Introduction to your digital SLR

Transcription

1 Introduction to your digital SLR So, this is what your lovely digital SLR looks like inside. Remember; it s a light-proof box with a lens on the front. Oh, and an incredibly powerful little computer. You can see from the lil diagram below, how the light comes in through the lens, bounces off the mirror, and then up into the pentaprism where it does some more bouncing, until it goes into your eye through the viewfinder. Light, of course, travels at 180,000 miles a second or something, so all this happens all the time, very quickly. When you press the shutter release button, the mirror flips up, the shutter opens and the image you ve framed up focussed on lands on your image sensor (also known as CCD). The image sensor records the image, and then writes to your storage card, and the sensor is ready for another shot. Brilliant. Here s another picture. This time, the mirror has flipped up and the light is going straight onto the sensor. 1

2 So you can take piccies like this: And this: Actually, we don t want pictures like the zebras, we want them to look more like the work of Ansell Adams (top picture). 2

3 HALF-PRESS! Half-pressing the shutter release button activates the camera s focussing system. Here s what the Canon focus points look like on a Canon 10D (different models have slightly different arrangements of focus points): Here is the focus point array for some Nikon models: 3

4 Just to prove that different camera manufacturers have different focus systems, here s the Olympus e-510 three-point system: So, with all the systems, you can choose which focus point to use, or you can employ all of them to help make sure everything in the frame is in focus. 4

5 METERING The half-press also activates the metering, that is the light reading. The camera will look at the scene, and produce what it thinks is the right shutter speed/aperture settings to give you a correct exposure. There are usually three types of metering available Below is the Nikon metering selector, from left to right, they are: spot metering, 3d Colour Matrix and centre-weighted. And here are the Canon choices: Evaluative metering takes a meter reading across the entire scene and attempts to deliver a decent exposure taking all the light in the frame into account. It may or may nor do a good job. It can often fail in back-lit situations, or where there is high contrast. Partial metering biases it results for the centre 9% or so of the frame. This can be useful when shooting back-lit subjects, or subjects where the peripheral light in the 5

6 frame may confuse evaluative metering. Spot metering takes its light reading from a very small (usually less than 3 or 4%) portion of the centre of the frame. This is the most accurate but most difficult metering method to use. Spot metering is useful when shooting close-ups (macro) and you want to be sure that a certain part of the subject is metered properly. It is also used in the Zone System of exposure. Centre-weighted average (Canon) takes a reading across the whole frame, but weights the exposure towards the centre. Nikon s 3D Colour Matrix system makes bold claims about its ability to make very accurate exposures and is highly regarded by many of its users. So, before you take a shot, you need to choose how your are going to focus, and how you are going to meter. Then you need to half-press to activate those systems, and then you can take your picture. 6

7 MODES Most dslrs have a mode dial. Here you can choose between: AUTO: The camera makes every decision apart from what to point the camera at. You are locked into what the camera wants to do. You often can t change very much at all. P: Programmed Auto gives you more menu items to choose from (like metering and focus types) and allows you to choose different combinations of aperture and shutter speeds. TV (or S): Time Value (or Shutter Priority): A semi-automatic setting that allows you you dial in the shutter speed you want and the camera does the rest. AV (or A): Aperture Value (or Aperture Priority): Another semi-automatic setting that allows you to dial in the aperture you desire, and the camera does the rest. M: Manual requires you to set both the aperture and shutter speed, almost as if you know what you re doing! A-DEP: Automatic depth of field (Canon only) makes sure everything in the image is within the depth of the field, ie: it ll be in focus. You also have some handy pre-sets on dial for taking portraits, landscapes, sports etc. Experiment with them and analyse the results to see if they are giving you the images you want. 7

8 APERTURE The aperture is the variable hole in the lens, and the size of the hole is measured in f- stops. The apertures has two functions: 1) Light gathering 2) Control of depth of field The larger the aperture, the more light lands on your sensor. Also the depth of field is restricted, leading to out of focus backgrounds. This is often used in portraits. The smaller the aperture, the less light lands on your sensor. Also, the depth of field is extended. You might use this for landscapes. 1) The top picture shows an extended depth of field, all the links of the fence are in focus. 2) Below is a limited depth of field version, you can see that the links are only in focus on the centre of the image. 8

