Monday, January 27, 2014

All about Sharpness - The Things You Need to Know

Sharpness, sharpening, sharp images versus blurry images, resolution... These are mostly different things (although interconnected), but many people - especially beginners - still confuse them. They also often confuse handshake, focus errors, or subject motion with lack of sharpness. People talk about a lens's "sweet spot", thinking that outside that range photos will inevitably be soft. Worse still, people think a certain lens of theirs is "not sharp" and rush to buy a more expensive one. And a more expensive, and a more expensive...

Well, I decided to give you all you need to know in a short, concise, easy-to-read text. My blog is not about MTF charts or other diagram-loving numbers. It's about photography. If you wanna read about "acutance", "line pairs per millimeter", or stuff like that, try another site. If you want to learn how to get the best of your photography, read on!

Resolution and Sharpening - what's the Difference?
First things first: we need to know what we're talking about. In a nutshell, and in layman's terms, it comes down to this:

- Resolution (or resolving power) refers to the ability of a lens (or a camera/lens combo if you wanna be picky) to resolve detail. In everyday terms we simply say "this lens is sharp" or "this lens is sharper than that one" to indicate this very thing: that, by virtue of its optical design, the lens in question can resolve a lot of detail (or more than another lens).

- Sharpening is the process that either the camera (through its sharpening settings) or the user (in post-production) increases the contrast between edges, in order to create what is, essentially, an increase in resolution perception.

Obviously enough, the sharpening process is limited by the resolving power of a lens. You can apply all the sharpening you want, if there isn't enough detail captured in the scene, it will take you nowhere.

NOTE: Sharpening is an art in itself, and so it will be the topic of a separate article

Typical example of a sharp image. There is an incredible level of detailed captured

So, Is my Lens Sharp enough?
There is an easy answer, worry not: For 95% of you, for 95% of the kind of photos you take, for 95%  of the time, any lack of sharpness on your images is not a result of your lens lacking resolving power, but of other factors:

1) Hand-Induced Blur
If your camera is not properly* secure on a tripod, then, depending on the shooting conditions, various degrees of hand-induced blur ("camera shake") might creep into the image. You need a shutter speed that is fast enough to counter this. One very, very generic rule is to use 1/focal length or faster (remember, by focal length here we mean the 35mm equivalent - read advice no.1 here). But this is not set in stone. Some hands are less steady than others. And if you travel to very windy places - like Iceland, to name my favorite destination - you'll need shutter speeds of 1/250 just for your 50mm lens.

* light tripods, imbalanced tripods, and other not properly secure... securing systems, work against you

Blur galore! Blur due to hand-holding, blur due to moving, blur due to subject moving. Possibly even focus error!

2) Subject motion
A remarkably misunderstood element. Most people believe that to avoid subject motion blur you simply need a fast shutter speed. Say, use 1/1000 for running dogs and you're fine. Technically, nothing wrong with that. The problem is, people often underestimate the...need for speed! Flowers trembling in a slight breeze, falling raindrops, and other subjects not reminding one of the "birds in flight" category, are not treated with the proper attention regarding shutter speed.

Wanna hear the most stunning example - one that makes the case best of them all? The moon! I won't get into the mathematics about it (it's a blog about photography, not optics), but even for an object so far away, that appears so stationary, shutter speed can be an issue! The motion of the moon passes unnoticed in most cases, but try a long tele (say, 600mm equivalent - so, 400mm for DX) with a high resolution camera - say, a 24MP one - and a shutter speed slower than 1/40, then you'll see what I mean!

3) Mirror-slap
A rather minor issue, but good to keep in mind. In certain shutter speeds - namely between roughly 1 sec to 1/40 sec - the mirror slapping just before the shutter opens, may introduce a vibration that will be registered as a blur in the final image. It is usually very minor, but for critical work it can be visible. The best advice is to use the mirror-up function of your camera (provided it is equipped with one). In speeds slower or faster than the range mentioned above, this is not an issue.

4) Focus Errors
This is by far the most misunderstood, the most eluding element related to pictures lacking sharpness. Countless of unlucky lenses have probably been returned by beginner photographers who erroneously believed their lens (or even camera!) was defective.

Focus error blur means that the camera has not focused on the distance the photographer intended. As a result, the picture appears to be lacking sharpness. If there is an area that is sharp, the perceptive photographer will realize what happened. If not, an inexperienced user will be at loss. With experience, it is easy to tell whether an image is out of focus - even if there aren't any other, sharp elements in the scene.

The problem is much more pronounced with fast (i.e. large maximum aperture) lenses, as the depth of field is much shorter. That is also a reason many beginners (being used to kit lenses, typically f/5.6 at the long end), believe their new lens is defective.

So, how can one detect and, mostly, avoid focus errors with these lenses? It boils down to two things:

  1. Understand the limitations of the camera/lens to track focus
  2. Understand your own limitations in regard to your holding/shutter-pressing technique
There is also a third reason, which is a rather special case: if you're using a very large aperture manual focus lens (say, f/1.4), some cameras do not allow you to estimate depth of field through your viewfinder for apertures smaller than f/2.8! In other words, when your lens is set at f/1.4, the image (and the depth of field) visible through your viewfinder corresponds to an aperture of f/2.8! There's no way around this, really, except by using some mirror/viewfinder accessory to help you with focus

But in most cases, and certainly with autofocus lenses, the problem is either the camera or you: the ability of the camera/lens combo to track focus of moving subjects (in very short distances, even a pro camera and lens cannot fully cope with a running pet or child that fills the frame of an f/1.4 lens); or, alternatively, your holding/shutter-pressing technique means that the way you hold the camera and press the shutter introduces a slight forward or backward movement, enough to throw you outside the focus zone of a stationary subject. Try to breathe out fully, and gently press the shutter to take the photo. If applicable, use a tripod.

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