Still Photography Camera Guide
Digital Zoom vs Optical Zoom Lens
Some digital cameras don't have a zoom lens yet it is marketed as a camera with 3X digital zoom and sometimes just the misleading "3X Zoom". Digital zoom is not really zoom lens in the strictest definition of zoom lens - they are "faked" lens.
Digital Lens and Digital Zoom
Before we go further on the subject of digital zoom vs optical zoom, we need to point out the difference between a digital zoom and a digital lens. Try not to get confused between these two. A digital zoom is a faked "lens" as it does not exist in physical form. Whereas a digital lens on digital SLR camera is an optical lens optimized for digital capture or designed specially for digital imaging.
Differences between Film and Image Sensor
Light strikes on film surface at an oblique angle.
Light strikes on image sensor at an oblique angle.
There are distinct physical differences between film and image sensor. Film has an almost flat 2 demensional surface whereas an image sensor has a 3 demensional surface formed by millions of tiny photodiodes or pixels.
Each pixel sits inside a shallow well-shaped surface on the silicon substrate. On top of each pixel there is a tiny microlens for gathering of light. The pixels are separated from each other by a tiny spacing on the silicon substrate and circuit area. Between the camera lens and the image sensor there are Low-pass filters and Infra-Red filters. Some cameras also have a wave filter on top of all these filters for auto dust cleaning.
Traditional lenses were designed for film's 2 demensional surface. They were not designed for digital capture so they didn't work as well on image sensor's 3 demensional surface. The design of traditional film lenses did not take into consideration the glossy surface of the Low-Pass filter, Infra-Red filter and microlenses of the digital camera. These glossy components cause internal reflection in the camera and sometimes create additional flare and purple fringing on digital images.
Image sensors' performance is at its peak when light strikes on their photo-cells at a perpendicular angle to the sensor surface. Image sensor performs poorly when light strikes them at an angle. On the other hand, films with their almost flat surface are effective and perform equally well at whatever oblique angle the light hits them.
When an optical lens designed for film is used with a digital camera it tends to have insufficient brightness, low contrast, inaccurate colour reproduction and vignetting at the edge and corner of the images.
This calls for a new approach in lens design. Lenses with the new design dedicated for digital capture with image sensor are called digital lenses. They are actually optical lenses designed for optimum performance for digital cameras.
There is nothing "digital" about the digital lenses. Though digital lenses do have IC chips embedded inside them, it is for information exchange between the camera body and lens about focal length, aperture setting and focusing, etc.
In fact there is nothing new in having a cpu chip inside a digital lens as many 35mm SLR film cameras already have this kind of chip inside their optical lens since 1990s. Maybe digital lens should rightfully be labelled as optical lens designed for enhanced digital imaging.
We shall now return to our topic on digital zoom. To make a long story short this is how digital zoom works: If you have used any imaging or graphics software on a computer, you will notice how you can blow up or "zoom" in an image with the magnifier icon of that software. That is exactly what those digital zoom cameras do. You will also notice how an image degrades as you use the magnifier icon on the imaging software. The same thing happens when you use a digital zoom.
The digital zoom function crops out the outer part of the image on the image sensor according to the amount of digital zoom you apply. The image on the centre portion of the image sensor is then "enlarged". The result is digitally "zoomed" and displayed on the LCD view finder.
In actual fact the pixel on the centre portion of the image sensor is "duplicated" digitally. This process is called interpolation. All these "enlarging", "zooming" and cropping of the images are done digitally instead of actually zooming with an actual lens and hence called digital zoom.
|4 times digital zoom Vs 4 X optical zoom|
|left -- Original photo taken without zoom. The two below are comparison of digital vs optical zoom|
Pixelated poor image quality
With the same zoom ratio an image zoom with a digital zoom is lower in image quality and tends to be "pixelated" than one taken with the same zoom ratio optical zoom lens.
The digital zoom or "faked zoom" (if you prefer) practically crops and throws image information away when you use it. No image details are added to your enlarged image unlike the actual optical zoom.
Every digital camera now comes with this digital zoom function. It costs next to nothing for the manufacturer to add a digital zoom to a digital camera as compared to an optical zoom. In order to compare apple to apple only use the optical zoom for comparison when comparing lenses of two cameras in your shortlist.
Even without using the digital zoom in the camera you can reach the same result once you load the picture on your graphic editing software in your computer. You will have better control doing it on your graphic software than using the digital zoom function of you camera.
Digital zoom is only practical if your digital camera has a resolution of 5 megapixels or higher. A digital zoom is fun to play with but not recommended if you are serious about image quality. In our opinion digital zoom is more of a marketing gimmick. If you are concerned with image quality turn off the digital zoom function from your digital camera's menu system.
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