I was out with a group of people on a photo walk the other day and one of those present – relatively new to digital photography – asked me what exposure settings to use for the location where we were standing. I gave a too short and relatively useless reply, so I thought to follow it up with an e-mail. When the e-mail reached 2000 words I thought maybe I should post it here instead:
“When we were out on the photo walk on Saturday morning you asked me what would be the best settings for making a sharp image. I gave you a short and wholly inadequate answer and thought to follow it up today.
So, while you asked what seemed to be a simple question, there are actually quite a few factors involved. I thought I’d give you a quick overview, but if you’re interested you can get MUCH deeper into this stuff. You can break it down basically into three parts – camera attributes, exposure choices and camera movement.
There are a few factors here. We’ll start by saying that there are two variables before we talk about anything else – what is ‘best’ and what is ‘visible’ or what is ‘adequate’. In mathematical terms people can get hung up on stuff that the human eye is incapable of seeing, and even within what we can discern there are variables. So…
We’ll start with the camera sensor. Every sensor has a certain physical size, and the sensor is then subdivided into sensels. The number of sensels on a given sensor determines the resolution of the sensor – if your sensor has 3000 by 5000 sensels for example, then you have 15 million sensels, or in common parlance, a 15MP camera. In theory one could have an infinite number of sensels on a sensor smaller than your fingernail but there’s a tradeoff between the number of sensels (more is better to a point) and the size of the sensels because at some point the smaller size means that the sensels aren’t capable of capturing enough light to render any information of value. Number of sensels (pixels) and quality of sensels (pixels) go hand in hand. This of course is determined by the camera you buy.
Next is the lens. Lenses are also physical things, and as such they’re bound by physics. The quality of the glass used in the lens elements, the arrangement and number of lens elements inside the lens barrel, the design and quality of construction are all factors. Again, this is determined by the lens when you buy it, and MTF graphs show the quality of a tested lens. Also fitting into this (as well as exposure) is the f/stop or diaphragm opening you choose as most lenses are sharper in the mid-range (f/8 to f/11) than they are wide open. If you stop down completely (say f/16 or f/22), then you can run into diffraction issues.
The third attribute, because most cameras today use auto-focus is the accuracy of the focus point. Cameras use one of two methods to focus, and this is also determined by the camera you buy. However, because focus is achieved by determining contrast of the scene based on several factors and then using a motor to turn the lens barrel to achieve point of focus, many cameras/ lenses have front focus or back focus issues. What does that mean? Basically it means that the lens is actually focusing on a point in front of or behind where it thinks it’s focusing.
Some cameras have built-in ‘micro adjust’ that allow the user to fine tune the focus point for each lens.
Exposure is a whole topic unto itself, but basically there are three aspects to exposure – a trifecta if you will – and they each have strings attached. For any given scene there is a certain amount of light reflected from the subject, and that’s how we see. That much is obvious. A camera’s light meter reads the amount of light reaching it and provides a readout based on that amount of light. The human eye responds the same way, by opening and closing the iris in the eye, except the human eye is far more advanced than any camera. So…
The first part of this trifecta is ISO (International Standards Association). This goes back to the film days, where each film had a certain sensitivity to light based on the number, the size and the arrangement of the silver halide crystals on the film. The sensitivity label on the film provided a way of calibrating the light meter to the film. There were actually two standards – ASA (American Standards Association) and a German standard called DIN. They used two different scales, similar to the differences between Fahrenheit and Celsius scales. At some point in the last century they decided to combine the two systems together so that a film box would read ISO 100/21o for example.
Now, digital sensors also have a certain sensitivity to light based on the design of the sensor and this is called the ‘base ISO’ of the sensor. Since the camera is taking those light readings, converting them to electrical signals and then to binary numbers, math can be used to change those numbers and this is basically what happens when you change the ISO on the camera. Film has grain – which is basically an attribute of the size/shape/distribution of the individual silver halide crystals themselves, but digital sensors have electrical noise – both luminance and chroma noise, but we’re not going to get into that. Noise is like the static when the radio isn’t quite on the right station. If the signal from the radio tower is too far away, it can be so weak it can’t be heard over the noise. You get the idea.
The second aspect of the exposure triangle is the size of the lens diaphragm opening, or the f/stop. F/stops are calculated based on the relationship between the physical size of the opening and the distance from the lens elements (the focal length of the lens) to the film or sensor. That’s all we’ll say about that for now. Obviously a bigger lens opening lets in more light in a given amount of time and a smaller lens opening lets in less light in a given amount of time. The trade-offs are several. As mentioned, lenses used wide open tend to be softer than lenses that are stopped down midway, and all lenses are sharper in the middle than they are near the edges. Since lenses are round and film/sensors are rectangles, you’re literally trying to put a square (rectangular) peg into a round hole. If the diameter of the lens is big enough and the size of the film/sensor is small enough you end up only using the center part of the lens and you get the best part of the lens to work with. If the sensor is bigger and the lens diameter is smaller, you get the opposite effect, which can include light falloff (vignetting) and softness around the corners of the image frame. If the glass elements within the lens barrel are not perfectly aligned you can end up with softness on one side of the image frame or the other – take the lens back and test a different one.
