Exposure: Imprinting light onto a light sensitive material
What is an exposure? In the early days of film an exposure was literally the process of exposing film to light creating a photograph. This can be thought of as imprinting the viewed light onto a material. Today we are still creating exposures but are doing them more and more in the digital realm. Instead of light being exposed onto film we are exposing light to a light sensitive sensor that interprets the light into a digital form.
At a high level we can then think of an exposure as the act of taking reflected light off of objects and recording their intensity and color onto some device that can interprets that light. Using this model we can also say that our own eyes and brain are constantly being exposed. Your eyes work to take in and focus light while our brains interpret and store the images into our memory. Cameras work similar to how our own visual system works
How images are created on film cameras: Basic
On a film camera an image is captured by taking in light and having that light focused onto a a piece of plastic that is coated with a substance ( tiny grains are contained inside this substance ) that undergoes a chemical reaction when exposed to light. This chemical reaction records the light that it was shown and will continue to undergo a chemical reaction as long as it is exposed to light. This is why you should never let your film be exposed to any light before it has been developed and developing film is done in dark rooms. If your camera should have a leak and light enters into where the film sits it will be exposed to light and record that onto the image.
Different types of film have different types of reactions to light. Film that is very sensitive have larger grains and are considered fast film. Films that are less sensitive are called slow film and have very fine grains. The film speed is written in ISO numbers generally ranging from 100 – 1600 and variations in-between. An ISO speed of 400 is twice as sensitive as an ISO speed of 200. When a very sensitive film is used in low light, you will notice that the picture isn’t very smooth and looks grainy. This is because of the larger grain size in the film. To attain very smooth photos you would want to use a film with smaller grain size but this now means your film isn’t as sensitive to the same amount of light and you must compensate in some way.
A very simple camera that can be made is called the pinhole camera. It is a light proof box that has a tiny pinhole to let light enter that can be closed or opened as needed. On the back of the box opposite this pinhole would be your film to record the image. This is all it takes to record an image using film.
How images are created on digital cameras: Basic
A digital camera creates images in a similar way as film except that the material to record the image is no longer based entirely on a chemical reaction but on a light sensitive sensor. This sensor interprets the light into digital form of 1’s and 0’s and the small computer chip inside takes this information and begins to process it before storing the final image on a memory card. The types or processing a camera may do might be some sharpening of the image, noise cleanup, color adjustment, and so on.
The key here is that there are multiple steps before a final image is stored. Two digital cameras with the same sensor can record two very different images based on how the camera decided to process the image or any user adjusted settings on the camera. Also note that unlike film cameras where you received a standard negative of the film, in a digital camera you must deal with the file format of the image. A file format is just a standardized way to store some information. For images we might have JPEG, GIF, TIFF, BMP, or other file formats. They can contain relatively the same image but based on how they store it the quality of the image can be very different. Memory and storage space are cheap and it is always recommended to store your images in the highest quality file format your camera will allow you to do. You can always reduce or compress the image into a smaller size or lower quality if you need to but you cannot go back the other way.
Another way that professionals save their digital images onto a camera’s memory cards are with what are called RAW files. Notice I did not say that this was a file format, just that they are called RAW files. The reason they are called RAW files is that this is the raw information from the cameras sensor before it has been processed by the camera’s internal computer. This is as close to a films negative that you can get out of a digital camera. This allows a professional to take this RAW file and “re-shoot” or re-take an image with different settings or adjustments.This is useful if a photographer happens to have a setting set incorrectly, misjudged a parameter, or wants to create several images in different styles from the same core image. Every camera manufacture has their own file format for RAW images and they usually include software to interpret the proprietary RAW files. There is however one RAW file format standard called DNG (Digital Negative) that will allow these files to be opened by any software that understands this standard. Creating your RAW files in a standard RAW file format helps preserve the files for archival purposes and backwards compatibility in the future. However, your camera’s proprietary RAW file format may provide more functionality and capabilities then the standard DNG RAW file format.
We talked about how using a faster film speed on a film camera created noise in the final image. In digital cameras do we still have noise as we are no longer using “grains” or film? The answer is yes we still do but this type of noise is not from the grain size and a chemical reaction but from signal noise of the sensor. Lets pretend I’m the sound guy at a local town hall and you’re giving a big important speech today. If I were to give you a microphone and have you speak into it softly you would probably think that the sound coming out of the speakers should be louder. I could either have you raise the volume of your voice, which would increase the sound volume at the input of the microphone, or I could turn up the sensitivity of the microphone ( called increasing the gain ). Lets pretend you lost your voice recently and can only speak at a low soft volume. I’m now stuck with a fixed amount of sound coming from your voice at a certain level. Well now my only choice is to increase the microphones sensitivity to make it pick up as much sound as possible from the microphone. But guess what? By making the microphone more sensitive I’m now picking up too many other unwanted sounds! I can hear your lips close, the person sitting in front chewing gum, the hum of the air conditioner, every small bit of breath you take as you speak and a tiny fly buzzing a few feet away! What can I do to reduce the amount of noise I’m now getting by making the microphone, which is interpreting sound, too sensitive? I would have to start adding filters that would try and cut out or smooth any odd sounds coming in. No matter how hard I try, it will never be perfect but I could get very close depending on how bad the noise levels are. However, there are real life limitations and you would most likely have to live with some noise in your final output when increasing the sensitivity of your input.
In digital camera terms, this means that even though we are no longer using film that has grain noise in lower light, we still have digital noise due to our sensor’s sensitivity to light being changed. In digital cameras we change our sensor’s light sensitivity by changing its ISO setting. The film ISO speeds have carried over to the digital world and work in a similar way. In film you had to change your film to change the ISO speed. In a digital camera you just have to hit a button and can now switch on the fly from exposure to exposure. All digital sensors have a native ISO speed or sensitivity. This is the ISO rating that your sensor produces the clearest highest quality photos. Think of this as using a film with the same ISO speed on a traditional camera. If my camera’s ISO is set to 200 but my camera’s native ISO is 100, that means I’ve now made my sensor be twice as sensitive as it normally is. This also means twice the amount of noise could be generated in the image. As we keep increasing the ISO we keep increasing the amount of noise as we start straying farther and farther from where the camera’s sensor produces its best images. Luckily our camera’s digital brains apply noise filters and do other tricks to reduce the amount of noise from high ISO speeds. But at some point there will be no avoiding noise when using higher ISO settings. To produce the cleanest images, always try and use the lowest or native ISO of your camera’s sensor.
For example, I have a digital SLR camera that has a native ISO of 200. This means that my sensor is as sensitive to light as a using a film speed of ISO 200. That means that I should try to shoot photos as if I was using a traditional camera with this type of film to get the highest quality images. In most digital cameras, the native ISO is 100. This means that they need a lot of light to produce a clean image as this film speed is considered slow. Slower film speeds require a longer amount of time (for the same amount of light hitting the film) to create the image versus a faster film speed.