Emulsion film, as a medium for capturing photographic images, has been around in one form or another for more than 150 years. But due to the rapidly growing popularity of digital cameras, newcomers to photography are sometimes surprised to discover that film is still being manufactured and sold for all-purpose photography. Once their first digital camera is in hand, and they have mastered the art of recording images electronically, and storing them as files, the idea of using film seems a strange concept indeed.
So why is film still being used by some photographers?
To understand the answer to this, we will need to take a look at the differences between film stock and the modern image sensors that make digital cameras possible.
Let’s start with the newer technology: the light sensitive electronic surface that sits at the center of a digital camera, and which is known as an image sensor. Basically these sensors are a specialized form of microchip. But they do not carry out any computations. Instead, all they do is measure the amount of light that falls on different parts of the sensor surface in a given time window. If you were to zoom in on one of these thumbnail-sized chips, blowing it up to the size of a football stadium, you would see that the surface of the sensor looked a lot like your tiled kitchen floor.
Each one of those square tiles represents a separate photodiode, capable of measuring just the amount of light that falls on it. In fact, sitting just above each photodiode is a colored filter. The filter is there to block all but the wavelength of light that it sends on to the photodiode. These filters come in red, green, and blue, so that any given photodiode will be measuring only the intensity of light for the particular color of the filter sitting above it.
Imagine for the moment that every tile on that kitchen floor is green. Now take one row of tiles and color every second tile blue. For the next row of tiles, color every second tile red, but shift the red tiles across by one space so that the red tiles appear beneath the green tiles of the previous row. Then repeat the process with blue tiles on the next row, and so on, until the entire floor is covered with these alternate rows of green/blue and green/red tiles. This is just what the surface of the image sensor looks like when seen close up.
In point of fact, this particular arrangement of colored filters, with twice as many green pixels as either red or blue, is known as a Bayer array. It is the most commonly used color filtering method employed in digital cameras, but it is by no means the only one used. Because the human eye is more sensitive to green light than red or blue, the green-favoring of the Bayer system actually works to its advantage when the colors are finally combined to produce printable images.
Every time a digital camera is used to record an image, this mosaic of colored tiles captures the various intensities of red, green, and blue light. So every photodiode, or “electronic pixel”, has associated with it a color and a number. If a perfectly green lime was being photographed, the red and blue pixels would have the number zero associated with them, while the green pixels around them would carry non-zero numbers. In reality, small contributions of red and blue light would be mixed with much stronger green contributions to produce the hues of green that color the skin of the lime.
All of this information is whisked away from the image sensor and stored in a temporary image buffer, later to be transferred to the memory card used by digital cameras to store images as files. Software is then used to manipulate the images (or not) before they are finally sent to a photo printer.
The resolution of the images is determined by the number of photodiodes (pixels) that can be crammed onto the surface of an image sensor. These days it is not uncommon to find about 4000 pixels along one edge of the sensor, and 2500 along the other. The product of these numbers is 10 million, or 10 megapixels in the digital camera parlance.
So, how does this compare with film resolution?
Well, these days the brand of film stock recognized as offering the finest resolution is Fuji’s Velvia 50 RVP. While it is not possible to ascribe a grain size to the emulsion that is used in slide film (there are a number of different sized grains in the dyes that make up the emulsion) an effective number of pixels per inch can be assigned to film stock based on how many closely-spaced “thin lines” can be resolved in the laboratory. The effective PPI, or pixels per inch, for Velvia is about 4000. This translates, assuming a 35mm frame size, to about 22 megapixels of image resolution. For better known film brands, like Kodachrome, the number is closer to 9 megapixels.
Clearly, with current top-end digital SLR models offering resolutions that surpass 20 megapixels, the numerical advantage that film once offered in terms of image resolution has all but vanished. Digital cameras now match the image resolution of even the very best film stocks, and will surpass them as image sensor technology continues to improve.
So now we are back to our still-answered question: why is film stock still produced when digital imagery is on a par with it, at least in terms of resolution?
The answer is that resolution is by no means the full story when it comes to creating a great image. As mentioned, emulsions contain a range of grain sizes and dye components that work together to produce effects that cannot easily be reproduced with three sets of red, green, and blue numbers. Velvia, for instance, adds more warmth to the actual recorded colors, and produces landscape images that look better than the real thing.
Fortunately for us, the makers of film stock, and the makers of digital cameras, are not, by and large, the same people. Otherwise film stock might disappear a lot more quickly from the world of photography. That probably will not happen for a while though. Not as long as film stock can produce images that surpass the quality of digital methods. Until that is no longer the case, photographic film will have a place in the world of photography.
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