The other aspect of f/stops comes with depth of field (DOF). Now, technically speaking if you focus a lens at a specific distance, only objects at exactly that distance are in perfect focus. You can see this by shining sunlight through a magnifying glass onto an object. If you line it up perfectly then the light rays become so focused they’ll burn whatever they’re focused on. A few mm higher or lower and it won’t. However, there will always be a range of distances in front of and behind the focus point that will appear to be in focus. This depends on the size of the image and the viewing distance as well as the viewer, but it’s known as the ‘circle of confusion’.
So, if we have a specific focus distance, there will be an area in front of and behind that point that will appear to be in focus.
The ability to discern focus will always be less at a distance, so the DOF will always seem to be greater behind the focus point than in front, and DOF will always be greater for distant subjects than close ones. For extreme macro work, DOF can often be measured in mm or microns. One answer to increasing DOF in macro work is focus stacking, but we won’t get into that here.
DOF is also a major factor in determining the ‘look’ of an image and so f/stop must be considered when shooting to determine what aspects of each image should be/should not be in focus. One of the challenges with really small sensors like those found on cell phones is that they have a huge DOF. Conversely, one of the things appreciated by those who shoot video with full-frame DSLRs is the ability to have really shallow DOF.
The third aspect of exposure is shutter speed. To overstate the obvious, the longer the shutter is open and exposing the film/sensor to light, the more light will reach it. Shutter speed selection depends a lot on the subject and the intention of the photographer when making an image. Do you want stopped action, a moment frozen in time, or do you want a sense of movement? There are no wrong answers to any of these choices, but the three options of ISO/f/stop/shutter speed must be used in concert for any given lighting situation.
The other thing to keep in mind is that light meters, by default, see everything as being a medium gray. Modern light meters can take samples from different points within the frame and average them out to give you an exposure setting, but a light meter can’t tell if you’re shooting coal or snow. This is where exposure compensation comes into play – increasing exposure to bring out shadow detail, or decreasing exposure to tone down highlights. Other factors such as split neutral density filters to even out a scene’s lighting or shooting multiple exposures to extend the dynamic range (the basis of HDR) can also be used.
As mentioned, shutter speeds can be used to accentuate or limit the perception of subject movement in an image, but in addition to subject movement there’s also camera movement. This can be intentional or unintentional. I did a blog post a while back on the ‘minimum handheld shutter speed’ that can be used with a given lens, and the best answer to that question is: it depends. Some cameras and/or lenses have built in compensation for movement, some don’t. I tend to have a ‘fine motor tremor’, so my ability to hand hold a camera steadily will be different than yours. The ability to actually see the effects of movement will depend on the size of the image, the quality of the image, the viewing distance, etc. Proper posture, bracing and technique can be important, but nothing is as steady as a good sturdy tripod. That again is a whole topic unto itself; aluminum tripods tend to be cheaper and heavier, but carbon fibre tripods tend to be not only lighter but offer better vibration dampening. Each tripod/head combination has a maximum weight limit it can comfortably accept, so choose a tripod based on the weight of your camera and the heaviest lens you expect to own. Remember also that while many/most tripods come with a center column, as soon as you raise the center column you no longer have a tripod, you have a monopod.
There are two other factors to consider regarding camera movement. The first is the movement induced when you click the shutter (squeeze, don’t jab). The other, with DSLRs anyway, is mirror slap. An SLR camera (film or digital) has the light entering through the lens, bouncing off a mirror and going up into the pentaprism at the top, then out the viewfinder. When you click the shutter that mirror has to swing up and out of the way, the shutter opens and you expose the film or sensor, then the shutter closes and the mirror drops back down. All of that creates movement. almost all DSLRs have a ‘mirror lockup’ function, often integrated with Live View, that allow you to move the mirror up and out of the way before you click the shutter. Some camera manufacturers like to think that actually asking the camera to lock up the mirror should be equivalent to a hunt for buried treasure but that’s a separate issue. When using a tripod you can circumvent the movement from clicking the shutter by using a remote release and/or by setting a timer to click the shutter after a few seconds.
So… how do you take a sharp image with your camera? Consider the tradeoffs of the camera and lens you’re using, the exposure choices you need to make for this image, and given those choices, whether or not a tripod/ remote release are possible or necessary.
Oh… one more thing to consider. If you’re out on the beach making pictures at sunset using a slow shutter speed, you have the camera on a sturdy tripod, you’ve locked up the mirror and you’re using a remote release to minimize camera movement, ask yourself if the wind is bouncing your camera strap around, and how much that’s going to affect your image. Should you remove it? No rights or wrongs, only choices.”
Now go out and make some pictures!