Optical zoom is used while taking a picture using a camera to get a close shot of the subject without moving physically closer. Digital zoom is a part of digital cameras, and camcorders, which helps to crop the entire image, and then digitally enlarge the size of the viewfinder of the portion that is needed to zoom in on

Comparison chart

Digital Zoom versus Optical Zoom comparison chart
Edit this comparison chartDigital ZoomOptical Zoom
About Digital zoom is a part of digital cameras, and camcorders, which helps to crop the entire image, and then digitally enlarge the size of the viewfinder of the portion that is needed to zoom in on While taking a picture using a Camera to want to get a close shot of the subject without moving physically closer, photographers use the optical zoom.
Function Digital zoom crops the image down to a centred area with the same ratio as the original, and also interpolating the result back up to the pixel dimensions of the original. this method involves cropping, hence the resolution and quality is reduced The optical zoom ratio of a digital camera measures how much the lens can actually zoom in to make subjects appear closer. Optical zoom, enlarges a picture while keeping the resolution and sharpness of the picture high.
Resolution and image quality Digital zoom, crops a portion of the image and then enlarges it back to size. And due to this, image quality is reduced in comparison with the original one. There is no relation between optical zoom and the resolution of the photo, as optical zoom only enlarges the whole image or the subject to a certain range. So image quality only depends upon the mega pixel (MP) of the camera.
Use Using the digital zoom allows the user to get closer to the subject when the photographer wants to be discreet about taking pictures, like taking a picture of a person in a graduation ceremony. Optical zoom is very useful, while taking a picture of a landscape, or to get a closer view of a subject, without reducing the quality of the entire image, like taking a picture of a Rainbow in the sky.

Function

While taking a picture, the photographer may need to focus on one area in the picture frame. For example, when the photographer takes a portrait photo he/she may want to make sure the object’s face fills the photo frame; and when he/she takes a group photo he/she may want to make sure everybody is in the photo frame. In this kind of situation, the photographer can either physically move closer to the objects or use the camera’s zoom feature.

When using the zoom feature the camera enlarges that area to fit the full picture frame. There are two types of zoom – optical and digital
Optical zoom works by physically moving the camera’s lenses and changing the focal length. Optical zoom lenses physically extend to magnify the subject. A motor controls the lens movement according to the user’s command. When the user rotates a special button or presses the switch to the subject is either magnified or reduced in size.

The user can take a photo of the subject from a greater distance and still get a clear, close-up image in a higher rating optical zoom camera. The concept of optical zoom is the same as the zoom on a non-digital camera. Optical zooms have adjustable focal length and every optical zoom lens has a range. By changing the focal length the photographer can make objects appear bigger and fit in the full photo frame. When satisfied with the zoom position he can shoot the photo by simply applying the shutter button.

Through digital zoom technique one can decrease or narrow down the apparent angle of view of an image. It is accomplished by cropping the image down to a centered area with the same aspect ratio as the original, and enlarging the result back up to the pixel dimensions of the original. This is done electronically, without any adjustment of the camera's optics. As enlarging disturbs the original pixel layout of the image, which is captured by the camera's image sensor, it reduces the image quality. With digital zoom, photographers use built-in software in the camera to define a portion of the photo.

After selecting the portion, the software crops the rest of the photo and enlarges the selected area to fit the complete photo frame. The software calculates new values for the pixels that were cropped in order to result in a full frame photo. The main disadvantage of this digital process is that the enlarged photo quality is lower than the original photo taken.

For example, lets us assume that a photographer has a 2MP (2 megapixels) camera, and he wants to zoom in 2X. He applies the digital zoom. To accomplish this zoom the camera crops half of the photo and enlarges the other half to create a 2X zoom effect. In the process a 1MP area is discarded. The other 1MP area is enlarged in a process that copies every pixel once to generate a 2MP photo. Although the new photo seems to include 2MP it really includes only 1MP of information. This results in a photo with a quality equivalent to a 1MP photo.

Advantage

Using optical zoom is preferred by professional photographers. The reasons are that

Optical zoom capabilities can make a big the difference in the final photo. The higher the optical zoom, the photographer can take the photo of the subject from a greater distance and still can get a clear quality shot. In digital zoom, enlarging the "zoomed" area reduces the image resolution and the image quality. So using digital zoom is not a good idea. The same kind of enlarging, cropping can be done later, using photo editing softwares, like Adobe Photoshop. In fact using PC software is always the preferred method to built-in digital zoom since it allows the photographer to try different zoom sizes, different zoom areas and different zoom algorithms while not losing the original photo.

Zoom factor

While cameras may have a digital zoom of over 700x, cameras are generally restricted to a zoom of 25x. In terms of simple zooming capability, it may mean that digital zooms bring a picture closer or zoom closer but since they lose picture quality so they do not capture the picture well. So a 25x optical zoom may turn out better results than a 700x digital zoom also. When cameras are advertised, they have much much higher digital zoom than optical zoom.

Video explaining the differences

This is a good video that explains what digital and optical zoom are and the differences between them:

References

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