Interpreting the meter reading

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The information given by the light meter after taking a reading is referred to as the 'meter-indicated exposure' (MIE). This is a guide to exposure only. The light meter should not be perceived as having any intelligence or creative sensibilities. The light meter cannot distinguish between tones or subjects of interest or disinterest to the photographer. It is up to the photographer to decide on the most appropriate exposure to achieve the result required. A photographer with a different idea and outcome may choose to vary the exposure. It is the photographer's ability to interpret and vary the meter-indicated exposure to suit the mood and communication of the image that separates their creative abilities from others. If light or dark tones dominate, the indicated exposure will be greatly influenced by these dominant tones. Using the MIE will expose these dominant dark or light tones as mid-tones. Minority light and mid-tones will be overexposed or underexposed. If you consider interest and visual impact within a photograph is created by the use of lighting and subject contrast (amongst many other things) the chances of all the elements within the frame being mid-tones are remote. The information, mood and communication of the final image can be altered through adjusting exposure from MIE.

Indian market (average tones) MIE

Average tones

A subject of average reflectance (mid-tone) is placed with equal dark and light tones. All three tones are lit equally by the same diffuse light source.

A reflected reading of the mid-tones will give correct exposure. An exposure using the reflected reading of the dark tone will render it gray and overexpose the mid and light tones. An exposure using the reflected reading of the light tone will render it gray and underexpose the mid and dark tones.

An incident reading will give 'correct' exposure regardless of which tone it is held in front of because it measures the light falling on the subject, not the light reflected from it. The intensity of the light source is constant but the reflected light from the three tones varies (see page 40, Hand-held light meters).

Dominant tones

If dark tones dominate the framed image the MIE will result in the dark tones being recorded as mid-tones. Mid-tones will be recorded as light tones and any light tones may be overexposed. If light tones dominate the framed image the meter-indicated exposure will result in the light tones being recorded as mid-tones. Mid-tones will be recorded as dark tones and dark tones may be underexposed. If the mid-tones present in amongst these dominant dark or light tones are to be recorded accurately the exposure must be either reduced (for dominant dark tones) or increased (for dominant light tones) from the MIE.

White wall (dominant light tones) MIE

Increased exposure

White wall (dominant light tones) MIE

Increased exposure

The amount the exposure needs to be reduced or increased is dictated by the level of dominance of these dark or light tones (see the chapter 'Contrast and Compensation > Exposure compensation').

Activity 6

Photograph a subject requiring adjusted exposure from that indicated by the light meter. State how the dominant tones would have affected the light meter reading and how the image would have looked if you had not adjusted the exposure.

Reading exposure levels

When taking a picture with a digital camera it is sometimes possible to check the exposure during the capture stage to ensure that the full tonal range of the image has been recorded. The most accurate indication of the exposure does not come from the image on the LCD screen but the histogram (all DSLR cameras and the better fixed-lens compact cameras are able to display these histograms). Some fixed-lens cameras can even display the histogram before the image has been captured. This 'live preview' is also available on a few DSLR cameras that have a second sensor designed to feed this live view to the LCD screen prior to capture.

The levels of brightness in the histogram are displayed as a simple graph. The horizontal axis displays the brightness values from left (darkest) to right (lightest). The vertical axis of the graph shows how much of the image is found at any particular brightness level. If the subject contrast is too high or the exposure is either too high or too low then tonality will be 'clipped' (shadow or highlight detail will be lost). Most digital camera sensors can only record a limited range of information when compared to the range of tones human vision is capable of seeing detail in. The tones that are out of the range of the image sensor of the digital camera record as black or white with no detail. We should attempt to adjust the exposure or reduce the contrast of the subject matter to ensure maximum information is recorded.

Note > When using the JPEG file format you should attempt to modify the brightness, contrast and color balance at the capture stage to obtain the best possible histogram before editing begins in the software.

Correcting exposure

The photographer can either increase or decrease exposure to ensure a full range of tones is recorded during the capture stage. Photoshop will not be able to replace information in the shadows or highlights that is missing due to inappropriate exposure or excessive subject contrast. The information should extend all the way from the left to the right side of the histogram if the subject contrast and the exposure are appropriate.

