Exposure is something beginners frequently struggle with. This invariably comes about as a result of the camera's meter being used incorrectly under certain lighting conditions. A lot of newcomers don't shoot outside of automatic mode. Some may venture as far as aperture priority but few ever opt for manual mode, which offers the photographer the greatest control. "Tried it once. Photos all came out wrong. Never again!"
The reason for this is that exposure is a mystery to many people and they are seemingly unaware that there are countless combinations of shutter speed, aperture and ISO that result in the same exposure and yet the images can look completely different. This combination of the three variables is often referred to as the "Exposure Triangle".
THE BEGINNER'S GUIDE TO EXPOSURE
The Exposure Triangle consists of the aforementioned 3 variables. Aperture, Shutter Speed and ISO. All are independently adjustable and yet all interact with one another to produce correct exposure. If any are too high or too low, then you'll end up with under or overexposure in your images. In the above diagram (which took me almost 2 hours to do from scratch in Photoshop), perfect exposure is represented by the white triangle. Anything inside of it is over exposure. Anything outside under exposed.
First we'll look at the aperture, also known as F-Stops. Below we can see a representation of a Nikkor 50mm f/1.4G lens. In the left hand image, no blades are visible as the diaphragm is at its maximum of f/1.4. The lens is regarded as being "wide open". In the middle, the lens has been "stopped down" to f/2 and lastly to f/2.8. The more the lens is stopped down, the less light is able to pass through. The results of stopping the lens down are increased depth of field (more things are in focus) and sharpening of the image. However, sharpness peaks around about 2 stops below the maximum aperture. Beyond this sharpness is actually reduced due to light diffraction.
As can be seen from the first diagram, f/1.2 is usually the fastest lens available for most cameras, and it's not uncommon to find lenses stopping to f/22. Some don't go this far, others go further. As you look at the numbers you will probably notice a pattern emerging. Every two stops you close the aperture down go on the lens, the number doubles. Do your best to memorize these numbers as it comes in very handy for flash photography, especially with manual strobes!
In between the whole stops we have half stops and third stops. You'll notice these on you cameras display or in the EXIF data of your photographs. For instance you might see f/6.3 or f/7.1. These are 1/3 stops between f/5.6 and f/8.
The lower the number on the lens, the wider the maximum aperture. The wider the aperture, the more light the lens lets in and the shallower the depth of field. They are also vastly more expensive. For example the Nikkor f/1.8G retails at around £155. The 1.4G is almost double that at £290. Nikon also a make a 50mm f/1.2 Ai-s lens, which is manual focus and costs more than the auto focus f/1.4!
If you own a kit lens with your camera, then your maximum aperture will most likely be f/3.5 or f/4. You'll also see numbers like f/3.5-5.6. If your lens is something like an 18-105mm zoom, then at the widest angle the maximum aperture is f/3.5 and f/5.6 at full zoom. This is known as a variable aperture.
This is pretty straightforward and doesn't require too much explanation. Most DSLR cameras go from 30 secs to 1/4000 sec. Generally higher specification and higher priced camera bodies will go up as high as 1/8000. All the cameras have a bulb feature which will keep the shutter open as long as your finger is on the button. The camera can often switch between moving in whole stops of light, e.g. 1/100 sec to 1/200 sec, or in half and third stops. Its a lot easier to work out the math with shutter speeds as you simply double or half the number to move a whole stop of light. A lot less confusing than the F-Stops in which a doubled number equates to two stops of light, not one. For example f/4 to f/8 is two stops. f/4 to f/5.6 is one stop. Makes sure you remember that one!
ISO like shutter speed is relatively simple to grasp. For best results, use the lowest ISO you can. The camera has the greatest dynamic range and image quality at its native ISO. For the Nikon D90 ISO 200 is the native. The D800 ISO 100. By upping the ISO you essentially make the sensor more sensitive to light and so it doesn't require as much of it to get the same exposure. The down side to this is that image quality falls and digital noise is created, which is most noticeable in the darker areas of a photo. We up ISO to enable us to use a faster shutter speed and thereby avoid unwanted motion blur from ruining our pictures.
Take a few moments and read the 4 settings examples below.
ISO 100, F/22, 2 SEC
ISO 100, F/4, 1/15 SEC
ISO 200, F/4, 1/30 SEC
ISO 800, F/2, 1/500 SEC
The first and the last of the 4 settings are wildly different but both share one common thing - EXPOSURE! Yes, all 4 examples, each varying from the last will expose your image exactly the same. What will differ is the depth of field and motion (if there is any in the frame).
Lets say for we were photographing a fountain or waterfall. We placed the camera on a tripod and were in aperture priority mode on the camera. We select f/22 on the aperture and ISO 100 and take a picture. The resulting shutter speed was 2 seconds. Looking at the image on the LCD screen we see the fountain with the water looking all soft and milky. Its been blurred due to the long shutter speed. Next we open the aperture up to f/2, ISO 800 and take a picture. The camera automatically chose 1/500 sec as the correct exposure and indeed everything in the second picture is a bright as the first. However, the background is nicely out of focus now and the falling drops of water are frozen mid air. Two identical exposures but two very different looking pictures.
To simplify what was done, we went from f/22 to f/2 which is 7 stops of light brighter! We also went from ISO 100 to 800, which is another 3 stops of light brighter. However, we shortened the shutter speed thereby decreasing the amount of light hitting the sensor by the same 10 stops of light from 2 seconds to 1/500th sec. This is the essence of the Exposure Triangle. Once we know one set of values that give us correct exposure on our subject matter, we can then manipulate them to create very different looks, whilst still being the same luminosity.
The two images above had the same exposure values but resulted in two very different looking images. The one of the left was shot at f/2.8 and the one on the right shot at f/16. The most observant of you may notice the right hand image being very slightly darker. While the exposure values of the camera were absolutely correct, the variation between the two images was as a result of the inconsistency of the output from the strobe I was using. Had this of been shot with constant light, the exposures of the bust would be identical. Technically you could argue that the camera settings aren't correct, due to the fact that only the aperture was changed and not the ISO or shutter speed, as I was using flash and shooting in manual mode. The right hand image should be 5 stops of light darker than the left but, this difference was countered by a 5 stop increase in output from the flash.
Manual mode is mainly used by professionals or enthusiasts who know what they're doing. It affords the photographer the greatest creative control over the look and exposure of their images. It's also the easiest way to screw up a photograph if you're oblivious to the exposure triangle and how to meter correctly. So when is best to use it?
Well once you know your exposure you can dial those settings into the camera. Provided the lighting doesn't change (Sun going down), your model doesn't move closer to your light source or vice versus, then your exposures will remain constant. So how do you find out your exposure? For the most accurate reading you can use an external light meter, like a Sekonic L-758DR. This is incredibly precise and accurate down to 1/10th of a stop. If you don't have a meter, put your camera into Aperture Priority (A on Nikon or Av on Canon) and switch your metering mode to spot. Then zoom in on your subjects face and take a picture. This should give you correct exposure. Preview the image on the LCD screen of the camera and make a note of the EXIF data. Once you the information, dial the same settings into the camera in Manual mode (M). You can then use the exposure triangle to create different looks with shallower or deeper depths of field, freeze their action or blur it to create a sense of motion.
It is important to remember that accurate exposure on your subject is dependent on you metering the scene correctly. Be sure to read the guide on metering with your camera, to ensure you get the best results.