The best camera is the one you have, according to the old adage. Now, smartphone makers are not content with giving you one rear camera in your gadget, but two.

When Apple launched its new iPhone 7 Plus last month, one of the much touted features was its rear dual-camera system.

The addition of another camera module opens up more possibilities for manufacturers to improve on the image quality, like having different focal lengths or adding a monochrome sensor, according to Miss Karissa Chua, consumer electronics analyst at research firm Euromonitor International.

The iPhone 7 Plus has a 28mm camera to shoot wide-angle photos and a 56mm camera for more close-up photos, with each having its own 12-megapixel image sensor.

But Apple is not the first to release a smartphone using a dual- camera set-up. LG and HTC were there first - in 2011. But the LG Optimus 3D and HTC Evo 3D used dual cameras to take 3-D still and moving images.

It was not until 2014, when the HTC One M8 was launched, that a true dual-camera system for still photography was realised. It uses an additional image sensor to capture depth information.

This allows users to re-adjust the focus point in post-processing to create a shallow depth-of-field or bokeh effect - whereby the main subject is sharp with other parts of the image blurred.

But the trend has gained momentum this year. In February, the LG G5 was launched with a dual-camera set-up that works almost like the iPhone 7 Plus. But the G5 uses a 10mm and a 24mm camera.

In April, Huawei launched the P9 - its first smartphone after announcing its partnership with German camera maker Leica.

Instead of harnessing two focal lengths, like the G5 and iPhone 7 Plus, the P9 uses one camera to capture colour information and the other for monochrome details.

Last week, Chinese smartphone maker Xiaomi announced its new Mi 5s Plus smartphone. It comes with a dual-camera system that works just like the Huawei P9.

Growth of the smartphone market is projected to slow to a compound annual rate of 5 per cent from this year to 2021, according to Euromonitor International. However, consulting firm Deloitte Global estimated that out of the 2.5 trillion photos to be shared online worldwide this year, 90 per cent of them will be taken using a smartphone.

Thus, smartphone makers are pulling out all the stops to get consumers to upgrade their devices ahead of their replacement cycle. And the camera becomes an important factor.

The use of a dual-camera system will increase the perceived value of smartphones. As such, said Miss Chua, expect more dual-camera designs - also referred to as dual-lens - to be incorporated into the higher-end smartphone models.

Mr David Yoh, mobile communications business director of LG Electronics Singapore, said: "The success of the dual-lens camera (in LG G5) has led to other models adapting and implementing this feature, setting a trend of more smartphones with the dual-lens feature to provide consumers with more photography options."

"It has been a success for P9 with its world's first Leica dual-camera," said Miss Ada Xu, global public relations director at Huawei.

She said global shipments of Huawei P9 had surpassed six million units as of end-August this year.

The technology in dual-camera designs is still quite new. So "there is still room for improvement in both the hardware and software to fully optimise the potential of such technology", Miss Chua said.

But consumers are already seeing the benefits. "I noticed better-quality images in low-light conditions and a wider dynamic range," said PR consultant Chong Jan Jee, 23, who uses an LG G5. She also felt her smartphone camera's auto-focusing was faster and more accurate.

Mr Nelson Tan, 40, a professional photographer and instructor, felt that photography enthusiasts will not rely on camera phones for their planned shoots, but having a smartphone with a good dual-camera system will not hurt. "The improving technology of smartphone cameras means that we can take even better pictures any time."