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Monthly Archives: July 2011

DSLR Next Steps – Understanding Exposure (Part 3)

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Carpe Camera, Continued

In the last article we ended with a question: Why would a photographer want to take control of exposure? We have established that shutter speed settings and apertures pair up in specific ways at different light levels in order to give us a good exposure and capture our image. However shutter and aperture controls have an even greater effect on our images.

Shutter speed effects are easy to see. If the speed is too low we get blurry pictures caused by subject movement or the tiny shakes and tremors of our hands. In general, most people can reliably hand hold a camera at 1/60 of a second. However this isn’t a fast enough speed to stop the motion of somebody walking fast let alone running.

Here are a few ‘Rules of Thumb’ for shutter speeds:
• In general, a shutter speed of 1/125 or higher will usually stop the motion of somebody walking or jogging. For a runner select 1/250, for a car or plane try 1/500 and higher.
• When shooting at speeds slower than 1/60 use a tripod or monopod. Below 1/15 shooting is only suggested with a tripod.
• In order to minimize the effects of hand tremor while hand holding, select a shutter speed that is faster than “1/lens length”. In other words, on a zoom lens set to 200mm choose a shutter speed higher than 1/200. (Most newer cameras will do this for you in Program mode provided there is enough light!)

So, selecting shutter speed is important. Setting the shutter speed can have a lot of impact on how we perceive the sharpness of the subject in the final image.

On the other hand, aperture effects can be quite subtle. Aperture selections contribute to the feeling of depth in a two dimensional photograph. As the aperture is opened wider, the background behind a subject will become increasingly blurry and out of focus. This is a very useful technique that can add a feeling of depth to a two-dimensional photograph.

Not only can aperture effect the perception of depth in our images but by selecting a wider aperture the photographer can create a sense of separation between subject and background.

The effects of aperture selection also follow a few ‘Rules of Thumb’
• Very few lenses are at their sharpest at the fully open aperture setting. Most cameras tend to have the greatest degree of critical sharpness at the middle range of their aperture scale – f 5.6 to f 8.
• A landscape photographer will more often work the aperture toward the smallest size possible in order to assure great depth of focus in their images. The opposite goal of a portrait photographer.

So back to the original question about why a photographer would want to shift the camera’s chosen exposure settings. From what we have just learned, a photographer shooting in Program mode now has a reason to choose between the exposure pairings that the camera’s meter has set.
• A faster shutter speed to stop action
• A slower shutter speed to blur action
• A wider aperture to blur backgrounds
• A smaller aperture to increase background sharpness
And all of these choices can be made “on the fly” by simply rolling the Program Shift wheel.

Moving On Up the Dial
The next step in using controls is to use two other advanced modes: Aperture Priority (Av) and Shutter Priority (S or Tv).

Aperture Priority and Shutter Priority are auto-exposure modes too, however the photographer selects one value and the camera responds by selecting the other value in order to achieve exposure. (Keep in mind that the photographer has also selected the ISO setting)

Selecting an aperture value in Av Mode

Suppose that we want to shoot a portrait. In order to capture consistent subject and background focus we select “Av” mode. We then select the aperture we want to use, essentially locking in this part of the exposure calculation. In this case f 2.8 is chosen to maximize background blur.

With this setting, no matter how the light changes our aperture will remain the same.
When the photographer selects Aperture Priority Mode the camera is given two of the three exposure components – the ISO sensitivity and the desired aperture value. The camera’s computer determines the correct setting for the shutter speed to complete the exposure calculation.

Aperture Priority exposure is the most commonly used auto-exposure mode by photographers. By controlling the aperture the photographer is also controlling the sense of depth and space in an otherwise two dimensional image.

Please keep in mind that if the aperture selected is too open or too closed for the available light the camera will flash exposure warnings in the viewfinder and on the LCD menu screen. If this happens simply select another aperture value or alter the ISO setting to maintain the chosen aperture.

Selecting a shutter speed in S (Tv) Mode

In this case imagine that we are shooting a sporting event. We want to remove as much motion blur from the images as possible. We select 1/250 as our shutter speed to help assure we get the motion-stopping capability we expect.

