By Hans Weichselbaum – www.digital-image.co.nz
In the last issue we talked about the size of the sensor in your digital camera and how it relates to image quality. We now look at some real world examples.
It certainly comes in handy to have a camera built into your phone, but why would anybody carry a kilogram of glass around in front of a sensor, the size of which goes back to the 1930s? It surely must have some benefit.
How Many Megapixels Do You Need?
There is a saying that 6 MP is all you need (just google ’Megapixel Myth‘). According to this theory, 6 MP is enough for any print size. How does it work? Simple: larger prints are viewed from a distance. Just prevent people from coming any closer. That might work with billboards that are 20 metres away, but that’s not how we look at our photographs. Size matters and larger prints get more attention. After taking in the whole picture, the viewer comes closer to examine the details, then moves back again to sample the entire scene, then comes back to look for more detail.
Even a modest-sized 12×18 inch print (30.5 x 45.7cm) needs 19.4 MP to be printed at 300 dpi. At 200 dpi you can get away with 8.6 MP, but the difference will be noticeable – if the photo is sharp and has lots of detail.
So, we need the megapixels, as many as possible, just to get decent quality prints. That takes us to the question of how many pixels can be squeezed onto a sensor of a given size. As we saw in the previous article, a smaller sensor with a given MP count will have smaller pixel sites. Smaller pixels gather fewer photons; the signal needs to be amplified more, giving more noise. Camera manufacturers counter this with more aggressive noise reduction, which again leaves you with less image detail. It’s a vicious circle.
However, for simple screen display the requirements are a lot more relaxed. A typical high-resolution 1080p display shows you 1920×1080 pixels, which is less than 2.1 MP. And even the UHD-1 (ultra-high-definition) format with its 3840×2160 pixels only requires 8.3 MP. The latest mobile phone models with their super-sharp displays sport around 2560×1440 pixels. When we look at photos onscreen, we get the very best quality if we have one image pixel per screen pixel. So an 8 MP camera is really all we’ll ever need. Of course, the moment you crop your images or zoom into the action, you’ll need some reserve.
In short, if your photos only go onto a website or Facebook page, your phone camera will do the job just fine.
Comparing Three Cameras
I did some practical tests comparing three cameras: Canon 5D Mark III, Canon PowerShot S90 and the Samsung Galaxy S4 phone camera. I used a sunny landscape for this demonstration.
Note that this test was not done under any strictly controlled settings. It is a simple comparison under conditions we experience every day. The goal was to compare overall image quality, in particular the image noise.
To give you an idea of the available light: The DSLR settings for the landscape were ISO 100, f/6.7, 1/250 sec.
The following table gives you the resolution and the pixel density of the cameras used:
|Camera||MegaPixels||Pixel Density (MP/cm2)|
|Canon 5D Mark III||22.3||2.58|
|Samsung Galaxy S4||13||850|
Image 1 shows you the entire frame, which was dictated by the Galaxy S4 lens of 31mm equivalent. The two other cameras were set to the same frame size.
Images 2, 3 and 4 are tiny crops from the centre of the entire scene. They are heavily enlarged, which was necessary to demonstrate the quality differences in print. On the monitor the differences were much more pronounced and very easy to see.
The full-frame camera has a pixel pitch of 6.2 microns, whereas its much smaller sibling weighs in at 2.06 microns. On top of that the S90 has the massive crop disadvantage of a 10 MP camera, but this is a real-life weakness of any camera with a lower pixel count. The S90 was set to ISO 80 and the noise level is well controlled. Overall, the significantly smaller sensor shows remarkably good results.
The tiny 1/3.06″ sensor of the Galaxy S4 shows its limitations. Even at ISO 50 the fine details are blurred, obviously due to heavy-handed noise reduction and subsequent aggressive sharpening in the camera software (note the halos around the edges). The 13 MP didn’t help it to match the 10 MP on a larger sensor.
Another comparison done under low light conditions showed an even more dramatic drop in image quality when going to a smaller sensor. The camera manufacturers have done a remarkably good job in giving us noise-free images. However, the price to pay is the compromised resolution that comes from aggressive noise reduction and over-sharpening.
Some readers might remark that both the Canon S90 and the Samsung S4 are fairly old models that have been superseded by a number of generations of more advanced models. This might be true for additional camera functionalities, but the sensor size vs. image quality ratio has not changed markedly in the last five years.
The latest just released Samsung Galaxy S7 phone is a good example. Its camera certainly represents the latest technology. It sports a 12 MP sensor, reduced from the previous 16 MP. The jump back to a lower pixel count is telling. Not surprisingly, Samsung promises us better quality photos taken under low light.
I hope that this excursion into sensor sizes and pixel densities has shown you the limitations that come with smaller sensors. If interested, an internet search will give you more in-depth information. Roger Clark (www.clarkvision.com) is a good source.