Understanding, Selecting and Executing Effective In-Camera Exposure

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1 Understanding, Selecting and Executing Effective In-Camera Exposure Graham Robinson, June 2006, Jan 2008, April 2009 Disclaimer: This is not a meticulously researched academic paper. It merely aims to give a flavour of my opinion. Many facts and subtleties have been knowingly excluded. This is not a complete tutorial and does not have all the answers. Hopefully it will encourage you to read other sources and experiment to find out what works for you. Audience: This is aimed at beginning to intermediate photographers who are using 35mm SLR film cameras and digital SLR cameras. To a lesser extent these notes also apply to users of advanced film/digital compacts. The first section is aimed at beginning/intermediate workers and the second section moves on a little and is more aimed at intermediate workers. The third section is for reference. Advanced workers will probably not find anything new here but seeing things from another s perspective may be of some interest. Film vs. Digital: It would be very tedious to always have to say film or digital image sensor. Consequently I will mostly only speak about film because there will be an equivalent implication for digital even though the technicalities may be different. Where there is a significantly different implication I will point this out. ISO vs. ASA: I will take these terms to refer to film speed only. For the purposes of these notes, I will treat them as interchangeable and stick to using ISO. Flash Photography: Flash is an extensive topic in its own right. I will not discuss the exposure considerations of using flash as a main light source. I hope these notes will be of some help. Thanks Graham Version 1.2 Page 1 of , 2008, 2009 Graham Robinson

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3 Section 1: Background Section 1: Background In this section I will give a definition of exposure, go over some basic ideas and do some practical experiments. In a later section I will bring these ideas together in to an end-to-end process. What is Exposure? The world is full of objects that radiate and reflect light. Even in everyday experience the variation in intensity is huge. Something very bright can very easily be several thousands of times brighter than something very dark. Our eyes and brain are very adept at coping with this huge variation and we barely even notice it. 1: When a film is exposed to light and then processed it will be lighter or darker depending on the amount of light that has fallen on the film. This level of darkness/lightness is referred to as density or tone. 2: It so happens that we perceive doublings of light intensity as equal steps in brightness. This is similar to the way we perceive sounds. Each doubling of pitch is an octave. The spacing of octaves sounds consistent yet the actual differences in pitch (or frequency) are much greater for the upper octaves that the lower octaves. 3: The laws of physics and chemistry dictate the relationship between the amount of light falling on the film and the resulting density (darkness). These laws also limit the range of density that a film can reproduce. 4: It varies but as a general rule film can only reproduce a range of five to six doublings of density before detail is lost. Selecting and executing exposure involves measuring, and where possible controlling, the actual and relative brightness of the different parts of the subject/scene and then making settings on the camera so that these variations are fitted on the available density recording range of the film for the best effect. The Need to Control the Process The creative responsibility lays with the photographer not the camera. Either you are in control of the process or you are not. If you want to control the creative outcome then an understanding of the end-to-end process of exposure is required so that you can intervene rationally to control this outcome. The exposure accuracy required varies with the medium. For slide film, exposure accuracy should be to plus or minus one third of a stop. For good results with other media a similar level of accuracy should be aimed for. Version 1.2 Page 3 of , 2008, 2009 Graham Robinson

