TECHtata

Exposure

Robert Dalton

If you are using your camera in the AUTO mode and still getting decent photos, why switch to the manual mode? Why bother learning the elements of photographic exposure if your camera does all the exposure calculations for you automatically? These are legitimate questions asked by many amateur photographers.

So, what's wrong with Auto Exposure, if you are getting decent results? First, let me say that you can get great photos on AUTO if the conditions are right. If you keep the sun behind you and your scene is comprised predominately of mid-tones with no bright sky, your photos will most likely be as good as a properly exposed manual shot. Of course, you will have to stay away from snow scenes , white sand beaches and any large body of water with light reflections. I would also avoid the mid day sky on a sunny day.

Notice what happens when you give up control of exposure for the convenience of the automatic mode? You limit yourself. You give up the chance to get proper exposures in difficult lighting situations. You will miss some of your best chances to get great photographs. If you are an amateur photographer and you want your photos to look professional, you need to start shooting the way the pros do: in manual mode.In order to start shooting manually, you need to understand three elements of your camera:

aperture diaphragm
shutter
reflected light meter

Let's start with the aperture and the shutter because they are the yin and yang of photographic exposure. Exposure is created by a mechanical relationship between the aperture and the shutter.

The aperture is a diaphragm that controls the volume of light that is exposed to the film/sensor. The aperture diaphragm opens from the center, using a series of overlapping leaves.

The larger the diaphragm becomes, the more light it allows through. The aperture controls the volume of light reaching the film or sensor. The shutter is a devise that controls the duration of time that the volume of light (controlled by the aperture diaphragm) is exposed to the film or sensor. On most cameras, the shutter is two curtains that open and close for a pre-determined amount of time. So, together, the aperture and the shutter control the exact amount of light hitting the film or sensor.

volume + duration = exposure

Now, the key to understanding this relationship between the aperture and the shutter is to know what a stop is. In photography, a stop is a relative measurement of light that either doubles the amount of light or cuts it in half. A typical 35mm camera will have aperture designations on the lens called f stops.

These are the aperture settings. They indicate how large the diaphragm opening is. The smallest number, in this case f/1.4, is the largest opening, allowing the most light through. The largest number, f/16 on the diagram but generally f/22 on lenses, is the smallest diaphragm, allowing the least amount of light through.

Each time you change your aperture by one full stop, you are either doubling the amount of exposure , or cutting it in half.

If you change your aperture from f8 to f5.6, you are doubling the amount of exposure because you are changing your aperture one stop to a larger diaphragm. Conversely, if you change your aperture from f8 to f11, you are cutting your exposure in half because you are changing your aperture one stop to a smaller diaphragm.

The shutter settings on your camera are also called stops because, like the aperture stops, they either double, or cut the light in half. The shutter settings on a typical 35mm camera look like this:

1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500

These settings indicate the duration of exposure or, the length of time that your shutter will open for exposure. They are measured in fractions of a second until one second, then in seconds. So, if you change your shutter speed from 1/125 to 1/60, you are doubling the duration of exposure and therefore doubling the amount of exposure. If you change your exposure from 1/125 to 1/250, you are cutting the duration of exposure in half therefore cutting the amount of exposure in half.

As you know, exposure is the product of the volume of light allowed by the aperture and the duration of light allowed by the shutter. So, let's put the two elements together. Lets suppose that the correct exposure for a scene is an aperture of f8 and a shutter speed of 1/125.

Depth of field and aperture have a direct and simple relationship. The wider open (the smaller the f-number) the aperture, the more shallow the depth of field.

Now let's suppose that we want to make an image of our scene that has shallow depth of field. We know that the larger our aperture diaphragm, the less depth of field we will have in our scene, so we change our aperture to f4. We have just increased exposure by two stops, if we were to make an exposure at 1/125@ f4, the resulting image would be two stops over exposed.

By changing our aperture from f8 to f4, without changing the shutter speed, we have doubled our exposure twice! Now what would happen if we cut our exposure in half twice with our shutter speed to 1/500? We would have equivalent exposure. A camera setting of 1/125@ f8 is equal in exposure to a camera setting of 1/500@ f4.

It doesn't matter what camera or lens you are using, or what the ISO is on your film or sensor; these two exposure settings will always be equal exposures.

