The Basics of Camera Exposure Controls
by Peter Zack
The word Photography is based on two ancient Greek words: Photo, for “light,” and Graph, for “writing” or “drawing.” So essentially the word means “Drawing with light”. Often we will hear people refer to Photography as ‘painting with light’. So to do this you need to understand the controls and how each will effect the final resulting image.
This is an easy subject that can be incredibly technical and complex. When we are out shooting, we really don’t care about ANSI Exposure Standards in the laboratory or how a particular camera maker, determines the exact metering levels they calibrate their meters to. You will often read statements like: Canon’s render the image too bright or Pentax is underexposed looking. The reality is partly that the metering systems are set up differently. We all discuss gray cards as 18% gray but exposure meters actually vary from that ‘standard’. Nikon’s meter is set at 12.5% K and Pentax at 14% K. Not 18% gray.
Hand held spot meters also have slight differences as well. I’ll stop there as this will get very technical at this point but just understand that each system will have variances in exposure metering so perfectly direct comparisons between brands is not completely possible. There will be slight variations on how each camera sees a scene’s lighting. You need to understand the basics of this to get the most out of your particular camera.
First is the aperture scale in 1 stop per step:
This is the shutter speed scale in full stops (in seconds):
1, 1/2, 1/4, 1/8, 1/15/ 1/30, 1/60, 1/125,
1/250, 1/500, 1/1000, 1/2000, 1/4000
Quite simply the lens has a series of aperture blades inside that could be anywhere from 4 to more than 20 in some older models. There are a select few that use 2 sets of blades like the Sony (Minolta) 135 f/2.8 f/4.5. This lens was built with Bokeh (background blur) as the primary feature in mind. A feature considered de rigueur by many lens buyers.
For the purposes of exposure settings, aperture controls the amount of light that will pass through the lens to the film or sensor. Stopping down a lens refers to closing the aperture and the numbers don’t often seem intuitive. A larger number is a smaller aperture. The reason is, it is actually a ratio. 0 (theoretical) being no aperture or wide open and 1:8 being a middle aperture with 1:22 a small aperture. It is a critical control that aids the final image in a number of important reasons. Apertures are displayed as f/16 or f/1.4 etc.
Some lenses allow partial or half stops between each standard setting and allow finer control. These are usually 1/2 stop adjustments and electronically controlled lenses can have 1/3 stop adjustments. Aperture controls are directly related to shutter speeds and if camera meter recommends an exposure of 1/125th at f/8 on any lens, changing the aperture to f/5.6 (1 stop) would allow 1/250th to achieve the same exposure. The lens type or length does not factor into the equation and the same ratios will render the same exposure no matter the focal length.
Aperture stops are exponential. f/2 to f/2.8 is one stop and requires twice as much light. f/4 then requires 4 times as much light and f/5.6 needs 8 times as much light compared to f/2 to achieve the same exposure ratio. So if you were to set the camera at ISO 100, the lens at F4 and the shutter speed of 1/1000th. Then you decide you want a smaller aperture to achieve a greater Depth of Field (DOF). You stop the lens down to f5.6. If the shutter speed is not adjusted, the image will be underexposed. So to allow enough light to expose the sensor or film the same as the first settings, you would need to leave the shutter open longer by one stop (1/500th). You could also increase the ISO to 200 (1 stop) if you needed to keep the same high shutter speed.
These considerations are important in shooting wildlife or sports. Increasing the DOF could be important and high shutter speeds will freeze fast movement. So Aperture controls the amount of light coming in. Shutter speed controls how long that light is exposed to the sensor or film and ISO controls how sensitive the sensor or film is to that incoming light.
I will delve further into aperture controls and uses in a future article. Aperture controls the collimation of light, depth of field, relative sharpness and Bokeh to name a few aspects of this important feature of a lens.
Film (ASA) or sensor speed number that actually refers to the sensitivity to light. They can be expressed in 1/3, 1/2 and full stops. The numbers also relate to ‘stops’ and is also exponential. ISO 200 is 1 stop more sensitive than ISO 100 and ISO 800 is 4 times more light sensitive than ISO 100.
