Everything in Balance
In my first exposure article I discussed how aperture, shutter speed and light sensitivity of the sensor work individually to affect
proper exposure, but I
|Exposure Control with the Histogram|
In previous articles I've talked about the use of the histogram to monitor and adjust exposure. There is an infinite number of ways in which the three factors controlling exposure can be balanced to achieve an acceptable result, and although all can appear the same on the histogram, the real challenge is to find the best balance among the secondary effects of these adjustments, so that the depth of field, motion control and image noise all conspire to achieve your vision. This week I will suggest one, step by step, approach to the balance of exposure control.
To briefly review from Part I, the camera controls of exposure include two which govern the amount of light and one which controls the sensor's sensitivity to the light which falls upon it. As the exposure triangle showed, each adjustment has a secondary effect that must be consider as the 3 factors are balanced.
Amount of Light:
1) Aperture: Effects Depth of Field
2) Shutter Speed: Effects the Ability to Stop Motion
3) ISO: Increased Sensitivity Increases Image Noise
Taken Individually, the effects of these basic controls are easy to understand, but the challenge and the real art of photography comes from how these adjustments are balanced in different situations of light, composition, action and the purpose of the image.
A Matter of Priorities
Ok, you are standing in front of the perfect vista. A glance at the Exposure Triangle would suggest that proper exposure is achieved through the simultaneous consideration of the effects of all three factors, aperture, shutter speed and ISO, but it can be unnecessarily complicated and confusing to try to balance the three components of exposure at the same time. Happily, most photographic situations inherently dictate which of these factors is most important and the resulting prioritization can provide a more reasonable step-wise approach to finding the best compromise among the factors contributing to exposure. The first step is to decide whether the control of aperture or shutter speed is the most important for the specific photographic situation.
This is usually referred to a Aperture or Shutter "Priority". Happily most cameras provide modes that allow you to lock in your priority.
|Aperture & Shutter Priority|
Baby Steps: A Systematic Approach
Ok, we start by considering the effects that changes in the aperture and shutter have on the final image, then step one is to study the scene and :
- 1) Decide whether aperture or shutter speed is most important to control for the desired effect.
Aperture Priority and Depth of Field
|Broad Depth of Field|
foreground to infinity is needed. With wide apertures, such as f/2 or
|Shallow Depth of Field|
In these situations, control of the depth of field may be the most important factor and therefore the "priority". You can set your desired aperture in "aperture priority" mode and then achieve optimal exposure with changes in the shutter speed. As predominately a landscape photographer I stay in aperture priority for about 90% of the time.
|Stopping the Jump|
Shutter Priority and Motion
Shutter speed controls exposure by governing the length of time that the sensor is exposed to the light and the shutter's secondary effect is on the ability to freeze motion. When capturing athletic events, running animals or birds in flight a fast shutter speed can keep the subject from becoming blurred, and in these situations the shutter speed adjustment may be the primary priority with the aperture allowed to drift to maintain the overall exposure. On occasions a
Considering the Side Effects
As we've seen, for any ISO, the Aperture and Shutter Speed settings are locked together to generate the exposure. Once aperture is fixed there is only one shutter speed which will result in proper exposure and the same fixed relationship is true when shutter speed is set as the fixed priority. In other words, once the priority setting is fixed the other "secondary" factor, whether aperture or shutter is strictly dependent. So step two is :
- 2) Whether it is Aperture or Shutter that is the "Non-Priority" or dependent setting, you must decide whether the effect of that setting on the image is acceptable.
|Deep Depth of Field|
- 3) Apply the ISO Fudge Factor if Needed
Adjustments in ISO is the new third control on exposure which digital photography has made practical. ISO has no effect on the amount of light hitting the sensor, but by managing the chip's sensitivity it can make possible a selection of aperture and shutter that does not force a compromise in either depth of field or control of motion. Of course the magic of ISO Adjustment is balanced by the effect of increasing ISO on the amount of digital noise in the image. As I discussed in Part One, digital noise can be muted in post processing, but in the end it must be balanced against all the factors that contribute to the quality of the final image.
Consider one more example. Assume you are set up to capture a
I would love to show you an picture to fully illustrate this scenario, but, give me a break!, I'm a landscape photographer. Use your imagination! The best thing I could do was to show Youk (Kevin Youkilis, for non-Red Sox or, ick, Yankee fans) diving for first.
There are many schemes and techniques which work to control exposure by modifying the light. Natural changes in light happen all the time, just wait for the sun to come out from behind that cloud, move from bright sunlight to the shadow of a tree or just come back when the light is better. Natural light can also be modified with filters, diffusers or reflectors and artificial light can be added with flash. These can all be effective, but the basic relationships between aperture, shutter and sensor sensitivity are always there and a system for balancing these fundamental adjustments makes achieving the perfect exposure much easier and sensible.
So to summarize this approach:
No triangle, but maybe a flow a diagram will help
1) Decide whether aperture or shutter speed is most important to control for the desired visual effect. It is usually easiest to use the Aperture or Shutter Priority setting on your camera to fix your desired priority.
2) Whether it is Aperture or Shutter that is the "Non-Priority" or dependent setting, your next step is to decide whether the effect of that setting on the image is acceptable.
3) If the resulting depth of field or motion effect is not what you want, it is time to consider applying the ISO Fudge Factor to shift the Aperture/Shutter relationship into a range that controls all of the important issues. Hopefully without an unacceptable level of noise in the image.
There are many other systemic approaches to achieving perfect exposure. The important thing is to pick one and start using it regularly. Happily with digital photography we get immediate feedback on the LCD screen, making the learning much easier. I hope that you will, quite quickly, find that the considerations, that I have so painfully presented, will become second nature. Your camera will feel comfortable in your hands, like a familiar hammer. So go out and pound some nails.
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