Understanding equivalent exposure is the most important lesson you will ever learn in photography; and it's as easy as counting stops. In the above example, we changed the aperture two stops MORE exposure from f8 to f4; then we changed the shutter two stops LESS exposure from 1/125 to 1/500, keeping our exposure equal. One stop of aperture change is equal to one stop of shutter speed change. If you change your aperture to one stop LESS light, you must change your shutter speed to one stop MORE light. Two stops for two stops, three stops for three stops, four for four and so on.

Using our example above, 1/125@ f8 is equal to 1/60@ f11, which is also equal to 1/500@ f4. Here are some other shutter speed / aperture combinations that are equal to 1/125@ f8: 1/30@ f16, 1/1000@ f2.8 and 1/15@ f22.

YOUR LIGHT METER

In the above example of equivalent exposure, how did we know that 1/125@ f8 was the correct exposure in the first place ?

The best way to know the correct exposure of a scene is to use your built in reflected light meter in an intelligent and thoughtful manner. I say "intelligent and thoughtful" because your build in reflected light meter is dumb. Well, it's not dumb about reading the correct exposure of "middle gray", but it's dumb about everything else.

Built in reflected light meters are calibrated in the factory to read the reflection of light on a surface of 18% gray (middle gray). So, when you are outside using your built in reflected light meter, your meter is telling you what exposure setting will produce middle gray. If you point your meter at a bright sky, your meter will give you the exposure to darken the sky to middle gray. On the other hand, if you point your meter to a dark shadow area, your meter will give you the exposure to lighten the area to middle gray.

So, here is where using your meter in an intelligent and thoughtful manner comes in. Start by pointing your reflected light meter at the mid tones in your scene, and only the mid tones.

If you fill your frame with mid tones, take an exposure reading, set your shutter speed and aperture according to the meter, then re-frame to the original scene that you want to shoot and make your exposure the highlights and the shadow areas will fall where they belong, and of course , the mid tone areas will be correctly exposed. By using this method, you are in effect, making your averaging meter into a spot meter. This is called selective metering.

What if there are no substantial mid tones in a scene to read with your meter? This happens quite often in snow or beach scenes. In this case, you can buy an 18% gray card at any photo store; I never go on location without one. You simply place the gray card in the same light as your subject and fill your frame with the card ( be careful not to let your shadow fall on the card), take a meter reading, set your exposure, remove the gray card, re-frame and make your exposure.

This is called a substitute reading. With more experience, you will learn to recognize other mid tones that can be used as a substitute reading. For example, I have found that a green lawn is very close to middle gray. I use green lawns as a substitute reading quite often because green lawns are very common where I live. Now is the time to test the principles of manual exposure versus your camera's AUTO mode.

Go to a location and take a series of photos with your camera on AUTO. Make sure that you take photos with a variety of lighting situations, including bright sun, high contrast scenes, as well as open shade, lower contrast scenes. Then shoot the same scenes using the concepts of manual exposure explained above. I think that you will find that manual exposure out performs AUTO exposure, especially in the high contrast situations. It is my hope that you will make the leap to using manual exposure more often.

Remember, if you want your shots to to start looking like the pros shots, you have to start shooting like a pro ... on manual mode.

A cropped screen shot from ted.photographer a truly
fabulous site for seeing how a camera works.

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TECHtata is a weekly technical column on the general topic of photography and is edited by Ryan Brenizer (carpe icthus). Photos used on utata.org are stored on flickr.com and obtained via the flickr API unless otherwise noted.

Questions, Comments, Discussion

On July 09 2006 Catherine Jamieson said ...

Comments are encouaged as are questions on the topic for the author who is, in this case Robert Dalton (slimdandy).

If you use your member page as your url then people can connect you up to your comments.

On July 10 2006 myla said ...

brilliant article, thank you so much for this.

On July 10 2006 Brenda Anderson said ...

Great article! I've been playing with exposure the last few days and have found that using my 'spot metering' setting on my point & shoot has really improved my shots. However, I've been doing that on Automatic... which means my spot and my focus must be in the same spot (in the center). I'll have to try the 'take a meter reading' and then set the camera manually method to give me some more creative control.
Thanks for the explanation.

On July 11 2006 Dancing star said ...

Extremely helpful.

On July 14 2006 Rick Deising said ...

Robert Dalton has added new depth to my experience in photography, I am thankful for someone with his experience and knowledge who has taken the time to share with us an article that can change the way we take pictures. Praise Life RD Juneau Alaska.

On July 27 2006 Jean Albus said ...

Crystal clear. Thanks.

On August 21 2006 joseph atallah said ...

