Digital SLR Basics 1

Transcription

1 Digital SLR Basics 1

2 When you re not using flash, there are three main components that make up your exposure. Aperture, Shutter Speed and ISO setting. It s called the exposure triangle. Aperture Shutter Speed ISO 2

3 These three legs of the exposure triangle must balance out for a proper exposure. If you have a balanced or proper exposure and you change one part of the triangle you must change one of the other parts to again achieve proper exposure. The aperture refers to the size of the opening that lets the light in. The shutter speed refers to the amount of time that you re letting this light in. The ISO setting determines how sensitive your camera s sensor is to light. In photography, the light that is exposing your sensor (the part of the camera that takes the picture) is most often talked about in terms of stops of light. More on that on the next slides. However, if you change your exposure to let one more stop of light in than what you previously had, you let in twice as much light. Conversely if you change your exposure to let in one stop less light you ve cut your exposure down by ½. 3

4 Aperture The size of the opening that allows light in. The smaller the number the larger the opening to let more light in. Aperture is usually a value that is dependent upon the lens and not the camera itself. The trade off is that the larger the opening is the smaller depth of field (part of the photo that is in focus) you have. Aperture setting is based on f-stops. The whole f-stops are: 2, 2.8, 4, 5.6, 8, 11, 16, 22, and 32. They go smaller and they go larger but that is the typical whole f-stops. Your cameras can probably be adjusted by 1/3 f-stops. For example the jump from 5.6 would be Smaller numbers (or larger openings or f-stops) are often desired for taking portraits because it tends to put the background somewhat out of focus (bokeh) and makes the subject stand out more. Lenses are often based or labeled by their speeds. The fastest lenses have the smaller numbers, 1.8 is faster than 2.0, 2.0 is faster than 2.8. They are labeled this way because as they let more light in you can shoot at a faster shutter speed. Several lenses have variable apertures usually meaning that as you zoom out the maximum aperture value changes. My Canon goes from a value of 3.5 zoomed all the way in to 5.6 zoomed all the way out. 4

5 Some lenses such as your Tamron mm zoom lens have a fixed aperture. That lens can stay at the same aperture throughout it s zoom range. 5

6 Shutter Speed Shutter speed refers to the amount of time that your cameras sensor is exposed to the scene or the photo you re taking. The slower the shutter speed the longer the time is that you re letting light into the sensor to record a picture. Slow shutter speeds however often show either motion blur (movement of the subject) or camera shake (you can t hold the camera still enough to get a sharp picture). At 1/60 th of a second you will most likely get motion blur if a person is just walking while at 1/1000 of a second you will probably freeze a hummingbirds wings. A slow shutter speed on a tripod is good for getting the dreamy water effect but not good for shooting sports. With Canon cameras, some lenses have built in image stabilization to help with the camera shake to some extent. I don t think any of your camera lenses have this. A good general guideline is to never shoot slower than 1 / your lens focal length to reduce camera shake. Example if you re shooting with your Tamron mm lens zoomed out to 200mm s you shouldn t shoot slower than 1/200 th of a second to minimize camera shake. 6

7 Shutter speed often goes in 1/3 stop increments as well. An example would be from 1/125 1/160 1/200 to 1/250 with the 1/125 1/250 being a full stop again. 7

8 ISO Settings The ISO setting is how sensitive your camera s sensor is to the light that is exposing it. The higher the number the more sensitive the sensor is to light. However the higher the number also means you get more digital noise or image contamination. An ISO setting of 100 will give you the cleanest images while an ISO setting of 3200 will give you the most sensitivity but the most digital noise as well. Shooting sports or faster moving stuff in low light usually requires a higher ISO setting. ISO settings also go by 1/3 rd stops on many cameras and is a function of the camera body itself. A typical ISO jump would be then to 200. The jump from lets one more stop (or twice) the amount of light in. 8

9 Balancing the exposure triangle will get you the proper exposure, than you can adjust the legs of the triangle to get the type of image you want. If you re shooting outside and sports your triangle might look like this. F 8 for the aperture 1/500 second for shutter speed 400 ISO setting 9

