Optical And Digital Zoom

More often or not, we'll see one of the main features of compact camera is the ZOOM range. Some compact cameras offer a simple 4x zoom, and some up to 12x zoom or more. Some even go to the extend of having 36x digital zoom. Without going to the showroom or a retail store to check out these cameras, how would you know the performance of them?

There are two types of zoom, Optical, and Digital zoom.

Optical Zoom

Let us focus our attention to Optical zoom. Optical zoom refers to the use of optics (lens) to zoom. The action is much like using a telescope, and there is little effect on the quality of the image when used.

In this example, we will consider this CASIO camera with 12.5x Optical zoom. You can calculate the zoom by looking at some of the text on the lens. Specifically, it states "f =  4.24 - 53mm". 4.24mm is the widest focal length (the wide angle), and 53mm is the farthest zoom (telephoto). Taking the longest reach divided by the shortest reach will yield you the zoom.

So, 53mm divided by 4.24mm, and you get exactly 12.5.

On the lens it also says 24mm WIDE OPTICAL. So what does it mean? It tells you the equivalent focal length when converted into full frame. Because different camera have a different sensor size, the lens will also have to be made differently to fit the camera. The only way to compare different cameras accurately is to convert it to a standard, the full frame (also known as 35mm equivalent).

Here's a pro tip: One piece of information that manufacturers will rarely tell you is the sensor size. This piece of information is one of the most useful in the selection of cameras because sensors is the heart of the camera, and bigger sensor usually means a better image. Fortunately, you can calculate the sensor size from just 2 piece of known information: The focal length of the lens, and the full frame equivalent.

Using the above camera again:

Focal length of lens at shortest: 4.24mm

Focal length of lens at shortest in full frame equivalent: 24mm

Focal length of lens at shortest in full frame equivalent / Focal length of lens at shortest = Sensor size (crop factor)

24mm / 4.24mm = 5.66x

This tells you the sensor of this camera is a crop factor 5.66x compared to a full frame. (The crop factor of a full frame camera will be 1.)

So the lower the crop factor number, the bigger the sensor.

Do DSLRs use the same system as well? Yes! The reason why DSLRs don't include the times of zoom in the spec sheet is because the lens can be changed and the zoom will also be changed.

Let's look at this example of a 18-55mm lens.

The zoom based on the above formula, will be 55mm divided 18mm, equals to approximately 3.06x zoom. (The zoom may pale in comparison to a compact camera, but in return you get excellent image quality!)

Since this lens is used on a 1.6x crop factor camera, the full frame equivalent of the zoom range will be 28.8mm to 88mm.

But you can't relay on the times of zoom to determine how far you can see into the horizon. If you want to see which camera has a better zoom, you must convert the lens' longest reach to full frame equivalent before comparing.

Below is an example to illustrate my point.

This lens is a 55-250mm lens. Based on the calculation above, the zoom will be approximately 4.55x.
(Doesn't seem like much, still lesser than the compact camera?)

The full frame equivalent of the zoom range will be 88mm to 400mm.

In contrast, the compact camera, has a full frame equivalent zoom range of 24mm to 300mm.

Even though the compact camera has a higher zoom times, the DSLR lens still can reach further at 400mm.

Digital Zoom

Digital zoom, on the other hand, is much less complicated compared to optical zoom. Digital zoom merely enlarges the image while maintaining the same aspect ratio, so you're getting a cropped photo enlarged onto
the normal pixel dimension of your camera.

It can be done manually by taking the photo using the maximum optical zoom, and then using photo editing software to edit, but if you're lazy to do it in post, then it might be wise to use the digital zoom function on the camera. Some say doing the manual method will not yield better results using the digital zoom function on the camera, but I don't see much difference in them.

Conclusion

When buying compact cameras, always look out for the optical zoom, not the total zoom, and if it doesn't come with digital zoom, you know it doesn't hurt much =)