If you find your self in a situation where you need to take a picture but dont ha ve a 18% gry card, use you palm of your hand to take reflected light reading.

Pro Photo Rochester NY

On October 10 2006 Brian said ...

Using a zoom lense, will the lenght of your lense affect your light reading?

For example, say my mid-tone metering point is small and 20 feet away. If I zoom in, to fill the lense with this mid-toned area to meter it, then zoom out to crop my photo, will the meter reading sill be correct?

On November 22 2006 Aditya said ...

Another important tool in the arsenal is bracketing, which consists of doing nothing more than taking an overexposed shot, an underexposed shot, and a normally exposed shot (all according to the built in light-meter of course).

This is useful for those when you just don't have the time for busting out a grey card or you want to be a little discreet.

and Brian, that technique would work.

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Hand Holding The Camera?

The ability of the photographer to take images without noticeable blurring by camera movement is an important parameter in the choice of slowest possible shutter speed for a handheld camera. The rough guide used by most 35 mm photographers is that the slowest possible shutter speed that can be used with care is the shutter speed numerically closest to the lens focal length. For example, for handheld use of a 35 mm camera with a 50 mm normal lens, the closest shutter speed is 1/60 s.

The rule can be a augmented with knowledge of the intended application for the photograph, an image intended for significant enlargement and closeup viewing would require faster shutter speeds to avoid obvious blur. Through practice and special techniques such as bracing the camera, arms, or body to minimize camera movement longer shutter speeds can be used without blur. If a shutter speed is too slow for hand holding, a camera support--usually a tripod--must be used. There are also stabilized lenses available.

Quoted from: wikipedia-shutter

Light Quality?

Light Quality is the most important characteristic of light for the photographer for it sets the mood of a shot. Quality of light breaks down into two types -- hard or soft light.

Hard light throws distinct shadows. It comes from a point light source such as the sun or an electronic flash. It is a harsh light and often can cause the subject if it is a person to squint or need to put on sun glasses.

Soft light is the opposite and comes from a reflected source or a broad source. It is often indirect light and is frequently found in the shade as light that reflects into the shade from the surroundings. Light quality is effected mostly by change in subject or camera position or the use of reflectors or fill flash.

Quoted from: scphoto

What's This DOF Business?

Everybody has probably seen photographs in which every element from foreground to background is in sharp focus, and other pictures in which only the subject is in sharp focus while everything else is blurry. The first picture is said to have more depth of field than the latter, which has shallow depth of field (see above). For those that don't know, depth of field, or DOF, is how deep the area in focus is, when you focus on a given subject. It's a very powerful artistic tool.

You can use your depth of field to make something in the foreground in focus, and blur out the background, thereby bringing more attention to your subject. Or you can open your depth of field way up and put your significant other and that mountain 10 miles behind her/him all into sharp, startling, crisp focus.

Quoted from: DPChallenge

Normal Exposures

Unfortunately (or fortunately, depending on your point of view), the human eye and brain have a superb autoexposure mechanism built in. This means that once your eyes have adjusted to the current lighting situation, and without clues as to what is causing the light level encountered, it is almost impossible to tell how bright things are on a relative basis. As long as the light level lies somewhere within that 10 stop range for most people it all appears the same.

This is why light meters, whether built-in or handheld, are such vital tools. But before exploring light meters and how best to use them it's worthwhile to have in ones mind a firm idea of what "proper" exposure settings are for the ten light levels normally encountered. This way you're not a blind slave to the meter.

Let's assume an F stop of f/8 and a ISO (film speed) of 400. Here's what these 10 light levels are and the shutter speed that would be needed.

A Sunny day outdoors -- 1/2000 sec
A hazy bright day -- 1/1000 sec
A bright cloudy day without shadows -- 1/500 sec
An overcast day, or open shade on a sunny day -- 1/250 sec
A heavily overcast day -- 1/125 sec
Deep shade. The woods on an bright overcast day -- 1/60 sec
Just before a thunderstorm or late on a heavily overcast day -- 1/30 sec
A brightly lit store interior -- 1/15th sec
A well lit stage or sports arena -- 1/8th sec
A well lit home interior -- 1/4 sec

Of course you would vary the F stop and shutter speed combinations to whatever would be most appropriate. In the case of a home interior, for example, instead of 1/4 second at f/8 you might choose 1/30 sec at f/2.8. The point is though that these 10 brightness levels represent 95% of the conditions under which we all do our shooting.

Quoted From: Luminous Landscape