Overexposure and underexposure

If the exposure is too high a tall peak will be seen to rise on the right side of the histogram (level 255). If the digital file is underexposed the peaks are crowded on the left-hand side of the histogram and there is little or no peaks on the right-hand side of the histogram. Some cameras can be programed to blink in the areas that are overexposed.

Solution: Adjust the exposure in the camera using either the exposure compensation controls or the manual controls. If exposure is too low due to bright backlights in the image you can try moving the camera to exclude the bright light source, locking the exposure by half-pressing the shutter release and then reframing.

Lowering contrast

If the contrast is too high tall peaks may be evident at either end of the histogram.

Solution: Decrease the subject contrast by either repositioning the subject matter or by lowering the contrast of the lighting. The light source can be diffused or additional lighting can be provided in the form of fill flash or reflectors. In the image above the camera's tiny built-in flash unit has been used to increase the exposure in the shadows. This allows the overall exposure to be lowered, which in turn prevents the sky behind the children from becoming overexposed.

Optimizing a histogram after capture

The final histogram should show that pixels have been allocated to most, if not all, of the 256 levels. If the histogram indicates large gaps between the ends of the histogram and the sliders (indicating either a low-contrast scan or low-contrast subject photographed in flat lighting) the relationship between the subject contrast and light quality could be reconsidered.

Small gaps at either end of the histogram can, however, be corrected by dragging the sliders to the start of the tonal information. Holding down the Alt/Option key when dragging these sliders will indicate what, if any, information is being clipped. Note how the sliders have been moved beyond the short thin horizontal line at either end of the histogram. These low levels of pixel data are often not representative of the broader areas of shadows and highlights within the image and can usually be clipped (moved to 0 or 255). Moving the 'Gamma' slider can modify the brightness of the mid-tones. If you select a Red, Green or Blue channel (from the Channel pull-down menu) prior to moving the Gamma slider you can remove a color cast present in the image. For those unfamiliar with color correction the adjustment feature 'Variations' (Image > Adjustments > Variations) in Photoshop gives a quick and easy solution to the problem. After correcting the tonal range using the sliders click 'OK' in the top right-hand corner of the Levels dialog box.

Note > Variations is not available for Photoshop users working in 16 Bits/Channel mode.

Color

Neutral tones in the image should appear desaturated on the monitor. If a color cast is present try to remove it at the time of capture or scanning if possible.

Solution: Control color casts by using either the white balance on the camera (digital) or by using an 80A or 80B color conversion filter when using tungsten light with daylight film. Use the available color controls on the scanning device to correct the color cast and/or saturation.

Raw format exposure considerations

One of the big topics of conversation since the release of Photoshop CS has been the subject of 'Raw' files and 'digital negatives'. This section of the chapter guides you through the advantages of choosing the Raw format and the steps you need to take to process a Raw file from your camera in order to optimize its exposure.

All digital cameras capture in Raw but only digital SLRs and the medium- to high-end 'Prosumer' cameras offer the user the option of saving the images in this Raw format. Selecting the Raw format in the camera instead of JPEG or TIFF stops the camera from processing the color data collected from the sensor. Digital cameras typically process the data collected by the sensor by applying the white balance, sharpening and contrast settings set by the user in the camera's menus. The camera then compresses the bit depth of the color data from 12 to 8 bits per channel before saving the file as a JPEG or TIFF file. Selecting the Raw format prevents this image processing taking place. The Raw data is what the sensor 'saw' before the camera processes the image, and many photographers have started to refer to this file as the 'digital negative'. The term 'digital negative' is also used by Adobe for their archival format (.dng) for Raw files.

The sceptical among us would now start to juggle with the concept of paying for a 'state-of-the-art' camera to collect and process the data from the image sensor, only to stop the high-tech image processor from completing its 'raison d'être'. If you have to process the data some time to create a digital image why not do it in the camera? The idea of delaying certain decisions until they can be handled in the image-editing software is appealing to many photographers, but the real reason for choosing to shoot in Camera Raw is QUALITY.

