Following on from my previous post where we talked about the parameters controlling an exposure... the immediate question that arises is.. ok.. so now I know aperture controls depth of field, shutter speed controls movement, and ISO the sensitivity of the film/sensor... how do I know at what to set each of these in any given situation..?
The answer is not an easy one to give because of the millions of variables that exist both in subject matter and light conditions... but setting aside for a moment any “artistic” requirements... and assuming we just want a “correct” exposure.. ie.. an even balance between highlights, mid-tones and shadows.. the question becomes a little easier to answer... and once we can get our heads around that... then we can start down the “artistic” effect road..
The first decision we have to make in any situation is what to set our ISO to... remembering our previous discussion..
lower ISO = lower sensitivity to light but better quality/detail, and
higher ISO = higher sensitivity to light but lower quality/detail...
The next decision (at what to set aperture & shutter speed) is governed by the subject matter we are shooting... let me try to simplify this completely for you into 3 basic categories...
There are of course many other categories of shot, but for the purposes of this tutorial I will use the above 3 to illustrate the decision making process for exposure settings... and you can then transfer that on to all the other situations...
Landscapes - are for the most part static (some exceptions being.. water, clouds, trees in wind etc.... but more on that in a follow up post) and for the most part we would also want the maximum possible depth of field, ie from foreground to horizon in sharp focus.. so following our ISO decision... we need to decide at what to set our Aperture to as priority over shutter speed;
Portraits - are almost static and for the most part we want the minimum depth of field to focus viewers attention on the eyes of our subject... so again following on from our ISO decision... we need to decide at what to set our Aperture to as priority over shutter speed;
Action - by definition is never static so no matter whether we wish to either freeze it or give some blur effect to show the movement, then following on from our ISO decision... we need to decide at what to set our Shutter Speed to as priority over aperture;
So that basically defines our decision making process...
- how much light is there?... set our ISO;
- what is it that we’re shooting?... prioritize either shutter speed or Aperture, then set the other to balance;
To help us know at which f/stop to set our apertures or at which fraction of a second to set our shutter speeds at any given ISO... we can use any one of several tools at our disposal...
The first is a light meter.. a small handheld gadget that can read light levels and convert them into photo-speak for us.. ie.. we tell the gadget what ISO we’d like to shoot at and it tells us what to set our aperture and shutter speed to... and by changing any one of our 3 parameters it will immediately make the necessary calculations and adjust the other 2...
The second is the camera’s internal light meter which does almost the same as a handheld meter only it is more immediate as it is “in-camera” and any adjustments you make are therefore immediate..
Important note: Whilst both of the above can be set to make:
“weighted average” readings... an average reading over the entire scene in front of you,
“spot” readings...reading of light on a particular part or point in the scene in front of you,
...the important difference to note between the above two is in the “kind” of light they both can read...!
...and it is important to understand that for the purposes of metering an exposure there are actually two types of light..!
- the first is called Incident Light... this is the light falling on to a subject direct from the source of that light;
- the second is called Reflected Light... which, as the name implies, is the light being reflected by our subject;
The difference between a handheld meter and the camera’s built in meter is that the handheld can read both Incident & Reflected Light but the camera will only read Reflected light... and as a rule... Incident Light readings are always more accurate.
The third tool, and by a long way the most commonly used tool.. is our digital camera’s back screen... ie.. we guestimate an exposure.. take a shot.. look at the little screen and make adjustments accordingly... by far and away the least accurate method of shooting... unless you also go to the effort of pressing the “info” button.. if your camera has one... and examining the histogram.. which you also have to know how to read. It is a useful tool though to help you know you’re on the right track but shouldn’t necessarily be relied upon..
The fourth tool... is yourself... the more you shoot and the more you shoot in different conditions and the more you shoot different subjects.. the greater your knowledge will become and you will find yourself setting exposures automatically.
There are also a few little tricks such as “the sunny 16 rule”... which states that... on a bright sunny day... at any given ISO... at f/16... your shutter speed should be equal to (or near to) your ISO... and you’ll get as near as damn it an accurate exposure...
eg. at ISO 200 at f/16 your shutter speed should be 200 or 250th /second... and you can make adjustments from there to suit your subject... so if you wanted to shoot at f/5.6 you’d re-adjust your shutter speed also by 3 stops up to 2000th/second...
In the same way as for the sunny 16 rule you can also set your exposures as the nearest reciprocal of the ISO and aperture according to the following table which you might find useful to print out and have in your pocket...
Aperture Lighting Conditions Shadow Detail
f/22 Snow/Sand Dark with sharp edges
f/16 Sunny Distinct
f/11 Slight Overcast Soft around edges
f/8 Overcast Barely visible
f/5.6 Heavy Overcast No shadows
f/4 Open Shade/Sunset No shadows
So having gotten our heads around the decision making process in setting our exposures we need to learn which apertures, shutter speeds and ISOs will give us the effects we desire to achieve for our images...
...what aperture will throw the background out of focus enough to give us a nice portrait... or keep enough depth of field to have from near-ground to horizon in focus? what shutter speed will freeze a bird in flight.. or blur the movement of a river..? what ISO do we set for for a sunrise.. or for photographing a small bird under a dark canopy of trees..?
All good questions..! and I’ll set up a few scenarios to illustrate in my next post....
Happy shooting... and if you have any questions please feel free to leave a comment or send an email and I’ll do my best to answer it for you...