One of the most challenging aspects of photography is getting proper exposure. Without a properly exposed image, our photos will always fall short of our own expectations. Exposure needn't be the weak spot in your photography, and hopefully this multi-part instructional will help you achieve the exposure you desire on film in a more consistent manner.

The Right Exposure

What is correct exposure? How is it determined? Basically correct exposure is any exposure that shows detail in either the white or dark areas on the film. We can all agree that an image is improperly exposed when the whites are "blown". By this I mean the viewer can not see detail in the whites on the image, and the whites are so bright that the eye is drawn to them. Dark areas will also appear to lack detail when an image is not properly exposed. A "1 stop" over or under exposed image is generally accepted by most photographers as improper. However, many photographers will not agree on perfect exposure. What is "perfect exposure" for me might not be perfect exposure for you. But varying concepts of perfect exposure should generally be within "2/3 of a stop or less, show detail in the dark or light areas, and not so bright as to be distracting or (in the case of whites) too gray or underexposed".

The first step to getting proper exposure is to view your slides on a color-corrected light box with a loupe. Slide film, also called positive film, will allow the photographer to evaluate their exposure more accurately than print or negative film. The reason being is that when print film is developed the person doing the prints will try to "correct" the exposures for the photographer; whereas with slide film "what the photographer exposes for in the photo is what the photographer gets". In simple terms, WYSIWYG! Only the photographer has control over the process and therefore can evaluate the results of their efforts and understanding.

Color-corrected light boxes can be purchased beginning around $25.00 at many office product or photography stores. The light box needs to be 5000K, so that the recorded colors hold true. It is also necessary to use a good quality loupe (magnification should be at least 4x). Some photographers use a second loupe of higher magnification for editing submissions in order to see more detail, however, magnification won't enter into our intended purpose for checking exposure - just remember when buying your loupe that any magnification greater than 4x will NOT allow you to view the entire slide at one time.

The next step is check your in-camera meter. Don't panic! This is actually very simple. On a clear day after 10am, set the aperture to F16. The shutter speed should be equivalent to the ISO when metering an 18% reflective (medium toned) subject. Medium tones are all around us in nature. Blue skies, green grass, the palm of your hand all work, but if you want to know that you have an exact 18% reflective subject, then purchase a gray card at your local camera store. They are inexpensive and easy to find. Just remember you should fill the frame with your 18% reflective subject. I use spot metering to check my in-camera meter in this manner.

If your meter is off you can have it re-calibrated by the manufacturer. Make a mental note for adjusting your exposures in the field, or simply adjust your ISO setting to compensate for the inaccuracy. Once we discuss stops of light you will be better prepared to make these adjustments.

What is Exposure?

Exposure is simply light hitting the film plane and creating an image. To obtain proper exposure, you have to control the light. There are three variables in the lighting equation that is in the control of the photographer:

The first is the ISO (or speed of the film). The lower the number of the ISO the more light it will take to create an image on the film. The larger the ISO number the less light it will take in the same circumstances to create the same properly exposed image. The film speed or ISO determines "How much" light is needed to make an image on film.

The next variable is Shutter speed. Shutter speed is measured in time, most often in one hundredths of a second, but for macro photography the time the shutter is open can be in entire seconds and for night photography even minutes or hours. Adequate shutter speeds are essential to capturing sharp images of wildlife since the subject is seldom stationary. Therefore, we want the shutter to open and close very quickly to freeze any movement of the subject. The faster the subject moves the faster shutter speed is required if we want a sharp image.

That leaves us with the third variable - the Aperture setting. The aperture determines the amount of light that will strike the film plane. By adjusting the aperture we can not only add or subtract light, but we can also control the depth of field (DOF). This use of aperture is why most nature photographers choose to shoot in Aperture Priority. We have control over the exposure, shutter speed, and the depth of field all at the touch of one button!

Shutter speed and aperture both affect the quantity of light striking the film plane - either by the length of time or the amount. They work hand in hand, and by reducing or increasing one you will have to change the other. These changes are referred to as stops of light.

Stops of Light

Light is measured in stops whether you are speaking about a change in aperture, film, or shutter speed. The difference in 100 ISO and 200 ISO is one full stop of light, whereas 1/125 of a second in shutter speed changed to 1/250 is also a one stop change. Below you will find a table with the shutter speeds and apertures listed in one stop increments. It is best to memorize these numbers, as it will make it easier in the field to understand your adjustments and think quickly before the subject moves.

Each change in ISO above is one full stop of light. Notice that each stop is a doubling of the film speed. This makes it easy to remember what the next stop will be for film.

Exercise: How many stops is it from 50 ISO to 400 ISO?
Answer: 3 stops

Each change in shutter speed is one full stop. One second is the longest and allows more light to hit the film plane due to the length of time the shutter is open. One one-thousandth is the shortest duration of time and allows less film to strike the film plane. Electronic cameras of today do have intermediate shutter speeds, but this is for a basic understanding of exposure and how it works. Don't get too caught up with the variations seen on your camera right now. Let's just build our understanding of how this works.

Exercise: ½ sec to 1 sec is how many stops?
Answer: 1 stop

Each change in aperture is a full stop of light. F/1 is the largest opening on the lens, thus allowing more light to strike the film plane. Hence, the larger the f/stop the less light will hit the film plane. Many nature photographers speak of shooting "wide open", meaning the lens is at its largest aperture or the smallest number for that particular lens.

Let's pretend we have Sensia 100 ISO film in our camera. My camera reads 1/125 of a second shutter speed, the aperture is at f/8 and the subject is a swimming Wood Duck. I want to decrease my shutter speed to 1/250th of a second to freeze the action. To get the proper exposure I now need to change my aperture. I decreased the duration of light, so I now have to increase the amount of aperture. A one stop reduction of light from 1/125 to 1/250 would necessitate increasing the aperture from f8 to f5.6 to allow one more stop of light in.

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Debbie Ferrell-Smith Nature Photography - you will be pleasantly surprised!.