Processing Raw data

The unprocessed Raw data can be converted into a usable image file format by Adobe Camera Raw (ACR). Variables such as bit depth, white balance, exposure, brightness, contrast, saturation, sharpness, noise reduction and even the crop can all be assigned as part of the conversion process. Performing these image-editing tasks on the full high-bit Raw data (rather than making these changes after the file has been processed by the camera) enables the user to achieve a higher quality end-result.

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Double-clicking a Raw file, or selecting 'Open in Camera Raw' in Bridge, opens the Adobe Camera Raw (ACR) dialog box, where the user can prepare and optimize the file for final processing in the main Photoshop editing interface. If the user selects 16 Bits/Channel in the Workflow Options, the 12 bits per channel data from the image sensor is rounded up - each channel is now capable of supporting 32,769 levels* instead of the 256 levels we are used to working with in the 8 bits per channel option. Choosing the 16 Bits/Channel option enables even more manipulation in the main Photoshop editing space without the risk of degrading the image quality. When the file is opened into the image-editing work-space of your Photoshop software, the Raw file closes. Any changes you make to the appearance of the image in the ACR dialog box will be applied to the image that is opened in Photoshop's main editing space but won't alter the original image data of the Raw file. The adjustments you make in the ACR dialog box are saved as XMP metadata in the DNG file as an XMP sidecar file or in the computer's Camera Raw database or cache.

*Note > Photoshop's 16-bit editing is 15-bit + 1 (15-bit processing gave the Photoshop engineers a midpoint). In Photoshop CS2 we had the option to set the Info palette to 16-bit values to confirm this but this option was removed for Photoshop CS3.

Distribution of data

Most digital imaging sensors capture images using 12 bits of memory dedicated to each of the three RGB color channels, resulting in 4096 tones between black and white. Most of the imaging sensors in digital cameras record a subject brightness range of approximately five to eight stops (five to eight f-stops between the brightest highlights with detail and the deepest shadow tones with detail).

Tonal distribution in 12-bit capture

-2 stops -1 stop Brightest Highlights

Darkest Shadows - 4 stops - 3 stops

75% of all levels

Distribution of levels

One would think that with all of this extra data the problem of banding or image posterization due to insufficient levels of data (a common problem with 8-bit image editing) would be consigned to history. Although this is the case with mid-tones and highlights, shadows can still be subject to this problem. The reason for this is that the distribution of levels assigned to recording the range of tones in the image is far from equitable. The brightest tones of the image (the highlights) use the lion's share of the 4096 levels available while the shadows are comparatively starved of information.

An example of posterization or banding

Shadow management

CCD and CMOS chips are, however, linear devices. This linear process means that when the amount of light is halved, the electrical stimulation to each photoreceptor on the sensor is also halved. Each f-stop reduction in light intensity halves the amount of light that falls onto the receptors compared to the previous f-stop. Fewer levels are allocated by this linear process to recording the darker tones. Shadows are 'level starved' in comparison to the highlights that have a wealth of information dedicated to the brighter end of the tonal spectrum. So rather than an equal amount of tonal values distributed evenly across the dynamic range, we actually have the effect as shown above. The deepest shadows rendered within the scene often have fewer than 128 allocated levels, and when these tones are manipulated in post-production Photoshop editing there is still the possibility of banding or posterization.

Accessing a broader dynamic range

The histogram displayed on the back of a digital camera may not accurately reflect the precise range of tones that is being captured when the photographer elects to shoot in the Camera Raw format. Most cameras provide information that is geared to the photographer capturing in the JPEG or TIFF format only. A photographer may therefore underestimate the dynamic range that is capable of being recorded by their camera. Instead of capturing a dynamic range of approximately five stops the Raw format may be capable of capturing images with a dynamic range that exceeds 7 or 8 stops when using a DSLR camera. The precise dynamic range will vary with the type of sensor being used as the smaller sensors fitted to the prosumer digicams do not enjoy the same dynamic range as the APS and full-frame sensors fitted to the DSLR cameras.

In the example above the highlights of the distant clouds were flashing when the image was reviewed in the camera (a common option on many digital cameras for indicating when overexposure has occurred). The histogram associated with the file clearly indicated that the highlights were being clipped. When the file was opened in Camera Raw the Exposure slider was moved to a value of -0.50 and usable detail (level 245) was returned to this area of the image. The Brightness slider was raised to compensate for the negative Exposure value. If the exposure had been lowered in camera the dark top that the boy is wearing would have required rescuing instead of the highlights. Most photographers find that it is preferable to rescue the highlights rather than the shadows although due care must be taken not to allow the highlights to become completely overexposed. Taking two spot meter readings will enable the experienced photographer to ascertain the subject brightness range and assess whether this is compatible with the dynamic range of the sensor they are using.

