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Pixel Peeping: Why You Should Care About Image Quality
Iain Anderson on Tue, February 16th 0 comments
In this enlightening article Iain Anderson explores the finer points of image details and pixel perfection. Just how much do you care about your digital images?

How do you judge an image? More cynically, why bother, if most of your audience doesn’t care about image quality? Well, if you’re a filmmaker or photographer, your job is to care more than the average person does about the details. If you do your job properly, you’ll produce a better, more professional image than they could, and even if your client can’t articulate exactly why your images look better, they’ll appreciate that they do. These days, though, there are many, many variables in play, and it’s not all as simple as it may seem. How do you make an image “professional”, and how far do you need to go?

Old manual glass has character, but you can add that in post too

Old manual glass has character, but you can add that in post too

 

Lens choice

A common refrain is that you can’t go wrong investing in “glass”, implying that lenses are eternal while camera bodies come and go. That’s not entirely true, though: lens standards change eventually, the quality of cheaper lenses goes up, and other factors have a big influence as well. Try not to get too hung up on “lens character”, though—much of the time that’s something you can add (a little sharpness, selective blur, color, and so on) in post-production.

But the right lens is still important: it’s hard to get a good image from a bad lens. If your lens is soft, or not fast enough for the available light, or the wrong focal length, or suffers from chromatic aberration, then it’s the wrong choice. It’s always going to be a balancing act, though, because you can’t have a very fast, super sharp, stabilized, long zoom lens.

Long (18-200mm f/3.5-5.6), Fast (30mm f/1.4), or the middle ground (17-50 f/2.8)? Each at the right time.

Long (18-200mm f/3.5-5.6), Fast (30mm f/1.4), or the middle ground (17–50 f/2.8)? Each at the right time.

 

Compromises

You can get a pretty sharp, fast (f/2.8) stabilized medium zoom from Canon or Sigma. You can get a very sharp, very fast, unstabilized short zoom (the well-reviewed Sigma 18–35 f/1.8) but it’s expensive and heavy. You can get many moderately sharp, slow, stabilized super zooms (Canon/Sigma/Tamron f.3.5-5.6 or slower) but focusing and sharpness aren’t perfect. And, of course, you can get many incredibly fast, sharp, unstabilized fixed lenses.

The power of an interchangeable lens system is all about letting you choose the best lens for the situation. If there’s very little light, you’ll want a fast lens; if the action is a long way away, you need a long zoom. Choosing a lens that can deliver a good image in the situation you face is critical, and while kit lenses aren’t all bad, they aren’t great, and they aren’t good at extremes.

This crop shows the difference between f/2.8, f4 and f/8 on the same lens — even the in-focus center is sharpest at f/8.

This crop shows the difference between f/2.8, f4 and f/8 on the same lens—even the in-focus center is sharpest at f/8.

Sharp vs Soft

Even if a subject is in focus, not every lens is perfectly sharp. Many lenses are soft at the extreme ends of their capability, and some complex superzooms have variable sharpness along their focal length and around the frame. A good lens should perform well enough that you don’t have to worry about this in the field, but a rough rule of thumb is that if you don’t push your lenses to the edge, they’ll do better. For example, a f/1.8 lens might look better around f/2.2, a little sharper still around f/8, then worse at f/13 as diffraction kicks in. There’s always a sweet spot, and if your lens can deliver as much sharpness as your camera’s sensor can record, you’ll be fine.

The difference between RAW and JPEG can sometimes be easy to spot — in this 200% crop, the RAW on the left (default processing) has more detail and less oversharpening

The difference between RAW and JPEG can sometimes be easy to spot—in this 200% crop, the RAW on the left (default processing) has more detail and less oversharpening

 

In-camera processing

Cheaper “consumer” cameras pre-process their images (extra sharpening, extra contrast) to make them look better at a glance, but harder to adjust later. If you’re going to massage the pixels yourself, you don’t want any of that. However, only fancier cameras (or third-party iPhone apps) let you turn those features off, so dig through your camera manual and see what you can do. Shooting with a flat “log” look can also give you extra options in post, but you have to be comfortable with color grading if you shoot this way.

For comparison purposes, here’s 4K UHD video, 12MP, 18MP and 50MP.

For comparison purposes, here’s 4K UHD video, 12MP, 18MP and 50MP.

How much resolution do you need?

Higher resolution can always be scaled down, but you can’t go far the other way. High resolution stills, printed large, will continue to look better and sharper given more resolution, but there’s always going to be a point of diminishing returns. If the sensor outclasses your lenses, you’re only going to record a blurry image more faithfully, and that’s one reason why resolution alone isn’t always enough.

While medium-format digital and high-end DSLRs can shoot around 50 megapixels, the mid market of 20–25 megapixels satisfies most clients and personal needs for still images. Why? The larger a print, the further back you stand to view it, and the less resolution you need. Only a large print that’s seen from close up needs extreme resolution. That’s also why you need to sit quite close to a 4K TV to distinguish it from HD—your eyes can’t see the detail from further away.

Finally, video isn’t even very high resolution: 4K video needs just 12 megapixels, read out many times a second. Shooting at a slightly higher resolution than you need is still a good idea, but mostly because of the flexibility you gain in post production, not because you need to deliver that higher resolution.

