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|Apr 21, 2009, 11:51 AM||#1|
Join Date: Jan 2009
Does anyone know of a good link to a video clip that goes over the basics of selecting a starting point for ISO and shutter speed and how to adjust shutter speed and fstop for the best exposure balance? Preferably explaining lower light conditions. I know the basics of it, but never seem to get started correctly and end up with not enough light so I just switch to auto mode. Thanks.
|Apr 21, 2009, 12:28 PM||#2|
Join Date: Aug 2004
don't know of a video, but here's a write-up I put together to answer this question before:
Exposure is controlled by 3 variables:
1.Aperture – how much light gets to the image sensor
2.Shutter speed – how long the sensor is exposed to the light
3.ISO – how sensitive the sensor is to that light
Aperture – how wide the iris in the lens opens to let light in. Measured in f-stops. The f-stop is a ratio of the opening to the physical focal length of the lens. Because it's a ratio, if the focal length stays the same, and the opening gets wider, the ratio (or f-stop) gets smaller.
What you need to know off the bat is – the smaller the f-stop the wider the opening and thus the more light that gets in. There is another resulting affect – depth of field. The wider the aperture for a given focal length, the less of the image will be in focus. The narrower the aperture for a given focal length (i.e. the larger the f-stop) the more of the image will be in focus.
Read here to get an understanding of depth of field:
F-stops are measured in FULL STOP or PARTIAL STOPS (usually 1/3 stop).
Full stop values: F1.0, f1.4, f2.0, f2.8, f4.0, f5.6, f8.0, f11, f16, f22, f32
(there's a pattern that makes it easy to remember – every other stop is a doubling f1 – f2 – f4 – f8 – f16 vs. f1.4 – f2.8 – f5.6 – f11 – f22)
Shutter speed – how long the shutter stays open and light hits the sensor. It is measured in seconds. In most cases the shutter is open far less than a second – so the notation is typically 1/x. Shutter speed is also often referred to in STOPS. Full stop values are typically 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, 1/8000. Notice every STOP is a doubling. The camera is usually capable of changing shutter speeds in 1/3 stop increments though.
Shutter speed is important for sports shooting because it is shutter speed that determines whether or not you see motion blur. Different types of motion require different shutter speeds to completely freeze. A major league pitcher throwing 95mph requires a very different shutter speed to freeze than a jump shot in basketball.
In most cases, as a sports shooter, faster is better. There are exceptions, but in general you want faster shutter speeds. For many human sports, 1/500 is considered the minimal shutter speed you want to have. For softball, however 1/500 will show a lot of blur in pitching, hitting and throwing.
ISO – how sensitive the camera's sensor is to light. Also measured in full and 1/3 stops. Full stop values are 100, 200, 400, 800, 1600, 3200. Notice again every stop is a doubling of value. ISO 200 is twice as sensitive to light as ISO 100. The negative affect of higher ISO though is you get a lot of digital noise in the images.
Now for the fun part – how they all work together.
Think of proper exposure as the image looking good – not too dark, not too light. Let's say you take a photo that is properly exposed.
Let's say the 3 variables for that photo are: ISO 200, f8.0 and 1/250.
Now, those exact settings aren't essential for the proper exposure – what is essential is their RELATIVE value to each other.
You can change the sensitivity of the sensor (ISO) up or down. If you change it up without changing the other two values, the image will be too bright. If you change it down without changing the other two values the image will be too dark.
The same holds true if you change only aperture (fstop) or change only shutter speed.
BUT, if you change ISO UP and one or both of the other two variables DOWN you'll get the same exposure.
SO, original shot was ISO 200, f8.0 and 1/250
If you move ISO up 1 stop to 400, to get the exact same exposure you need to move one or both of the other 2 variables DOWN by a total of 1 stop.
So ISO 200, f8.0 and 1/250 has same exposure as ISO 400 f16 1/250 has the same exposure as ISO 400 f8.0 1/500
Similarly if I bump ISO up from 200 to 1600 (3 stops), I can change the other 2 values by a total of 3 stops. So I could gain 3 stops of shutter speed –
ISO 200 f8.0 1/250 has same exposure as ISO 1600 f8.0 1/2000
So, how do you decide which combination of these 3 variables you want?
Ah hah, that's the million dollar question - and no one can tell you what value to use
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