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Cameras aren't just a smartphone feature now: they're practically the killer app. As phone camera quality keeps accelerating, upgrading to a new phone can often be a question of, "Is the camera worth spending more for?"

There are plenty of flagship phones with cameras worth comparing head-to-head, but for now, we wanted to dive into a simple comparison: How good are the iPhone 6 and 6 Plus as cameras versus the iPhone 5S ? Some of this has already been answered in our reviews, but this more detailed deep dive was a chance to compare them in the same way we compare point-and-shoot cameras. Is the 6 a worthy step up? And does the 6 Plus, with its optical image stabilization, make a significant difference?

The answer is, yes: but, the step up isn't as massive as in previous years. But if you value better autofocus and low-light shooting, the 6 and 6 Plus are both improvements.

For these tests, we chiefly used the rear iSight camera, and concentrated on still shots. (We'll look at video recording in a future evaluation.) But the improved autofocus, stabilization and added 240fps slow-mo and 60fps video modes make a big difference as an everyday video camera.

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5S vs. 6/6 Plus

If the camera is one of your main reasons for upgrading to an iPhone 6 or 6 Plus, go ahead, it's definitely worth it. The 5S certainly has a very good camera, but the 6's photos are a bit more usable for enlarging and cropping, despite having similar specs to their predecessor.

Color and exposure are solid across all three phones, but the iPhone 6's image processing is noticeably better. Not mind-blowingly better, but it's improved enough that details are visibly sharper and cleaner and the HDR mode seems to have further tamed blown-out highlights and rescued detail that otherwise would be lost in shadows.

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As with picture quality, the camera sensor's Focus Pixels -- aka phase-detection autofocus (AF) system -- isn't a huge leap in speed, or at least it likely won't feel like much of one. Frankly, the iPhone 5S's camera focused more quickly than most smartphones, so speed never felt like an issue.

However, in our tests, the AF was more accurate, especially when trying to hit a moving subject and in low light. Too often with the 5S we would end up with pictures that would be just off the mark or focused behind our subject. That wasn't the case with the 6 and 6 Plus.

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The 6, the 6 Plus and optical image stabilization

So, along with screen size and battery life, the optical image stabilization (OIS) of the 6 Plus is one of the big upsells. The question is, "Is it worth it?" It really all depends on what you shoot, how you shoot, and what you typically do with your photos.

With the 6, you do get improved low-light photo quality in part to Apple taking advantage of its new sensor and processor. Just like past iPhones used to do for HDR pictures, the new phones in dim conditions will quickly shoot multiple photos and stack them, in the process removing minor blur from hand shake as well as noise (a technique Sony and others have been doing for years in their point-and-shoot cameras).

The problem is, you might still need a high ISO, which adds noise, and the extra processing adds some softness. Instead of this stacking, the OIS can compensate for slower shutter speeds, allowing the camera to go with a lower ISO setting that introduces less noise and resulting in a cleaner, sharper-looking photo.

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Also, the OIS helps with camera movement in general. It's always going to be best to be perfectly still when shooting, but there are times when that's not possible such as snapping from a moving car. There's only so much OIS can help with this situation, though.

But, back to our original point: If you shoot a lot of low-light photos and frequently find yourself enlarging and cropping in on things, it might be worth going with the larger 6 Plus.

Similarly, if your hands shake a lot when taking pictures (or video for that matter) or you have way too many blurry shots in your collection from chasing your subjects around or shooting while moving, you might find the OIS beneficial.

Yes, the 6 Plus is a better camera for having the OIS. But, overall, unless you regularly enlarge and heavily crop your pictures before you share them, you probably won't miss the OIS. At least not enough to deal with a phone that might be too large for you to use or for your wallet. Surprise, they're both just really good smartphone cameras.

What else will you notice?

In everyday use, here's what we learned: there's more clear detail in most photographs taken in normal lighting, which means that the camera focuses better on important details and also adjusts exposure better. It's still not perfect: take a quick snap in a normally-lit room and photos still have noise and blur. It's not as crisp as an SLR, or even a really great small camera, which should be obvious. But the point is, this is better -- just don't expect miracles.

Front-facing photos were better in low light, but they offered less photo detail: again, because the FaceTime camera isn't as good. It's perfectly suitable for the average selfie.

Panoramas, which we didn't compare side by side, are obviously and clearly better on the 6 and 6 Plus versus the 5S: they're larger file sizes, capture more detail, and look better zoomed in. However, they still don't handle scenes with moving subjects particularly well.

Conclusion

The 6 Plus is worth it for hardcore iPhone photographers. But by no means does it scream out a huge difference in everyday picture quality over the 6, which will be more than good enough for most. Consider your pockets and how large you want your phone to be. And if you're an iPhone 5S owner, know at least that your phone still takes really good photos: it's just that the new phones have autofocus abilities you'll likely envy.