Exposure Explained. - Go Manual - Exposure, Shutter Speed & F Stop Explained. By Kelly Heck

Transcription

1 Exposure Explained - Go Manual - Exposure, Shutter Speed & F Stop Explained By Kelly Heck

2 Photography is a great joy of mine and I love to share it with anyone interested. If you have an SLR Camera, but just wish you could control it a little more, this is a must read! This article covers the elements that control our exposures: Shutter Speed, F Stop/Aperture, and ISO/ASA, among other photography details. Once you understand these elements, you are on the path to mastering MANUAL mode! I hope you learn a vast amount, find your inspiration, practice, and come back for more! Exposure Before we talk about shutter speed and f stop, we must know exactly what exposure is. Exposure is the amount of light needed to properly expose film. Following are three photos of the same scene; the first is overexposed, the second is perfectly exposed and the third is underexposed. See the difference? When we deal with exposure, 2 key components are at play: shutter speed and f stop, also known as aperture. Imagine that you are in control of a faucet that provides water to a bucket. In doing so, you are in control of two things: how fast the water comes out (f stop), and how long you allow the faucet to run (shutter speed). Your end goal is being able to fill the bucket just to the top without spilling over (overexposing). Think of f stop as being a measure of how fast the water comes out. Shutter speed is a measure of how long it takes before you have to shut the water off. For example, you can let the water come out very fast (a more open f stop) for a short period of time (a faster shutter speed), or you can let the water drip slowly (a more closed f stop) over a long period of time 2

3 (a slower shutter speed). There are several ways of doing this, but you must first understand how the variations will affect your end product. So how will these variations affect your photograph? Are you a traditional photographer? *NOTE THAT DIGITAL PHOTOGRAPHY EXPOSURE GOALS ARE OPPOSITE OF FILM In film you expose for your shadows and develop our film accordingly to bring detail back into the highlights and you can even burn in your exposure s highlights until they display on your sensitive paper in the darkroom. In digital photography, you have to expose for the highlights because if you overexposure your bright areas, there is no way to 'burn' in that data. Shutter Speed Shutter speed measures how long the actual shutter stays open to capture a photograph at the correct exposure. An example of how shutter speed may be displayed on a camera is as follows; 4 seconds, 2 seconds, 1 second, 1/2 second, 1/4 second, 1/8 second, 1/15 second, 1/30 second, 1/60 second, etc. Need a longer exposure? Keep winding your exposure back and you will hit your BULB setting. Changing your shutter speed to BULB will allow you to keep the shutter in your lens open as long as you hold the button down. Note that your shutter speed time doubles/halves in length by two times from one stop to the next. This is important to understand when you may want to change your correctly exposed settings for a different effect, capturing more or less motion. Shutter speed not only affects your light intake, but the motion you capture. A faster shutter speed (1/200th of a second) will stop motion blur for many subjects while a slower shutter speed (1/15th of a second) will capture blur. For slower shutter speeds, you may need a tripod to avoid capturing camera shake, which may result in an undesired blur. 3

4 In some instances, you may want to capture movement. A beautiful technique you should try is called panning. While using a slow shutter speed, follow a moving subject with your camera, trying to mimic its pace. In a successful panned photograph, the subject will be in focus and the surroundings will be blurred. F-Stop (also called Aperture ) In our example of the bucket of water, f stop is a measure of how fast the light comes in, but what exactly does f stop mean? F Stop is a combination of two factors, dividing each other to form a ratio: F-Stop = (diameter of the) aperture / focal length As most of you know, we're familiar with these common f stop values: On this scale, something like f/1 intakes twice the amount of light as f/1.4. These values seem very random don't they? Look closer. 4

5 Aperture is a physical measure of how "open" a lens is. We all know that a lens' opening can expand and contract, much like how our eyes adapt to brightness and darkness. When it's bright, our pupils contract to let in less light. When it is dark, our pupils expand to gather more light. Aperture is not just about light intake, however. Your f stop also affects your depth of field. In the diagram above, more open f stops will have a shallower focal depth while more closed f stops will have increase focal depth, sometimes resulting in infinite focus! For those of you that enjoy mathematics and such: A lens is circular. We can measure the amount of light entering through the lens by determining its area. We all remember that the area of a circle is Pi * r^2. To double the area, r simply needs to be multiplied by the square root of 2, because the square root of 2 squared is 2! That's exactly why the values above differ by that seemingly arbitrary number. ISO / ASA (also known as Film Speed ) There is a final factor that comes into play, your ISO or Film Speed. Embedded in film s emulsion are microscopic, radiation sensitive, silver halide crystals. When x rays, gamma rays or light rays strike the crystals or grains, the crystals reflect light and in turn, burn into the film an image that is revealed during development. There are different film grades you can select between based on your needs: 1. Lower film speeds (100, 200, etc.) have smaller halide crystals, slowly capturing finer grains of information lower film speeds are ideal because you will have finer grain in your image (in the digital age, this is called pixels ). 2. Higher film speeds (800, 1600, etc.) have larger halide crystals, quickly capturing larger areas of light, producing a grainy image. In the instances where you cannot use a longer shutter speed and a more open aperture, higher film speeds will allow you to capture more light. The downside is that your photo will likely be very grainy. 5

