Archive for stops

Stops

Posted in Tutorials with tags , , , , on October 1, 2008 by thtroyer

It’s time to start tackling exposure, and you’ll need to understand what a stop is.

A stop is a dimensionless measurement of light — it needs a reference to be quantified. Stops are most often used to measure relative brightness (“this is 2 stops brighter than that”) and to describe exposures of photos (“that photo was underexposed 1 stop”). The easiest way to see it work is in examples.

So, lets say I have a candle in a dark room. If I add another candle, I would double the light in the room. Doubling or halving the light creates a 1 stop difference.

1 – 2 – 4 – 8 – 16 – 32 – 64 – 128 – 256 – 512 – 1024

If those numbers represented candles in a room, the difference between each number is 1 stop, regardless of where we started and which direction we went.

One stop brighter = light doubled.
One stop darker = light halved.

The shutter is used to allow light through at a certain interval.  On manual cameras, it is often between 1 second to 1/1000th second, and is laid out something like this:

1 – 2 – 4 – 8 – 15 – 30 – 60 – 125 – 250 – 500 – 1000

There is approximately one stop between each number.

Now, you have more control over exposure than just the shutter.  This is where the aperture comes in. I mentioned it in the Depth of Field article. It’s primary purpose is to block light to help one get a correct exposure. The effect on DoF is secondary.

In any case, the aperture value is a ratio of focal length over aperture diameter and represented as f-numbers. The actual size of the aperture isn’t important, but the ratio is — that’s what the f-number is. Important to note: the f-number is a fraction, the smaller the bottom number, the larger the aperture and the more light that is let in. So, f/1.4 is brighter than f/2.8.

The f-numbers on lenses are often arranged in a stop pattern to make them easy to use and understand:
1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16

The numbers seem arbitrary — don’t worry about them right now. More importantly is the concept of an f-stop, in that between each of the above numbers is a difference of one stop of light. When dealing with the aperture altering the light one stop, it is called an f-stop. So, the difference between f/4 and f/5.6 is one f-stop.

Finally, there’s one last common variable beyond shutter speed and aperture.  That’s the ISO, or sensitivity of the medium you’re using.  A digital camera can change ISO on the fly, making its sensor more or less sensitive as appropriate.  On a film camera, the film type dictates speed.  Common modern film speeds vary from 50 to 3200 (consumer film is usually between 100 and 400).  The scale looks like this:

50 – 100 – 200 – 400 – 800 – 1600 – 3200

Again, there is one stop difference between them all, meaning that 200 is twice as sensitive to light than 100 (one stop).  1600 is 16 times as sensitive as 100 (4 stops).

There are definite advantages in picking different film speeds.  With faster speed often comes greater grain (noise on digital) and slower speeds capable of higher resolution.  Faster speeds have obvious advantages in low-light, but slow speeds have advantages when wanting a limited depth of field.  There is no obvious hands-down winner because they are all useful for different applications.

Just this morning I just unloaded a roll of Kodak Tri-X I pushed to 3200 to shoot freehand at a night festival and loaded Fuji Velvia 50 that can require a tripod in daylight!

All sorts of combinations can be used to get a proper exposure:
– f/1.4, 1/1000 sec, ISO 400
– f/2, 1/500, ISO 400
– f/2, 1/250, ISO 200
– f/8, 1/60, ISO 800

These would all provide the same exposure given the same scene.  They would not all look the same, given that there are consequences of changing these variables, but the point is, if one was exposed properly, they would all be exposed properly.

Faster (larger) apertures = less depth of field
Slower (smaller) apertures = more depth of field

Faster shutter speeds = more motion stopped
Slower shutter speeds = potential blur from subject/background movement, handshake, camera vibrations

Fast ISO = more grain
Slow ISO = less grain

All of these things can be used creatively — depth of field, grain, blur. However, understanding stops is fundamental to predicting and being able to consistently get the shots you want.

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