Firstly, let us take a look at the different between auto focus and fixed focus. Usually, a well focused image would be the sharpest in details. To be precise, the light comes in from a focused point makes the image directly on the sensor, whereas that which comes from an unfocused point creates the image either at the front or behind the sensor. This is the importance of focus while capturing an image.
Auto Focus
In the past, users of DSLR cameras came with manual focus technology that requires them to focus the image manually by rotating the focusing loop on the lens tube. However, the digital cameras with auto focus changed this necessity of adjusting the focus manually. In auto focus, the lenses get adjusted automatically to render a sharp output. Simply, the technology would focus the lens till the sharpest image is projected on the film. Auto focus increases the depth of field that will make the image clearer and crispier. Consequently, it is a vital aspect of modern DSLRs, point-and-shoot cameras and smartphones.
While there are two types of auto focus, active and passive, the high end smartphones and those premium SLRs arrive with passive auto focus only. On the other hand, the point-and-shoot cameras make use of active auto focus mechanism.
Fixed Focus
In fixed focus the distance between the lenses are maintained the same that is the lenses are fixed and cannot be adjusted. To be specific, the depth of field is kept minimal, blurring the front and rear of the point of focus. It is also impossible to focus the foreground and background simultaneously in fixed focus devices. If the focused point is far, the depth of field will be high and for this reason, the fixed focus cameras are made to focus to infinity ensuring that zoom settings and aperture are small.
Optical Zoom
Optical zoom is used by photographers while taking snaps if they want a close shot of the subject without moving closer to it physically. The optical zoom ratio of a digital camera calculates the zoom capacity of the lens in order to make the subjects appear closer. It enlarges an image by maintaining its sharpness and resolution high. Moreover, optical zoom has nothing to do with the resolution of the image as it only enlarges the image either completely or a part of it. On the whole, optical zoom is pretty useful when taking a landscape snap or a closer shot of the subject without minimizing the quality of the image.
Usually, optical zoom is preferred by the photographers as it allows them to capture a subject from a large distance and still click a clear shot. However, in digital zoom, enlarging the zoomed region will reduce the image quality and resolution.
Which is Better?
Auto focus and optical image stabilization are the terminologies that we find in most of the high end smartphones. Primarily, auto focus lets users to tweak the lenses to focus an object, which is not possible with fixed focus. Owning to this, the fixed focus cameras will be rendering crisp images only when they are focused properly, whereas auto focus units render crisp and focused images at almost all striations.
Furthermore, optical zoom makes a big difference in the final output. High optical zoom levels will let the photographers to click a subject from a far distance despite degrading the quality of the shot.
Conclusion
For smartphones, auto focus camera units with optical image stabilization are the best ones as such units can render focused results without requiring the users to adjust the focus manually or to move closer to the subject. Of course, there is an issue with the passive auto focus that is used in the smartphones when it comes to low light performance, but this is eradicated with the presence of LED flash as it will light up the subject and the environment.
