Why the iPhone/Android Camera Focal Length Matters

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When it comes to smartphone cameras, the majority usually use them like point and shoot devices. Nothing wrong with that, but the phones themselves they are becoming ever more sophisticated, so it doesn’t harm to expand your knowledge a little, especially when it comes to knowing the camera’s focal length.

All the principles below can apply to any smart phone made in the last year or two. These latest crop of smart phone cameras feature two forward facing lens, one wide-angle and one telephoto. With more options available, it pays to have that bit more camera knowledge to get the best from your images.

What’s a Focal Length?

No matter if your camera lens is in a smartphone or a full-frame DSLR, the basic principles are the same. The focal is usually measured in millimeters (mm) with one of the standard lengths being 50mm. The reason for this is that 50mm is more or less the same field of view as we see with our eyes. Well, that’s the way it works on old film cameras or full frame DSLRs. On crop sensor cameras devices with smaller sensors, that same focal length is bigger. So, A 50mm focal length on a crop sensor can end up looking more like 80mm (more telephoto.)

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Wide-angle lenses on a full frame camera body are usually 24mm and less. On a crop sensor, it can be 18mm. Telephoto lenses on a full frame camera body are 70mm and upwards. That’s about 45mm on a crop sensor. Once you get used to the millimetres thing on a lens, you’ll immediately have an idea of how wide angle or telephoto your image will be.

So, what’s the focal length on an iPhone X? The wide-angle lens on the iPhone X has a focal length of 4.25mm, which is a 26mm equivalent. The telephoto lens is 6mm, equivalent to a 52mm full frame camera. The rear-facing camera on the iPhone, now called the TrueDepth camera, has a 2.87mm focal length. Earlier iPhone models had the equivalent of 28mm and 33mm. These are all quite wide angle lengths, which is understandable for a smart phone as it’s easier to fit more in the frame and get more depth of field (less chance to mess up the image.)

If you spot a DSLR lens from now on, you now know what all those ‘mm’ numbers mean…

Why We Need to Know These Things

Wide-angle and telephoto focal lengths all have their own characteristics, not just different numbers. Wide-angle focal lengths are great for fitting a lot into the frame, such as for landscapes. But this also means close-up objects become very pronounced, looking almost bigger than the actually are. The very extreme of this is a fisheye lens.

The telephoto lens on an iPhone will magnify things, but not by a great amount. Images in this range look a little flatter and are better for things like a portrait. You can zoom in on your phone by 10 times, but its digital zoom, not true optical zoom. This means that a smaller part of the sensor is being used to make it look like an image is more zoomed in. In this way, you’re trading image quality for magnification.

There are other caveats when it comes to the focal length you choose. The longer the focal length/telephoto/zoom in range, the more susceptible you are to handshake and blurry images. The rule for a full frame sensor is the shutter speed should be the same or faster than the focal length, e.g. 200mm 1/200th second and 70mm 1/70th second. Optical image stabilization is coming in to allow slower shutter speeds, but it’s always good to be mindful of these rules. When in doubt, use a tripod, clamp or stabilize your phone against a wall or solid surface.

Depth of field is also affected depending on the focal length you use. Depth of field increases when the focal length and sensor decreases. An iPhone X has a sensor measuring 7.01 x 5.79mm. Quite small compared to a DSLR with a 36 x 24mm sensor. With quite wide-angle lenses on the iPhone means you’re always going to get a deep depth of field, meaning everything is going to be in focus, front to back. If you want an image with some pleasing background blur/bokeh, then you have to switch to Portrait Mode, which adds the blur via software.

Smartphone manufacturers are now allowing you to change the aperture on the camera lens which controls the depth of field/amount of light hitting the sensor and background blur/bokeh. These are measured in ‘f’ numbers from the likes of f/1.8 (wide open hole to the sensor) to f/16 (a small hole open to the sensor). Changing your aperture number has the knock-on effects of changing other things like shutter speed, but your phone usually handles this side of things for you.

As smart phone cameras get more sophisticated, they’re also going to adopt more features from full-blown DLSRs. ISO numbers, adopted from the film days which used to be how sensitive the film was to light. The higher the ISO number, the lower the light images can be taken in, but also the grainier or noisier the images. The iPhone X has an ISO range from ISO 24 to ISO 2304 with wide-angle lens, with a telephoto lens having a range from ISO 1200 to ISO 1440. The lower the ISO number is generally the way to go due to the least amount of noise.

Video

Focal length isn’t just a factor for stills but also for video shooting. The iPhone can produce 4K video and in play 8MP stills, so knowing how focal length affects footage is a good tool to own. The iPhone has optical image stabilization which helps with telephoto shots and the telephoto lens can use 2x optical magnification and 6x digital zoom.

Getting to know the basics like focal length will pay dividends in the long run. Your eyes will always be the best to gauge what you are taking, but also knowing the numbers will never let you down when you want to get the best exposure. Experiment for a few hours, taking images and videos with different focal lengths and noting down other important numbers such as shutter speed, aperture and ISO. You can go a long way with just these settings.

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