What do we mean when we say 'stop down'?


Defining 'stopping down'–the first entry in an occasional Photocritic glossary

Every so often, one of the students at the Photocritic Photography School will pop up with a question that seems to have a blindingly obvious answer, if you know it. If no one has ever explained the term or the theory to you then it's as clear as mud. It doesn't just happen with respect to photography, either, but any discipline or activity with specialised knowledge. 

The term 'stopping down' cropped up the other day.

So for anyone who's ever heard the term 'stop down' being bandied about but isn't quite certain what it means, this is for you. (And flippin' heck, do not feel embarrassed for not knowing, either. Everyone starts somewhere.)

To stop down means to make your lens' aperture smaller.

 Want a good depth-of-field? You'll need to stop down. In this case, ƒ/8.

Want a good depth-of-field? You'll need to stop down. In this case, ƒ/8.

Yep, that's it. There's no black magic involved or complicated procedure to follow. It's just a case of moving from ƒ/1.8 to ƒ/8, or from ƒ/11 to ƒ/16, for example.

And why might you want to stop down? Quite a few reasons, as it happens.

First, your image is over-exposed and you need less light on the scene. Stop down, you'll decrease the size of the aperture and thereby reduce the quantity of light reaching the sensor.

 Low light balanced against small aperture: ƒ/11, 1/80 second, and ISO 3,200

Low light balanced against small aperture: ƒ/11, 1/80 second, and ISO 3,200

Second, you want a deeper depth of field. Large apertures with lots of background blur work fabulously for some photos, but terribly for others. If you want more of your scene to be in focus, stop down as necessary.

Third, lenses are not at their sharpest wide open, or at their largest apertures. They also don't tend to be at their sharpest when they're fully stopped down, or at their minimum apertures, either. This is down to spherical aberration and diffraction. The optimum aperture for sharpness is often somewhere between ƒ/8 and ƒ/11, but it's worth experimenting with your lenses to see which apertures give you the best results. 

So longer do you need to 'decrease your aperture', you can 'stop down,' instead.

 Mount Etna at ƒ/11

Mount Etna at ƒ/11

If we've piqued your interest in lenses, then you might want to check out Haje's article on everything you might ever want to know about them. And there's also a look at using smaller apertures creatively, too.