Technical Help Please -- ASA f/stop focal length etc.

[Jean Kichenbrand]

Hi Guys,

This forum has been a vital source of information to a newbie like me. I’ve done the search but would still like to get some more solid answers on the following:

1. What is ISO/ASA?
2. Where would I be able to learn topics such as f-stops, focal length, shutter speeds etc?

Any advice would be highly appreciated.

Thanks
Jean K

[Jeff Donald]

Hi Jean,

The Wranglers here are all working professionals in video production or related fields (various computer and web related professions). Some of us have dual careers, I teach photography at a local arts schools. So, feel confident that the answers you receive will be accurate and informative. With that in mind, feel free to ask any questions relevant to video production and media in general.

ISO/ASA are the acronyms for the International Standards Organization and American Standards Association. These are groups that meet to make and define agreed upon standards. In film, they relate to the sensitivity of the film or coated emulsion. They are referred to by the film speeds, ISO 100, 200, 400 etc. The lower the number, the less sensitive the emulsion is to light. Because of difference between film and CCD's (imaging devices in video cameras) a direct comparison can not be made.

I would be happy to discuss f-stops, focal length, shutter speeds etc. Would you like the explanations to to be centered around video or film? This information is covered on many of the film sites, but with a very heavy film slant. I hesitate to provide links to those sites because they may confuse some of the issues.

Jeff

[Christopher Go]

For the absolute basics on photography in general, visit Kodak's website on how to obtain better pictures:

http://www.kodak.com/global/en/consu...ng/index.shtml

If anything else that resource should allow you to get a good idea on what the terms mean at least. The more experienced members can reveal how they're applied to video and filmmaking. I'm only learning too.

[Jean Kichenbrand]

Hi Jeff,

Thank you for the kind words.

It would be great if you could explain the basics to me regarding F-stops, Focal Lenghts etc. in DV terminology.

Any books/websites you could reccomend?

Do you think that if I took some still photography classes it would help?

Thank you for the advice!

Jean

[Jeff Donald]

I'll define a few terms for you and then see if there are questions.

Aperture - Clear opening in the lens through which light passes to hit the CCD. It's size determines the amount of light the lens will transmit or allow through.

Effective Aperture - the diameter of the clear opening , expressed as a dimension (inches, MM), the size of which is determined by the iris (diaphragm). This number is normally only used in certain optical calculations.

Relative Aperture - The number written as a fraction "F" followed by a numerical value. The ratio of lens focal length to diameter of effective aperture.

Iris Diaphragm - works like the iris in the eye, a variable opening in the lens that controls the size of the aperture.

F Number - or Diaphragm Opening, an effective opening in the lens (iris diaphragm) that transmits 1/2 the light of the previous smaller number. The scale is as follows:

F/1.0, F/1.4, F/2.0, F/2.8, F/4.0, F/5.6, F/8.0, F/11, F/16, F/22, F/32, F/45 etc.

To determine the relative values of two F Numbers, square the numbers and compare the resultant numbers. For example, F/2.0 squared (2 X 2 = 4) and F2.8 squared (2.8 x 2.8 = 7.84 In our example F/2 lets in almost twice as much light as F/4 (4 vs. 7.84). All the F Numbers work in a similar manner, with smaller F Numbers letting in twice as much light as the next larger number. Inversely, the larger numbers let in half as much light as the next small F Number.

Maximum Aperture - The largest iris diaphragm opening on the lens. The smallest numerical number on the aperture control on the lens.

Optimum Aperture - The iris diaphragm opening that result in the sharpest image quality. Usually two opening smaller than the Maximum Aperture. On an F/2.0 lens the Optimum Aperture is usually F/4.0

T-Stops - The Diaphragm Opening determined by the light the lens system actually transmits. It takes into consideration reflectance and absorption of the light by the multi-coatings, lens elements and internal construction. The term is commonly used in the motion picture industry.

The F Number is one of the two basic controls the camera operator has to control exposure (amount and duration of light falling on the CCD). The other control is shutter speed.

I'll stop here and answer any questions on Lens Openings. Part Two will be shutter speeds.

[Jeff Donald]

Shutter speeds are the second component in determining the proper exposure. In modern 35mm still film cameras the shutter speeds range from 30 seconds to 1/2000 of a second. In some advanced amateur or professional models the speeds range as high as 1/8000 of a second or higher. The scale of shutter speeds is as follows:

30 secs. 15 secs. 8 secs. 4 secs. 2 secs. 1 sec. 1/2 sec. 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, 1/8000

Video cameras typically have a range of 1/8 second to as high as 1/15,000 a second shutter speed.

If you carefully examine the shutter speed scale you'll notice that the speeds or settings are exact multiples of the next fastest or slower speed. For example 1 second is twice as slow as 1/2 second and 1/2 second is twice as fast as 1 second. Look at the scale again and see how each speed is exactly twice as fast or half as fast as it's next faster or slower speed.

