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Sunday, November 23, 2014

Exposure - The Basics Part II

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
never explored what proper exposure actually is or how it is evaluated.  A simple definition might be that proper exposure is achieved when the recorded image matches the appearance of the original scene in brightness and tonal range. Simple enough, but no photographic media can fully reach this goal.  Pictures are always a compromise, but with the tools available in modern digital cameras, there is no longer any excuse for poor exposure. The challenge comes from the choices you make to get there.  

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

Sensor Sensitivity
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.

"Priority" Settings﻿

Aperture & Shutter Priority
Most cameras have settings for aperture or shutter priority. In "Aperture Priority" the aperture is fixed and adjustments in exposure compensation affect only the shutter speed. In "Shutter Priority" it is the shutter speed which is fixed. Of course these same adjustments can be done in Manual mode, but in manual, the exposure does not automatically change when the meter detects a change in light. The Manual setting is great when you are learning about the functions on your camera, but many photographers shoot on Aperture or Shutter priority based on the requirements of the subject. On Canon cameras Aperture Priority is marked as "Av" and Shutter Priority "Tv" on Nikons it is "A" and "S".  

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
As I discussed in my first article on exposure, the camera aperture affects exposure based on the size of the opening allowing light to strike the sensor. I went into an involved discussion of why increasing aperture numbers (f/stops) resulted in smaller openings and less light. Happily, the aperture's secondary effect on depth of field follows a more intuitive progression with larger numbers resulting in larger bands of sharp focus. Small apertures with high f/stops, such as f/ 22 or f/32, lead to broad depth of field which is often preferred in landscape photography where sharp focus from
foreground to infinity is needed. With wide apertures, such as f/2 or
Shallow Depth of Field
f/4, the narrow depth of field can help to isolate and draw attention to a subject, de-emphasizing distracting elements in the foreground and/or background. Traditionally, narrow depth works well in portrait photography where the attention is usually centered on the eyes, but it can also be effective in many other situation to mute distractions such as in macro or wildlife photography. 

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
Slow Shutter
slow shutter speed might be preferred, such as when capturing a soft image of flowing water or when a blurring of motion can be used to imply movement. In many of these situations the specific depth of field may not be critical and setting the camera on Shutter Priority would be the best first step.

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
Consider an example that I frequently experience. I'm shooting a grand landscape and I want to get the maximum possible depth of field to record both the foreground flowers and the distant
mountains in sharp detail. I set my camera to aperture priority and stop down to a small fixed aperture of f/22.  Great, everything from foreground to background looks sharp, but when I take the picture I find that the shutter speed that was required to maintain the desired exposure for that small fixed aperture had drifted to 1/4 second and, what seemed like a gentle breeze, had been enough to smear my beautiful flowers into a messy abstract blur. I could wait for a, usually nonexistent, lull in the wind or I could consider focus stacking (beyond the scope of our current discussion), but it is at this point that an ISO adjustment could save the day.

  • 3) Apply the ISO Fudge Factor if Needed

As in my example, it often happens that the optimal adjustment of your priority setting leads to unacceptable side effects from the resulting secondary or dependent factor. In my landscape scenario I might be able to get by with a wider aperture, perhaps f/16 or f/18, which would allow a faster shutter, but these "compromises" too often yield unsatisfactory results in both DOF and shutter. A change in ISO is often a better solution. Adjustments in sensor sensitivity or "ISO" shift rather than break the aperture/shutter connection. In my example an increase of ISO from 100 to 800 (3 stops) will allow an increase of shutter speed to 1/30th of a second which may be fast enough to freeze the flowers without affecting my exposure, aperture or depth of field.

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

baseball player streaking down the line to beat out a base hit. You set a fast shutter speed of 1/1000th of a second, in shutter priority, to freeze the motion. Your ISO is set to 100 and the resulting f/stop is at f/4. This wide aperture and resultant narrow DOF works well to isolate the runner against the blurred grandstands. A great choice, but if you want to see the faces of the screaming fans in the background you could increase the ISO to 1600 causing the f/stop to shift 4 stops to f/16. Both the runner and the fans would then be in dramatically sharp focus. The only question would be whether the increased image noise at ISO 1600 is acceptable.

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.

Light Modification

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.

Simple !?

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.

Other Articles About Exposure

Jeffrey Newcomer


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    They have an EFFECT upon...