Fundamentals of Photography: Optimum Exposure (Part 2)

Last month we examined the elements that influence photographic exposure: ISO, aperture and shutter speed. These are they keys to photographic creativity, so it is helpful to understand:

  • how they interact with each other to create an exposure
  • the other effects each element has on your imagery

Why bother“, you might reasonably ask, “if my camera can do all this for me?”  Three reasons:

The camera won’t always get the exposure right – it can be fooled by a variety of scenarios – and if you don’t know why it is under- or over-exposing you will not be able to fix it. For example, we have probably all shot portraits outside where the subjects’ face is in shadow whilst the background is nicely exposed.

In the previous post I talked about correct and optimum exposure – the latter stems from your interpretation of a scene, and may differ from what the camera determines to be correct. In other words, you may want the camera to get it ‘wrong’.

Lastly, when you proudly show off your next meisterwerk and envious friends ask which camera you used, you can tell them “the camera didn’t create this, I did”.

The Relationships

gratuitous cute kitten shot
Gratuitous cute kitten shot representing the idea of cozy relationships.

From here on in I will be employing standard photographic ‘jargon’ that was introduced in the previous tutorial; if you are unsure what a stop is then take a quick look back at that post.

ISO, aperture and shutter speed combine to create an exposure. Whether your picture ends up too bright, too dark, or just right is determined by all three elements together.


Change one of those settings and make the image lighter or darker.

e.g. widen aperture from f8 to f5.6 = exposure increased by 1 stop


Change two settings by the same amount but in opposite directions and the exposure stays the same.

e.g. widen aperture from f8 to f5.6 + increase shutter speed from 1/30 to 1/60 = no change to exposure


Change all three settings by equal amounts, and you change the exposure again.

e.g. widen aperture from f8 to f5.6 + increase shutter speed from 1/30 to 1/60 + decrease ISO from 800 to 400 = exposure decreased by 1 stop


The combinations, clearly, are manifold, but why might you want to fiddle with any of these settings at all?

The most obvious reason is that the current exposure does not look pleasing to you – it is either too light or too dark. That may be the overall exposure, or it may be an important part of the image is too bright or dark.

Remember also that each of these settings also has ‘side effects’ (again, see descriptions in Part 1).

To freeze or blur a moving subject the shutter speed may need to be changed. Freezing the freerunner below required a shutter speed of 1/500.


parkour frozen mid air


To reduce noise or to shoot hand-held in low light the ISO may need to be adjusted. The LED lights at the venue below were not particularly powerful, and I needed to hand-hold a particularly heavy lens, so an ISO of 1600 was required to facilitate a fast shutter speed of 1/200. The aperture was also wide open at f2.8.


high iso 1600


To make your subject stand out from the background or to achieve front-to-back sharpness you may need to alter the size of the aperture. The Portuguese Squill below was shot twice in rapid succession, once at an aperture of f36 and once at f3.2.


If you are not sure which image was shot at f36 and which f3.2, refer back to Part 1 of this tutorial – or better still, have a play with apertures on your camera.

But wait just a minute, as a novice lacking the finely-tuned nose for exposure of more seasoned snappers, what values do I set as a starting point?


Once upon a time no self-respecting snapper would be caught without his or her light meter. In fact these wonderful ‘old skool’ gadgets still find employment, especially in the studio. But your digital camera is a clever beastie and contains its own meter, plus it lets you see what you have produced immediately (jumping back in time again, the humble Polaroid used to serve this function for pros).

In auto and semi-automatic exposure modes the camera uses its meter to establish an exposure for you. Even in fully manual exposure mode the meter still functions and most digital cameras will indicate whether your exposure is correct (note I said correct, not optimum which is what you want it to be).

But as already mentioned, sometimes you want the whole image to be nicely exposed, and sometimes there is just one key area that needs to be exposed well. Digital cameras offer a range of metering options for different situations. The following terminology will vary slightly depending on the make of camera.

A generalised representation of metering options offered by many digital cameras.


Average / Matrix / Evaluative metering methods are the all-rounders. The camera uses information from most of the frame to establish correct exposure. Where lighting across the entire frame is quite even it will produce a balanced image with subject and background nicely exposed.

With Centre-partial / Centre-weighted metering, the camera again uses information from most of the frame, but gives considerable priority to the centre of the frame. The size of this central area can sometimes be modified via custom settings menus. This form of metering is frequently used for portrait photography where you are primarily interested in lighting your subject, the background being less important.

With Spot metering a very small area of the frame is metered. That area can be in the very centre of the frame, and sometimes a number of other selectable focus points as well. Use this to correctly expose the subject (or part of a subject) when the background is considerably lighter or darker.

Check your camera manual to see which of these metering options are available to you, how you select them, and which shooting modes they can be used with. Check whether you can take spot readings only from the centre or from various locations around the frame.

In the next tutorial, the final part of this intro to exposure, I’ll go through the various shooting modes and reveal how you can take complete creative control over your photography.

To chat about photography or Photoshop tuition, call me on 07757 259390 or send me a message via email.

