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Photography 101: The Basics of Exposure


I decided to write up a couple of blog posts for people just getting into photography, specifically people who just picked up a shiny new Nikon D3000, Canon T2i, Sony Alpha 390 or a Pentax K-something-or-other, and would like to do more than simply shoot in "Auto Mode". Of course, not everyone who buys a DSLR is interested in studying photography; some people simply want a camera that's better than a point-and-shoot, and camera manufacturers are more than happy to both claim explicitly that DSLRs are better than compact cameras , and also imply that using DSLRs will make you a better photographer. To the first point, DSLRs are indeed "better" than compacts in many respects, chief among them the larger sensors and quality lenses, which will always be physical limitations that compact cameras can never overcome, not matter how many megapixels are jammed into their tiny sensors. The second point is, as you probably already know, hogwash. Buying a Steinway & Sons grand piano won't make you a better pianist, piano lessons and practice will. The same is true of photography, and there are several concepts to learn, both technical and artistic. This post on exposure will be the most technical of those concepts, but a critical first step to taking control of your camera and crafting the photo you want, instead of settling for whatever the camera has decided on.


1. Intensity, Time, and Sensitivity

My Nikon FM - a fully manual film camera. If you can shoot with this, you can shoot with any camera.

Photography is all about recording light, and exposure is the method by which a camera achieves this. At it's most elemental level, a camera is an opaque box with a small hole on one end for light to pass through and project an image onto the inner wall on the opposite end. Artists throughout history would use camera obscuras to project a scene, such as a landscape, onto the opposite wall, tracing the contours of the image. The introduction of light sensitive chemicals in the 19th century would transform the camera from a mere projector into an actual recording device. To the small hole would be added optical glass to focus the light, further clarifying the image, and since the photo-chemicals would render a pure white image if exposed to light for too long a time, measurements were devised for both the amount of light let in and for how long it should expose the photosensitive surface. These are the basic elements of photography; the intensity of the light, the time need for exposure, and the sensitivity of the film/sensor.

As photographic technology progressed, photo-chemicals were designed to be more sensitive to light. This led to the need for faster exposure times (early photographs were taken by removing the lens cap, counting off the minutes or seconds needed, and putting the lens cap back on), and mechanical shutters were introduced to accommodate these split-second exposures times. As fast as these new shutters were, photographers were still encountering situations where the light was too intense for proper exposures. An aperture, which is a metal diaphragm that opens and closes, was added to lenses to further control the amount of light let in.


2. Intensity is measured in f-stops

A Nikon 50mm f/1.8 Series E lens - a cheap, yet good quality "fast" lens from the 1980s The intensity of the light coming through the lens is affected by both the focal length and the aperture. The focal length is the distance light rays are brought into focus by the lens. A short focal length, like 18mm, will have a wide angle of view; a long focal length, like 200mm, will have a much narrower angle of view. As focal length increases, the less light is let in, decreasing the overall intensity of the light reaching the sensor (If you've ever used binoculars or a telescope, you might remember how much dimmer the view looked). Apertures are constructed to open and close their diaphragm blades at set distances, each distance creating an opening that is double the size as the previous setting. The wider the opening, the more light is let in; the narrower the opening, the less light is let in. In order to have a uniform measure of light intensity for all lenses, a ratio of the focal length and the aperture opening was created, called an f-stop. One f-stop is equal to a doubling or halving of light's intensity (the term "stop" is also used to refer to the doubling or halving of light for shutter speeds and ISO as well). With modern DSLRs, the lens aperture is electronically controlled, but in prior decades the lens aperture was set by rotating a ring which clicked between each f-stop (the Nikon 50mm lens pictured here is one such lens). A typical range of f-stops you will encounter will be: f/1.4, f/2, f2.8, f4, f5.6, f8, f11, f16, f22, f32. f/1.4 is the most wide open of this range, and is found on a select few lenses with optics capable of letting in a large amount of light. f/22 and f/32 are available on most lenses, as the they are the result of a more closed down aperture.



3. Time is controlled by shutter speed

The shutter speed is currently set to 1/125th of a second. The ASA (a precursor to ISO) is set to 200, which allows the built in light meter to calculate exposure against this value.Just as f-stops are delineated in increments that double of halve the light, shutter speeds are set at intervals that roughly double or halve the time the shutter opens and closes. The picture you see here are the shutter speeds available for my Nikon FM (a fully manual film camera from the 1970s), and illustrates what I mean by "roughly" - 1/15th of a second isn't exactly "half" 1/8th of a second; likewise 1/125th of a second is slightly more than half of 1/60th of a second. This slight discrepancy is due to the mechanics of shutters and what delineations are possible in the timing. Still, the speeds are close enough to the 2:1 ratio mirrored in f-stops to make them reciprocal of each other; one click of the aperture ring is equal to one click of the shutter dial. For todays DSLRs (as well as more modern film cameras), f-stops and shutter speeds are further delineated into 1/3 of a stop, so that instead of going directly from f/4 to f/5.6, it goes f/4 > f/4.5 > f/5 > f/5.6. Likewise shutter speeds are divided into thirds of stop so that 1/60th to 1/125th progresses as 1/60th > 1/80th > 1/100th > 1/125th. While this gives the photographer a much greater degree of control over exposure, it does make it more daunting for someone just learning. You can set your DSLR to only progress in full stop increments, but in the long run, it's more advantageous to just bear in mind that 3 clicks of wheel equal one full stop.





