Digital Photography 101 - Part 4

Photography 101 – Part IV and final

The Digital Image

The Digital Image – after capturing the image using the camera's body, the digital sensor and the lens system, now we have a Digital Image. Let's see what that means in this final episode of photography 101.

a. Pixel – stands for "Picture Element" and that's the most basic unit of an image. Each photoelectric cell on the sensor creates one Pixel in the image. A Pixel has only one color and no details, it's a simple square.

b. Resolution – an indication to the amount of detail in the image. The more pixels you have in an image the sharper it will appear and we can make larger prints without losing quality. The resolution is calculated by multiplying the amount of horizontal pixels with the vertical pixels and since the results are in millions we use the word "Mega" which stands for million (in Greek), and that’s where the term "Mega Pixels" got coined. An image that is 3,000 pixels wide and 2,000 pixel high is actually a 2,000x3,000 = 6 Mega Pixel image. A 2 megapixel camera can print a satisfying 10x15 cm image at 300dpi, for a 20x30 300dpi image we will need an 8 Megapixel camera.

c. Interpolation – some cameras, usually the cheap ones, use interpolation to extend the pixels count. For example a 12 megapixel interpolated camera with 4 million real pixels is a lie, since the camera has only 4 million pixels on the sensor, which are taken and then multiplied in order to produce the said 12 megapixels image. There are several methods for interpolation and here are the most common three.

d. Digital Zoom – based on the same principal of interpolation as mentioned. It essentially involves cropping and interpolation back to the original resolution. You should avoid the digital zoom and apply crop and interpolation (if needed) with a good image editing software.

e. Calculating Image Size – Let's see what's the real size of a digital image: If I photograph an image that is 3008x2000 pixels as in the Nikon D70, I produce an image of 6,016,000 pixels (we talked about it before), if I shoot at 24 bit per pixel (we also mentioned that) then the final file would contain 144,384,000 bits. In order to change it to Bytes we need to divide by 8 (8 bits=1 Byte) and so 18,048,000 Bytes, or in short 18.048 Megabyte.

f. Jpeg – Since it's not reasonable to save files in the size we calculated just now, the JPEG algorithm was developed. JPEG stands for "Joint Picture Expert Group" and here’s their homepage. This algorithm manages to decrease the file size substantially by erasing unnecessary information from the image, usually small hue changes that the human eye doesn't even notice. This method decreases the file in a 1:6, 1:10 or any other ratio, so you get a 3 Megabyte file instead of the original 18 MB file. This method is irreversible and the information erased is gone forever. JPEG is now the main format in digital cameras.

g. RAW – This means the image’s raw data, as retrieved from the sensor, is saved in its original high quality and with no compression. The information contains greater dynamic range, 12-bit per channel (instead of 8), and before any type of manipulation. All the definitions of white balance, sharpness, brightness, saturation etc. are done on the computer. Raw files take much more room than JPEG and need more work on the computer.

h. White Balance – every light source has different color temperature, which is measured in Kelvin degrees. Low Kelvin (1,500-3,000) indicates red light source (Candle, Tungsten); medium Kelvin is for sun light (around 5,500) and high Kelvin is for shades and cloudy weather (8,000K). if you look at the flames in your home stove you can see that in low temperature the flame is red and in higher temperatures it is blue. The human eye adapts very well to different light sources, but the camera lacks this amazing ability and needs various calculations to achieve the correct white balance in every frame. In auto white balance mode, the camera searches for something white in the frame and can use that as reference for all the other colors. But when the image doesn't have anything white, or it has several light sources, it can mess up, and we can help by adjusting the White Ballance (also written WB) manually or by selecting from a variety of presets.

i. Sharpening – every image goes through some kind of sharpening. That's done by increasing the contrast in edges of forms. In photoshop you can use the Unsharp-Mask filter, which is a tool that lets you apply contrast in a very efficient way. It's important not to over-do the sharpening, because that could create visual artifacts you’d want to avoid.

j. Dynamic Range – that's the amount of details we will see in the highlights as well as shadows in the same image. The human eye has an incredible dynamic range, you can sit in a room and look outside a window and see the interior and exterior properly. The camera is much more limited, and we will either see the interior as silhouettes or the exterior washed out with no details. You can increase your camera's dynamic range by using RAW or by taking several images differently exposed and combining them into an HDR (Hyper Dynamic Range).

Summary – after concluding the four parts of photography 101 we know that the camera and the digital systems are very complicated. Here I described the main basic terms, Knowing and mastering the theory can help you gain more from your camera and achieve better results. There is always more to learn and I hope you've made the first step successfully

I Hope you enjoyed part IV of Photography 101 and that you gained new information from the series.

Previous Parts – The Camera, The Digital System, The Lens.

Yours truly,

Roie Galitz

For more photography articles – composition, Depth of field, Filters, Flash Photography, infrared photography, Sunset photography, Blue Channel, Panning