BetterPhoto Q&A
Category: New Answers

F-Stop, ISO, Aperature

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November 13, 2007

For starters I don't know what kind of camera you have, so a term or two might be off in my explanation..
Might want to check your owner's manual for explanations specific to your camera type.

F-stop is the actual aperture (f) value, ie: f/20, would be a really narrow range of acceptable focus on your subject. (think of it like a cone either facing your camera or your subject) Large F/number is a small range of acceptable focus on your subject and vice versa- hence the cone facing one way or the other.. (sort of easy way to remember it)

As my Better Photo instructor explained it to me recently, (Thanks Charlotte!- Rebel XTI Class) the choice of Aperture, or F-stop, is the primary factor that affects the Depth of Field. A wide aperture, or a small F-stop number, gives shallow depth of field, blurring background elements so that they don't distract from the subject. A narrow aperture, or large F-stop number, gives an extensive depth of field so that elements in the image from front to back appear acceptbly sharp provided that you are reasonably far from the subject.

Affects will differ from camera to camera and between the different types of lenses used. You'll have to toy around with it yourself to get a handle on it.

"Acceptably" sharp focus is just what it says- not 100% sharp, but with details that are clearly discernable, and you determine the amount of acceptable focus by setting the aperture (f-stop).

ISO speed is the sensor's sensitivity to light to suit the ambient light level. When you increase the ISO speed (higher number) for low light (to increase the effective range of the flash), a faster shutter speed can be used and camera shake will be less prone to occur.

Remember though, that the higher the ISO speed the higher the noise level will be. Noise is graininess showing in the image.

As an example if you were shooting in a bright sunny area then you'd only use ISO 100/200 , overcast skies would be 400/800, night or dark conditions would be 1600.. See the trend?

But again you'll have to toy around with it to see what works for you and the effects you're trying to achieve.

Have fun, hope this helped!

You should think about a course through BP- they're really helpful.

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November 13, 2007

The camera is a darkened chamber (box) with a lens at one end and light sensitive media (chip) at the other. The job of the lens is to project an image of the outside world onto the surface of the chip. Between the lens and the chip is a doorway (shutter). Most of the time the shutter is closed making the inside of the box pitch black. When you press the go button (shutter) the shutter door opens briefly allowing a projected image to play on the surface of the chip. During this brief moment a picture is taken, the deed is called the exposure.

The chip inside the camera is persnickety. If the camera settings are OK the picture will be good. If the chip receives too much or too little light energy the picture will be bad. Getting the light energy correct is what camera settings are all about.

What must be taken into consideration?

ISO: We need to know how much light energy the chip needs. Sometime we call this the chip’s speed. We have appointed a scientific group to take on this task. ISO is an abbreviation for International Standards Organization. Both film and chips are rated as to their sensitively by this organization. Small numbers translate to low sensitivity whereas high numbers denote high sensitivity. Generally film and chips with low sensitivity provide higher quality images. Most camera chips are adjustable by the user. Typical values are 100 ISO – 200 ISO – 400 ISO – 800 ISO. Note each doubling of the ISO denotes that the media is two times (twice) as sensitive.

Shutter speed: We are using a unit of time you know as the second. This value tells us how long the shutter trap door will remain open for the exposure. The chip can accumulate light so if we allow light to play on the chip for a longer period of time, we can take pictures under dimmer condition. We generally set the camera shutter so it is only open for a fraction of a second. Typical shutter speeds available are: 1 – 1/2 – 1/4 – 1/8 – 1/15 – 1/30 – 1/60 – 1/125 – 1/250 – 1/500 – 1/1000. Note the sequence above starts at 1 full second and each subsequent charge is twice as quick meaning the shutter speed doubles (gets faster or quicker) with each faster increment. On this list, 1/1000 (one thousandth) is the fastest. Because we photographers tend to hold our camera unsteady, we generally work in the 1/60 ~ 1/1000 range.

f/number: A lens acts like a funnel gathering light. The larger the working diameter of the lens the more light it draw into the camera. More light entering translates to a bright projected image at the chip’s surface thus we can take pictures even under dim conditions. The light path inside the camera, lens-to-chip distance is important. If the travel length is far the lens magnifies (telephoto). If the light path is short the lens is a wide-angle. We must take both travel distance and diameter into consideration. This value is the lens focal length. To figure out how much light the lens will gather, we divide the focal length by the working diameter. When we divide we get a value known as the focal ratio and we write it abbreviate f/8, this is the f/number. f/nubers are important as the are universal meaning any camera set to the same f/number has the same image brightness at the chip’s surface regardless of lens diameter or focal length in use. The f/number is sometimes called f/stop or stop. When you change f/umbers you are changing the working lens diameter.
The number set most often used:
1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22 -32. This is an odd looking set of numbers with f/1 being the biggest diameter. f/1 is very fast meaning the image projected on the chip will be bright. f/32 is produced by a tiny lens diameter resulting in a dim projected image. The number set above is based on the geometry of a circle. The idea is to allow the photographer to adjust lens diameter to image brightness at the surface of the chip. Significant: Each value going right cuts down the light by half. Each value going left doubles the light energy received by the chip. Again the increment of adjustment is a doubling or halving of the light energy reaching the chip. This method seems somewhat primitive and confusing. We have tried and failed to come up with a more simplified method. Perhaps if you study the problem, you will be the one that shows us how to present these facts in a better light.

Alan Marcus (dispenses marginal technical gobbledygook)
ammarcus@earthlink.net

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November 14, 2007

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November 14, 2007