Why the iPhone 4 takes good low-light photos: BSI CMOS sensors explained!

Backside-illuminated CMOS sensors can be found in everything from the Apple iPhone 4 and HTC Evo 4G to camcorders and point-and-shoots. Here's why you want one--and why you don't.

Sony

When Apple announced that it used a backside-illuminated (BSI) CMOS sensor in the iPhone 4, it was likely the first time most people had heard of the technology. Frankly, I doubt most iPhone users care about the sensor or why it was worth mentioning. What they do care about is that it apparently takes great photos (for a smartphone) in less than ideal lighting.

Apple and HTC were the first to put this sensor type in mobile devices, but they've been popping up in digital still cameras and camcorders since Sony rolled out its HDR-XR500V and HDR-XR520V camcorders featuring the company's own Exmor R sensor in February 2009.

Just about all of the major camera manufacturers have models that use BSI CMOS sensors now, which has triggered quite a few e-mails from readers asking for more information about them. Below is my attempt to answer those questions. If you have others, please feel free to leave them in comments.

What makes a BSI CMOS sensor better than a conventional CMOS sensor?
The simple answer is that the design makes it easier for light to reach the photo diodes on the sensor. In a regular, front-illuminated (FI) CMOS sensor the light has to travel through metal wiring and circuit elements before it hits the photo diodes, what's labeled as "light-receiving surface" in the illustration above. In a BSI CMOS sensor, the wiring is moved behind the light-receiving surface. Doing this makes the sensor more light-sensitive. The more sensitive it is, the less light is needed to get a properly exposed photo and the less noise is created.

Click the image to see more side-by-side photo examples of the iPhone 4 versus the iPhone 3GS. Carrypad

My compact camera has a CCD sensor in it. Is a BSI CMOS sensor better?
CCD (charge-coupled device) sensors are typically used in compact cameras because there's nothing obstructing the light and keeping it from hitting the photo diodes. That means even at smaller sizes, CCDs are more sensitive than FI CMOS sensors. BSI CMOS sensors solve that problem, though, by moving the obstructions. Add in the fact that CMOS sensors--regardless of type--offer faster performance than CCDs as well as better battery life and heat dissipation, and you can see why BSI CMOS sensors are a popular choice.

If BSI CMOS sensors are so great, then why aren't they in all cameras?
If you're talking about smartphones and point-and-shoots, it's because the sensors are still somewhat expensive to make. However, production has definitely ramped up, so that's changing. Though you won't see them in sub-$150 cameras just yet, you will find them in compacts priced around $200.

As for interchangeable-lens cameras (ILC) and digital SLRs, there's no need. The real benefit is in the case of the smaller sensors used in the bulk of point-and-shoot cameras and in smartphones. The larger sensors in ILCs and dSLRs don't suffer the same amount of loss in sensitivity from the obstructing circuitry. Producing large BSI CMOS sensors is currently very expensive, too, so the cost would definitely outweigh the benefits. Sony, for example, uses its larger APS-C-size Exmor CMOS sensors in its NEX ILCs and Alpha dSLRs, and smaller Exmor R BSI CMOS sensors in its Cyber-shot point-and-shoots.

OK, so they add to the cost. Any other issues?
That sort of depends on what lighting conditions you regularly shoot in. In our testing, we've noticed that shooting in full sunlight can make photos look washed out or slightly overexposed. Some we've tested seem to correct for this by adjusting exposure, but that in turn makes colors look darker. If you do all of your shooting in full sun, you may prefer a camera that uses a CCD sensor. Again, the benefits of having a BSI CMOS sensor are the increased sensitivity and lower image noise. This means they typically perform best outside in partial sun or cloudy conditions and indoors/low-light situations.

So are all BSI CMOS sensors the same?
Yes and no. They all have the same general design (moving the wiring from in front to the back), so the advantages that come from that--better low-light photos with less noise--will stand regardless of the BSI CMOS sensor used. However, just because two things use the same design concept doesn't make them equals. For example, the sensors in camera phones and smartphones are several times smaller than those in digital cameras, and with larger sensors comes better sensitivity and, hopefully, better photo quality.

It's important to keep in mind, too, that the sensor is just part of what goes into producing photo quality. While the iPhone or another smartphone might use a BSI CMOS sensor, that doesn't automatically guarantee a good picture. The lens and image processor are just as important. And, of course, the photographer.

 

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