The imaging surface
The imaging surface in a camera, of any kind, is the surface that ultimately collects the light and defines the image. This could be film or a digital sensor, and can be a variety of sizes.
There are a few details about this object that we, as photographers, need to fully understand.
Size
Yes, size does matter. The size of the sensor, all else being equal, defines how a given lens will affect the final image. Generally speaking, the bigger the surface the wider the viewing angle the lens will produce. The standardized size of the surface is known as 35mm, which is the diagonal measurment of the surface. Larger size surfaces are in cameras known as medium format or large format cameras. Long ago, all the way through to today, photographers have used these large format cameras for all kinds of photography. Common sizes for these are 4″x5″ and 8″x10″. There are many advantages to these cameras, the most significant being the incredibly clarity of the image no matter how large you print it up.
But, I digress, for the moment, the camera in your hands is likely to have a 35mm or smaller sensor. These smaller sensors are known as crop sensors, and the smaller that sensor the less wide the lens will be when compared to the same lens on a 35mm sensor camera. One important detail here, not all lenses are built for full frame cameras. This is because of the popularity of the less expensive smaller sensor cameras.
The important facts here are:
1) Standardized sensor size in an SLR/DSLR is 35mm.
2) Smaller sensors reduce the wide field of view possible from any given lens.
3) Not all lenses are compatible with full frame sensors due to the size of the projected area from the back of the lens to the sensor not fully covering that sensor. Conversely, all full frame lenses will work, typically, with a crop sensor camera.
Sensitivity
This is not about emotions, this is about how sensitive the imaging surface is to light. There is a rating known as ISO or ASA which is the sensitivity rating. The lower the number the less sensitive the surface is to light. A typical starting number is 100 ISO, and modern cameras range up to 12,800, and even higher. Generally speaking you want to use the lowest possible ISO in a given situation. Mainly this is because color takes time to absorb whether you are talking about digital or film. It requires more photons to define a color than to simply illuminate. Photons actually have no color, its complicated, but it takes more than one to perceive a color. Anyway, moving on.
When we worked with film the ISO was set according to the roll of film you loaded into your camera. Need multiple ISO settings to shoot an event, well, you will be carrying multiple cameras or get very very good at rolling the film in and out and keeping track… yeah, multiple cameras. Now with digital we can simply twist a little dial and pick our ISO. I tend to pick my ISO based on the light where Im shooting. If it is outside and sunny, with slow moving targets, I will use 100. If the action is very fast, or it is darker, 400.
The easiest way to visualize the effect of cranking the ISO up, for me, is to imagine ISO as volume on a radio. If there is static (poor lighting), turning up the volume just creates louder static. This is essentially what happens as you crank up the ISO. Instead, tune the situation using other options, better lighting, wider aperature (smaller value) and slower shutter speeds. If none of those are an option then sure, move the ISO up, but its the last resort. This was true of film as well. The film, to capture more light and thus a higher ISO rating, used larger ‘grains’ of material. Without very meticulous processing this grain could really degrade the image. Even with perfect processing it would still never produce an image with all the potential detail as the tighter grained low ISO film.
Doubling the ISO value doubles the sensitivity. Its that simple. Double the sensitivity means 1 stop more. A stop is a unit of exposure. Each stop is a 100% change in light level. Reducing by 1 stop or increasing the stop, can be accomplished in many ways. Where ISO is concerned 1 stop = double or half the current value. If you are at 400 ISo, and it is too dark, you can double the brightness level by increasing it by a stop, or going to 800 ISO. This ‘stop’ concept will be covered in detail in a later article.
So, the key facts here are:
1) Use the lowest ISO your camera permits
2) In any situation think first, low iso or high iso, with high being 4x that of the low value.
3) Avoid very high ISO’s no matter how awesome your camera is. Very high ISO’s are a sales brochure trick.
