Maximum aperture

The maximum aperture of a lens represents its light-gathering ability, and it's an important selling point. The smaller the number, the wider (or 'faster') the lens aperture.

This is the scale of aperture values. These are whole 'f-stops', but there are apertures in between ('half stops' or 'one-third stops'):

• f1 (very rare)
• f1.4 (some fixed focal length lenses)
• f2 (some zooms)
• f2.8
• f4 (most standard zooms start at f3.5, which is mid-way between f2.8 and f4
• f5.6 (the maximum aperture of most standard zooms at their maximum focal length*)
• f8
• f11
• f16
• f22

* Zoom lenses rarely offer the same maximum aperture throughout their zoom range, which is why the lens specs will quote something like 'Nikkor 18-55mm f3.5-5.6'. That means the maximum aperture at 18mm is f3.5, but at 55mm it drops to f5.6.

Each aperture value in the list above is one stop (or 1EV) 'faster' than the one below it. F1.4 is two stops 'faster' than f2.8, which means you can use an ISO setting two stops lower or a shutter speed two stops faster.

There are zoom lenses with constant maximum apertures, but these are much bigger, heavier and more expensive.

Fixed focal length lenses like this Nikon 50mm f1.4 have wider maximum apertures because their optical design is simpler and the designers can push the envelope in different directions.

Neutral density filters | Can be used for good and evil

Neutral density filters reduce the amount of light entering the lens without altering it in any other way. It sounds an odd sort of thing to do, but ND filters have two important uses: (a) cutting the amount of light in daylight to allow slow shutter speeds; (b) darkening bright skies (graduated ND filters).

Graduated ND filters can be positioned carefully over the lens to darken the sky without affecting the rest of the scene (see the example above). They come in various strengths, usually expressed as a number. A 2x ND filter darkens the image by one stop, a 4x ND filter darkens by  two stops and an 8x ND filter darkens by three stops. It does take a bit of experience to gauge what strength of filter you need in any given situation.

Graduated filters slide up and down in the filter holder so that you can get the position exactly right. The filter holders have extra slots which allow the use of two or even three filters in combination.

You can also find 'neutral density' filters in some compact cameras. Here, they're used to simulate the effect of a smaller lens aperture (presumably it's cheaper than actually including a lens aperture mechanism in the lens). It does the job, adjusting the exposure just like a real lens aperture adjustment, but it will not produce the varying depth of field effects you might be expecting from aperture adjustments.

EV (Exposure Value) | How exposure works

EV, or Exposure Value, is a numerical measurement of the amount of light in a scene. We all tend to think of exposure in terms of shutter speeds and apertures, but in fact it's the exposure value that it all starts from.

When you use a handheld light meter like the Weston Master V below, it quotes an EV number which you then translate into shutter speed and aperture combinations using a rotating dial. When the camera measures the light level, it too starts with an EV number, which it then uses to decide shutter speeds and/or apertures, depending on the mode you're working in.

If you want to use manual exposure control, or take more control over what shutter speeds and apertures the camera uses, a basic knowledge of exposure values is really useful. In fact, you might not need an exposure meter at all, because it's possible to predict the EV values for different scenes surprisingly accurately. The table below gives some examples:

Once you know the EV number, either from a handheld light meter or using the table above, you can use it to work out what shutter speed and aperture you're going to use with the table below.

One thing to be aware of is that the EV values change at different ISOs. It's not something you need to worry about if you're using the camera's internal light meter because if you change the ISO the camera will allow for this automatically. But if you're using EV values from the table above, or transferring them from a handheld meter, you will need to match the EV to the ISO - you can do this with the table.

What you can see from this table is that for any one exposure value there are lots of different shutter speed and aperture combinations you can use. When you use the 'program shift' function on a camera, this is what it's doing.