The Lens Aperture or Iris Explained

The Aperture: What is it and what does it do?

Closed Iris on a Lens Aperture

It's all about light entering the aperture! Kind of like your eye and how it works. The apertures in your lenses work in much the same way as the pupil in your eye. Too much light and the pupil will close to block it, not enough light and the pupil will widen to allow more light in!

It is found in the lens of your camera and in modern cameras is adjusted via a control wheel or dial. If you ever owned a manual film SLR you will remember that the lenses had an ring that you set manually. It moved with a "Click-click" as you turned it to alter the apertures.

One of the most important things to learn about apertures is Depth of Field or DOF plus the more technical "hyper-focal distance".

Depth of field is the amount or "depth" of the image that is in focus in your images. The rule is that small apertures of say F11 or F16 will give good depth of field or more of your image is sharp (as in the image below). A large aperture of say F2.8 will have shallow DOF or very little in focus.

Yachts in Weymouth Harbour

Apertures and Shutter Speeds

Another thing to remember is that as you adjust the DOF one way, the shutter speed has to be adjusted the other way to compensate.

For example, if your camera meters a sports scene for 125th/sec at F8 and you want an setting of F2.8 to create shallow DOF, you are letting more light in the lens. You will therefore need to have a faster shutter speed to compensate.

In this instance, you have opened the aperture by 3 stops (F8 – F5.6 – F4 – F2.8). You will need to increase the shutter speed by 3 stops (125th – 250th – 500th – 1000th).

The difference here is that 125th/sec @ F8 would give a reasonable "action-stopping" shutter speed and good DOF. However, 1000th/sec @ F2.8 will give a fantastic sports "action-stopping" speed but very little DOF. This could result in focussing and sharpness problems.

If I were shooting sports, I would opt for either of the middle settings (250th @ F5.6 or preferably 500th @ F4).

Aperture Priority

Personally, I almost always shoot in Av or Aperture-Priority mode. This is where you set the aperture and the camera will automatically set the corresponding shutter speed. I would rather have control over the depth of field for work such as;

  • Weddings
  • Portraits
  • Commercial
  • Still Life and Macro

But if I need a faster shutter speed for sports or nature photography, I would whack the iris right open. To around F4 and see what shutter speed I get.

If it is still too slow, I simply up the ISO to 200 or 400 to again, allow more light in. Actually, I am making the sensor more sensitive but it is the same principle. This would give me one or two stops faster shutter speed whilst retaining the aperture setting. It is all a matter of personal preference how you make your adjustments.

Apertures with Different Lenses

The depth of field that a certain aperture gives you will dramatically change depending on what lens you are using, this is quite an important lesson for creative and consistent photography.

Wide Open F2.8

Ok, you want very shallow depth of field in order to isolate your subject and "blow out" or blur your background.

NOTE: Remember here that a wide aperture will give a much faster shutter speed. If it is particularly sunny and bright, you may find that you don't have a fast enough shutter speed on your camera. The speed won't block the light sufficiently to warrant the use of F2.8.

Here to compensate, you could either close the aperture a little. you could also keep the same one and use a polarizing filter to block out some of the light.

Wide Angle Lenses

When using a very wide angle lens such as a 15-20mm for landscapes, you won't see too much of an effect. For general scenes most of the shot will remain in focus using F2.8.

The only way to get a shallow DOF with a wide angle lens is to get close to your subject and focus on it ensuring the background, or some of it, is still in view.

Orange Lifebuoy in Weymouth Harbour

Telephoto Lenses

With a telephoto lens however, it is a very different story. The longer the focal length (200-500mm), the shallower the DOF obtained, or less of the shot in focus.

This is why most portrait photographers like to use 85mm to 200mm lenses. They will generally blow out the background enough to make the model or subject really stand out. Even at F10 for example as in the image below.

Machine Gun on Top of WW2 Tank

The only problem here is when shooting wildlife or sports with a long telephoto lens. With something like a 500mm, your depth of field is severely shortened. You will need more precise focussing or smaller apertures of say F8 - F11. Even then you have to be spot on with your focussing.

Closed F11 - F16

A closed iris at these settings will invariably give good depth of field whatever you are shooting except for macro shots. Macro photography is an art unto itself and requires a lot of patience, practice and precision.

NOTE: Once again, beware. Opposite to the above note, when using small apertures, you will invariably get slower shutter speeds. The use of a tripod is recommended for shutter speeds of 60th/sec or slower.

Again, the depth of field at these lens settings is affected by the lens that you use.

Wide Angle Lenses

For landscapes and architecture work, a wide angle lens used with small apertures is just about right. With a decent quality lens and a camera with a 1.3X crop factor or more, you should end up with some pretty sharp images with good DOF.

Why does the crop factor help? Quite simple really, you are cropping out the edges of the frame. These are normally subject to a bit of barrel distortion or poorer quality of focussing/DOF. Chopping it out at the point of exposure rather than later in an editing program is a bonus).

Telephoto Lenses

For sports and wildlife photography you really need a fast shutter speed of 250th/sec or 500th/sec minimum. By closing down the iris to F11 or F16, you are going to struggle in most situations.

The only way to get round this is to increase the ISO to 400/800/1000 to increase the sensitivity. This will also allow a faster shutter speed to be used with the same apertures. The downside here is that you will start to see more grain or "noise" appear in your shots.

For portrait work, using these apertures on a telephoto lens of 150mm or more will still result in a nice "Bokeh" or background blur especially if you close in a bit on your subjects face.

For more information on Depth of Field use the following link: Depth of Field

  • […] of light and how to control. Which is fundamental to one’s success as a photographer The aperture of most lenses is designed to allow control over the intensity of light falling onto the film or […]

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