CS 39J > Schedule & Notes > Session 4 Detailed Notes

CS39J Notes: 02/13/2009

The basics of the class:

Tradeoffs—there are a number of things we juggle around when it comes to taking photos. We know what is more important and what is less important for the specific photograph. A camera is fundamentally a sensor that captures photons.

ISO

How sensitive is the image capturing sensor? The more sensitive it is, the less light should be let in. ISO means the international standard organization. ISO refers the sensitivity of the film, with arbitrary scale. Let’s say the number is 100, which is a typical number for film. Digital cameras take the basic concepts on film and give them a digital context. Although the actual mechanism of digital cameras is different from that of film cameras, the concepts and effects are copied over.

The numbers are only arbitrary in terms of scale, but they are significant in terms of their relationship to each other. For example, ISO 400 is four times as sensitive as ISO 100.

Exposure is darker (less sensitive ISO; good in bright setting)

25

50

100

200

400

800

1600

Exposure is bright (more sensitive ISO; good in dark setting)

Shutter Speed and Aperture

Both factors have to do with how much light is let in. The aperture determines the opening of the light-letting hole. The shutter speed determines how long the light-letting hole is open for.

Exposure Time (Shutter Speed): F/Stop (Aperture)

1/1000 second : f/32

1/500 second : f/ 22

1/250 second : f/16

1/125 second : f/11

1/60 second : f/8

1/30 second : f/5.6

1/15 second : f/4

The typical sensor dimension is 24 mm and 36 mm.

The “normal” lens is the lens which corresponds to the focal length.

The diagonal of the rectangular sensor correspond to the diameter of the cone-shaped focal length.

In a 35 mm film camera, anything that is 43 mm (rounded to 50 mm) is a “normal” camera.

Example: 200 mm is telephoto. 18 mm is wide angle short focal length.

The number of the f/stop is determined by focal length divided by the aperture (the diameter of the sensor). It is important to know that the numbers are relative. For example, 40 mm of focal length divided by 5 mm of aperture is equal to f/8.

For next time: why choose 1/60 s (f/5) as opposed to 1/30 s (f/8)? It has to do with the motion of the subject and the depth of field.

More advanced questions:

Chris: Does a high shutter speed in film… relate to digital….grainy

Sean: Zone system

Assignment this week:

Play with the shutter speed and aperture. Use dramatic combinations. Take pictures of the same subject but with different pairings of aperture and shutter speed. It is more of an exercise than an assignment.

Assignment

I just wanted to recap the assignment for this week.

The idea is to do the exercise of shooting two photographs keeping everything identical except for the exposure settings; the different exposure settings should let the same amount of light fall onto the image sensor or film. Adjust both the shutter speed and the f/stop, but adjust one in the "opposite direction" of the other such that the net result has the same amount of light fall onto the sensor or film for all images. BTW, this "amount of light fall onto the sensor or film" is sometimes referred to as the Exposure Value, or EV for short. In order to get the benefit of this assignment, I would recommend using widely varying exposure settings rather than exposure settings that are only slightly different.

Just to be clear, for example, the exposure settings of 1/1000th sec. @ f/2.8 and 1/30th sec. @ f/16 provide the same Exposure Value. Note that difference between f/2.8 and f/16 is 5 stops, in photography vernacular, and the difference between 1/1000th sec. and 1/30th sec. is a factor of about 33.

Puzzling about this a bit more: (1) Note that if the number of stops were an exponent with 2 as the base, that is, if we considered 2 raised to the 5th power, we get 32, which is more or less the same as the ratio of the shutter speeds which about 33. That is not just a coincidence. Think about it and we can discuss the reason on Friday. (2) Note that if the number of stops were an exponent with the base being the square root of 2, that the result would be the same as taking the ratio of higher f/number divided by the lower f/number; in this case, the square root of 2 raised to the 5th power is about 5.7, the same as the ration of f/16 divided by f/2.8. We can also discuss the reason for this on Friday.

Opt-out assignment: Reminder that if some of you are already experts on these concepts that you are free to do an alternative "subject-based" assignment. In that case, please have two photographs handed in on Friday.