras have aperture 'clicks' on the 1/2 stop as well.
A light meter is a very handy (but not absolutely necessary!) piece of photographic equipment. OK, ok, there are some situations where achieving an appropriate exposure would be extremely difficult to impossible without one, but for general purpose outdoor or indoor flash snap shooting a light meter is only a convenience. Ok, an extreme convenience.
There are two basic types of light meters -- reflected-light and incident-light. Reflected light meters come in two basic flavors -- average and spot. A spot meter measures the light reflecting off a very small portion of the subject. The average meter takes all the light from the scene and gives you an average.
Through years of statistical analysis it has been discovered that the average scene reflects 18% of the light that falls on it. This 18% is what reaches your camera. Your light meter is programmed to know that given a certain sensitivity of media and a given aperture, a specific shutter speed is necessary to correctly register an image at an average reflectivity of 18%. Herein lies the rub. Not every scene reflects exactly 18%. Let your light meter do all the work on the slopes during a day of snowboarding and you may be a bit disappointed in the results. I recommend memorizing, yes, memorizing that little chart printed on the inside of the film box. Before you release the shutter, double check to see that the exposure being registered by your light meter is reasonable. If in doubt, bracket. Bracketing means take a couple more pictures, manually overriding the exposure a stop or two, correcting what might be a mis-interpretation of the reflectivity of your scene on the part of your light meter.
The spot meter measures specific portions of the scene, allowing the photographer the opportunity to gather more specific information about lighting conditions and, in capable hands, produce a more accurately calibrated exposure. This method takes more skill and time. Learn more about Ansel Adam's 'zone' system and you'll want a spot meter of your own.
The incident-light meter measures the actual light falling on the subject. Thus, it would seem possible to produce accurate exposures every time. This is entirely true, except say, when your subject is a black bear standing in a dark forest. I've heard if you ask politely, a bear will stand quietly by while you take your readings and won't eat you until you turn to leave. (The incident light meter is most effective in the studio where subjects are less likely to eat their photographer.)
The distance between the closest object and the farthest that are in acceptable focus is considered the 'depth of field'. Depth of field increases with smaller apertures (bigger f numbers) and decreases with larger apertures (smaller f numbers).
Depth of field is either a by-product of shutter priority automation or the primary objective of aperture priority automation. Many times an exposure is a delicate balance between the desired depth of field and a shutter speed necessary to arrest undesired motion.
Some rules of thumb - Wider angle lenses (shorter focal length) have an inherently greater depth of field at a given aperture than longer lenses. Closer focus results in shallower depth of field than more distant focus.
Depth of field can be used to great effect. Use a large aperture to accentuate a the main subject in an image, leaving the background out of focus. Stop down to the smallest aperture possible for broad panoramas with foreground details preserved intact. Depth of field is an important photographic tool.
In summary, exposure is the delicate balance among a number of factors. At times, the best image possible is a tradeoff between the ideal and the practical. Understanding all the options and their implications is all that is necessary to make informed decisions about the final image.
Advanced Stuff:
Flash photography seems straight-forward enough - Shine a light on your subject and take a picture. That's what it would seem, anyway. And, actually, the advent of the electronic 'strobe' flash has revolutionized artificial light photography. But, there are still things to be learned. First of all, here's how it really works - Open the shutter. Turn on the light. When your subject is adequately illuminated, turn off the light. Close the shutter. And, it all happens with the push of a single button. Automatic flash exposure takes a lot of the guesswork out of the process. But, what if conditions aren't perfect?
Your subject sits in a large space against a very dark background. Your automatic flash floods the area with light, draining every last electron from its capacitor. Its sensors still believe the image is underexposed. Your picture reveals quite the opposite - the subject is grossly overexposed, beyond retrieval. You need to calculate a manual exposure. Here's what you do. Get out your flash's manual. Read the fine print back in the back where it tells what the 'guide number' is. This number tells you how much light your flash puts out. Sort of. The guide number is also based on a film speed - the ISO rating.
A flash with a guide number of 45 meters with ISO 200 film means that a properly exposed image would be achieved at a distance of 45 meters at f1.0. To calculate other exposures for other distances, divide the guide number by the distance to the subject yielding the appropriate aperture or f number.
That's simple enough. What if you're not shooting ISO 200 film? A new guide number must be calculated. That formula is:
Throw a calculator in your camera bag and get over it. You're going to need it anyway if you get into...
I can hear it now - 'You mean macrophotography, don't you?' - No, I don't. The term 'macro' as defined by Webster - of, involving, or intended for use with relatively large quantities or on a large scale. Thus, macrophotography is photography with huge cameras using giant sheets of film. A long time ago Kodak was roaming the country with a camera that produced three foot by two foot slides that were enlarged to 30 feet by 20 feet and displayed with backlighting in Grand Central Station in New York. That is macrophotography on a grand scale. Photomacrography, on the other hand, takes very small subjects and enlarges them life size or larger. Knowing what we do about depth of field, it's easy to understand in extremely close quarters a very tiny aperture will be necessary to focus a suitable portion of the subject unless it is perfectly flat.
The terms are similarly confused in the micro world as well. Micro photography produces images that require a microscope to view, as in spy cameras and silicon chip production. Photomicrography is the process of taking pictures through a microscope.
As if this weren't challenge enough, we start encountering another phenomenon of photographic theory. Previously, we were taking light reflecting off a large object and concentrating it into a smaller point. All the light (well, that magic 18% anyway) that was reflecting off of the entire side of your house was focused down onto a tiny piece of film barely an inch square, or a CCD the size of your fingernail. Now, we are taking a very small subject and spreading that light around a much larger area, sometimes larger than the object reflecting the light itself.
You guessed it. Another formula. This one allows you to correct for the amount of extra light necessary to properly expose the image on the media. Now, unless you have rather sophisticated digital equipment, this stuff is just for fun, right? Here we go...
Exf = (M+1)f
There, that wasn't so bad was it? OK, a little more detail...
Exf is the effective aperture
M= image size / subject size
f is the aperture marked on the lensM is the most difficult number to derive but basically means 'magnification'. Measure the size of the object on the film and divide it by the size of the actual subject. The only problem with this approach is by the time the image is on the film so you can measure it, it's too late and underexposed. So, if you're really fussy, you can try this version:
M= focal length / (subject distance - focal length)
Many advanced cameras have a mark on the top plate of the body marking the exact location of the film plane. This is really handy for calculating effective aperture for close-up photography. A tripod is really handy for making measurements. (Toss a tape measure in your camera bag along with the calculator.) A simple estimation of object size through the viewfinder may be adequate. The image visible in the viewfinder in the standard 35mm single lens reflex camera is approximately 35mm wide. A well informed estimation of size is probably adequate.
So, what does this all mean? If the image on the film is one inch long, and the subject is also an inch long, then M=1 and the effective aperture of a lens at f2 is (1+1)*2 = 4. In other words at a magnification factor of 1:1, you're losing 2 stops of your expected exposure. If you were trying to improve your depth of field by shooting at f16, then expect to need to add enough light to expose the media as if you were shooting at f32. If your macro lens stops down to f32, then plan on flooding your subject with enough light to adequately expose a normal image at f64! Be careful. A large flash at close range is capable of incinerating small insects and igniting fabric and paper.
Remember, cameras don't take pictures. People do. You are responsible
for the final outcome. It's ignorance of the theory and techniques
that create bad pictures.
Your input and suggestions are welcome...
© Terry Blackburn, 2001
