Silver B&W Camera has manual focus option you can use when autofocus does not work the way you like. Touch button M and then select Focus. If your device has more lenses than the single wide angle lens you first need to select a supported lens. iOS devices allow manual focus when wide angle lens or tele lens is in use.
You may let the autofocus system first focus to subject you want and then touch the Auto switch do disable autofocus and freeze the focus distance. You can then frame the photo the way you like.
If the target is close by but so small that autofocus does not detect it you can use something bigger to help focus. Even your own hand if you manage to place it on the right distance.
And of course you can use the focus slider and use the camera preview to focus your subject.
Note that you can move the manual settings window by touching its top frame and moving your finger.
There is one important thing to remember about manual focus. You may want to focus to infinity when shooting distant objects or scenery. Be aware that the rightmost position of the focus slider may not be the infinity. Typically lens focus system extends beyond infinity. This guarantees that lens can focus to infinity but does not define physical position for the infinity. So when manually moving the focus slider to focus to infinity carefully check the preview window and don’t simply move the slider to the right end.