There are some major differences between the mobile camera on your iPhone and DSLRs. Many of these differences are related to the mobile phone’s objective of having the smallest, least intrusive, but also most powerful camera possible. This combination results in a number of trade offs that make certain types of photography less suited for an iPhone out of the box. There are lots of tricks that can be done using software to try and make up for these limitations which have varying degrees of success. In general, many photographers opt to eschew these built in tricks for pure sensor data that they can manipulate to their taste after the fact.
Aperture very simply means the hole that the light passes through to hit your camera’s sensor. DSLRs have a big advantage over iPhone cameras in that they can change the size of their aperture to control the amount of light hitting the sensor. Unfortunately on iPhone cameras, the aperture is fixed. This means that each lens has it’s own aperture, and there’s no way to change that value while using the same lens.
Aperture is often denoted as f-stop in photography, this refers to the ratio of the length of light hitting the lens (focal length) to the size of the aperture hole itself. Some apps, including Apple’s Camera app, do appear to have a setting to control the f-stop, however this is simply a software approximation of that change, not a physical change to how the camera works, as in a DSLR.
A shutter is kind of like a window shade that you can open and close really quickly. It covers the aperture until the shutter button is pressed, then opens to allow a certain amount of light into the sensor. The way iPhone cameras handle this is drastically different from a traditional DSLR. iPhones have what is called a “rolling shutter”. This means that instead of the light from your scene all being captured by the sensor at once, it’s captured in sequential strips. While this still happens very very quickly, it can lead to some interesting artifacts if there’s movement during your shot. For instance, the top half of a car being further to the left than its wheels as it moves across your scene from left to right.
Format choice was honestly worth it’s own article, but it’s also relevant to the choice between an iPhone and DSLR. You can find it here.
This is really more than one thing, but it’s an extremely important one. An iPhone fits in your pocket, and a DSLR… won’t. If you’re on the go and want to travel light, an iPhone’s compact size makes it extremely attractive. However that small package comes with not only the limitations listed above, but a few more relating to it’s physical form.
It’s impossible to ignore the world of interchangeable lenses for DSLRs. They allow you to have a multitude of effects and adapt to changing lighting situations. On the other hand, a bag full of lenses is big, bulky, and can get very expensive.