4 Section 1: Background Camera Exposure Controls The amount of light required to achieve correct exposure depends on the sensitivity of the film. The more sensitive the medium the less light is required to obtain a correct exposure. There are two ways to control the light falling on a film. Therefore, we have three key values to control. Film Speed: Aperture: Film speed is a measure of the light sensitivity of the medium. For film, this means choosing a suitably ISO rated film. For digital, this means choosing an appropriate ISO value. Crudely put, we need to control the size of the hole in the front of the camera. A big hole lets in a lot of light, a small hole much less. Shutter Speed: How long the film is exposed to the light. Obviously, the longer the light falls on the film the greater the exposure. One significant advantage of digital is the ability to select the ISO setting on a shot by shot basis rather than being tied to one film speed for 36 frames. Double Trouble It seems that stops, f-numbers and ISO cause a lot of confusion. Photography has a hundred years of historical baggage and part of this is the reason for some of the apparently arcane numbers and systems we have to use. As noted earlier, it is handy to work in doublings of light. One reason for this is that it matches the way we perceive things. The second is that by using doublings a very large range of light intensities can be covered in a few steps. We refer to these doublings as being one stop more and halvings as one stop less. Typical ISO values: 50, 100, 200, 400, 800 As the number doubles so does the sensitivity of the film and therefore it requires half the amount of light. Note: other values of ISO are used but the arithmetic remains the same. Typical shutter speeds: 15, 30, 60, 125, 250, 500 These indicate the length of time the film is exposed in fractions of a second i.e.: 1/15 th, 1/30 th, 1/60 th, 1/125 th, 1/250 th, 1/500 th Each of these double or half as we move from one to the next thereby halving or doubling the exposure. Typical aperture values: f2.8, f4, f5.6, f8, f11, f16 Each number in this sequence represents an effective aperture that lets through half as much light as the previous one. Now, (1) yes, the bigger the number the smaller the aperture; (2) yes, the numbers double every other number instead of every number. Sorry but that is the way it is. You can now choose (a) to be confused; (b) to just accept it; or (c) read some technical stuff about diameters and ratios of focal lengths. I will assume that you went for (b) and are just prepared to accept that the amount of light hitting the film halves each time as we move through the sequence Version 1.2 Page 4 of , 2008, 2009 Graham Robinson

5 Section 1: Background Equivalent Exposure Values Because each of the controls can be adjusted independently and by the same measure (i.e. stops) we have the situation where several combinations of settings give equivalent exposure values. For example, all the following give the same exposure: 250 f5.6; 125 f8; 60 f11. By using longer shutter speeds with smaller apertures the same amount of light hits the film creating the same density on the film, i.e. the same tone (or shade of grey). If we were to use a one stop faster film, say 200 instead of 100, then the same tone would be produced by using less light and we could use the following values: 250 f8; 125 f11; 60 f16. For most picture taking situations we are at liberty to use any of these equivalent combinations. The great beauty of this is that by changing films (or ISOs), shutter speeds and apertures we can achieve a huge range of creative control. Note of caution: at the extremes this nice relationship breaks down. For example, many films require significant extra exposure once shutter speeds start to exceed one second. Give Yourself Creative Options Each of the controls used to create the correct exposure have side-effects beyond achieving the correct tonal range. These side-effects need to be understood in order to better know the creative options available. With the options available to you personal creative selection becomes possible. These notes are primarily about exposure so I do not have time to go into this in detail. I suggest that you read more about the topics once you have the basics. Film Speed/ISO (a) Film This means choosing a film whose characteristics suit the task. Slow slide film gives beautifully saturated or realistic rendering of colour; but it needs good light or stable camera support. Fast colour negative is easier to use in hand-held or poorer light; but there are difficulties with colour fidelity and definition. Mono film has huge potential with the same film capable of being rated at different ISOs and developed to suit a particular need; but this all needs experience and craftsmanship to achieve. Film grain is usually much coarser in faster films. This may be a problem with highly detailed record work but it might be just the right thing for a moody pictorial image or a gritty documentary shot. (b) Digital Noise is the random electronic variation that occurs in the capturing and processing of signals in the digital image sensor. Turning up the volume of a cheap audio cassette player makes the background hiss (noise) more apparent. Likewise, turning up the ISO makes image noise more likely and more intrusive. Noise is more apparent in darker areas of the picture and shows up as speckles of colour and brightness. Noise is generally bad. It is much better to start with a clean image and add grain or similar effects when subsequently manipulating the image in Photoshop. Sensors with small photo-sites are much more prone to noise. Hence small compacts suffer most badly with image noise problems. Version 1.2 Page 5 of , 2008, 2009 Graham Robinson

