Photography Terms Explained for Mobile Phone Cameras

November 5, 2014

Phone camera terms explained

Many people pay a great deal of attention for the camera features when buying a new mobile phone or even a tablet device. The fact that this will be the camera that goes with you everywhere you go and an alternative to a compact point-and-shoot camera, makes the camera phone an important feature.

We all love taking pictures with out mobile phones, and indeed, there are differences between one phone and the other, and it’s important that we understand what each feature means. Understanding the technology terms and their meaning will help you buy the phone with a camera that fits your particular needs.

In this article I will talk about many of those features and explain about each one in-depth. This will help you understand the camera capabilities when reading an article or review about the camera or reading its tech specs – so lets begin!

Lens Aperture  — the lens aperture is the pupil, a hole, though which light is transmitted to the sensor and it’s designated with an f-number value (e.g. f/2.0 or F2.0). The smaller the f-number, the larger the hole and the higher the amount of light that is transmitted through the lens, and the better the low-light capability of  the camera is in general-terms. The f-number that you see in the specs refers to the maximum aperture f-number. This is the largest size of the aperture possible for a given focal length (See focal length explanation below).

For for example, the camera can shoot at f/5.6, which means less light, but the maximum aperture that the camera can shoot is f/2.0.  For example, the iPhone 6 has a 29mm f/2.2 lens for its rear camera. This is a relatively “fast lens”. By saying “fast”, it means that you will be able to shoot at faster shutter speed for a given exposure.  The “faster” the lens (the smaller the aperture f-number) is the better the low-light performance of the lens is. So you should be looking for a camera that has the lowest maximum aperture f-number (f/2.2 is better than f/2.8).

In zoom camera phone like the Galaxy K zoom and Galaxy S4 zoom, you get two pairs of focal length numbers in most cases. You might see a camera with zoom with constant aperture, but it usually in conventional digital cameras, not smartphones.

So for example, the Samsung Galaxy K Zoom has a 24-240mm f/3.1-6.4 lens. This is referred to as a variable aperture lens. The first f-number (f/3.1) signifies the maximum aperture when shooting at the widest angle (24mm) and the second f-number (f/6.4) signifies the maximum aperture opening when shooting at the tele-end (240mm). The aperture changes incrementally when you zoom in, change the focal length length.

 Focal Length — I mentioned the term focal length earlier, and it’s important that you understand what it means. The focal length measures in millimeters the distance between the optical center of a given lens to the image plane, in smartphone cameras, this means that image sensor.

So when you zoom in with a zoom lens, the optical center changes and therefore the focal length value changes.  The focal length also tells us about the angle of view, and this is the most important thing to understand. The focal length of the lens is fixed to the lens, but the angle of view changes based on the crop factor of the sensor. To keep things less complicated, you should be looking for the equivalent focal length of the lens, that takes the sensor size into account and gives you the 35mm equivalent focal length that can than be compared between various cameras.

The equivalent focal length than tells us how wide the lens is. You can use this converter to see how the focal length (in 35mm terms) translates to a field of view angle. The smaller the focal length number, the wider the field of view. So for example:

iPhone 6 / iPhone 6 Plus: 29mm (35mm equivalent)
Galaxy S5: 31mm (35mm equivalent)

So we can say that the iPhone 6 and iPhone 6 Plus have a wider field of view, as 29mm translated to 73.4 degree field of view and 31mm translated to 69.8 degrees.

With a smaller focal length number, the camera can capture a wider area of the scene (both vertically and horizontally). This is great for shooting group shots, interiors, architectural building, selfies, etc. This is also why phone manufacturers use a smaller focal length lens for the front-facing camera, to make it more suitable for self-portrait shots.

A camera with a single fixed focal length lens is called a prime lens. This means that the camera has a 1x zoom, or in other words, it lacks the option to zoom in. The Galaxy Zoom smartphones have a variable focal length. For example, the Galaxy S4 Zoom has a 24-240mm f/3.1-6.4 lens. The 24mm signified the focal length at the widest angle and the 240mm signifies the focal length at the tele-end (when zooming all in), and of course the aperture as we mentioned above, tells use the aperture opening at both the wide and tele ends.

*video by Mike Browne.

By the way, the optical zoom is calculated by dividing the longest focal length value to the shortest one. So in the case of the S4 Zoom, we divide 240 to 24 and we get 10. So in other words, the S4 Zoom has 10x optical zoom.

Sensor Size — The sensor size plays a significant role in the camera’s performance. It is said that the larger the sensor is the better the image quality, and in almost all cases this is indeed the case. With larger sensor, sensor manufacturers can utilize more advances technologies that are not possible or expensive to implement in smaller sensors.  However, the most important spec is actually the sensor pixel size on most part.

The pixel size is measured in micrometer (µm) or microns (µ).  You might won’t find this spec in the official specs sheet, but some phone manufacturers do provide this value as more people are becoming aware of this and the importance of the pixel size towards image quality and low-light performance.

iPhone 6 sensor size comparison

iPhone 6 sensor size comparison

The larger the pixel size (also referred to as photodiode, the light gathering pixel) the better the light gathering capability, which translated to more data  to construct the color for this specific pixels and its neighbor pixels. This is more complicated than that, but I try to keep things simple.