9 The standard f-stop sequence is: 2.8 = wide open aperture (limited depth of field, most light) = small aperture (extended depth of field, least light) Each of these increments either doubles or halves the amount of light coming into the camera. Each of these increments is called a stop. Our clever dslrs also have one-third of a stop increments between the standard fullstop increments, like this: 2.8 (3.2, 3.5) 4 (4.5, 5) 5.6 (6.3, 7.1) 8 (9, 10) 11 (13, 14) 16 (18, 20) 22 The aperture range varies from lens to lens. Most kit lenses that come with dslrs are capable of f3.5 when fully open. Bear in mind that zoom lenses will shut the aperture down a little when you zoom in, usually to f5.6 when fully zoomed. Also, you may find you can get f-stop values up to f32 when zoomed in, but only f22 when not zoomed in. Above: A limited depth of field (large aperture, small f-number) has thrown the background out of focus, and the subject is thrown into sharp relief. Looks nice. 9

10 SHUTTER SPEED The shutter is like a pair of curtains that opens and closes to expose the sensor to the scene you have focussed on. You can control how long the shutter stays open. The shutter has two functions: 1) Light gathering 2) To control the sense of motion in the image The slower the shutter speed, the more light lands on the sensor, and more motion blur is captured. The faster the shutter speed, the less light lands on the sensor and motion is frozen in the image. Above: A slow shutter speed and a moving subject creates motion blur in the image and gives the sensation of speed. This image has also been taken using the panning technique; following the subject with the camera and then pressing the shutter release button while still following the movement. This has thrown the background into one big motion blur, emphasising the sense of speed. 10

11 Above: This image was taken with a fast shutter speed, and has frozen the droplets of water splashing off the apple. Above: A slow shutter speed has created light trails from the moving cars. 11

12 Shutter speeds are measured in these standard increments: 1/4000 of a second (very fast shutter speed, less light, frozen motion) 1/2000 1/1000 1/500 1/250 1/125 1/60 1/30 1/15 1/8 1/4 1/2 1 (the denotes a full second, rather than a division of a second) Bulb (with this setting the shutter stays open as long as the shutter release button is depressed, used for very long exposures needed when photographing the stars etc). As with aperture settings, each of the above increments halves or doubles the light, and each is called a stop. Our dslrs also have one-third shutter speeds between the above standards, like this: 4000 (3200, 2500) 2000 (1600, 1250) 1000 (800, 640) 500 (400, 320) 250 (200, 160) 125 (100, 80) 60 (50, 40) 30 (25, 20) 15 (13, 10) 8 (6, 5) 4 (0 3, 0 4) 0 5 (0 6, 0 8) 1 and so on. By the way, 0 3 means 0.3 of a second, roughly 1/3, 0 4 is 0.4 of a second or 2/5, 0 5 is 1/2, 0 6 is about 2/3, 0 8 is 4/5 of a second. 12

13 EXPOSURE Exposure is one way of describing the amount of light that the sensor needs to make an image. An over-exposed image has too much light, highlights will have no detail, colours will be washed out, blacks will look grey. An under-exposed image doesn t have enough light and will look dark with grey whites, drab colours and no detail in the shadows. The correct exposure is determined by the camera s metering system, and is a combination of shutter speed and aperture. The ISO setting can also be used to influence the exposure (more about that later). You can have different combinations of shutter speed and aperture to create the same exposure. For example, let s say your camera indicates that a correct exposure for a given scene is a shutter speed of 1/125 and an aperture of f8. If we remember that each standard increment of both shutter speed and aperture halves or doubles the amount of light, we can then calculate that the same amount of light will hit the sensor with a setting of 1/60 (doubles the light) and f11 (halves the light) or 1/250 (halves the light) and f5.6 (doubles the light). These settings are called equivalent exposures. If you use the MODE DIAL to put your camera into TV (or S) mode, you dial in the shutter speed you want, and the camera will calculate the correct aperture setting. As you change the shutter speed, you will notice the aperture setting changing automatically as the camera sets equivalent exposures. If you use the MODE DIAL to put your camera in AV (or A) mode, you dial in the aperture setting you want, and the camera will calculate the correct shutter speed, and as you change the aperture settings, you will see the shutter speed changing automatically as the camera sets equivalent exposures. 13

