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.

 

depth-of-field-comparison
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?

Metering

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.

metering
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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