Decent Exposure Part 2

In an earlier post I discussed the various controls available to you for controlling how your camera exposes the scene before you. However, I didn't talk about how to read or wisely adjust what the camera tells you via its light meter, or what decisions it will be taking behind the scenes.

If you're relatively new to the whole camera thing, you're probably using scene modes or even the full auto 'green' mode present on many cameras. The level of control afforded you in these modes varies, but they all have in common that they will tell you what decisions regarding shutter and aperture they have made, but tend not to allow you to change them. Some allow ISO adjustment, some don't. I would therefore argue that one of the best ways to learn how to take pictures is to find out what the scene modes do, then make those decisions or break those rules as you learn.

I should qualify this with one quick note. The actual decisions taken by the camera are way more complicated in many cases than I'm about to suggest. My description is more of an approximation or ultimate simplification of reality. The camera will usually use a clever metering mode, compare its results against a library of saved 'standard' scenes, and choose what it considers to be the best selection of variables. You (I presume) don't have such a library, so you'll be using rules of thumb mostly instead.

Landscapes
The landscape mode will be attempting to get as much in focus as it can. It will therefore be using the smallest aperture it can get away with whilst maintaining a fast enough shutter speed to give some chance of avoiding camera shake. If auto-ISO is enabled, it may increase the sensitivity to achieve the two aims. Some compact cameras lock the focus distance on infinity in landscape mode, then rely on their relatively high depth of field to make the foreground look acceptable too.

Portrait Mode
In many ways this is the opposite of landscape mode. The camera will select a large aperture (small number) and will usually be focussing pretty close. The aim is to blur the background, thus emphasising the subject. ISO will be kept as low as possible, while maintaining hand-holdable shutter speeds. This is easier in this mode due to the wide aperture in use.

Close-Up (macro) Mode
True 'Macro' photography is defined as producing an image on the sensor which is larger than life-size. This requires specialist gear and is not what the 'close up' mode is designed for. However, the principles are the same. This mode will be expecting you to be focusing as close as you can, often using flash to add light and allow the use of small apertures to maximize the extremely shallow depth of field at close range. Shutter speed is considered to be of secondary importance although at this range, any shake at all will make the picture unusable unless the light is bright enough to allow very fast speeds. The camera will therefore play its usual game of compromise and trade some depth of field for a safer shutter speed for handholding.

Night Portrait
Usually shown as a person's outline against a night sky on the mode dial, this is a mode which allows the use of a relatively advanced feature - slow sync flash - within an easy to use 'beginners' mode. The camera will meter so as to expose the dark scene properly, as if the subject person was not present. With this part out of the way, flash will be added so as to give extra emphasis to the subject, lifting them out of the darkness of the scene as a whole. Due to the potential slow shutter speed needed, this mode may well require the use of a tripod. Think of it as landscape mode (maximising depth of field for distant scenes) combined with a blip of flash. This mode will provide the means for you to get snapshots of family or friends or whomever, in front of darkened scenes such as night cityscapes or fairgrounds. Once you move on to manually controlling your slow sync flash, you'll find it's usually quite customisable for more natural results than the 'stuck on' effect that some auto modes create.

Sport/Action
This one is all about shutter speed. The camera will do its usual compromise calculations to work out how to get the fastest possible shutter speed without making the image unusable. It will do this by opening up the aperture as wide as it can, increasing ISO in some cases etc. It will sometimes also affect the focus mode, changing it from 'one-shot' to 'servo' or 'continuous' autofocus, meaning that there is no longer any concept of locking on - the camera will be expecting you to track the action, shooting as you follow. It will therefore follow as best it can by keeping the AF moving with the subject.

DEP and A-DEP modes (canon only)
if you have a canon SLR then you may well have access to one of these modes, and sadly there's a good chance that it will be A-DEP - the less useful of the two. In this mode the camera attempts to choose an aperture and a focus distance which will render the subjects under the closest and furthest focus points (in the viewfinder) sharp within the depth of field available. DEP mode is much more useful (though still hardly an everyday staple in my experience), allowing you to individually select, using the shutter button, the closest and most distant objects you want to be sharp, then allowing you to recompose before the shot is taken.

Not the Length of the Grass...

Yes, with a terrible piece of punnage I'd like to introduce a mini topic worthy of being thrown into the mix at this point, which is Depth of Field. Also known as Depth of Focus, this concerns what I can only really summarize as 'front-to-back' sharpness. Look in any magazine where there are both portraits and scenery shots, and the main difference you will usually see is that in the landscape shots, the whole of the scene will be sharp, from the closest sheep to the distant hills. Look at a portrait and you tend to find that the only sharp part is actually the subject, and the background is thrown out of focus, thus partly or entirely rendering it a blur.

The distance between the closest and furthest elements which are rendered 'acceptably sharp' is described as the depth of field. If you're using a digital compact or small bridge camera then your control over this effect will be limited, and can best be demonstrated when shooting as close as possible to a detailed subject, using the longest setting on the zoom lens. Some may show the effect on the LCD screen before you press the shutter, while on others you will need to compare captured images. If you're using a reasonably well specified SLR camera, whether film or digital, it's much simpler to demonstrate the effect, due mostly to the additional control available over the settings, and the 'depth of field preview' button which I personally regard as essential on any film SLR camera and highly desirable on digital SLRs. The circumstance where the resulting differences will be most noticeable will be the same scenarios as for the digi-compact i.e. close subject, long lens.

So to the demo.

Set your camera or lens to a wide aperture (small number) such as f/4 or as wide as your kit will allow, put the camera on a tripod or beanbag, focus on a nearby subject such as an ornament - something with some front-to-back depth to it. Look through the viewfinder or at the screen. You should see that only a small part of the subject is in focus. At the widest aperture, what you see is what you get. The image will record like this if you fire the shutter. Take the photo.

Now 'stop down' the lens or camera to the smallest (large number) aperture available. This may be anything from f/8 to f/32 depending on your kit. The image in the viewfinder will not change, so if you're shooting film, use of the depth of field preview at this point is your only route to check the result before your prints or slides come back, and it's well worth a look if using a DSLR too. As you press the button or lever the viewfinder will darken significantly. As your eye becomes accustomed to the reduced brightness you should be able to see that much more or all of the scene is now sharp. What the button or lever is doing is closing the lens down to the aperture you selected, allowing you to see what the film or sensor will 'see' when the shutter fires. On a well specified digital compact I would expect the LCD screen to show the result of the change, but many may not and you will have to take the photo to see the difference. The results should be obvious in these circumstances.

Which brings me back to my earlier comment, about depth of field control being limited on digital compacts. DoF is affected by three main variables. The focal length of the lens :- shorter lens = more DoF, the aperture of the lens :- smaller aperture = more DoF, and the size of the recording medium :- smaller size = shorter lenses = more DoF. On a digital compact, the sensor tends to be very small, increasing the depth of field. Due to the small sensor size, lenses tend to be very short, increasing the depth of field again, and the available aperture range is limited, thus having a further limiting effect on your level of control. These factors all conspire to make it difficult to achieve shallow DoF on many digital compacts unless you are very close, zoomed in, and using a wide aperture - using portrait mode will achieve this if your camera does not have aperture controls.