Friday, July 9, 2010

Elements of Photographic Exposure (it's simple, have fun!)

In the world of photography, exposure stands for the amount of light collected by the light sensitive material of the camera (film or digital sensor). The same term is also used as a synonym for the words picture, image, shot (eg. “I shot multiple exposures” mean “I shot multiple pictures”). In this article when I use the word exposure I’ll be referring to the amount of light collected by the digital sensor.

So, exposure is controlled by (only) 3 factors:
1. ISO sensitivity
2. Aperture
3. Shutter speed
These factors work in conjunction to form the final exposure.

1. ISO sensitivity: In the old film days (when the grass was greener and you could still hear the birds singing), each film loaded into the camera had a fixed ISO sensitivity. In the digital era, you change the sensor’s sensitivity by altering the sensor’s amp circuit (boosting a chip). But how does ISO affects exposure? Well, it’s quite simple actually; the higher the ISO value, the more sensitive the sensor is to the light. In other words, higher ISO’s values require less amount of light to be capture by the sensor. Straightforward ha? When you need more light (eg. inside a church) you increase the ISO value, when you need less light (eg. outside the church), you set a lower ISO value.
Side effects: By now you should be wondering why the hell not to set a high ISO value all the time and get away with. Well, there are some drawbacks. Best image quality is recorded at the base ISO. As you increase the ISO the image quality starts to decrease. With film, higher ISO values produce more grain. With digital, higher ISO values produce more digital noise (artifacts). Please note that digital noise is not as pleasing as grain (grain is perceived by the eye as “more natural”. Sometimes we may even want to add some amount of grain to our black & white images, in order to reproduce the feeling/look of film).  
The amount of noise each digital sensor produce varies. In general, the larger the sensor’s size, the lower the noise it produces at a given value (eg. full-frame cameras tend to produce less noise than crop-factor cameras). Also the image processing engine of the camera plays a role in noise handling. For instance, when Nikon D90 came out, its EXPEED processor proved to have better ISO handling performance than Nikon’s more professional model D300. So as the technology keeps moving forward, cameras are gradually becoming better in such issues.   
Another drawback of high ISO values is that the sensor’s dynamic range is reduced (in other words, you may not be able to capture the scene’s dynamic range as you would with a lower ISO value), colors are shifted and contrast is increased. So, what is the higher acceptable ISO value you can assign? In terms of specific numbers that depends on the camera model you are using (eg. Nikon D3s is an exceptional ISO handler). In terms of a general principle, it’s up to the point above from which noise is dominating true image detail.    
The ISO scale, in terms of full stops (each stop doubles the amount of light):
50, 100, 200, 400, 800, 1600, 3200, 6400, 12800, (25600, 51200, 102400)
(The above values correspond to the values available at the moment this article in being written. Depending on the camera model you are using, the limits of the above scale may vary).

2. Aperture: This is simply the hole (opening) of the lens (which is not constant and it’s controlled by the user). Bigger openings let more light get into the sensor, thus allowing faster shutter speeds and vice versa (this is the reason why lenses with wide apertures, eg. f1.4, f2.8, are called “fast lenses”).  The Aperture scale, in terms of full stops follows this sequence:
f: 1.4,  2,  2.8,  4,  5.6,  8,  11,  16,  22,  32
(In order to remember the scale you can just memorize the first two numbers and that each number doubles every two. Notice! the above pattern is only a way to memorize the scale. Remember that every next number doubles the amount of light reaching the sensor. Also note that smaller numbers represent bigger openings: eg. f1.4 lets in double the amount of light than f2 and four times the amount of light than f4).  

3. Shutter speed: The heart of every digital camera is the sensor which collects the light to form an exposure. The sensor is not always exposed to light. In front of the sensor is located a “devise” called the shutter. You can imagine it as a window with a number of shutters! (although we call it “the” shutter). Every time you press the button to take a picture (called the shutter released) the shutter is being released for a specific amount of time in order to let the light reach the sensor. Longer exposures (the time the shutter remains open) let more light hit the sensor, than shorter exposures. The Shutter Speed scale, in terms of full stops follows this sequence:
30”, 15”, 8”, 4”, 2”, 1”, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, 1/8000.   
(Most cameras today have a highest shutter speed of 1/8000 of a second and a slowest speed of 30 seconds. If you want to set a slower speed than 30”, you have to set the camera in “bulb” mode. In bulb position the camera opens the shutter as soon as you press the shutter-release button and keep it open until you press it again. In the above scale note that each time you lower the shutter speed to the half, you lower the amount of light by half as well and vice versa).    

Putting it all together: With that being said we have to see how in practice we combine Aperture, Shutter Speed & ISO to come up with the desired exposure (I intentionally avoid using the word “correct” because exposure is a subject of aesthetics and personal interpretation of the scene). For any given exposure there are several combinations of the above factors that form it. For example A: ISO 100  f:8  1/125 will result to an image that has the same exposure as B: ISO 100  f:11  1/60, or C: ISO 100  f:5,6  1/250, or D: ISO 400  f:11  1/250, Don’t panic, I’ll make it clear to you right away!
Combination B: I closed the Aperture from f8 (combination A) to f11, in order to get greater depth of field, but in terms of exposure, my action results in half the amount the light hitting the sensor (1 stop). So, in order to keep the exposure constant between the two combinations I doubled the amount of light by reducing the shutter speed by 1 stop (from 1/125 to 1/60).
Combination C: I opened the Aperture from f8 (combination A) to f5.6, in order to get more swallow depth of field, but in terms of exposure, my action results in double the amount the light hitting the sensor (1 stop). So, in order to keep the exposure constant between the two combinations I reduced by half the amount of light by increasing the shutter speed by 1 stop (from 1/125 to 1/60).       
Combination D: In order to get more depth of field and retain the same shutter speed as combination C, I increased the aperture by 2 stops (from f5,6 to f11) and thus I also doubled the ISO value (from 100 to 400) to keep the exposure constant.  

Exposure is controlled by (only) 3 factors: ISO sensitivity, Aperture, Shutter speed. The combination of these elements affects the final exposure. Although it may seems difficult at the beginning, it’s not! Move straight away from “Auto” and start becoming more creative (that’s why you bought a DSLR in the first place)!
 Upcoming articles will deal with two related subjects: exposure metering and exposure modes.
Live well & take beautiful pictures!

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