Full stop!



Historically, the unit of exposure in photography has always been measured in "stops". Stops can be set for the lens, called f/stops; this is done by adjusting the aperture. But stops can also be controlled in camera by adjusting shutter speeds. Both f/stops of the lens and shutter are interrelated and represent an identical effect on exposure.


Shutter Speed Stops

Imagine your camera has adjustable shutter speeds in one stop increments (instead of 1/3). Each click of the dial going upwards (faster) is twice the value of the one before it (e.g. 1/125, 1/250, 1/500 and so on) and moving downwards (slower) is half the value of the one before (e.g. 1/2000, 1/1000, 1/500 and so on).


What may not be explained fully is that 1/8000 represents a fraction of a sec. In this case it is one eight thousandth of a second. As we move down through the scale (1/4000, 1/2000, 1/1000 and so on), we begin to slow the shutter speed down until it reaches full seconds (e.g. 1", 2", 3" and so on).


Each of these individual steps from fastest to slowest is known in photographic parlance as "stops" and it represents doubling the amount of time that the light is affecting the cameras digital sensor.


Aperture Stops

An aperture stop (or f/stop) is something that often confuses novice photographers. It is important to note that aperture f/stop notation is fractional and is actually expressed as f/4 or f/11 etc. The "f" in f/stop actually represents focal length of the lens, usually expressed in millimeters.

For instance, if we are using a lens with 80mm focal length and have the aperture set to f/8, this simply means that the opening in the lens (the aperture) is 10mm (or "f" divided by 8 = 10mm in diameter).


The sting in the tail is that on the surface it looks like changing the aperture to say from f/8 to f/4, which doubles the size of the aperture would let in twice as much light, but it actually lets in four times as much. This is because the area of the circular opening of the aperture is actually quadrupled (x4). Or for you maths buffs out there:

- Pi times the diameter squared and divided by four.

If you are beginning to panic, don't. You don't need to remember formulas, you just need to understand the basic concept.


From the largest f/stop value of f/128 (smallest opening to the sensor) to smallest f/stop value of f/0.7 and beyond (largest opening to the sensor) in full stops looks a little like this: F/128, f/90, f/64, f/32, f/22, f/16, f/11, f/8, f/4, f/2.8, f/2, f/1.4, f/1, f/0.7


A mistake made by many is to forget that the f/stop is a mathematical fraction and instinctively assume that the smaller the f/stop value, the smaller the aperture, but in fact the opposite it true. I hope you can see that as the aperture f/stop numbers get smaller, the opening of the aperture itself gets better and vice versa.


ISO

ISO ("International Standards Organisation") has its humble beginnings when photographic film was given a rating of sensitivity to light (the lowest rating being less sensitive to light). In that respect, ISO is seen as a camera's sensitivity control, whereby the camera sensitivity to light can be adjusted to suit the lighting conditions. Low ISOs (like 100) yield the best quality, where higher ISOs contribute to a grainy image.

Each doubling of the ISO value corresponds to an increase in the digital sensor's light sensitivity of one stop. For example an ISO 200 setting will give you one stop more exposure than an ISO 100 setting. What does all this mean? I'll explain in my next post, but for now all you need to retain is that the higher the ISO number, the grainier the image will be (specifically where there's low lighting conditions).


My next post will cover Shutter Speeds, Aperture and ISO relationships.


Happy snapping!


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Tel: +64 27 5284566

eMail: Kathrynnobbsphotography@gmail.com

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