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The Basics of Digital Cameras Part 2: Autofocus, Exposure Modes, Live View, Viewfinders and Saving Images

In the previous 2 lessons in this Digital Photography Basics series we have looked at: the main types of cameras, the fundamentals of how cameras work, the role of lenses and the main facts about image sensors. Before we start getting practical, let's get clued up on the way autofocus sytems work, exposure modes, Live View mode, viewfinders and saving photos to memory cards...

Autofocus systems

Although it's important to get the hang of using the manual focus ring on a DSLR (for when the autofocus struggles, like at night) most of the time we will be using autofocus. We'll explore autofocus later in this series, but let's take a quick look at some important basics: The quality of autofocus is primarily determined by its sensitivity to edges in a scene, its ability to lock on to a subject quickly and the number of autofocus zones.

Cross sensors v linear sensors:

Autofocus works by detecting edges and focusing on them, hence why it's impossible to lock on to a clear blue sky or blank sheet of white paper. But some cameras are more sophisticated at spotting edges than others. This is partly dependent on the type of autofocus sensors being used. So called 'cross sensors' are superior to the alternative 'linear sensors', because they can detect vertical and horizontal lines, whilst linear sensors only pick up on horizontals.

Focus zones:

The number of focus zones is worth looking out for. Focus zones are the points that you see through the viewfinder with which you lock on to a subject. On the whole, the centre zone is the most accurate and many photographers use it all the time, simply re-composing an image when the subject is off-centre. But if you prefer to scroll through the focus zones to select one for an off-centre subject without the need to recompose, the more zones the better.

Continuous focusing:

Most DSLR cameras also feature a 'continuous' focus mode, which allows us to lock on to a moving subject, by half pressing the shutter button, and remain focused on it even as it moves around so long as the shutter is kept depressed. This is a really clever system that works by anticipating the direction of an object's movement in order to keep up with it. It's a very useful feature if you are keen on sport and wildlife photography.

 

Exposure modes

The exposure modes of digital cameras is something we will explore in practical terms later on, but it's worth having a quick overview here. The various exposure modes give us control over the way in which we determine the brightness of our images, along with several other visual characteristics.

digital camera settings

Program:

This is the fully automatic mode, which most of us used without thinking about it - before we realized what we were missing! It's still useful when you need to be incredibly fast and don't have time to think and prepare shots with any care. So candid photography and photojournalism are good examples of when program mode is useful.

Manual:

Manual mode gives us total control of the aperture and shutter speed. We will discover how these 2 things impact your pictures soon. In a nutshell, a big aperture lets in lots of light and vice versa, whilst a slow shutter speed lets in lots of light and vice versa. Aperture also influences the amount of a picture that is in sharp focus, and shutter speed influeces the appearance of motion.

Aperture priority:

Aperture priority mode lets us control the aperture size without having to worry about shutter speed. The camera works out the required shutter speed to produce an image that is the correct brightness. Lots of photographers work in aperture priority mode the most, because it gives them control over depth of field whilst still being a fast way to shoot.

Shutter priority:

This mode is the reverse of aperture priority. It lets us choose a precise shutter speed, to determine how motion appears, whilst the camera selects an aperture to produce a bright picture. This is useful when you need to freeze motion, like at a sports game, or capture blur, like traffic lines or beach waves.

Scene-specific modes:

Many compact cameras have really useful scene modes, like portrait, landscape and macro. Selecting one of these tells the camera to choose the settings that are more less right for the respective type of scene you are shooting. For example, portrait mode will use a nice wide aperture to throw the background out of focus and draw attention to the subject.

Exposure compensation:

Exposure compensation is a great feature of digital cameras that lets us dial in a specific amount to increase or decrease the brightness of pictures by. Image brightness is measured in 'stops', so if we dial in a positive value of 1 stop, images will be 1 stop brighter. Meanwhile a negative value of 1 stop would make images 1 stop darker.

The camera automatically selects an appropriate shutter speed and aperture based on the exposure compensation values. If we are shooting in manual or semi-manual mode, then the light meter reading is simply altered to guide our choice of settings.

Exposure bracketing:

I'm sure that you've taken photos at sunset where either the foreground is really bright but the sky is burnt out, or the sky looks lovely but the foreground is totally dark. It's these kinds of situations that exposure bracketing lets us rectify. Basically, it involves taking multiple shots of the same scene, with different exposre settings so that the right brightness is achieved for all areas. All we have to do is select the number of photos we wish to take, and the difference in brightness (measured in stops) between them.

For example, we might take 3 photos, each one stop apart. These pictures can then be blended together to create the perfect single image in post-processing. Some cameras fire off all the bracketed exposures with just one press of the shutter button, whilst others require you to press it for each picture that is taken.

 

Live View and viewfinders

All compact cameras project the image live on the LCD screen. Some DSLR's also offer this feature, called Live View, but many simply have a viewfinder to look through when you are composing a shot. There are pro's and con's to Live View:

Pros: It provides a very simple way of shooting, with no need to raise the camera to eye level. Plus, some DSLR's use an articulated fold-out LCD screen with a Live View display, which is a really handy feature. It allows you to shoot at some tricky angles - without having to get into crazy positions - all the time with a clear view of the image.

Con's: That said, relying on Live View can be a problem, especially when bright light reflects off the screen, making it almost impossible to actually see the image. Live View tends to result in slower autofocus too, which is a problem for action photography. Finally, perhaps the biggest flaw with Live View is that it drains battery power at a pretty fast rate.

The viewfinder on DSLR cameras generally provides a clearer view of the scene in front of you, so is the best option most of the time. Although it's worth bearing in mind that viewfinders typically only have about 95% coverage (meaning you see about 95% of the actual image).

 

Saving images to memory cards

When we take a photo with a digital camera the image sensor converts light into electric current, which is then saved in the memory card. Not all memory cards are created equal though - some can save more information and at higher speed than others. The capacity of a card is always given in GB. Meanwhile the speed is measured in MB/second, which is sometimes shown as a number followed by 'x' (e.g. 200x), with 100x being 15 MB/second.

The most popular memory cards with compact cameras are called SD cards, whilst DSLR users often tend to use CF cards. Many cameras support both types though. One of the chief benefits of digital photography over film is, of course, that we get to preview images we have just taken and delete the bad ones to free up space on the card. When all of the images on a card need to be deleted, the 'Format' button should be used (instead of deleting one-by-one) to preserve the reliability of the memory card.

What happens when you are shooting at a faster rate than your memory card can save at? Well, DSLR's these days do indeed offer rapid shooting rates (frames per second, or fps), but fortunately they also feature a small amount of built in memory, called the 'buffer'. So, when you're firing off dozens of images, they are immediately saved to the buffer, which then sends them on to the memory card. This buys you the time to keep shooting whilst your card is busy saving all the images in the buffer.

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