If a given scene showed a meter reading of ISO 100, f/8 and 1/250, you change the ISO to 200 and now the settings could be changed to f/11, 1/250 or f/8, 1/500. The exposure will remain the same. In the first adjustment, we have compensated for the 1 stop increase in sensitivity by closing the lens down 1 stop. In the second adjustment, we have adjusted for the higher light sensitivity by a one stop shutter speed adjustment. In both cases this keeps the exposure ratio between aperture, ISO and shutter speed the same to avoid over exposure.
The speed at which the camera fires the shutter. The smaller the fraction, the less light allowed to expose the film or sensor. The metering ratio is directly connected to the aperture setting on the lens. Also raising the ISO will allow faster shutter speeds. Like the other 2 controls, the ratio is exponential. 1/60 allows double the light of 1/125 and 1/250 is 4 times less light. 2 primary factors in using a higher shutter speed are freezing fast action and using a shutter speed that equals the lens’s focal length. Action shots may require a speed of 1/2000 to stop the action where a landscape shot might be fine at 1/30.
Second, before the anti shake systems in most cameras, the rule was “shutter speed matches lens focal length”. So if you had a 100mm lens, your minimum shooting speed would be at least 1/100 to ensure a sharp image. It’s still a valid rule. I have taken sharp images at 1/60 on a 400mm lens using the SR of my camera but if I can take the same shot at 1/500th, the results will still be better. If using a DSLR with an APS-C sensor, then the crop factor needs to be taken into account for the rule of thumb, which then becomes “shutter speed matches lens focal length times the crop factor”. So for the same 100mm lens your minimum shooting speed would be at least 1/150.
A simple practice of taking 2 or more shots of the same scene at different settings. That does not mean shooting the scene at f/8, 1/250 and the bracket shot at f/11, 1/125. This is the same exposure result. There are generally 2 methods of bracketing. Either the camera has the feature and when activated, it will adjust the camera by the user selected exposure increases or decreases in exposure. The other method is understanding the ratios previously discussed and shooting in manual. Thus adjusting the shutter speed up or down by the required Ev change desired. You would not adjust the aperture to control this normally, because that will often change the DoF and how the shot looks.
The idea is you might take 5 different exposures of a scene at -1 Ev, – 1/2 Ev, 0, +1/2 Ev, +1 Ev. This will give you a large latitude of exposures to assist in getting the scene correctly captured.
|0 EV||+2 EV||-2 EV|
Above you can see the same scene shot at different EV settings.
Click on the thumbnails for a larger view.
If your camera has this feature, use it. This is probably one of the most important aids that a modern digital camera has to help get a good exposure. I read all sorts of wild explanations on the perfect exposure and perfect histogram. There is no such thing. We are trying to get the best possible result out of a given scene but the camera sensor does not have enough dynamic range (ratio between dark and light areas) to expose a scene perfectly. There will always be compromises. The sky might be too bright against a subject in the foreground for example. What you are hoping to do is avoid clipping or peaks on the left or right of the histogram.
Too much light or exposure on lighter areas of a scene will ‘blow out’ that area and a white area of a shot has no data. There is nothing there to recover. Similarly with black areas, too little light renders these areas as black with little or no data.
If you look at some histograms and also your editing software, the histogram goes from 0 to 256. 0 being black and 256 being white. We want the histogram to be peaking right in the center if possible which also corresponds to the reading off an 18% gray card. But certain scenes just have too much dynamic range for this type of histogram. You eyes and brain combined have a dynamic range of around 20 stops (a very hotly debated number, which is often quoted higher). The average camera now has a dynamic range of 5-7 stops. So you will have scenes that just can’t have that center ‘hump’ in the reading.
You can bracket the shot as described above and if you used a tripod with cable release, the frames can be combined in software like HDR or Photoshop to get the most out of each region of the frame. You could use a graduated neutral density filter to balance out the exposure levels but that can be difficult if the scene does not have a relatively straight line, defining light from dark areas. Additional light from a flash or other source could fill the dark areas, allowing a more balanced light level to the scene for exposure. Also some methods of shadow or highlight recovery in software can give you a 1 or 2 stops of adjustment.
So try to get a balanced histogram, without clipped peaks at either end, that would tell you the whites or blacks are over or under exposed.
Cheers and good shooting –Peter Zack