Understanding f-stopsf

Transcription

1 Understanding f-stopsf When you buy a new DSLR there is a mass of technical information for you to get your head around. Here I will explain what you need to know in order to get an understanding of f-stops. f It takes time and practice to understand which f-stop to use for the type of image you want to create.

2 Each f-stop f will be represented by a number, depending on your lens. The smaller the number the larger the aperture, or opening. Each f-stop f increases or decreases the amount of light entering the lens by a magnitude of two. These f numbers increase by the following increments: f1.0, f1.4, f2.0, f2.8, f4.0, f5.6, f8, f11, f16, f22, f32, f45, f64, f90 Beginning generally at f5.6 and going as far as f22. Remember that as the number increases the smaller the aperture re in the diaphragm, and therefore, the less light is being let into the camera. c So, f5.6 allows x2 as much light to enter the lens as f8, this means m that f8 allows x2 as much light in as f11, and so on. Each f-stop f can be divided into 1/3 or 1/2 stop increments, which are as a follows: (1/3 stops): f5.6, f6.3, f7.1, f8, f9, f10, f11, f13, f14, f16, f f18, f19, f22 (1/2 stops) f5.6, f6.7, f8.0, f9.5, f11, f13, f16, f19, f22

3 If the largest f-stop f on your lens is f2.8, it will be allowing the most light into the camera as the opening will be large, and you will also have the shallowest depth of field. This means that when you are focused on the main subject you want to photograph, the objects in front or behind it will be out of focus depending on how far they are from the main subject. As the f-stops f decrease, the f numbers get bigger but the aperture size decreases, the surrounding objects become sharper.

4 Take a practical example to get a better understanding of this. Take two similar objects place them on a table, one being about 30cm in front of the other, making sure that they are both visible in your viewfinder. Mount your camera on a tripod to keep the camera still and in the same position throughout. Once you are set up, focus on the front subject for each shot. Set the camera to f5.6 and do a light reading to determine the shutter speed, camera metering will suffice. If there is not enough light or too much light at this point, you can increase or decrease the shutter speed time. To demonstrate the effect of f-stops f affecting the depth of field (DOF) you will need to take approx 6 photographs of the subjects. In the following examples I took one set of images at f2.8 then f4.0 then f5.6 The second set at f11 then f16 then f22.

5 f2.8 f4 f5.6

6 f11 f16 f22

7 You will notice that as you decrease the f-stops f the shutter speed will double, for instance if you photographed the first image at 1/500 sec. at f5.6, for your second image at f8.0 the shutter speed will have doubled to 1/250 sec. this is because the size of the aperture has decreased by half, f11 would end up 1/125 sec., f8 1/60 sec.. A hand held camera at 1/60 second, unless you have a steady hand, will be shaky so use a tripod where possible. For each of the photographs note the f-stop f used and the shutter speed. Once you have photographed the subjects at the 5 or 6 different exposures, load the images up onto your PC. Once you view the images and identify which image goes with which exposure, you will see a slight difference between the first image at f5.6 and the second image at f8 having the background subject sharper than that in the first image. Then compare image 1 to image 6 at f22 where both the items are sharp.

8 To get the full benefit of f-stops f and DOF (Depth of Field) you will need to understand the basics of the built in light meter in your camera: In most digital cameras the light meter is represented as the following on both the LCD screen and when you look into the viewfinder:

9 The represents the current light reading. If it is positioned under the centre [0] it means that the camera is set to the correct exposure. Should the be positioned under the -2, -1, +1, +2 this means that the camera is set one or two f-stops f under or over, for the ideal exposure. So when you are out photographing, for instance if you have your camera set to f11 and the light meter marker is at -1, change the f-stop f to f8 to open up the aperture, likewise if the marker is at +1, change the f-stop to f16. Additionally, it is also good practice to take images one stop s under, one stop over, and one at the correct exposure. You can view the results and see which image has the best exposure on screen or when printed, this is known as bracketing. Bracketing gives you some safety when lighting conditions are difficult or you are using flash. Some camera models have an auto-bracketing mode, which automatically takes 3 or 5 images at various f-stop f intervals, so that you can later choose the photo with the most pleasing exposure. It should also be noted that the built in light meter in your SLR is not accurate 100% of the time, as it generally just takes an average light reading. Hand held light meters are much more accurate as you can n do readings on various areas on your subject. The two small dots between each number ( [0] ) represent 1/3 stops. If there is only one dot between each number, this would mean that they are 1/2 stops increments.

10 This should have shed some light on the subject of f-stops f and exposures. It takes time and practice to fully come to grips with lighting, shutter speeds, depth of field etc... The more you get accustomed to using the manual settings on your camera, the more you will enjoy your photography and the more you will be impressed with the results.