6 Section 1: Background Shutter Speed The longer the shutter is open for the more apparent any camera or subject movement will be. This movement will be especially pronounced with high magnification as with close-up subjects or long focal length lenses. (a) Camera Movement Camera movement whilst the shutter is open is generally a bad thing that leads to blurred or shaken pictures. Even at moderate shutter speeds proper use of a good tripod will improve the quality of pictures. (Note: poor use of a dodgy tripod is a bad thing). An important exception to this is photography of fast moving objects such as motorcycles. Skilful tracking of, or panning with, the subject during exposure will hold the main subject still in the frame whilst blurring the background. (b) Subject Movement The clichés are all there to be had: waterfalls with water rendered as milky-white vapour; runners with blurred hands and feet; and all the rest. Experience with and understanding of the effect shutter speed with subject movement provides many creative options from which to select provided that you have the craft to execute it properly. Aperture At any one focus setting a lens only brings one distance into to true focus. However, a zone in front of and behind the actual point of focus will appear acceptably sharp. Many factors will play into what is deemed as acceptable such as the size of the reproduced image and the viewing distance. There are also some physical limits to sharpness such as the quality of the optics in the lens and the resolving power of the film. This zone of acceptable sharpness is known as depth of field (DoF). Simple experiments will show that this zone of acceptable sharpness varies quite dramatically depending on the aperture set. With small apertures (high f-numbers) the DoF will increase. With large apertures (low f-numbers) the DoF will reduce. In addition to being affected by aperture, DoF is affected by camera to subject distance. DoF is reduced for near subjects and increases for distant subjects. Also, due to the change in perspective, apparent DoF is reduced when using long focal length lenses. Creative use of DoF is critical to the success of many pictures. Be it creating front to back sharpness in a sweeping landscape or picking out a single point of interest using selective focus. Additional Exposure Controls In a backlit shot you may want to deliberately overexpose by a stop so that the main subject gets the right exposure. This can be achieved by setting a +1 of exposure compensation. In different situations you may want to deliberately over/underexpose by known amounts. A similar effect can be achieved on film cameras (not digital) by deliberately setting the wrong ISO for a film. For example, to knowingly overexpose a 400 ISO by one stop override the ISO to 200. Typically this is done when re-rating a whole film. Version 1.2 Page 6 of , 2008, 2009 Graham Robinson

7 Section 1: Background Automatic Exposure A modern camera is able to use a variety of metering modes to measure light levels and decide for you where to place the tones on your film. It can then go on to decide which combination of shutter speed and aperture to use. Metering Modes (aka: Patterns/Methods) Metering is the measuring of light intensity. Most cameras now offer a variety of ways to measure the light intensity of the scene/subject. Average: Centre-weighted: Partial/Semi-spot: Spot: Measures the light level across the whole frame and comes up with an average reading. Like average but with a bias in favour of the centre of the frame. Take an average reading from a fairly small area, typically at the centre of the frame. Meters a very small area so that specific readings of particular tones in the subject can be measured. Matrix/Evaluative: A very sophisticated mode that takes readings from large number of different areas of the frame and then looks up or calculates the required exposure. On quality cameras the mode is so effective that it does a pretty good job most of the time. There are a few problems though. Firstly, when shooting a sequence of similar shots the nature of the evaluation may lead to significantly different exposures from frame to frame. Secondly, unlike with the simpler metering modes, it is hard to predict what kind of metering the camera will come up with. And finally, the camera is making all the creative choices for you. Exposure Modes Having calculated the correct exposure the camera can now go ahead and help set shutter speed and aperture values. Subject Modes: Program: Shutter-priority: Aperture-priority: Manual: Entry level SLRs often offer special modes for specific subjects like portrait, landscape, close-up and so on. Based on your selection the camera will try to set the most appropriate shutter speed and aperture based on the available light. A major problem with this mode is that the camera is again making all the creative choices for you. The camera again chooses both shutter and aperture for you. Some cameras have a dial that allows you to flick-through the acceptable combinations. The photographer chooses the shutter speed and the camera comes up with the aperture. This is a good choice where camera and subject movement need to be controlled. The photographer chooses the aperture and the camera comes up with the shutter speed. This is a good choice where depth of field needs to be controlled. The photographer chooses both aperture and shutter speed. The camera will often show its estimate of over/under exposure. This is a good choice when the photographer is choosing the correct exposure. Version 1.2 Page 7 of , 2008, 2009 Graham Robinson