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Fundamentals of Photography: Optimum Exposure (Part 1)

Many of the people I tutor come to me because they know their camera boasts many great features to help them unleash their creativity, but they are stuck on the auto settings where the camera does almost everything. So this month (continuing over the next two) we’re getting back to fundamentals, looking at the things your camera gets up to when you use auto or preset exposure settings, encouraging you to grab some control back, and busting a bit of jargon along the way…


three exposures of the same subject
These photographs were taken within seconds of each other in the same location, but with different exposure settings. Which is the optimum exposure?

Correct exposure often means your principal subject and (maybe) its background are photographed such that detail is visible in both shadows and highlights.

This is not always possible. If you photograph a black statue against a white wall you will struggle to get much detail in both statue and wall.

Correct exposure may also not be what you want to achieve. Maybe you want to over-expose the wall to surround the statue with pure white.

Camera auto or preset exposure modes may produce either an over-exposed wall or an under-exposed statue – the camera will decide which you get.

So rather than correct exposure, I prefer the term optimum exposure, meaning the exposure that serves your creative intent rather than some objective standard.


tonal range


Ignoring colour for the moment, every scene you will ever photograph will feature a range of tones that will vary with the strength, direction and quality of the light. In camera (and Photoshop) speak, that tonal range is represented by 0 to 255 where 0 is pure black and 255 pure white, with greys in between. Photographs of many scenes will feature the entire range of tones – whether or not the actual scene did. This is because your camera is not as sophisticated as your eyes, and can only deal with a limited tonal range in a given scene.

If the tonal range present in a scene exceeds the range your camera can record you get clipping; detail in either (or both) highlights / shadows is lost. Commonly this is seen when shooting outdoors on a sunny day; if the sun is in or near the frame of your composition blue sky is bleached white – or the foreground may become a silhouette (common with sunset shots).


poorly exposed landscape


A lack of detail in shadow areas (blocking) is conventionally more acceptable than a lack of detail in bright areas (blown out highlights). But you might actually want a high key look where some elements are blown out – or lots of deep dramatic shadow. Once you understand the ‘rules’ you can break them and create your optimum exposure. But before we get on to the rule breaking let’s bust some of the most common jargon you will encounter.


Stops and Exposure Value (EV)

These are terms for the basic units of exposure – often used interchangeably, although stops is more common.

Exposure can be increased or decreased by adjusting the camera’s ISO, aperture and shutter speed. A change of plus or minus 1 stop is a doubling or halving of the amount of light entering the camera.

Although camera manuals frequently employ this terminology, camera displays frequently do not, relying instead on numbers or graphic representations that relate to the three terms covered below.



ISO is the sensitivity of the media (film / digital sensor) recording the image. Low ISO settings require more light to render an image, and vice versa. Low ISO also produces ‘fine grain’ images, whilst high ISO produces courser grain (or noise) and reduced contrast.

Digital SLRs offer a range of ISO values, often between 100 – 3200 or 6400; the range can be bigger or smaller depending on make and model.

The scale is linear so ISO 200 is half as sensitive as ISO 400 and twice as sensitive as ISO 100. In other words a camera would need twice as much light to enter through the shutter to record the same exposure at ISO 100 as it would produce at ISO 200.

Aperture and f-stop (f-numbers)

f-stops express the range of lens apertures available. The aperture is the size of the lens pupil at the moment a shot is taken and (along with shutter speed) controls the amount of light hitting the sensor. Aperture size also influences depth of fieldthe area within a photo that is sharp – the subject of a future post.

Intuitively, a larger aperture lets in more light but the terminology can be confusing.

An aperture of f5.6 is larger than f16.

The scale is also confusing as it is not linear. f5.6 is twice as big as f8, while f11 is half as big as f8.

To add to the confusion, your camera may allow you to alter aperture by 1/2 or 1/3 stops. So, between f8 and f11 you may also see f9 and f10.

The good news is that you do not have to remember all this. Remembering a few of the most commonly used apertures (f5.6, f8, f11, f16) and the direction of travel (f5.6 wide – f16 narrow) is sufficient. It can help to think of the f-stop as a fraction (f/8, f/11, f/16 etc) where f is a constant.


Shutter Speed

More good news – shutter speeds are MUCH more intuitive than apertures. A slow shutter allows in more light than a fast one; and the scale is (more or less) linear, so a shutter speed of 1/30 second lets in twice as much light as 1/60 second.

However, your naughty camera may try to confuse you; many displays do not show the whole fraction, they just show the denominator, so 1/30 is shown as 30 and 1/60 as 60. Shutter speeds of 1 second or more may be marked by the symbol ” after the number (e.g. 10″ = 10 second shutter speed).

A fast shutter can freeze objects in motion, whereas a slower shutter may blur them.

The range of speeds available commonly stretches from thousandths of a second to 30 seconds. Some cameras also have a bulb setting that allows you to open the shutter indefinitely. Apparently a German photographer has achieved shutter speeds of several years… the mind boggles!


In the next tutorials post we’ll look at how ISO, aperture and shutter speed interact to create an exposure, and critically, how you can wrestle control of them away from your camera.

To chat about photography or Photoshop tuition, call me on 07757 259390 or send me a message via email.

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