Session 3 - "F" stops and shutterspeeds

And so to the nitty-gritty of "F" stops and shutter speed.

Aperture and "F" stops

I'm not going to go all technical here as I'm no expert. The "F" number is calculated from the relationship between the focal length of the lens and the lens diameter. Oops! That sounds somewhat technical. Ho hum.

F = focal length (mm) / lens aperture (mm).

So a 100 mm focal length lens at an aperture of 25mm will be f4.
         100 mm                                                               10mm will be f10
         100 mm                                                                  5mm will be f20

So, the higher the "F" number the more light is being blocked out.Each whole "F" stop aparently cuts out half the light available to the previous "F" stop.

Image source

The aperture size also has an effect on the sharpness of the resulting image. A small aperture will provide a sharp image as the light is concentrated and less scattering occurs. A larger aperture increases the amount of light scatter and results in a less coherent image. Aperture can be used to control the "depth of field" in a picture. According to Wikipedia "depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image". A shallow depth of field will only have a very limited focus. The image below was taken at F4

A smaller aperture provides a sharper image with a much greater depth of field. This image was taken at F22.

The human eye in daylight conditions is around F8

Shutter speed

The shutter speed controls the amount of time that the sensor/film is exposed to light.
A slow shutter speed would be needed for poorly lit conditions as more time is required to obtain the light. Conversely bright conditions require high shutter speeds to avoid the sensor being swamped.

According to Wikipedia

"The agreed standards for shutter speeds are:

• 1/1000 s
• 1/500 s
• 1/250 s
• 1/125 s
• 1/60 s
• 1/30 s
• 1/15 s
• 1/8 s
• 1/4 s"
• 1/2 s
• 1 s

1/60th of a second is the mid value for shutter speed.

Shutter speed can be used to either blur motion or to freeze it. A fast shutter will freeze an image while a slower shutter will result in blurring.

original source

The See-Saw connection

Aperture and shutter speed work in unison. As the aperture increases in size then the shutter speed must also increase to avoid over exposure. Similarly when the aperture becomes smaller then the shutter speed needs to slow to avoid under exposure. The best pictorial representation I've seen of this is the see-saw diagram. 

See here for the full article 

The initial diagram shows a camera set at F4 and a shutter speed of 1/125th.

The left hand side of the diagram provides the approximate aperture size.

The right hand side provides a clock view representing the shutter speed. The "open" part of the pie represents the fractions of a second that will pass at the particular shutter speed.

ISO

So far in my photography I've played around with apertures and shutter speeds but left my ISO on auto. Now it seems I need to work in full manual mode, so I'd better become acqainted with ISO. I remember once buying a high ISO black and white film for my old Olympus Trip in an attempt to capture some moon shots and star trails. However, I didn't have a tripod or a remote shutter release,  so the results were a blurry, grainy mess!

The ISO number is a measure of how sensitive the sensor / film is to light.

A high ISO (1000+) is recommended for low light conditions.

A low ISO (160) is recommended for strong light.

A medium ISO (400 ish) for dull rainy days in England. Looks like I'll be using 400 a lot!

High ISO generates a lot of digital/pixellated noise giving a grainy texture to the image. Low ISO privides a much sharper image.

ISO can be used in combination with aperture and shutter speed to control the amount of light entering the camera. The basic rule is:

1 "f" stop = 1 shutter speed "stop" = 1 ISO value.

original link

So if an image is overexposed at f8 and 1/60th with an ISO of 400 you could ...

A. Reduce the aperture to f7           OR

B. Increase the shutter to 1/200ths OR

C. Decrease the ISO to 200

All the above would have similar results depending in what you are taking a picture of, and what you want in the final image I.e. frozen motion / Depth of Field / sharpness