8 Section 1: Background Practical Experiments The followings experiments can be carried out with your camera set to average metering mode and program exposure mode. If you are taking a picture of a flat mid grey, a pure-black and pure-white subject or a complex scene then the exposure offered by the camera will look pretty good. In the black and white picture the black will look black and the white will look white. So far, so good. If you take pictures of a white card, a grey card and a black card then all three of the pictures will come out as grey. Large areas of predominantly high or low tone will lead the camera to render these areas as a mid tone and thereby in-correctly exposing the picture. This is the source of the problem with back-lit subjects turning into silhouettes when taken on automatic settings. Automatic settings are great in many cases. However, they will let you down in tricky situations. Summary of Section 1 Light levels are measured in stops. Each stop is experienced as an equal increase in brightness. Each stop is in fact a doubling/halving of actual brightness. To fully control the pictures produced the exposure should be controlled to plus or minus half of a stop (or preferably to +/- one third of a stop). Exposure is controlled by selecting the appropriate ISO, aperture and shutter speed. Different combinations of these three values can give the same exposure value. This flexibility should be used to realise the creative aims of the picture. Automatic settings often work well; especially modern evaluative/matrix metering systems. These are particularly useful in difficult or fast moving situations. Automatic settings can fail; especially in quality-critical work in difficult lighting. Selecting and executing exposure involves measuring, and where possible controlling, the actual and relative brightness of the different parts of the subject/scene and then making settings on the camera so that these variations are fitted on the available density recording range of the film for the best effect. Version 1.2 Page 8 of , 2008, 2009 Graham Robinson

9 Section 2: The Process Section 2: The Exposure Process The setting of an exposure is partly a technical operation based on light measurements. However, the reasoning behind choosing a particular exposure is determined by many factors throughout the whole picture making process. Choosing the Film The choice of film and setting exposure is a two way conversation. The needs of a picture taking situation (such as light levels, colour/mono, image quality requirements, etc.) will lead to appropriate choices of film being made. Having made the choice of a film this will inform decisions made during the exposure process (required ISO, filtration, whether to protect highlights or shadows, etc.). Many of these decisions are purely technical or practical. However, many of the choices can (or should) be made for creative reasons. For example: using high speed slide for soft pastel colours; low speed mono for high quality record work; or, high speed mono for a photojournalistic style. Having made these choices they need to be followed through with sympathetic and supportive decisions being made during exposure. For digital, film choice is equivalent to setting a range of parameters during the picture taking and image development stages. An important picture taking setting is obviously ISO rating. Choosing high ISOs means potential noise problems and careless choices made during exposure can seriously compound this problem. One important development parameter is white balance (equivalent, say, to choosing between tungsten and daylight film). Many other contrast, saturation, etc. options are available. All of these to a greater or lesser extent affect the attitude to the correct way to expose the sensor. Are You Using a Direct or Indirect Process? I had some difficulties in coming up with a good way to describe this distinction. In the end I have settled on the terms direct and indirect. The distinction is important because the choice of exposure is dictated by which parts of the end-to-end process are (or should be) under your control. Direct processes Direct processes are those where all the material decisions and controls that affect the outcome of the final image are carried out either before or during exposure. In this case any downstream processes either do not exist or have been standardised. The classic direct process is colour slide. Press the shutter, that s 40p and game over. Anywhere that light has touched the slide film will be washed away during processing; which is why overexposure is a bad thing. We have no good means of intervening later. Therefore the exposure preparation and execution must not only make sure that the full range of tones is captured; we must also ensure that the tones fall in the right place, i.e. whites should be white and greys should be grey. I will call this placing the tones. If tones are technically exposed but incorrectly placed then whites might be grey and greys would be black; we would have an underexposed slide. The other direct processes are JPEG shooting (protect highlights), colour negative (capture shadows) and batch RAW (protect highlights). In all these cases we must be thinking about not only exposing adequately but also placing the tones in their final resting place. Version 1.2 Page 9 of , 2008, 2009 Graham Robinson