With this setting, no matter how the light changes our shutter speed will remain the same.
Just as happened in Av Mode, when the photographer selects Shutter Priority Mode the camera is given two of the three exposure components – the ISO sensitivity and the desired shutter speed. The camera’s computer determines the correct setting for the aperture in order to complete the exposure calculation.

While Aperture Priority may be the most common auto-exposure mode selected by photographers, Shutter priority is the number one choice for sports photographers. In sports photography rendering a fast moving player in crisp, sharp focus is the goal. Capturing a sense of depth and dimension is an important but secondary concern.

Once again, if the shutter speed selected is too fast or too slow for the available light the camera will flash exposure warnings in the viewfinder and on the menu LCD. If this happens simply select another shutter speed or alter the ISO setting to maintain the chosen shutter speed.

An Important Note: Please be aware how your specific camera alerts you to a situation where you have picked an exposure combination that is outside the capabilities of the light that is present! Even in auto exposure modes it is possible to get over or under exposed images if you aren’t aware of your camera’s exposure warning symbols!

Final Review
• In order to capture images that are better than “above average” the photographer can use Program, Aperture and Shutter priority modes.
• The photographer can choose these modes to select a specific component in the camera’s exposure calculation in order to create the effect desired.
• Program mode allows for shifting between various combinations of aperture and shutter that yield the same exposure. The “Program Shift Wheel” is the dial that provides this control.
• For even tighter control the photographer can select to control two of the three exposure controls allowing the camera to automatically respond with the correct setting on the third control.
• Aperture Priority Mode to control depth of field
• Shutter Priority Mode to control motion blur

Next Steps
There are many next steps that can enhance your photography:
• Learn about exposure compensation
• Add an automatic flash into your shooting
• For sport shooters, learn about the “peak of action” and how it can help stop motion
• For portrait or still life shooters learn how the distance between subject and background further increases the blurring effect of a wide aperture.

Future articles in this series will cover exposure compensation as well as discuss aperture and shutter priority plus full manual exposure modes in greater depth.

Editor’s Note: We tried to avoid using the phrases ‘proper exposure’ or ‘correct exposure’, this was not an omission. While there are images that are obviously under- or over-exposed, the judgment of good or bad is entirely subjective. Exposure variations are important tools in creative photography often used to focus attention on (or direct it away from) elements in the scene.

DSLR Next Steps – Understanding Exposure (Part 2)

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In the previous article we discussed the three tools that control exposure: lens aperture, shutter speed and ISO sensitivity. This article begins the discussion on putting these three tools to work.

The Exposure Controls

Now that we have our three main characters in place (Aperture, Shutter, ISO) we can begin to put them to work for us. You’ve heard of the “Food Pyramid”? Now meet the “Exposure Triangle”.

Since all three of the controls are linked and equally essential to create the desired exposure, if the value of one point changes one or both of the other two points must change too for the exposure to be maintained.

With three related but separate controls, where do you start?

First, take a slow breath in, and a slow breath out. Repeat as needed.

In general, our goal in photography has always been to use the lowest ISO setting the lighting will allow so that we capture images with little to no grain. ISO 100 produces images with less grain than ISO 400, even in digital.

Armed with the knowledge that we want to use low ISO ratings, that makes ISO the perfect place to start. (Even if after reading this article you choose to stick your camera in good ol’ “P” for Program mode you’ll understand why you at least want some control over the ISO setting.)

The reason we have the ability to change our ISO settings is that we need to be able to adjust the sensitivity of our cameras for all of the different environments we shoot in.
• Outdoors on a sunny day, 100 ISO is perfect.
• Indoors with flash 400 ISO may be better
• At a basketball game where the light is poor and the flash can’t reach to the players we choose 800 ISO or maybe even 1600 ISO.

As photographers we work the ISO scale to our advantage. We choose higher ISO settings in order to capture images in dimly lit settings, yet we are mindful that we don’t want to go any higher up the scale than we need to since we don’t want to capture images that look too grainy. It’s a balancing act, but an easy one with only a little practice.

Relationships

Because of the special relationship of the three exposure tools, a change to one of them has an effect on all of them. As the ISO sensitivity goes up we require less light to achieve our exposure. The camera can use a faster shutter speed, a smaller aperture – or both.

The first step in exercising control over our exposures is to set the desired ISO. In the examples that follow we can assume that the ISO is the same. To start, we’ve chosen 100 ISO as a base setting. This leaves only shutter speed and aperture controls available to select a good exposure.