4. Sensitivity is measured by ISO

The ISO rating of 400 for this black & white print negative film allows for faster shutter speeds in lower light situationsISO refers to the "International Organization for Standardization", which sets engineering standards for manufacturers to follow, allowing for more interoperability.  For photography, this includes a set standard for a film's sensitivity to light.  An ISO value is also referred to as the "film speed", since the lower the ISO value, the slower the shutter speed needs to be to properly expose the film, and vice-versa.  Like aperture and shutters speeds, ISO is also delineated in "stops", where each value is double the previous value.  The typical range of film speeds for most films is as follows: ISO 25 > 50 > 100 > 200 > 400 > 800 > 1600.  A fast film speed like ISO 1600 will need less time to expose, but the resulting image will be grainier in appearance, due to the larger silver crystals in the emulsion needed to gather more light.  Likewise, ISO 25 will take the longest to expose, but will have a finer, smoother appearance due to smaller, more evenly distributed silver crystals.

When shooting with film, the ISO (or ASA as it was called before ISO adopted this American standard) value meant that the photographer essentially makes his or her first decision about exposure when choosing which film speed to use before loading a roll of film.  Loading a roll of ISO 400 film locks in the first of your 3 exposure variables, with aperture and shutter speed then adjusted up or down to account for the sensitivity of your film.  In DSLRs, the ISO is a setting that can apply to the image sensor at any time between shots.  DSLRs typically have a "base" ISO of 100 or 200, which is the optimal setting for that image sensor for the least noise and best color saturation.  Turning up the ISO in your DSLR will amplify the sensitivity of your sensor, allowing for shorter shutter speeds, but at the cost of a noisier image (similar in effect to film grain) and less color saturation.  The limit at which ISO of a DLSR sensor can be boosted before noise overwhelms the image varies with each camera model, with the latest digital sensors being capable of the highest ISO settings.  My Nikon D70, which was first manufactured in 2004, has a range of ISO 200 to ISO 1600, though I rarely go above ISO 800.  Conversely, a Nikon D3s has an incredible range of ISO 200 to ISO 12,800, meaning a setting of ISO 6400 on a D3s produces a cleaner, more vibrant photo than ISO 1600 does on my old D70.


5. Light is measured with light meters…or an educated guess

Before the introduction and widespread use of light meters in the mid 20th century, photographers had to rely on shared experience through trial and error to determine the correct exposure for a given scene.  It helped that sunlight is a consistent source of light, particularly on clear sunny days, from which to base a rule of thumb; that rule of thumb being the Sunny 16 Rule.  I'll delve more into this in a future post; suffice it to say photographers back then relied on judgement and experience to set their apertures and shutter speeds.  With the innovation of light meters, photographers had a more consistent method of measuring light and determining exposure.  At basic level, a light meter measures the light, and then computes the combination of aperture, shutter speed and ISO setting needed to produce an exposure equivalent to a middle grey tone.  Middle grey is the tone that marks the midpoint of the tonal range of exposure from completely underexposed (pure black) to completely overexposed (pure white).  To simplify for the sake of conceptualizing, if your light meter thinks f/5.6, 1/125th, ISO 400 will achieve middle grey, then you can reasonable expect expect your scene to expose properly.  It's actually not quite this simple, which accounts for one of the reasons more experienced photographers don't use "Auto Mode".  As with the Sunny 16 Rule, I'll expand on the vagaries of metering a scene in a later post.  For now, recognize that when you camera automatically determines the settings for you, it does so by first measuring the light with it's built in light meter, then by adjusting the aperture, shutter speed, and, in digital cameras, the ISO in order to get a shot that is properly exposed.  In a DSLR, you will know how well your photo exposed fractions of a second after you press the shutter, as the camera records the image on it's sensor, processes it into a jpeg, writes the jpeg file onto your memory card, and displays on the LCD screen on the back of your camera.  If you were using a film camera, you wouldn't know until after your film has been processed, something I continually need to remind myself as I reflexively stare at the leatherette back of Nikon FM after each shot I take.


6. How it all ties together

What the inside of my viewfinder looked like when taking the shot for the first photo seen in this blog post. The difference in appearance is largely due to the off-camera flash I used when taking this photo.If you switch your camera into manual mode and look through your viewfinder, you will see numbers that represent your shutter speed and aperture (and in some cases your ISO as well) and probably as couple of symbols to indicate things like battery life, flash, etc. You will also see a scale indicating your exposure meter. This is the camera's built in light meter telling you how overexposed or underexposed your photo will be at the given aperture/shutter/ISO setting you currently have your camera set at. In this picture here, showing what my viewfinder looked like at the time, you can see that my camera was set to f/5.6 and 1/250th (the ISO was 200, but that isn't displayed in my viewfinder). At these setting, the meter is showing my photo will be severely underexposed, with little rectangles progressing towards the "minus" side of the metering scale. If I had opened up my aperture to as high as it could go (f/2.8 in this case) and/or slowed my shutter speed considerably, the little rectangles would regress toward the "0" in the middle. If I kept dropping my shutter speed even further, they would then start progressing to the "plus" side of the scale, indicating overexposure. A "correct" exposure is one where there are no little rectangles (or hash marks, or whatever symbol your camera's meter uses) on either side of the "0" at the midpoint of the scale. The "0" indicates an EV (Exposure Value) of zero, which means middle grey. You can do the same with your DLSR, and I encourage you to give it a try. As you do so, you will start to notice that several different combination of aperture/shutter/ISO settings will yield a correct exposure, but that these different combinations have different effects on your image aside from how light or dark your image turns out. These effects, specifically blurriness at slow shutter speeds and the depth of focus at differing apertures, will be the subject of the next Photography 101 blog post.

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