The Sun on Your Face

Aperture Size

A smaller value on the camera indicates a larger the aperture size. F-stops are a measurement of aperture size. The diameter of the aperture is expressed as fraction of the focal length of the lens.

The aperture diameter of a lens with a focal length of 40mm at a setting of f 8 will be 5mm.

  	Aperture size 	=	focal length ÷ f stop

			=	40 ÷ 8

			=	5
   

For then same lens with an f stop setting of 16, the aperture diameter will be 40 ÷ 16 or 2.5mm.

On a camera the aperture setting is often referred to by the f stop number e.g. f8 or f16. As the calculations above show:

• A setting of f8 will give an aperture of 5mm.
• A setting of f16 will give an aperture of 2.5mm.

Thus the larger the f stop number, the smaller the aperture. The largest aperture on my camera is often f3.5 whereas the smallest is f32, depending on the focal length being used.

Changing the Focal Length

The focal length of a lens is a measurement from the optical centre of the lens to the image censor.

Basically, the longer the lens, the longer the focal length.

Altering the focal length will either widen or narrow the field of view

• The shorter the focal length, the wider the field of view.
• The longer the focal length, the narrower the field of view.

This is why telephoto lenses have narrower field of view.

Focal Length: 20.0mm

Focal Length: 135.0mm

Focal Length: 180.0mm

In the above photos of balloons, the closer we get to the balloon by using a telephoto lens, the narrower our field of view becomes.

Factors Determining What ISO Setting to Use

The ISO setting determines the sensitivity of the camera sensor and therefore, in conjunction with shutter speeds and aperture sizes determines how much light is captured by the camera.

To increase the amount of light captured ( e.g. if an image is underexposed )

• Increase the size of the aperture (i.e. Decrease the f number ).
• Lengthen the shutter speed.
• Increase the sensitivity of the sensor

To decrease the amount of light captured ( e.g. if an image is overexposed )

• Decrease the size of the aperture (i.e. Increase the f number ).
• Shorten the shutter speed.
• Decrease the sensitivity of the sensor

For general photography, or where a tripod can be used, the standard ISO setting (50-100) should be used.

Reasons When You Might Change the ISO

1. Take photographs indoors without using the flash.
2. Freeze action with faster shutter speeds in lower light conditions.
3. Increase the shutter speed to avoid camera shake in lower light conditions.
4. Increase the effective range of the flash, or in low light conditions where a flash is prohibited e.g. Museums.

Cost:

Increasing the ISO increases the noise on the photograph leading to a graininess that is more noticeable the higher the ISO is set. It should only be used as a last resort.

Altering The Depth of Field

A lens focuses precisely on only one plane at a time, but there is a also a certain distance before and after this point that also appears sharp. This is the depth of field and it varies predominantly depending on

• The size of the aperture.
• The distance between the camera and the subject.
• The focal length being used.

To achieve a narrower depth of field:

• Increase the aperture ( decrease the f number ).
• Get as close to the subject as possible.
• Increase the focal length.

Focal Length: 55.0mm Aperture: F/5.6

To achieve a wider depth of field:

• Decrease the aperture ( increase the f number ).
• Move further away from the subject.
• Decrease the focal length.

Focal Length: 100.0mm Aperture: F/14

Using Preset Focus Modes

Most DSLRs have a number of preset focus modes that can be used quickly depending different circumstances. When using these modes the camera will automatically choose what it thinks are the most appropriate settings to achieve the best results depending on what the context is.

Most DSLRs offer the similar preset modes. On my Canon EOS 450D, the following settings are available.

1. Full Automatic

   A half press of the shutter button will set the focus and the camera automatically takes care of all other settings.

   If you are focusing on a moving subject, the Al Servo AF will lock focus on the subject automatically as long as you keep the AF point on the subject and the shutter button pressed halfway.

2. Portrait

   This sets the aperture as large as possible under the conditions to narrow the depth of focus so that the face stands out from a blurred background.

   Focal Length: 34.0mm Aperture: F/5.6

   

3. Landscape

   This sets the aperture as small as possible given the conditions to increase the depth of field to sharpen as much of the scene as possible.

   Focal Length: 31.0mm Aperture: F/9.0

   

4. Sports

   This sets the highest shutter speed to freeze the action. Focus is set to continuous while the shutter is pressed halfway.

   Shutter Speed: 1/320 sec Aperture: F/3.5

   

5. Close Up

   This sets a medium aperture so that there is some depth of field while the background is slightly out of focus.

   Focal Length: 31.0mm Aperture: F/4.5

   

6. Night Portrait

   This balances the flash on the subject with the existing light in the background.

   Focal Length: 18.0mm Aperture: F/3.5

   

7. Disable Flash

   This disables the flash.

Manipulating Manual Focus Modes

There are a number of exposure modes that you can set that allow you to set certain properties while the camera will automatically adjust the others. My Canon 400D has the following:

• Program Mode

  The shutter speed and aperture are set automatically to give a good exposure of the subject.

• Shutter Priority

  You set the shutter speed and the camera automatically sets the size of the aperture.

  It is used predominantly to freeze the action or achieve a motion blur.

• Aperture Priority

  You set the size of the aperture and the camera automatically sets the shutter speed to give the best exposure.

  It is used predominantly to affect the depth of field.

• Manual Exposure

  You set both the shutter speed and the aperture size. You can also set the bulb exposure..

  It is useful under some flash photography conditions e.g. When photographing fireworks.

• Automatic Depth of Field

  The depth of field is set automatically - all AF points will function to detect the object, both in the foreground and the background, while the aperture will be set to attain the necessary depth of field.