10 Section 2: The Process Indirect processes Indirect processes are those where the final image is produced by a process after the film has been exposed. (a) The classic indirect process is monochrome print work. The aim of in camera exposure mono negative is very different than the aim for slide. The aim for mono negative is to produce the best possible starting point for a good print; i.e. a good printable negative. Digression about black and white negative photography. Black and white film photography is a massive and hugely complex subject. I am not qualified to and have no intention of exploring the zone system, Ansel Adams or his massive volumes on the negative and the print. A lot of this is about the individual exposure and custom development and printing of individual negatives. We are talking about Camera Club photography where images are not handled individually but in batches of 36. The ability of a negative to record subtle variation of tone varies across its recording range. At the ends of the range to maximum and minimum density the change in brightness gets squashed into a smaller range of changing density. The best separation of tones occurs in the middle of the density range of the film. The aim of exposing mono negatives well is to place the most important tones on the sweet spot but without losing required tones off the ends of the recording range of the film. (b) The RAW - digital indirect process is very similar (aside from being in colour) to the mono negative process. However, the best tonality of 16-bit RAW files is way up in the highlights and at its worst in the low tones (where the noise also lives). This creates an interesting dilemma: the best tonal separation is up in the highlights but (as with slide) a blown highlight is gone forever. So the strategy in this case is to overexpose as much as possible without blowing any needed tones. (The technical reason for this is linear gamma.) An additional strategy is that unlike with film you can develop your exposed film as many times as you like. It is possible to blend two developments one for the highlights and one for the shadows and mid-tones. This is only usually needed in difficult situations. Look at the Subject The first thing to remember is that the eyes and brain conspire to prevent you from seeing the world as it actually is. Pretty much they try to decide as quickly as possible whether something is prey, predator, edible, in-edible, friend, foe or potential mate. The act of really looking is quite hard work. When looking at someone s face the eyes and brain will be deciding up whether you know them, working out if they are friendly and assessing their emotions. It will take conscious effort to see the shadow of the nose, whether there is light in the eyes, whether there is a gentle highlight on the cheek away from the main light source, and many other things that make a picture with balanced exposure. (Let alone compositional, emotional and artistic considerations.) Decide how you are going to render the scene/subject. Are you going to create a natural/realistic rendering? Make it high-key or low-key? What is the contrast range? What are the important highlights and shadows? Is the scene/subject a good mix and range of tones? Does it need to be brighter than average or darker than average? Think about the important tones compared to a middle grey. For slide and digital, pay attention to any dangerous highlights. For negatives, watch out for large heavy shadow areas. Version 1.2 Page 10 of , 2008, 2009 Graham Robinson

11 Section 2: The Process Measure the Light Levels There are basic, advanced and everything in between approaches to measuring light. Advanced landscape workers (esp. those using large format slides) will probably use a spotmeter and make many readings of the parts of a scene and mentally calculate the separation on tones. Studio photographers will assess were shadows fall using modelling lamps in the flash units and flash-meter the relative power of each of the lights at the subject to check the lighting ratio. Social/wedding photographers will be avoiding harsh top lighting and trying to find some nice light that gives good modelling. They may use an incident lightmeter to avoid the fancy matrix metering in their posh SLR being fooled by white dresses. Many cameras now have a spot metering mode that allows individual parts of the scene to be quickly checked and with practice spotting a few areas will give a good mental impression of the final outcome. This was my preferred way of working with slide film. I would pick out my most important mid/highlight and spotmeter that for my setting. If the scene was predominantly dark I would pick a mid to dark tone and make that either one or two stops under. As an aside, I would very rarely bracket exposures; I wanted my mistakes to hurt so that I would learn. If you have done a good job of looking at the scene then in many situations you will be able to assess what correction a simple centre-weighted average reading might need; though this does get harder in difficult or confusing light. If you are going to use the built-in matrix/evaluative metering mode then you are probably best just to go with its recommendation; though you might want to bracket exposures in very tricky light. Try to Balance the Exposure After assessing and/or measuring the scene you may decide that there are going to be problems. More brightness range than the film can cope with. The scene is too contrasty or too flat. The lighting of portrait is harsh or un-flattering. The lighting is not giving the atmosphere/drama you need. If you have got problems then fix them in front of the lens before you take the photo. In pictorial and landscape photographs polarizing and graduated filters will help a lot. All serious landscape workers have a range of different types of graduated filters. These are most often used to control the exposure of skies in relation to the foreground. In a studio or still-life situation the lighting is all under your control; adjust it as required. Consider using light modifiers; such as diffusers. With natural light portraits, move the subject. Soft daylight though a window is much more pleasant than bright overhead lighting. You might consider fill-in flash, but don t over do it. If using monochrome then sometimes using red, orange, yellow or green filters at the taking stage will help separate tones and help get a more balanced exposure. Fix all the problems the best you can. If required, re-assess and re-measure the situation. Version 1.2 Page 11 of , 2008, 2009 Graham Robinson