14 EXPOSURE COMPENSATION Bear in mind that so far, the camera is deciding the EXPOSURE, whether you re in AUTO, P, TV (S) or AV (A) mode. You can dictate a certain amount, based on whether you want to control the sense of motion in your image, or the depth of field, but you have no control over the actual exposure. What if your camera gets it slightly wrong, or what if you want the image to appear darker (or lighter) than the camera does? This is where exposure compensation comes in. The function varies from model to model, but commonly there is this button somewhere on the camera s body: Above left: EV Compensation button on a Nikon. Above right: EV Compensation on a Canon 400D You can use this function to override your camera s automatic exposure setting. Take your picture, review it on the LCD screen. Is it too dark? Press the EV Comp button, and dial in a larger aperture (or a slower shutter speed, depending whether you re in AV or TV mode). You can usually go plus or minus two stops in one-third of a stop increments (the Olympus e-400 can go plus or minus five stops!) This is a hugely useful function that gives you much more control over how your image looks. Remember; you re in charge, not the camera. The camera isn t always right. REMEMBER: Once you ve used the EV compensation function, put it back to 0 when you re finished, or all your pictures will be under/overexposed by the same amount! 14

15 ISO We mentioned ISO earlier. It s a simple concept, the ISO rating refers to the sensitivity of the image sensor to light. A high number means the sensor is more sensitive to light, a low number means it s less sensitive. You might find using a higher ISO helps out in low-light conditions. Lower ISO numbers generally mean better quality images, because the higher you push that ISO, the more NOISE you introduce into the image. Noise is a form of digital distortion that often looks like splotches of purple or green, you ll see them in blocks of tone in high ISO images. ISO increments halve and double the sensor s sensitivity to light (a bit like aperture and shutter speed increments). And they also have one-third increments between the standard ones: 100 (125, 160) 200 (250, 320) 400 (500, 640) 800 (1000, 1250) 1600 Noise is rarely seen in ISOs of up to 400, but it starts to be a problem after that, unless you are using a FULL-FRAME image sensor camera. Full frame refers to the size of the image sensor, and means it is the same size as a 35mm negative. The cheapest Canon full-frame camera is the 5D (around 1200 in June, 2008), the cheapest Nikon is the D3, which is their only full frame dslr ( 2,500). Above: An example of noise in an image. Ugly. 15

16 WHITE BALANCE If you take a picture by candlelight, it s going have a different colour cast than one taken outside on a sunny day. An image taken under fluorescent light will have a different (usually a ghastly green) cast from one taken with flash. Different light sources create different colour casts, and we need to correct this if we don t like the pale orange cast that we re seeing across our image that we took indoors with household light bulbs providing our lighting. The White Balance setting allows us to correct these problems with some pre-sets. Most of the time, your camera will be on AWB (Auto White Balance), but it isn t always accurate. Move on to the pre-sets if you want a better result and learn how they affect the image. Pre-sets usually include Daylight, Shade, Cloudy (twilight, sunset), Tungsten and Fluorescent. For optimum control the Custom White Balance setting allows you to create a custom white balance for a particular lighting situation. This is useful in mixed lighting scenarios, where you might have, say, fluorescent lights, and daylight coming in through a window. Usually, you need to take a photo of something white (preferably a white balance card, but a piece of white paper will do). Try not to get glare or shadow on the paper, and try to fill the frame with the white paper. You may need to put your lens into MF (Manual Focus) mode, otherwise the AF (Auto Focus) system will have a hard time locking focus on a white tone. Some cameras allow you to set the actual Kelvin temperature of any given light source, and on the page 19 there s a scale which might be useful if that s the route you want to take. You can adjust white balance afterwards in Photoshop, particularly if you are shooting in RAW format. However, you might want to ask yourself how much time you really want to spend sitting at a computer with each image colour correcting it. 16

17 Below: An example of how different white balance settings will render colours of a flower. 17

18 Below: The various colour temperatures of different light sources, in case you re interested 18