Camera Improvements

Arguably the second largest hardware change (with the A5 SoC being the first and largest) in the 4S is the inclusion of a much improved 8MP camera. In case you’ve forgotten, the iPhone 4 previously included a 5 MP camera. Back when the 4 was introduced, Apple talked for the first time about backside illumination, and pixel sizes. In a later update, the camera got even better with the ability to buffer three full size images and merge to HDR in real time. This time, Apple brought up F/# and backside illumination again, and added one more thing.

Though Apple never talked about any of their optical design for the iPhone 4 camera, to the best of my knowledge the design likely was close to reference designs reported on a few lens lists consisting of four plastic elements. For the 4S, Apple has mixed things up by including its own optical design front and center, and made special note of a five plastic element design. I’ve put together a table showing the 4 and 4S in comparison based on what information is available.

Note that many have speculated that Apple is dual sourcing the CMOS sensor which seems likely, and given the sensors out there the two most likely choices are Omnivision’s OV8830 and Sony’s IMX105. Both of these have almost identical specifications, including 1.4µm pixels, a 1/3.2“ format, and an improved backside illumination process over the previous generation wafer-scale process. Omnivision’s BSI–2 process cites some specifications that seem to line up with what Apple talked about in their presentation, including better quantum efficiency (ability to convert photons into electrons), low-light sensitivity, and larger well capacity (which translates to increased dynamic range). You’ll note that the 4S uses the same sensor format as the previous generation - 1/3.2”, and includes more pixels, which results in the pixel size going down from 1.75µm to 1.4µm.

iPhone 4 vs. 4S Cameras
Property iPhone 4 iPhone 4S
CMOS Sensor OV5650 OV8830/IMX105
Sensor Format 1/3.2" (4.54 x 3.42 mm) 1/3.2" (4.54 x 3.42 mm)
Optical Elements 4 Plastic 5 Plastic
Pixel Size 1.75 µm 1.4 µm
Focal Length 3.85 mm 4.28 mm
Aperture F/2.8 F/2.4
Image Capture Size 2592 x 1936 (5 MP) 3264 x 2448 (8 MP)
Average File Size ~2.03 MB (AVG) ~2.77 MB (AVG)

Everybody likes talking about sensors (and I see lots of attention given to them), but any good photographer knows that it’s a combination of optical system and sensor that matters to performance. Optical design is important, and having studied as an optical engineer I find it interesting that Apple would draw attention to having a custom design of their very own with an additional plastic element. For a while I’ve held off on really talking about smartphone camera optics, but while we’re here, let’s touch briefly on them.

The iPhone 4S camera module

Thus far this generation and the one before it have primarily used 4 plastic elements, and virtually everyone but Nokia uses nothing but plastic (Nokia famously uses Zeiss-branded designs, often with glass elements). Optical design is generally driven by material availability, and there are only a few optical grade (read: transmissive in the visible) thermoplastics out there - Styrene, Polystyrene, ZEONEX, PMMA (Acrylic) and so forth - the list is actually relatively short. Thankfully polystyrene and PMMA can be used to make something of an achromatic pair, with polystyrene as a flint, and PMMA as something of a crown. Plastic provides unique constraints as well though - coatings don’t stick well, not very many have great optical properties, they have a high coefficient of thermal expansion, high index variation with temperature (which oddly decreases with increasing temperature), and less heat resistance or durability among others. With all those downsides you might wonder why smartphone vendors use plastic, and that reason is simple - they’re cheap, but more importantly, they can be molded into complicated shapes. Those complicated shapes are aspheres, which are difficult to fabricate out of glass, and afford much finer control over aberrations using fewer elements, which is an absolute necessity when working with very little package depth.

Apple's 4S versus 4 infographic

So what does adding another element get you? Well, when you’ve faced with limited material choices, adding more surfaces gives you another opportunity to balance aberrations that start blowing up rapidly as you increase F/#. That said, there are tradeoffs as well to adding surfaces - more back reflections, increased cost, and a thicker system. In the keynote, Apple notes that sharpness is improved by 30% in their new 5 element design, and MTF is what they’re undoubtably alluding to.


Genius electronic optical - 5P lens. Compare to above.

Genius electronic optical has a page on their website with a lens system that seems likely to be what’s in the 4S, as the specifications include 8 MP resolution (same size), same sensor format, F/# (2.4), 5 plastic elements (5P) and looks basically like what’s in the 4S. Other than that, however, there’s not much more that I can say about this Apple specific design without destructively taking things apart. One thing is for certain however, and it’s that Apple is getting serious about camera performance, something that other handset vendors like HTC (with its F/2.2 systems) are also doing.