10 However if you re getting motion blur because the kids are moving too fast, this will give you the exact same exposure with only a decrease in the depth of field your focus is. I m changing the f-stop from f8 f5.6 which would be one stop lighter and the shutter speed from 1/500 to 1/1000 which would be one stop darker which would make the exposure pretty much the same. F 5.6 for the aperture 1/1000 second for shutter speed 400 ISO setting 10

11 Let s say that I needed that depth of field. I could go back to f8 for the depth of field, keep the shutter speed at 1/1000 of a second and move the ISO from 400 to 800 and still get the same exposure. F 8 for the aperture 1/1000 second for shutter speed 800 ISO setting 11

12 So all three of those exposures would give you roughly the same exposure. ETTL and how it works (basically) Newer Canon camera work in ETTL mode when used in one of the automatic settings. ETTL means evaluative through the lens metering. In a gross nutshell ETTL takes parts of the image that you re trying to shoot and weighs each part of the image for it s significance. ETTL then tries to balance that all out to what is considered an 18% gray. That s supposed to give you a perfect exposure. If the scene is pretty average with minimal bright and minimal dark spots it does fair. However, if you re shooting something that is extremely dark like a colored man or women in dark clothing and a black background it will try and turn that scene into 18% gray and overexpose it and the image will be gray. Same thing if you shoot a fair colored person in white clothes on a white background it will try to reduce all that white to an average 18% gray and underexpose it and the image will be gray. 12

13 To compensate for ETTL s short comings when shooting in one of the auto or programmed modes you might need to use the exposure compensation on your camera to adjust the exposure till you get it right. If you re shooting in manual mode and you re shooting a very dark scene or a very light scene, you most likely won t be able to use the little meter that s built in the camera as it won t give you a perfect exposure. You ll need to check out your histogram to get it right. A histogram can give you a good idea of where your exposure is at. 13

14 Histogram A histogram gives you a rough measurement of the amount of pixels (your cameras sensor has pixels in it that record the picture) in each brightness level of your picture. The higher the spike the greater number of pixels in that brightness level. Levels of brightness in a digital camera usually go from 0 (black) to 255 (pure white). 14

15 These are three different histograms for three different photos. The top one has a lot of information in the middle or just to the right of the middle in the brightness level and a bit of information at both ends. The middle histogram is probably overexposed and has almost no information at the dark side of the histogram and way too many at the bright end of it. There most likely is detail lost in the brighter parts of the image because of this. The bottom histogram is probably underexposed because there s too much information on the dark side of the photo and you re probably loosing some detail there. Histograms are useful to get proper exposures, however if you re trying to shoot something dark and keep it dark your histogram might be like the bottom one and that s how you want it. Histograms are useful in that you can tell if you re loosing detail if there are way too many pixels on one side of it. 15

16 16

17 When shooting sports it s usually wise to use the AI servo mode for focusing. AI servo will continually adjust your focus for objects that are getting closer or further away. Most people don t recommend using the AI focus mode for anything. When you re shooting still shots or portraits where you might want to focus on the main subject and then recompose the shot you want to use the one shot focus mode or when you move your subject off of the focus point your camera will most likely focus on the new spot and the subject will be out of focus. Shooting in RAW mode will give you more room to edit or adjust your image. Adobe Cs4 will not read your T3i camera raw files but Adobe Cs5 and up will. However you can get a free Adobe DNG convertor, convert the T3i s raw files to the DNG(digital negative) format and then use Cs4. I have Cs6 so I don t have to do this but I would rather shoot and convert to DNG to use Cs4 than shoot in JPEGs. When you shoot JPEG s your camera is throwing a lot of information away that you cannot recover. Opinions differ on this though but you would be amazed at what a difference it can make to even properly exposed images. 17

18 When editing photos in Cs4 or any editing program, try to use layers whenever possible. Sure it will make your image file size bigger but if you are working with layers and doing it properly you re not destroying your image and you can always go back to somewhere in your editing if you want or need to. 18