Sunday, September 22, 2013

iPhone 5s Review: Camera

Since the launch of the 4s, Apple has delved deeper and deeper into photography with each generation, turning "iSight" into a branded concept so that the consumer is not left with the more pedestrian concept of  "iPhone camera." Though there were a few software feature additions (burst mode, square format pictures, filters), there were two big additions this year: improved flash white balance and electronic image stabilization. Few were expecting a new flash system, but many were hoping for image stabilization. A final improvement was not in the camera itself, but in the new iOS7 camera app, which moved to a more user-friendly gesture-based interface.


Much has been said about how the mainline camera companies seem to be caught in a rut, constantly iterating, but seldom innovating. DSLR users have been accepting of this; until recently, camera performance had yet lived up to people's wildest expectations, so constant improvement was something most DSLR users looked forward to. Apple faces a different dynamic; as a consumer electronics company, it's not enough to say that something is better. These days, it's not even enough to say how it is better.... instead companies now must tell as the name of the thing that makes it better.  So with that said, do the two biggest new features deliver?


True Tone Dual LED Flash: White Balance


The iPhone 5s uses a novel LED flash design with two LED units, one warm in colour temperature and one cool. The idea is that for any given white balance situation, the phone will analyze the ambient colour temperature and apply the appropriate mix of either flash bulb to produce an ideal image. As far as anybody can tell, the LED units are both similar in size to the LED units in the previous iPhones. This does not necessarily imply that the 5s flash power is twice as bright, nor did Apple make any such claims at launch time. So how does the new flash system work in practice?

Left: iPhone 4s                 Right: iPhone 5s

Guess which one is the more accurate depiction? Correct, it's the 4s. It's flash rendition is just a tad bit cool for the Apple Store lighting, but the 5s is very much too warm. You can see that the 4s is producing accurate more whites where as there's a warm colour cast in the 5s. (The camera in the iPhone 5 has similar, if not identical white balance to the 4s.) A closer look:

iPhone 4s
iPhone 5s

My initial theory was that the 5s image processing didn't handling the halogen lights well. The ambient colour temperature in an Apple store can be a bit on the warm side; the white-oak tables often come out yellow-looking on an iPhone when shot without flash (see the picture of iPhone 5s at the very top of the post). With the cooler temperature of the 4s flash, the colour balances out. However, with the 5s, you would expect that it would use predominantly the cooler LED, but it appears to be trying to mix in the warm LED as well.... as the light is already warm, you end up with almost something of an Instagram-like effect. (Apologies for the different camera angles. An Apple Store on iPhone launch weekend is not exactly the most serene place to be... or stand still.)

It's worth noting that Gizmodo has a test set up showing the same phenomenon of an overly-warm flash image from the 5s. The ambient light conditions look cooler than that in an Apple Store but the results are the same, suggesting that it's not just a case of the camera not being able to parse the ambient colour temperature. Shooting a picture with the flash turned on but covered produces interesting white balance results under halogen pot lights:


iPhone 4s

  • Flash Uncovered: Neutral-cool
  • Flash Covered: Neutral with slight pink cast

iPhone 5s

  • Flash Uncovered: Orange-yellow cast
  • Flash Covered: Strong pink cast


What's going on? Ordinarily, a camera will adjust its flash white balance by compensating for the cool colour cast of the flash by shifting the colour of the output towards the warm end of the scale. By covering the flash, the camera is being tricked into shifting the colour without the influence of the flash light. On the 5s, the strong pink cast with the flash covered does indicate that colour temperature management is indeed being managed by the dual LED system and not just by adjusting the colour electronically inside the phone. The problem is that the 4s colour balance doesn't show this large discrepancy either way. One wonders if the iPhone 5s camera would have been more consistent had they remained with a conventional flash system.

