Photography Principles - 02 Exposure
Alright guys. I know you're dying for me to get to the good stuff about using your compact cam/camera phone to capture awesome photos, but for now you'll have to bear with a few more definitions.
These are words I'll be using in future when I start re-shooting the Photo Challenge assignments, so I'll be typing them here and referring back to this thread if need be.
I also need these to explain some of the effects I was asked about by molona and others in the General Chat thread.
Ready? Let's take a look.
EXPOSURE: This is the total amount of light allowed to fall on the film or sensor, during the process of recording a photograph. If you ever used a point-and-shooter film camera, and carried it to your one-hour photo store to develop, you may remember they referred to your pictures as "exposures". Well... That's why.
OVER-EXPOSURE: This is when too much light is allowed to fall onto the film or sensor to create a pleasing image. Over-exposed photographs will be brighter or look more "washed-out" than what you'd possibly want. At extremes, an over-exposed photograph would also start losing details in the brighter parts of the image.
UNDER-EXPOSURE: The opposite of "over-exposure". This is when too little light is allowed to fall onto the film or sensor to create a pleasing image. I use the phrase, "pleasing image", by the way, because there are times when the background or parts of the image may be over or under exposed, but the photograph as a whole still looks great or better than if you "got everything in". It becomes a judgement call.
Under-exposed photographs tend to be dark, and lose details in the darkest parts of the image.
An under-exposed image.
So, what do I mean by, "allowing light to fall on the sensor" ?
Well, there are two main ways to control that. Remember I said last week, in reply to kohoutek, that a camera is a light-tight box with a hole at one end? Well, the first way to control light is in how long that "hole" is allowed to remain open.
There is a mechanism that opens and closes this hole. That mechanism is called a "shutter".
The length of time the shutter is allowed to remain open is called the "shutterspeed". Usually, in most situations, shutterspeeds are in fractions of seconds; That is, when the room or location you are photographing in is well lit, You can expect speeds of 1/125 th of a second, maybe down to 1/60 th. In very bright light, you might get up to 1/1000 th of a second or higher! Think of that like the blink of an eye.
But that all depends on the second way of controlling light which is...
Aperture. This refers to how big the aforementioned "hole" is. (So it's; How big the hole is, and how long the hole is open for.)
The opening is measured in "f-numbers". The reason for that is technical and I never remember it myself (something to do with the ratio of the size of the opening to the focal length), but the important idea is a larger f-number means a smaller hole, and a smaller f-number means a wider hole. This may sound counter intuitive, but remember it's a ratio, so it's like a fraction; A fraction with a massive denominator is a tiny sliver.
In most situations, you may use an f-number around f/5.6 to f/8 . The actual size of that opening depends on the lens you're using (again, an f-number is a ratio; Calculated based on other dimentions of the lens).
An f-number of f/22 is considered "slow"; Meaning a very tiny hole, with very little light passing through, therefore often coupled with a longer shutterspeed to compensate.
An f-number of f/1.4 is very wide (maybe an inch or more wide) and is considered "fast". When you hear camera manufacturers referring to a "fast" lens, this is what they mean. An f-number of f/1.4 is good for low-light situations; Shooting in dark rooms or at nighttime outdoors under dim lights.
Have I lost you yet?
Don't worry. Play with it a little on your own cameras (whether SLR or compact cam set to Manual) and you'll get a feel for it. Turn the camera around and try to look at the lens if possible. If you're using a Canon Powershot in live-view, you may be able to actually see the display getting brighter and dimmer as you scroll through the f-number settings.
All of photography is basically a balancing act between shutterspeed and aperture.
When your camera is set to Auto or "Easy", the camera does this balancing act for you. That's fine for maybe 95% of the images you want to capture. If you want to really get creative and take control over how your images look though, you'll need to understand the principle behind it.
SENSITIVITY / ISO
There's a separate concept that affects exposure, but it's to be thought of as a last resort. That is, how sensitive the film or sensor is to light.
If more sensitive, then the less light required to create an exposure, and the faster a shutterspeed or smaller an aperture you can use. And the inverse is true.
Sensitivity is usually rated by "ISO".
ISO refers to the "International Organisation for Standards", which us webby people might recognise from,
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Back in film days, films were rated by their sensitivity according the ISO/ASA (American Standards Association) specifications. Lower numbers, eg. ISO 100, referred to less sensitive film, high numbers, eg. ISO 800, more sensitive.
When digital camera sensor developers began rating their own sensors, they adopted the same standards (de facto, I think) and rated sensitivity on a comparable scale to the ISO film standards. The awesome thing about digital, of course, is that you don't have to be stuck to one sensitivity until your film roll is done. With most cameras, you can freely go in and choose a different sensitivity as the need emerges.
So now you may be wonder why anyone would WANT a less sensitive sensor. Well, the problem is grain. Higher sensitivities are more receptive to light, but as an offshoot, are more grainy. By the time you reach ISO 1600 and higher, the quality of the image (with most sensors) just declines rapidly.
So I say again, ISO is to be thought of as a last resort. In general, one would try to use the lowest ISO possible, and slowly increment up the scale as the day progresses and light gets dimmer.
If you've ever looked at your night-time photos from your point and shooter or camera phone and wondered why they are so grainy, well, that's your answer. In those cases, the camera set on auto will choose a higher sensitivity/ISO to compensate.
DEPTH OF FIELD
The final definition for this week is Depth of Field, or the area in front of and behind the point of focus (or more correctly, "plane" of focus), that is acceptably sharp.
This region of sharpness is dependant on the aperture. A smaller opening/aperture, will result in a larger region of sharpness. A larger opening will result in a smaller region of sharpness.
Think of it like squinting your own eyes. When you squint, things appear sharper; Much for the same reason, in fact.
This is difficult to describe in words, so please have a look at the two images below for an example;
So that's all from me for this week. I hope it was a help, and sheds some light on what some of those camera advertisements mean.
Next week I'll be looking at focal length as I start shooting some of the Photo Challenge assignments.