How to add ImageJ to list of apps that can access system’s camera (or other USB wecams)

Sometimes, even when we have tried "everything," a reset of the device can help. This 'reboots' the phone.

Hard Reset

• Hold the Home and Power buttons down at the same time until the  logo comes back to the screen. (The screen will go black, keep holding.)

• Let go of BOTH buttons when the logo comes back. (Continuing to hold the Home button will put the iOS device into Recovery Mode)

Recovery mode will re-download iOS and then give you the option of restoring from a backup.

If this doesn't work, and a restoration of the phone from scratch in Recovery Mode (which is time consuming for the download) doesn't work, if you iPhone is under warranty see if Apple will be able to help. They want you to be happy and will do what they can within reason.

From a thread in Apple support community:

Soft = Turn phone off and on. Hard = BOTH buttons held until you see Apple Logo.

Difference: I like to explain by thinking of a laptop. On a laptop (or even computer) you can go to hibernation mode (memory gets written to disk then powers down). When turned on, it loads back up from memory and thus everything that was loaded in memory before shutting down is put back. That is what a Soft reset is (when you do the power button the slide to turn off). Thus if there was a memory leak for example in some program causing issues (sluggish, crashes, etc)....then if you just turn off and on, it will still have issues. A hard reset is like pulling the plug on your computer. It will have to turn back on and reboot fully. It does write an error log out to itself (nothing you see) but causes no harm to your phone or data. It is just a way to do a full clean boot up. This actually fixes about 90% of the issues most people have.

Has no effect on battery outside of the fact it has to boot up thus processor and read writes will be happening.

This question over on StackExchange lead me to Apple's iOS Developer Guide, specifically, the background section. This lists the activities apps are aloud to run in the background, camera access is not one of them.

This would also be a terrible condition for battery life and so it is not surprising that it is not supported. While this doesn't guarantee that none of your apps will try to get access to your camera, you can rest assured that if anyone tried to, Apple would reject their app.

This also ensures that there's no API for background camera access, so unless someone has figured out a way to hack non-jail broken iOS at a very low level, we can all sleep safely with only the watchful eye of Siri upon us.

Edit: Per my discussion with timothymh below, I realized you may have been asking about if you are still using a given app, having take a picture without switching to a new app. With more interesting reading from Apple's Developer Guide, we find that the camera API won't hand you data until a picture is taken or a video completed. Because the camera isn't handled by a daemon, it would be very difficult to be constantly polling the camera for input, even if you found a way past the API to get data without user input. Note I'm not an Apple developer, but from an official standpoint, it appears what you're talking about shouldn't be possible.