In our first article on Lighting, we talked about properly exposing an image, bringing the right amount of light into our camera, giving either the film or digital sensor inside the correct information it needs in order to reproduce what we see. Now the next step is to get it all in focus.
Usually when we reject images for focus, it is because of one of three main problems:
In this case, absolutely nothing is in focus. Either you had the wrong shutter speed for your exposure, your subject moved slightly, or your hands were just too shaky. Whatever the cause, this is too blurry to be useful for anything.
You attempted an artistic blur, or played with the depth of field for compositional reasons, and it just didn't work.
In this case, the 'experiment' is a failure. It doesn't look good, and doesn't communicate anything other than 'out of focus'.
So in practice, if you're shooting with an 80 mm lens, your longest exposure time for a hand-held shot would be 1/80th of a second.
This rule applies to full-frame cameras. For digital cameras with sensors smaller than 24 x 36 mm (aps-c for example, with 1.6 crop), you need to multiply your focal length by the difference (1.6 in this case) and adjust your exposure time accordingly.
Note: this is a general guide and is dependent on many other factors. Image stabilizing (IR/VR) lenses will allow shooting at much slower shutter speeds, as will good posture and shutter pressing technique. Too much coffee or a rapid pulse will cause you to shake more and a faster shutter speed may be required.
The longer your focal length, the steadier you'll have to be. That's why a 24 mm lens doesn't need any image stabilization, and your favorite 300 mm tele lens probably has it built-in.
Each new generation of modern digital cameras now comes with bigger and bigger maximum output files. At a certain size, as long as the overall image is sharp, a minor focus misalignment, where the primary focal point may not be in the ideal place for the composition, can be tolerated. However, if the overall focus is soft and the main focal point is away from the ideal compositional spot, the shot won't work. It's all about overall context.
In the case of flat textures, the entire field must be tack-sharp for it to be useful. If you've spotted a rusty metal wall that would make a great background, focus is going to be the most important consideration, as any kind of softness will leave it unsuitable for the kind of use people will want it for.
Does it look good? Does it make good use of the frame? Is it an attractive abstract, or a jagged, confused jumble? Does it look attractive, or just incorrect? If there isn't a particular 'in focus' area, is the eye instead given a compelling direction to lead it through the image? These are the questions you need to ask yourself when attempting to move beyond conventional focus. Be tough on yourself.
Panned images would be one of these occasions. That means following your subject in action across the frame with a longer exposure time (usually horizontally), giving it an extra feel of motion, leaving everything but your subject, or part of it, in focus. Camera focus IQ in servo mode is your best friend here and it's rather easy to master.
Another nice trick with these long(er) exposure shots is to use a flash with its second curtain sync, meaning the flash will shoot just before the exposure time ends, freezing the action and leaving you with the last moments in focus.
You've probably seen lots of great long-exposure shots of streaming water of all kinds (rivers, sea waves, usually with a focus on some rock or fixed object somewhere in the frame), motion blur shots of large group of people walking and doing their everyday stuff — rush hour crowds, underground stations with blurred trains, traffic, cycle races — anything that holds a message of motion, action and so on works as long as the technical side of it covers all the bases.