The two scales below list apertures and shutter speeds. The apertures get smaller as we move down the list while the shutter speeds get faster as we move up that list.

If our constant is the 100 ISO sensitivity setting, then we only have to worry about using two controls to balance the exposure.

Lets make the assumption that our camera’s light meter reads f 8 at 1/60 as the correct exposure for the light in our scene. We can accept this setting of f 8 at 1/60 but maybe we are shooting a runner and need a faster shutter speed to prevent motion blur.

If we select a faster 1/125 shutter speed to help stop the action, we need a wider aperture to keep our exposure triangle balanced.

Looking at the chart above we can see that a move to 1/125 shutter matches up to the aperture value of f/5.6. We “lose” one stop of exposure for the faster shutter and “gain” one stop of exposure with the wider aperture so our exposure triangle remains balanced.

As the light changes we need to adapt. The last example was based upon shooting a runner outdoors. But what happens if we move inside to a living room? Since we are using light sensitivity (ISO) as the constant, this means that the initial meter reading in this new environment will change.

Since there is less light in the indoor scene and our ISO sensitivity remained the same, the aperture and shutter scales had to shift against each other in order to achieve an accurate exposure. The meter sets the camera aperture to f/4 in order to use the same 1/60 shutter speed. In this situation it is said that the scene has two stops less light than the outdoor scene.

Though we could shoot our indoor images with this meter reading, there isn’t a lot of room for error. We are operating right at the aperture extreme of our lens. We can change our ISO setting from 100 to 400 and the scales will once again shift against each other in order to maintain exposure.

The change from 100 ISO to 400 ISO is a two-stop change. To maintain the exposure value we can now close down (stop down) the lens aperture value from f/4 to f/8 – also a move of two stops and the exposure remains balanced.

Lets take a moment for review. We have learned that the camera’s meter will measure the light in order to choose settings for the lens aperture and the shutter speed in order to ‘properly’ expose the image sensor. (Proper as defined by the camera manufacturer’s programmers) The camera meter uses the ISO sensitivity setting (aka film speed) as the basis for the exposure reading.

Reactive vs. Proactive Control

To this point we’ve allowed the camera’s meter to measure the light and set the initial aperture and shutter speeds. This means that we have examined what we call a reactive approach to shooting images. The camera is pointed at a subject and the built-in processor reacts to the scene placed before it. Let’s explore why as a creative photographer we might want to take back some of that control and also take a look at the tools that allow us to.

In The Zone

If we set the camera in the “Easy” mode or “Green Zone” we are telling the camera to make all of the decisions for us. We cannot choose ISO, shutter speed or aperture. The camera’s computer will generally select a combination that favors the fastest shutter speed possible with a lower ISO. The camera is programmed to do this to help prevent blurry images from hand-held camera shake and to mitigate image grain.

If your camera has icon modes like Portrait, Sports, Macro or Landscape these modes also prevent the photographer from taking any control of the exposure.

There is nothing wrong with taking pictures in the Easy mode or any of the Icon modes. They perform as advertised and can take a lot of the guess work out of taking a picture. In addition, these modes will give the photographer a very good chance at getting above average pictures. And the photographer doesn’t have to spend any time at all making settings on the camera. But what if sometimes we want more than “above average” pictures?

Carpe Camera

Yep, you have to seize your camera. Grab a hold of it and turn a dial, and do it with a little understanding. Photography is truly a pursuit where a little knowledge goes a very long way.

A good first step is to set the camera in the “P” for Program mode. Program is very much like the “Easy” mode in that the camera will suggest an exposure for the image about to be captured. The big difference between Green Zone and the “P” mode is that the photographer can choose to override the camera’s settings. Plus the photographer can set ISO sensitivity.

In Program mode the photographer can first see the camera’s suggested starting exposure and then using a Program Shift wheel to move up and down the table of shutter / aperture pairings until a preferred setting is found.

Sounds easy, but why would a photographer want to take that control?

We have established that shutter speed settings and apertures pair up in specific ways at different light levels in order to give us an acceptable exposure and capture our image. However shutter and aperture controls have an even greater effect on our images. In the final article on Exposure we will take a look at shutter and aperture effects and answer the question “Why?”