Everything You Need To Know When Buying An iPhone – Part 3
By You can find the part 1 here and part 2 here. Further in our incursions to buy an iPhone, we look at the screen. Unlock the phone and look if it has this problem (ie, “leakage” of light), which is a general problem on iPhones, especially on the white models. Put a white background with maximum brightness so you can see as clear as possible if it has this problem or not. Another general problem that appears to iPhones screen is the yellowish tinge. The picture below illustrates this point perfectly.
The next step is to check both cameras, this in case you buy an iPhone 4/4S/5. In the main camera, first test the autofocus function. Open the application and click on any part of the screen. After testing if it try and take some pictures, test the video too. Shoot a little, play with the video and see if it’s ok. Look carefully at the main camera, to be free of impurities, since the factory iPhone sealed in theory. Front camera (FaceTime) should not have any problems. Switch off the main camera and see if the secondary camera takes pictures.
Now get on the network (very important). If the iPhone you want to purchase is neverlocked, i.e. decoded from factory, borrow a microsim card from a friend which is from another network than yours. If the two received signals, it is neverlocked. If it’s coded, only works on a network or even none, it depends on the baseband.
Do not forget to give a reboot or hard reset the phone to see if it starts, whether if is having jailbreak or not. Another important aspect is to test if the iPhone is charging.
The international warranty of the iPhone is based only on its IMEI for a period of one year from the date of activation. You can check on the Apple website the IMEI or serial number if it is out of warranty or not. This is the site where you can check.
To see exactly what the differences between iPhone 4 and iPhone 4S, the picture below is very suggestive and should be able to distinguish between the two very simple models, without any further checks.
Buying An iPhone 