You might find two cameras with the same sensor size, but with different resolutions, which translated to the smaller sensor having smaller pixels. This is one place where you need to ask yourself whether you prefer going with a camera with a lower resolution but with larger pixels (e.g. HTC One UltraPixel camera) or one that have higher resolutions but smaller pixels. Of course this changes from one camera and the other, and different cameras have different sensor sizes and resolutions.

You might find a camera that has bigger pixels but with inferior low-light performance to another camera, because sensor technologies and image processing also take an important place here.

For example, Backside-illuminated sensors or BSI in short, are sensors that utilizes a unique sensor design that significantly improves the sensor’s sensitivity to light. In BSI sensor (compared to FSI = Front illuminated), the wiring that responsible for transferring the data out from the sensor are behind the light-sensitive area, allowing sensor manufacturers to build small sensors with larger pixels.. This compared to FSI that has the wiring at the front, which take space and therefore there is less space for larger photodiodes.

New generation sensors proven to outperform older generation as sensor technology continues to improve. We’ve seen that the HTC One UltraPixel 2.0 micron pixels doesn’t always translates to much better low-light performance compared to sensors with smaller pixels.  The fact is that the iPhone 6 Plus with its 8MP and 1.5 micron sensor currently holding the top first place (as of the time of writing) in DxOMark. The HTC One M8 is in the 18th place. It’s not in the same generation, but even so, it cannot compete against cameras like the Samsung Galaxy S5 (3rd place) that have 1.12 micron pixel sensor and 16MP resolution.

The size of the sensor also have an effect, accompanied with the lens specs, over the depth of field. With larger sensors for a given aperture, those will produce an image with the ability to achieve shallower depth of field, or you might know it by the term “defocused background”. Being able to produce shallower depth of field allows us to create better separation between the subject and disturbing background elements.

In order to get better defocused effect, you should be looking for a camera phone with larger sensor and smaller f-number. In terms of exposure, there isn’t any difference between a f/2.0 lens with a full frame sensor and f/2.0 on a 1/2.3″ sensor, aperture wise — but there is a difference in the depth of field.

You might find that the sensor size is written in a fraction number like 1/2.3″, 1/2.5″, 2/3″, etc. This measured the diagonal size of the sensor, but doesn’t really helps to easily compared sensor sizes. You can check this helpful sensor size comparison online tool, or check out this article on Wikipedia that lists the most popular sensor types with their equivalent width and height in millimeters.

You can see that the Nokia Lumiz 1020 has relatively very larger sensor (2/3″ = 8.80x.6.60mm) compared to the Nokia Lumia 720 (1/3.6″ = 4.00×3.00mm).

So the next time you purchase a phone and look at the cameras specs, remember to take a look at the sensor’s pixel size and dimensions. Most camera phones have BSI, some with more advanced technologies than others, but almost all of them utilize BSI technology.

Image Stabilization — The image stabilization is an important aspect of many modern phone cameras. There is a digital image stabilization and an optical image stabilization. With optical image stabilization, the camera compensate for hand movements and shakiness by moving the lens elements in side the lens to the opposite direction of the movement – this results in sharper images.

When you shoot handheld, you cause small movements that can lead to a blurry image. This occurs because the light travels to the pixels, and if the sensor moves slightly, it can overlap some parts of the image. In other words, light that should be targeted to a specific pixel will find itself in two or more pixels where it originally should be in.

If you put your phone on a stable surface you wouldn’t have to worry about it, but it’s a mobile phone, and therefore you probably be shooting handheld most of the time. In order to get a sharp image, you should be shooting at 1/focal-length equivalent shutter speed – that’s according to the shutter speed rule of thumb. So for example, you need to shoot at 1/30 sec shutter speed when shooting with a 30mm equivalent lens to get a sharp image.

The starts when shooting in low light situations when we need to shoot at slower shutter speeds, or at times where in auto mode, the camera automatically changed to slower shutter speed to compensate for the lack of light. Because the shutter is now open for a longer period of time, there is a greater change for blurring in the image.

Digital stabilization works differently by using software adjustments in real-time to compensate for those movements. It takes a photo or record  video with a smaller area, and use the outbound image area to move it to compensate for any movement. For stills, optical image stabilization is the preferred method as it more effective and doesn’t lead to any changes in the resolution of the image. Optical image stabilization also works great for videos, but we’ve seen that digital stabilization can do wonders as well and has a very positive effect on the final output.

The effectiveness of the optical IS is measured in steps (e.g. 5 steps or sometimes written as 5 stops). You probably won’d find this info in the official specs and anywhere else, and it usually available when buying interchangeable lenses, not for phone cameras. I hope that phone manufactures will provide this detail because it helps in comparing the effectiveness of the IS between two or more camera phones.

Check out this Samsung Galaxy S5 (digital stabilization) vs LG G2 (optical image stabilization) comparison by PhotoArena

So we’ve talked about each term (aperture, focal length, sensor size and image stabilization) in-depth, and you know have more understanding what each spec means. In future articles I will explain more about other terms that you should be familiar with. Don’t forget to subscribe to our Facebook page to be the first to know when we publish a new article. Thanks for reading and don’t forget to share this article if you like it!

To compare phone sizes (in high-res), check out this phone size comparison tool.

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