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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  • metafor - Tuesday, November 1, 2011 - link

    Fair enough. But that really doesn't take away from the fact that the A5 is a relatively large chip and from the UV-scans of it, looks to use quite a bit of that die area for the GPU.

    I don't know if a similar scan has been done of Exynos but one can't safely say both chips are far bigger than SoC's traditionally used in this space.

    Though that trend appears to be moving forward with MSM8960 and Tegra 3.....
    Reply
  • PeteH - Tuesday, November 1, 2011 - link

    That leads to an interesting question: will Apple always have the largest SoCs, and thus (most likely) the highest performance in the mobile space?

    The reason I could see this happening is that Apple doesn't have to sell their SoC's at a profit, so they're paying closer to cost for the chips (excluding the fab mark up). Other manufacturers (like NVIDIA) need to make a profit on their chips.
    Reply
  • name99 - Thursday, November 3, 2011 - link

    "I'm not entirely sure why they had to use such a powerful GPU, though. "

    And you know EXACTLY how Apple use the GPU do you?
    Does Siri run some of its workload on the GPU? Does the faster camera stuff (eg fast HDR) run on the GPU? Does Apple already have OpenCL running (for internal use) on iOS?
    Reply
  • doobydoo - Friday, December 2, 2011 - link

    He must be an Android fan.

    Androids new marketing campaign will offer a revolutionary 'new' feature - the ability to have a slower GPU than other phones!!!

    Magical.
    Reply
  • InternetGeek - Monday, October 31, 2011 - link

    They might give AMD and nVidia a run for their money if they ever tried creating desktop products... Reply
  • sprockkets - Monday, October 31, 2011 - link

    Kyro 2 was a good chip, but obviously went to focus on the desktop market. Reply
  • tipoo - Tuesday, November 1, 2011 - link

    Maybe, but there's a reason such crossovers usually take so long. Look at Intel trying to get into this space, I don't doubt they will be good at it but it takes years of development. Imagination specializes in low power, it would take lots of development effort to get into the high power desktop game. Reply
  • _tangent - Tuesday, November 1, 2011 - link

    I think this might be intentionally ironic given they got out of that game a long time ago :P

    On point though, anyone would given AMD and nVidia a run for their money with the right up front cash and expertise. I imagine the barrier to entry into that market is truly colossal though. Point is, the SGX543 MP2 is no evidence one way or the other.
    Reply
  • lurker22 - Monday, October 31, 2011 - link

    Before buying many people who got a 4s on AT&T told me how much better it was than their prior AT&T iphones.

    Anand, thanks for confirming and explaining the reasons.
    Reply
  • LordSojar - Monday, October 31, 2011 - link

    Can't we have reviews as detailed as this for the really big name Android phones? They are always far less detailed and lack a lot of the testing put into this.... thing....

    Apple makes a few adjustments, tweaks a few things, adds in the same processor that's in the iPad 2, and we have a highly detailed, scientific review that covers every single aspect, even if said aspects are the same. Samsung releases a new phone that has overall better features, faster CPU, faster NAND, a different and arguably better (or at least equal) screen, and mums the word?

    The bias is getting a bit out of hand at this point... We get that you're big time Apple fans, but cmon... At least do a major review of this caliber for the Droid RAZR and the Samsung Galaxy Nexus and the Galaxy S2 Skyrocket (LTE on AT&T!). Even if you combine them into one review, just make it THIS detailed for once instead of giving Apple the huge, super detailed ultra review!
    Reply

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