Lowering exposure in ACR

For those digital photographers interested in the dark side, an old SLR loaded with a fine-grain black and white film is a hard act to follow. The liquid smooth transitions and black velvet-like quality of dark low-key prints of yesteryear are something that digital capture is hard pressed to match. The sad reality of digital capture is that underexposure in low light produces an abundance of noise and banding (steps rather than smooth transitions of tone). The answer, however, is surprisingly simple for those who have access to a DSLR and have selected the Raw format from the Quality menu settings in their camera. Simply be generous with your exposure to the point of clipping or overexposing your highlights and only attempt to lower the exposure of the shadows in Adobe Camera Raw.

1. The first step is the most difficult to master for those who are used to using Auto or Program camera exposure modes. Although the final outcome may require deep shadow tones, the aim in digital low-key camera exposure is to first get the shadow tones away from the left-hand wall of the histogram by increasing and NOT decreasing the exposure. It is vitally important, however, not to increase the exposure so far that you lose or clip highlight detail. The original exposure of the image used in this project reveals that the shadow tones (visible as the highest peaks in the histogram) have had a generous exposure in-camera so that noise and banding have been avoided (the tones have moved well to the right in the histogram). The highlights, however, look as though they have become clipped or overexposed. The feedback from the histogram on the camera's LCD would have confirmed the clipping at the time of exposure (the tall peak on the extreme right-hand side of the histogram) and if you had your camera set to warn you of overexposure, the highlights would have been merrily flashing at you to ridicule you of your sad attempts to expose this image. The typical DSLR camera is, however, a pessimist when it comes to clipped highlights and ignorant of what is possible in Adobe Camera Raw. Adobe Camera Raw can recover at least one stop of extra highlight information when the Exposure slider is dragged to the left (so long as the photographer has used a DSLR camera that has a broader dynamic range than your typical fixed-lens compact digicam).

Adobe Camera Raw rescues the highlights - sometimes automatically

'Exposing right'

When the Auto checkbox in the Exposure slider is checked, Adobe Camera Raw often attempts to rescue overexposed highlights automatically. With a little knowledge and some attention to the camera's histogram during the capture stage, you can master the art of pushing your highlights to the edge. So if your model is not in a hurry (mine is watching a half-hour TV show) you can take an initial exposure on Auto and then check your camera for overexposure. Increase the exposure using the exposure compensation dial on the camera until you see the flashing highlights. When the flashing highlights start to appear you can still add around one extra stop to the exposure before the highlights can no longer be recovered in Adobe Camera Raw. The popular term for this peculiar behavior is called 'exposing right'.

PERFORMANCE TIP

If the highlights are merrily flashing and the shadows are still banked up against the left-hand wall of the histogram, the solution is to increase the amount of fill light, i.e. reduce the difference in brightness between the main light source and the fill light (see Lighting on Location > Fill flash).

2. Before we massage the tones to create our low-key image we must first check that our tones are smooth and free from color and luminance noise. Zoom in to 100% magnification for an accurate preview and look for any problems in the smooth dark-toned areas. Setting both the Luminance Smoothing and Color Noise Reduction sliders to 25 (found in the Detail tab) removes the noise in this image. I would also recommend that the Sharpness slider be set to 0 at this point. Rather than committing to global sharpening using the Adobe Camera Raw dialog box, selective sharpening in the main Photoshop editing space may help to keep the tones as smooth as possible.

3. Create the low-key look by moving the Exposure and/or the Brightness sliders to the left in the Basic tab. You can continue to drop these sliders until the highlights start to move away from the right-hand wall of the histogram. Select the 'White Balance Tool' and move your mouse cursor over the deeper shadows - this will give you an idea of the RGB values you are likely to get when this image is opened into the editing space. Once you approach an average of15 to 20 in all three channels the low-key look should have been achieved.

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