Apple made mention that it also included an IR filter in the 4S optical design. If you recall back to our Kinect story, I used the 4 camera to photograph the IR laser structured light projector that Kinect uses to build a 3D picture. The 4 no doubt has an IR filter (though not a great one), but it’s probably just a thin film rather than a discrete filter right before the sensor. The 4S includes what Apple has deemed a ‘hybrid IR filter’ right on top of the sensor, which is possibly just a combination of UV/IR CUT filter (UV is a problem too), and an anti-aliasing filter.

If you try and take the same Kinect (IR source) picture with the 4S, thankfully all those non-visible, IR wavelength photons get rejected by the filter. This doesn’t sound like much until you realize that silicon is transparent in the IR and will bounce around off the metal structures inside a CMOS or CCD and create lovely diffraction effects on fancy sensors. I digress though since that’s probably not what Apple was trying to combat here. On a larger scale, IR will generally just cause undesirably incorrect color representation, and thus people stick an IR filter either in the lens somewhere or before the sensor, which is what has been done in the 4S. The thin film IR filters that smartphones have used in the past also are largely to blame for some of the color nonuniformity and color spot (magenta/green circle) issues that people have started taking note of. With these thin film IR filters, rays incident on the filter at an angle (as we move across the field) change the frequency response of the filter and the result is that infamous circular color nonuniformity. I wager the other effect is some weird combination of vignetting and the microlens array on the CMOS, but when I saw Apple make note of their improved IR filter my thoughts immediately raced to this ‘hybrid IR filter’ as being their logical cure for the infamous green circle the iPhone 4 exhibits.

Another minor difference on the 4S is that the LED flash is improved. The previous LED flash had a distinctively yellow-green hue, the LED flash on the 4S seems slightly brighter and also has a temperature that’s subjectively much closer to daylight, though I didn’t measure it directly. I habitually avoided using LED illumination on the 4 and will probably continue to do so on the 4S (and use HDR instead), but it does bear noting that the LED characteristics are improved. Unfortunately the diffuser and illumination pattern still isn’t very uniform or wide. It also seems that all this talk of moving the LED flash to the other side of the device to combat red eye turned out wrong as well.

Display Improved ISP in A5
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  • doobydoo - Friday, December 2, 2011 - link

    You don't have to be concerned with something to care about getting an improved version.

    For example, I may be quite happy to work for a salary of £1,000,000 a year, but that doesn't mean I wouldn't care if my salary was doubled.
    Reply
  • Grandpa - Wednesday, November 9, 2011 - link

    Anyone heard of staticgate yet? How come nobody gives a review about the static issue on the iphone? Reply
  • mymomentummedia - Thursday, November 10, 2011 - link

    Here is a funny iphone 4s review... iphone 4s plus a 50.cal Barret sniper rifle. what do you get?

    http://www.youtube.com/watch?v=RsVEbRt6g94
    Reply
  • chillstatus - Monday, November 14, 2011 - link

    I made sure to switch my dataplan over to the "Unlimited Data Plan for iPhone 4S" and when I run the field test, it still says it's using wap.cingular. I live in Los Angeles, and am wondering how I can get it to use the faster network. Anyone else have this issue? Reply
  • rhch - Sunday, February 5, 2012 - link

    I just found out that my 4S wifi range is not as good as my old iPhone 4 or iPad 2.
    4S gets much earlier "no connection" with Safari, Appstore etc when i take distance to my hotspot.
    Can someone confirm this?
    Reply
  • AlexD - Monday, February 6, 2012 - link

    With this commend I ask for help to find a developer for an iPhone-App that enables to use the 4S for the catching of light below 380nm (UV) and above 780 nm (NIR). The purpose is to identify light emission of materials (inks, plastics) outside the visible range and to visualize it with the smartphone. Even a slight range (e.g. >320nm or <850nm would help me.
    I would also be very greatful if somebody could explain what range of wavelength the 4S camera sensor is catching, if there are filters which can be removed or exchanged or the like to enable the above function. Thanks in advance for your support.
    Reply
  • meace - Sunday, February 12, 2012 - link

    Is the POP memory attachment to the A4 permanent?
    As in... could that pop ddr sdram of 128 *2 be replaced with 256 *2 ?
    I'm guessing there's no way that it's coming apart without damage
    (maybe even in a clean room) but I'll ask.
    Reply
  • vitaprimo - Saturday, August 18, 2012 - link

    I don't think it's the phone's fault that much. I have two iPhone 4s (not 4Ss) on different carriers (Telcel and Iusacell) and the never lose signal, no matter how tightly I grip them and I don't use cases. Now, one of the carrier is relatively new in my state and out of the major cities I've had a few dropped calls but they always connect back. I use to have iPhone 4Ss and I noticed no difference; I lost a bag with the phones in it so I'm using my old 4s until the sixth comes out--hopefully in less than two months.

    This whole antenna issue seems to just be affecting carriers in the US--that being AT&T.
    Reply
  • phero - Friday, January 18, 2013 - link

    Thank you very much for this extensive review, especially the part about power consumption. It has given me all the information I needed for a well informed decision on which phone to buy. Reply

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