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Exposure Strategies - part 2 of 1 2 3 4 5 6

by Mike McNamee Published 01/12/2013

The Exposure Triangle

The exposure of an image is governed by three things - the ISO speed setting, the shutter duration and the aperture. All three are inter-related.

Old (very old!) hands will remember the 'Sunny 16 rule' which expresses the relationship as: 'for a sunlit scene, using an aperture of f16 requires a shutter speed of the reciprocal of the ISO setting' - in other words if you have a setting of ISO 500 then you need 1/500s at f16 as your exposure for a sunny day. This simple rule formed the basis of the setting of ASA speed which eventually morphed into ISO speeds and served its purpose for many years (remember it used to be printed on the inside of the film box).

The relationship between aperture and shutter duration may be compared to a ball of dough. Imagine that the ball of dough represents the amount of light required to expose the scene correctly. If the ball is rolled out to a thin cylinder (small aperture opening) then the length of dough is long (long exposure) and vice versa, ie they are reciprocally related. The equivalent of ISO is the density of the dough, a high-density dough allows for shorter exposures and smaller apertures while providing the same 'weight' of light.

The obvious choice would therefore be to bump up the ISO setting but this brings with it an increase in noise - the relatively shorter exposures and/or smaller apertures bring less light to the sensor and so the background noise makes a larger contribution to the image, especially in the shadows where there is less light in any case. This is also why shadows are noisier than highlights and why under-exposure is dangerous - all you do when you lighten up the image in Photoshop is bring out the noise!

The trick in all this is getting the best single-shot compromise between the competing factors and the purpose of this feature is to discuss the strategies for choosing the exposure settings. In essence, it is a case of deciding where to place the histogram in relation to the highlight and shadow end points of the graph.


The Next Move

We need to measure the effect of different exposure strategies to have any chance of understanding the price that you pay for the inevitable compromises. The Macbeth Chart makes a good starting target because it is of relatively low intrinsic contrast and well within the dynamic range of a camera - we can thus push it up and down the exposure scale and measure how it fares. To do this we employed the Rags Gardner' audit routine which assesses the errors in all the patches of the chart under the different conditions. There are three conditions in common use to consider; they are:

1. Expose perfectly for a mid-grey - in other words the out-of-camera value of a 50% grey patch is 50% luminance.

2. Slightly under-expose and bring back the image in Photoshop to the correct value while watching the highlights for clipping. This is a very safe option and often employed by wedding photographers who must, above all, not over-expose the bride's dress! It does require more post work and carries the penalty of noisier shadows and reduced effective bit depth.

3. Expose to the right - the so-called ETTR. Here the exposure is adjusted so that the important highlights are just placed at the point of clipping. It is a considered technique which can be used in the studio or for leisurely activities such as landscape photography, but it is more difficult to get right in the heat of a wedding! Technically it produces the least noise and maximises the dynamic range of the camera. It has many devotees amongst the fanatical fraternity with time on their hands. (Jeff Schewe has a well-considered appreciation of the method at http://schewephoto.com/ETTR/ )

1. Expose for the Mid-grey

This is the 'least work' option - you pin your exposure to the 50% grey which effectively places the average of the scene in the average position. It is the exposure a well-adjusted camera should deliver in automatic mode. It is the least-work option because the images may be used without further ado, ie straight from Lightroom to Loxley's! Done well it enables the user to employ a JPEG workflow as no downsteam adjustment is anticipated. Its critics say (in a derogatory tone) that it produces 'average' pictures. The aim RGB values for mid-grey depend upon the colour space used and are 121 points for sRGB and Adobe RGB, and 102 Points for ProPhotoRGB. The histogram for such an exposure should lie in the middle of the graph, indeed an image of a grey card should have a histogram consisting of a single vertical line, bisecting the histogram space. Experience of placing an exposure at this mid-point teaches us that it might not result in a 'preferred' image - as we have already said some like 'em lighter, some darker!

2. Slight Under Exposure

Here the aim is to avoid clipped highlights at all costs. The histogram always lies just slightly to the left and in many instances it can be achieved by setting the exposure compensation to -0.5EV and then using the automatic camera exposure (eg aperture priority) safe in the knowledge that most highlights will be kept under control. Light-bulbs in the scene will still be clipped but such effects are usually ignored. This method provides the fastest and simplest way of working at the time of exposure, the penalty is paid downstream in post production. A downside is that if you are driven to using high ISO values and you also under-expose, then noise can become an issue. It is a method favoured by some (but not all) wedding photographers, who have little time to chimp their histograms and no opportunity to catch some moments twice.


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1st Published 01/12/2013
last update 21/07/2022 08:46:24

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