6 Pictured below is an example of the grains mentioned above. As you can see, the film far to the left would have smoother transitions from grain to grain or pixel to pixel than the HUGE pixel in the film to the far right. There are many different ISO options: 100 Speed 160 Speed 200 Speed 400 Speed 800 Speed 1600 Speed So there are two factors to selecting the right ISO film speed. How much light do you want to capture, and what photo quality do you need? Depth of Field Depth of Field is directly connected to focus, but not to be confused with it. DOF refers to the amount of an image which is in focus at any given time. To put it simply, a shallow depth of field means a small portion of a shot is in focus, whereas a deep depth of field means that a larger portion, sometimes all of the shot, is sharp and in focus. 6

7 When you use this technique on your subject, it is called selective focus, and draws the viewers attention to the subject. So how can we alter DOF in order to produce the desired effect? A camera lens contains what is called an aperture ring: an adjustable iris just like the ones found in the human eye, which controls the amount of light reaching the camera. So, a smaller or narrower aperture lets less light get to the camera than a larger or wider aperture, making the whole scene darker. This relates to DOF in the following way: the wider the aperture of a lens, the shallower the DOF. Again, conversely, the narrower the aperture of a lens, the deeper the DOF. Unfortunately, while professional cameras tend to have the technology to manually adjust the aperture of the lens (especially SLR cameras in which the lens is separate and interchangeable), consumer cameras generally lack this function, making it more difficult to control DOF. Another factor affecting DOF is the magnification of the subject in the frame, i.e. the size of the person or object you are photographing. It is for this reason that when the camera is left in the same position, a wide angle lens will produce much deeper DOF than a telephoto lens. On a camera with a built in zoom lens, simply remember that zooming in will reduce DOF, while zooming out will increase it. This also applies to simply moving the camera closer and further away (a much preferred option for filmmaking). Histogram Many photographers use their camera's digital light meter to render a good exposure. The light meter is reading a variety of points in your composition to render an average mid tone across the entire image (18% gray). This meter can really help you as a beginner using Manual or Semi Manual settings; however it is a rough guess for a good exposure, and is often not the best option. So how do we fine tune our exposures? This is an important time to begin understanding and viewing your camera's histogram! In statistics, a histogram is a graphical representation showing a visual impression of the distribution of data. In digital photography, that data is light. The light distribution in one 7

8 photograph is unique to any other photograph. To the left of the histogram are your pure blacks and to the right are your pure whites. All stages in between these two extremes represent the photographs' grayscale values. The histogram shows us how the tonal range of our image is currently being distributed between pure black and pure white. When I shoot and teach, I am using a digital SLR and am frequently asked why I look at the LCD after taking a shot. The answer is that I m barely even aware of the image on the LCD, it s the histogram that commands my attention. In fact, looking at your photo on the LCD screen to see if the image is exposed is an inaccurate depiction of correct exposure for two reasons. First, many LCD displays (even in cameras) are set to adjust their brightness to the environment; in bright rooms the LCD brightens so you can see better and in dark rooms it darkens. Second, because the human eye automatically makes corrections for a better, real life exposure... so your eye might think there are good highlights and shadows, but your brain may be fooling you. To really fine tune your exposure, always begin by reviewing your histogram. Your typical image would have an almost perfect distribution of tones, covering the dynamic range capabilities of your camera, from deep shadows on the left to bright highlights on the right with many values inbetween. Your histogram edges (aka, the deepest and brightest values in your photo) should just barely touch the edges of the graph. By doing this, you avoid loss of detail within your shadows and highlights. In other words, if you histogram goes over these edges, these areas or areas of lost data it's so dark or so bright, that the camera could not record what it saw in those areas and you will have areas that are actually flat white or flat black. In instances like these, you have exceeded your cameras' dynamic range capabilities. This is because if a subject is exposed too close to either extreme you will run into the limitations of the camera sensor s ability to record the image. Too close to 0 (absolute black) and there won t be an image at all, or it will be very dark and noisy, and too close to 255 (absolute white) and there will be nothing there except oversaturated pixels with no image information 8

9 Though you are taught to avoid poor exposure causing loss of data and to strive for a fair distribution of values throughout your image, don t hesitate to break the rules. Remember that the histogram is an electronic measure in some instances you will be taking a photograph that does not have deep shadows or bright highlights, or maybe it should have a lot of one or the other or both! You are the final judge... does it look right to you? The histogram will help you get to a good exposure, but you have the final say. You are the artist! GO MAKE ART! As an artist, I encourage you to go out and have fun! Photography is an experimental medium, and as you begin to take control of these settings and take advantage of the digital tools you have, you will find a balance for your shooting preferences. Shooting in Manual Mode opens whole new worlds for you to explore... now you can control the motion, the depth, the focus! Go create! 9