Shutter Speed

How does shutter speed relate to photographic exposure?

Shutter speed is the easier of the two main factors to consider. Simply put shutter speed is how long the camera is open to light. The faster the shutter speed the less light let in and the slower the shutter speed the more light let in (of course, all else being the same).

Now when converting to stops of light it is much easier twice or half the shutter speed is a stop difference.

However, shutter speed plays an important roll. The faster your subject is moving the faster shutter speed you need to freeze the subject in the photograph, otherwise the subject will blur (which may be an effect you want). Another time you want faster shutter speeds is when you use longer focal length lenses (that is lenses that bring distant subject closer to you). Longer focal length lenses shorten the apparent distance between you and your subject, but they also magnify shakes and other movement. The rule of thumb to remember is for instance, if you are using a 200 mm lens you need to shoot at 1/200th of a second or faster to hand hold.

When you use slower shutter speeds you can use smaller apertures thereby increasing your depth of field.

Next installment putting it all together.

Exposure Control - The F/Stop

Ever notice on SLRs and higher end digital cameras labels or numbers like this: f/2.8, f/3.2, f/4.5, f/5.... Those are called f-stops and give an indication of how wide open the lens is. The smaller the number the larger the opening. a thorough discussion of the f-stops is here.

The smaller the f-stop the more light reaches the photographic plane (film, chip, etc) and vice-versa the larger the number the smaller the opening the less light that pass through the lens to form your photograph.

Wikipedia provides the following chart:

f/2.8

f/4

f/5.6

f/8

f/11

f/16

f/22

f/32

provided equal shutter speeds each step to the right is 2x the light (or +1 stop) and a step to the left is 1/2 the light (-1 stop).

So, given the same shutter speed we now have a way to increase or decrease the exposure.

Another interesting consequence of f/stops is called depth of field. Depth of field is the amount of +/- distance of error you have when focusing. The more depth of field the more room for error you have in focusing (very helpful for moving subjects). However, depth of field comes with a cost and in order to get more depth of field you must use larger f/stops which often means slower shutter speeds, still with auto-focusing cameras being so prevalent now depth of field is less important.

Next lesson shutter speed.

Exposure Control Lesson 1 - The Basics

Every now and then I critique photographs usually on the technicalities, as that is where most photographs fall down on.

The #1 problem people have with photographs is exposure. What is exposure? Exposure the amount of light allowed to fall onto the development plane (whether it be old-fashioned film or a charge-couple-device ie. digital).

Cameras control exposure by a combination of two different mechanisms. The first is by shutter speed. The faster the shutter speed the less light there is and of course the opposite holds the more time the shutter is open the more light the greater the exposure. The second mechanism is by aperture that is how much the lens is opened up.

The combination of the aperture and shutter speed gives the total exposure. A common term associated with exposure is stop which is a relative measure of light. One stop is twice or half the light. To increase exposure by one stop you can double your shutter speed or increase the aperture in such a way to double the light admitted. Let us say the radius of your lens is opened to 1" (for instance) and you opened it up to 2" note your exposure increases by four (the old pi-r-squared formula from your geometry lessons).

Next lesson the f-stop.