Image Stabilization and Detail


Here's a 100% crop from the above table shot:



By camera-phone standards, this is fairly good output. The edge definition is certainly there. You can't make out the smaller text on the sleeve, but given how tightly we are zoomed in and the physical constraints of the optical system, this is expected. However, if you look closely, there's something amiss that's sitting in plain sight... it's the table. You can see it even on the scaled samples above, but here are the 100% crops.

iPhone 4s: ISO 50, f/2.4, 1/44s, 4mm focal length. Flash brightness: 4.54
iPhone 5s: ISO 32, f/2.2, 1/45s, 4mm focal length. Flash brightness: 4.96

Admittedly, the camera angle is different in both, but the loss of fine detail and micro contrast in the 5s is marked. What's going on? Isn't the 5s supposed to be an improvement... bigger pixels, image stabilization and what not? First, recall that in the change from the 4s to the 5, Apple increased the amount of noise suppression. JPEGs from the 5 tend to have the same amount of edge acuity, but fine details tend to get lost. As the 5s uses both a slightly larger aperture and has a sensor with a larger light capturing area, there would be more leeway for the 5s to back off on the noise suppression. However, that's not what's happening in the above example; noise suppression wouldn't produce this much of a difference. Part of the difference is that the exposures aren't equivalent. The 5s is using a lower ISO but more aperture, so that part is a wash, but the flash power is higher, which would tend to de-emphasize finer detail.

Also note that these crops were taken from the center of the images. Because of the slight difference in vantage point, I didn't want to compare near the corners, but the 5s does appear to have worse corner performance in terms of lens sharpness as well, which understandable given that it has a larger sensor but the same camera depth.

However, it's possible that the electronic image stabilization might have something to do with it. The way image stabilization works on the 5s is that the camera takes 4 images in quick succession and then merges them together. As described by Apple, the image processing examines each one and looks for the best detail to merge together into a final image. In other words, the cameras is taking a multiple underexposed images, tone mapping them to look for peak contrast, and then image stacking the images to produce the correct exposure. The downside to this type of processing is that hard lines are easy to catch, but soft lines and details can blur.... and that may be what we are seeing. It's a bit reminiscent of the fine detail loss you get with HDR processing... the hard edges and outlines are all intact, but the texture loses out to a straightforward single-exposure capture.

DPReview has a studio set that doesn't clear things up. It's never a good ideal to completely rely on studio tests for sharpness, as setup and mechanical tolerances in focus tend to muck up true equivalences. However you can tell that the 5s is exposing brighter than the 5. On the resolution scale, the 5 can resolve to 2000 vertical lines per picture height (lph) before moiré creeps in. On the 5s, you can  a bit further to 2100 lph and moiré is not as ugly. However, if you scan across the rest of the studio test, the iPhone 5 produces better contrast when the detail gets soft or fine. (Look at the pencil drawings).

The problem is that the image stabilization is always "on," and in real world shooting where you can't completely eliminate hand motion there may be some give and take to the image stabilization system. For most people the trade-off in soft textures for a reduction in blurriness of hard edges is probably a beneficial one, but if you zoom in and crop before posting, you may notice that the textures are a bit more plastic-looking than they were in the past.

Conclusion


Overall, the camera system does what it needs to do, but the new technical features don't live up to the promise... at least not completely. It's not elitist to say that the majority of iPhone users won't scrutinize their images in this way, and in most ways, the technical improvements will aid their picture taking skills. However, at least at the time of launch, it's hard to say that the 5s camera has better image quality than in previous phones, as the two fundamentals of white balance and exposure are not better than before, and are arguably worse. The wider aperture and larger sensor area would make for an incremental improvement of  roughly 1/2 stop combined, not enough to be perceptible by most smartphone users. So to sum up: yes the 5s camera is better, if imperceptibly so, but it's image processing could use further tuning up.






2 comments:

  1. As a photographer, I have to point out that your analysis about the dual-LED flash is a little off.

    Of the two photos taken with 4s and 5s, you may feel that the 4s photo is more natural. Maybe. But colored LED flash really matters because now the flash more resembles the color of the ambient light.

    Everyone has experience of adjusting the white balance (with a software afterwards) but either the background is too orangish or the skin is too blue. You can't avoid one, because the ambient light and flash had different color temperature. With iPhone 5s, the colors of the two light sources are more closer, and you can change the white balance afterwards without having to worry about skin tone becoming blue or the ambient becoming orange.

    To be fair, you should have included peoples' face in the 4s v. 5s comparison. With that, try to make the person's face as natural as possible by changing the WB (with a photo editor in iPhone or PC), and see how that turns the background.

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    1. I should have been a little more clear that I was describing a specific circumstance, that for whatever reason, the 5s doesn't handle that particular light environment well, not that it's always overly warm.

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