How does the iPhone 5s manage to take HDR photos at full-frame rate?

Question

The iPhone 5s has a burst mode where it takes 10 photos per second. According to this review:

Even HDR shooting doesn’t slow things down much, and there’s no waiting for the phone to digest a long burst either.

Since the iPhone takes multiple exposures to make a HDR photo, how can it do that without slowing down the burst speed? In other words, if the phone can actually take 20 or 30 exposures per second in HDR mode, why doesn't Apple let us take photos at this speed without HDR?

A comment on the aforementioned page says that:

It's just that the two frames are captured in a very rapid succession - I think during the photodiodes' filling with photons.

What does this mean, and how is it different from how cameras normally take photos? Does it mean that the iPhone is taking a burst without closing the shutter (physical or electronic) in between the two photos? If so, won't the data from the first image show up in the second image, defeating the point of taking two photos?

Footnote: If you are wondering that the iPhone does single-photo HDR, it doesn't. You can see the artefacts to the right of the poles in this HDR photo, while the normal version of the photo does not suffer from this problem.

Answer 1

Bear in mind that the iPhone can also record video, which is usually at least 24 frames a second. Many cameras these days can record video at 60 frames a second. 20-30 shots a second should be a piece of cake if you have enough processing power or choose a lower resolution. There's nothing to slow it down, really.

The limit of 10 images a second is reasonable, if only because that way they can provide a more consistent experience. Even if the sensor's capable of taking 100 pictures a second, it's simpler to limit it to several a second, which will suffice for most users, and provide a feature that'll behave similarly regardless of the lighting conditions.

Apple probably won't tell us how the iPhone takes HDR exactly, so let's make some logical assumptions by going through the process step by step. Consider how HDR is made. It's 2+ images with the same composition but different exposures. The exposure is affected by 3 factors - the sensitivity, aperture size, and shutter speed, which affects the duration of the exposure.

The aperture is constant on the iPhone, I believe. For simplicity, let's assume so is the sensitivity. Therefore, an HDR image from the iPhone is 2+ images with different shutter speeds.

Let's say we'd like to take 3 pictures for an HDR with the shutter speeds of 1/10, 2/10, and 3/10s. It sounds like it'll take 6/10s, but it doesn't have to: multiple exposure. You can take a picture at 3/10s, copy it, add another 3/10s to it, and so on.

The article you referred tries to explain it further ("I think during the photodiodes' filling with photons."). What you can do is take a picture for 3/10s, but also save snapshots of the sensor's state 1/10s and 2/10s in. By the time 3/10 elapses certain pixels may have become overexposed, but you have taken those intermediate snapshots, and you can combine them to produce an HDR image.

Answer 2

The reviewer is wrong so the question is irrelevant - iPhone 5s can't make burst HDR photos at all. Even if you force the HDR mode ON, it only produces regular, single exposure photos when holding the shutter button. The reviewer probably just looked at the HDR switch and assumed it also works in burst mode without actually checking the result.

So, your initial suspicion was correct, the situation described in the question seemed unrealistic and it is in fact unrealistic.