Here’s a situation where a simple manual focus lens can do something a modern zoom lens can’t. I’ve dug this dusty old Pentax SMC 50mm f/1.7 lens out of the garage to demonstrate it, but there are an increasing number of modern manual prime lenses which can do the same thing – setting the hyperfocal distance.
Depth of field is a bit of a lost art with modern zooms and for two reasons. First, because they’re zoom lenses, they lack the depth of field index markings you get on prime lenses. The annotation shows just how these work. Second, modern lenses mostly have short focus distance scales with nowhere near the precision needed for depth of field control.
So to show how this works, I’ve added annotations to this shot of my Pentax 50mm.
They show it set up for hyperfocal focusing, which is designed to give maximum depth of field in landscape shots in particular, where objects at infinity are right at the far limit of depth of field. This means that your focus point is as near to the camera as possible without losing sharpness in the distance. It also means that objects stay sharp-looking as close to the camera as possible…
01 Line up the infinity symbol
You’re not lining it up with the regular focus distance marker. Instead, look for the pairs of depth of field markings either side of this. Let’s say you’re going to shoot at f/16 – what you need to do is look for the f/16 index marker and then turn the focus ring until the infinity symbol lines up with it. What you’ve just done is set the focus distance so that objects at infinity are right at the far limit of depth of field at f/16.
02 Check the focus distance
Now if you take a look at the focus distance marker, you’ll see that the lens if focused at 5m. This is the ‘hyperfocal distance’ for this lens at f/16, where objects at infinity are just within the lens’s depth of field at that aperture. You can look up hyperfocal distances for different lenses and apertures in tables, but if you’ve got a manual focus prime lens like this one you don’t need to.
03 Check the near limit
Now if you check the f/16 depth of field marker on the other side of the focus index marker, you’ll see it’s somewhere between 2m and 3m. This is the closest distance at which objects will still look sharp at this focus distance and lens aperture.
Easy, isn’t it? Modern AF systems are designed for super-accurate focus on a single plane, not this kind of depth of field control. It’s a bit of a lost art, though there does seem to be an upsurge of interest in manual focus prime lenses, so maybe this skill will return.
Depth of field is not a precise thing, of course. What the designers of my Pentax 50mm lens though was ‘acceptably’ sharp may have made sense in the days of film, but it might not be sharp enough now.
And depth of field is not an ‘on/off’ thing, even though the numbers given in tables and calculators appear to indicate a precise cut-off. In practice, the sharpness falls away progressively from the point where your lens is focused, and depth of field calculations are simply a guide to what you’d consider ‘sharp enough’.