12 Section 2: The Process Now Place the Exposure You should now understand what kind of exposure is required and now you need to put it on the film. The way to do this varies with the medium. Direct methods The tones must be placed so that they are in their correct and final place. For colour slide, JPEG, and batch-raw processes be careful with highlights. For colour negative (and trade processed & printed B&W) ensure shadow detail is protected. Indirect methods The tones must be placed so that the give best tonality (and information) for the subsequent processing into the final image. For mono negative use the centre of the range of the negative s tonal range for the important tones and ensure that important shadows have enough density to print well. For indirect-raw process overexpose as much as possible without blowing the highlights. Make sure that you understand how to understand and read your camera s histogram display. What Comes Next? If you are using a direct process then for this picture all that is required is to put the pictures through the standardised process. If you are using an indirect process then the fun of realising the final picture is about to start. Whatever method you use always compare the final outcome with what your thoughts were at the time of making the exposure. Think about whether there was anything you missed at the time that you with do better next time. I hope these notes were helpful or interesting. Good luck with your photography. Version 1.2 Page 12 of , 2008, 2009 Graham Robinson

13 Section 3: Process Comparison Medium Key Properties Colour Slide ISO; Daylight/Tungsten; Saturation Type JPEG (digital slide) Set camera settings (i.e. film choice) ISO; tag srgb; contrast; white_balance; sharpening Colour Negative ISO Daylight/Tungsten Saturation Type Batch RAW (digital c-neg) ISO tag Adobe-RGB Basic Mono ISO; film type Advanced Mono ISO; film-type; grain-structure. RAW (digital indirect) ISO tag Adobe-RGB Lighting On-Lens Filtration Exposure Development Printing / Digital Prep Avoid high contrast Avoid high contrast Can cope with moderate to high contrast Can cope with moderate contrast Can cope with moderate to high contrast Can cope with high contrast Can cope with moderate to high contrast Use grads e.g. skies in landscapes. Polarizer for reflections/saturation. (Colour correction filters in strange light) Use grads e.g. skies in landscapes. Polarizer for reflections/saturation. Use grads e.g. skies in landscapes. Polarizer for reflections/saturation. (Colour correction filters in strange light) Filters less appropriate as likely to be using this process in a Portrait/Product shoot or Wedding/Social photography situation. Can use filters as per JPEG Grads & polarizers (as above) Use tone control filters as required: red, orange, yellow and green. Grads & polarizers (as above) Use tone control filters as required: red, orange, yellow and green. Use grads e.g. skies in landscapes. Polarizer for reflections/saturation. Do not blow the highlights. Use exposure to accurately place tones on the film. Do not blow the highlights. Expose to accurately place tones. Also avoid underexposure of shadows esp. at high ISO to avoid noise problems. Ensure that you capture shadow detail. Highlights will mostly take care of themselves. Manually fix exposure for each set-up probably use studio flash-meter or incident meter for outdoor work to give batches of consistently exposed images within a shoot. Ensure that you capture adequate shadow detail. May choose exposure to suit planned development. E.g. under-expose & over develop to boost contrast. Over-expose as much as possible without blowing important highlights. Watch histogram or use spotmeter. Standardise e.g. use process paid film Development occurs in camera based on camera settings. Standardise e.g. use pro lab Develop on computer. Apply any required global changes to all batches in a shoot. Assess representative image in a batch. Apply required changes to all images in batch. Standardise to create printable negatives. May choose developers & process to suit specific contrast range & exposure. Develop on computer. Do WB and basic levels & curves in RAW conversion. Multiple developments are possible. N/A Standardise Should print as is. For screen use: down-sample and resharpen as required. Standardise e.g. use pro lab Standardise e.g. use pro lab Should print as is. For screen use: down-sample and resharpen as required. Multigrade RC paper Grade & exposure time for overall tone. Fix locally by burning in skies and corners. Specialist Papers, Local contrast control, Split grading, Toning, &c. Do reqd. manipulation. Do not sharpen master file. Re-sample and sharpen copies for each print size Version 1.0 Page 13 of Graham Robinson