Most camera sensors use a Bayer R-G-B-G pattern, but count subpixels as full pixels.

Most camera sensors use a Bayer R-G-B-G pattern, but count subpixels as full pixels.

 

Dirty secrets

Even though most cameras claim to record some number of megapixels, they might not be using all those pixels. The greatest dirty secret is that a 20-megapixel sensor doesn’t include 20 million full pixels. It includes 20 million SUB-pixels, each of which records only red, or green, or blue, and fills in the blanks with algorithms. Mostly, the dots are too small for us to notice this, but consider this: an iMac’s 5K Retina display, at 5120x2880 is around 14.7 million true, full-color pixels. If it was a sensor in a camera, it would be marketed as 44 megapixels.

These zoomed-up 100% crops of the same object, both in focus, are obviously different (BMCC on the left is flat but has detail, Canon EOS M on the right does not).

These zoomed-up 100% crops of the same object, both in focus, are obviously different (BMCC on the left is flat but has detail, Canon EOS M on the right does not).

  

Stills vs Motion

Compounding the issue is that in video, many cameras take shortcuts. The processors in many cameras can’t read all their 20 million pixels and scale them down to video size quickly enough. Instead, they read just a subset of the pixels, skipping most of them and effectively reducing the image resolution.

It’s not all bad news, because dedicated cameras will do the right thing. A Blackmagic Cinema Camera reads its full sensor at 2400 x 1350, then downscales to 1920x1080 to record in ProRes, and produces an image that actually has that resolution. A mid-range Canon EOS records far less information in video mode than the 1080 lines claimed.

It gets even worse when shooting at high speed for quality slow motion. At higher frame rates, the resolution falls right away, which is why the highest 240 FPS on an iPhone only claims to be 1280x720. The new iPhone’s full 4K resolution is only available up to 30 FPS.

At the end of the day, all resolution scores have to be taken with a huge grain of salt. A Blackmagic Cinema Camera can sometimes produce a sharper, less noisy image than an iPhone 6s with four times the (theoretical) resolution. Not all pixels are equal, and this is especially true in video.

Catching both shadow and highlight detail at once is a rare trick, but all the shadow detail and almost all the highlights were captured here.

Catching both shadow and highlight detail at once is a rare trick, but all the shadow detail and almost all the highlights were captured here.

 

Dynamic Range

Often overlooked by a casual observer, if a camera can capture both dark shadows and bright highlights at the same time, that extended dynamic range allows for far more flexibility in post-production and a cleaner look. Dynamic Range specs can be a bit fuzzy, so opinions on the “true” range of a camera can take a while to surface. Again, it’ll be a tradeoff: the 4K Blackmagic sensors have reduced dynamic range when compared to their own lower-resolution HD sensors, for example.

Fine details in complex images are the first to fall apart with heavy compression—note how the edges of these leaves and the helicopter aren’t clean.

Fine details in complex images are the first to fall apart with heavy compression—note how the edges of these leaves and the helicopter aren’t clean.

Compression

Most cheaper cameras use a tightly compressed version of H.264, while most higher-end cameras use ProRes or a flavor of RAW. Lower compression means greater flexibility in color grading, and fewer blocky artifacts in fast motion and detailed images. It also means higher space requirements, and for RAW formats, you’ll need a powerful computer. Trade-offs.

Motion considerations

There’s a whole lot more to video too. Shutter speed, if poorly controlled, can ruin a shot’s feel. Also, the cadence of an image is how it looks in motion; not just the shutter speed but something more subtle. We’re used to the way that film looked, and some digital cameras are better than others at matching it.

How the camera moves can also be important. A stable image helps a lot with a professional look, but a stabilizer that can handle a heavy camera is not cheap nor trivial to use. If you can’t afford to do a shot well, keep it simple, on a tripod.

A nice tripod is still key to a rock-solid shot

A nice tripod is still key to a rock-solid shot

Conclusion

When you consider all the variables, you can be left with a tricky decision indeed. Spending a fortune on the very best gear isn’t worth it if you can’t carry it, and the most stable gimbal on Earth is of no use if it requires a separate focus puller you can’t afford. And of course, the requirements for stills are often very different to those for video. In lens choice, camera choice, compression, resolution and everything else, you need to find the sweet spot that helps you deliver what you need to, not just the most expensive gear you can afford. In some situations, the sweet spot might even be an iPhone.

Remember that while there’s always a higher quality image available for more money, it may not mean a better end product. Unless you’re printing high quality at large sizes or cropping heavily, extra pixels from super-high-res stills are just wasted. Even a super-crisp video image may not be so important if the end goal is online, heavily compressed video—and today it often is. If your clients equate a shallow depth of field with a “professional” look, buy a fast lens, not a fancy camera.

A great camera can recover from situations that lesser cameras can’t, while a convenient camera that’s “good enough” can capture shots you would have missed. Pixel peeping is worthwhile, up to a point, but can rapidly become an expensive game with few winners. Finally, the answer is simple: make your clients happy, because a great image you can’t deliver, or that nobody else ever sees, has little value.

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