This relationship of twice as fast a shutter speed and half as fast a shutter speed is the same as the F Numbers being half as large and twice as large as the next largest and smallest F Numbers.

In a film camera, a mechanical device, the shutter opens and closes for the selected exposure duration (shutter speed). In a video camera the CCD is constantly be exposed to light. No mechanical shutter is used. The CCD is sampled for a specific amount of time and repeated at that rate. For example if a shutter speed of 1/60 of a second is selected, the CCD is sampled 60 times per second. If a slower speed is used there are fewer samples per second.

The use of a faster shutter speed will stop action and a slower speed will blur action. Why is this? Think of an object traveling a certain distance in a specific amount of time, lets say 1 foot per second. If we set the shutter speed to one second the object travels one foot and produces a blur or streak. If we choose 1/60 of a second, the object travels only 2/10's of an inch (12 inches divided by 60 samples = 0.2 inches per sample or frame). This would produce a slight blur, but probably not noticeable to the human eye.

The combination of shutter speed and aperture (lens opening, F stop) produce the proper exposure. In the morning I'll discuss exposure and focal lengths and answer any questions.

[Jeff Donald]

The amount of light reaching the surface of the CCD is controlled by the Iris Diaphragm. The duration of the sample of the CCD becomes the Effective Shutter Speed. The proper combination of these two controls allows the camera operator to select the best exposure. Exposure is any combination of F Number and Shutter Speed that produces the correct ratio of brightness and contrast within a scene. Since brightness and contrast can be subjective, correct exposure is usually not an absolute value. Exposure is part personal taste, creative control (sets mood for example) and technical requirements (minimum levels need for proper playback or broadcast).

F/16 - F/11 - F/8 - F/5.6 - F/4.0 - F2.8

1/60 1/125 1/250 1/500 1/1000 1/2000

The combinations of Shutter Speeds and F Numbers above, are all correct exposures for a given amount of light. If I double the amount of light falling on the CCD surface (changing the F Number from F/16 to F/11) then the sample rate (effective Shutter Speed) can be cut in half (Shutter Speed changes from 1/60 to 1/125). This is the Law of Reciprocity.

If one component changes and the other is not changed, over exposure or under exposure may result. Over exposure is too much light reaching the CCD or the light being sampled for too long (slow shutter speed). Under exposure is not enough light reaching the CCD or the rate of sample (shutter speed) being too short.

Again, questions are welcome. This afternoon I'll post more information about exposure.

[Jeff Donald]

A few definitions:

Contrast - The difference in density of areas of an image. Areas of an image may be dark (black with no detail), and referred to as D-Max. While other areas are light (white with no detail), D-Min. An extremely high contrast image would contain only white or black and no middle shades or tones. Low contrast images contain only middle shades and no distinct blacks or whites. A low contrast image is sometimes referred to as flat. Fine detail will be completely lost in high contrast images. The contrast in the scene is used by the autofocus system to correctly focus the lens system. It may be impossible to AF in some low contrast scenes.

Sharpness - The steepness or abruptness of change between light and dark areas of an image. The steeper the change the sharper the image appears. DV is not only limited by optical quality of lenses, but tape formulation and the methods used to convert analog (light) to digital (electronic values). DV uses 8 bit electronic signals. Some formats or cameras use 10, 12, 14 or more bits. The more bits may allow for sharper images with more contrast.

Resolution - This may also be referred to as resolving power. A measurement of the ability to image sharply defined, closely spaced black and white lines. The measurement is an indication of the camera systems (camera, format, tape etc.) capacity to record very fine detail.

Acutance - A somewhat subjective measurement of contrast, sharpness and resolution. The measured density of a gradient across a boundary (black to white).

[Jeff Donald]

Exposure - The proper combination of F Number (lens opening, aperture) and Shutter Speed so that enough light falls on the CDD to provide enough light for correct scene illumination. Exposure is subjective, there is not right or wrong exposure if the photographer is happy with the effect. Certain minimum levels are required to obtain a broadcast legal signal. Very often the photographer chooses a level of exposure (combination of lens opening and shutter speed) that illuminates the scene in a manner similar to how the scene is actually viewed. The scene is then neither over exposed or under exposed but accurately exposed.

The combination of lens opening and shutter speed is often times picked on a basis of creative control or intent. Lens openings (size of opening or F Number) directly effect the Depth of Field (DOF) in the scene. DOF is covered in a separate post and article. Numerically small F Numbers (F/1.4, F/2.0) decrease the DOF. Large F Numbers (F/11, F/16 etc.) increase the DOF. Faster shutters speeds will show less blur of moving subjects. The wings of a hummingbird are shown sharply with a 1/1000 of a second shutter speed.

Focal lengths and more this evening.