Exposure Basics

The next exercise in the Lighting 102 course deals with balancing flash exposure with twilight.  Completing this exercise has been difficult for me because of the time of year.  We are nearing the winter solstice and where I live that means that twilight is around 4 in the afternoon.  Since I work a day job, that happens to support my photography hobby, the time of day the sun sets limits me to working on the weekends.  This is my long-winded way of saying that I can’t write a post about my progress through the next Lighting 102 exercise because I haven’t actually done it.  I also thought that before I write a post about balancing flash exposure with ambient light exposure that I should give a quick overview of exposure in general. 

When I began this blog a few months ago, I had told myself that I wouldn’t write a post about exposure.  It’s not that exposure is a complicated concept but it takes a while to discuss and there are literally hundreds of books on the subject.  I convinced myself that there wasn’t a need to repeat a subject that has been so thoroughly covered by others.  I have since thought differently but today’s post will only be a little review on exposure and will be in layman's terms. 

Simply said, exposure is the quantity of light captured on your camera’s sensor (or film if you’re old school).  Regardless of the type of camera, only three things determine exposure: Aperture, Shutter Speed and Sensitivity.  If you understand these you can say good-bye to full-auto modes and "creative modes" forever. 

Aperture

Aperture is the opening in a lens that allows light to enter the camera.  Aperture works much like the iris (colored portion) of an eye.  As an exercise, stand in front of a bathroom mirror and stare at one of your eyes. Then cycle the bathroom light on and off and watch how the iris expands and contracts as it adjusts to the changing light. Camera lenses do the same thing except instead of a pretty iris they have a series of fins, or the diaphragm, to restrict the incoming light.  If you have an SLR with a “depth of field preview” button you can see the lens’ diaphragm by looking through the front of your lens and hitting the button (as long as your aperture isn’t set to the lens’ maximum). *  Below is a series of shots to illustrate.

 

f/2.8                                    f/5.6                                       f/11

Depth of Field Preview Button

Aperture is expressed in f-stops and they are fractions. Therefore, f/2.8 is a larger aperture than f/11.  A larger aperture (smaller number) will allow more light to enter the camera than a smaller aperture (larger number).  Aperture affects more than just the amount of light entering the camera but I'll discuss that later in this post.

*The “depth of field preview” button is on the front of the camera to the right of the lens just under the lens lock release button (at least on my Canon).

Shutter Speed

Cameras have a shutter made up of multiple little doors. The shutter is opened for a given amount of time and allows the light to expose the camera sensor (or film).  You can think of the camera’s shutter as your eye lids if you’re like me and love analogies.  On a SLR you can see the shutter by taking off your lens and lifting up the pentamirror or pentaprism.  If you choose to do this just be careful, that’s a sensitive piece of machinery that you’re sticking your finger into.  If you don't know what a pentamirror or pentaprism is, I don't suggest doing this at all, just trust me that the shutter is made up of multiple pieces. 

Shutter speeds are measured in seconds or fractions of seconds.  Longer shutter speeds allow more light in and shorter speeds less light.  The important thing to remember is that shutter speeds, much like aperture, are also expressed in “stops”.  I’ll get into why after I get through a little sensitivity training.

Sensitivity

Remember this stuff? (wiki)

Sensitivity is measured in ISO.  The sensitivity of a digital camera’s sensor is adjustable.  In contrast, a roll of film has a set ISO sensitivity rating.  On film cameras the only way to adjust the ISO is to remove the roll of film and load another with the desired ISO rating.  ISO actually stands for International Organization for Standardization, (I figure that if I took the time to look that up on Wikipedia I might as well share it) and is a way to measure how much light a type of film needs to become exposed. When digital cameras were developed someone smart decided to measure the digital sensor sensitivity using the same system.  ISO is measured in whole numbers, finally whole numbers!  What’s even better is that a lower ISO value is less sensitive than a higher ISO number so it’s intuitive.  This means for instance that ISO 100 needs more light to get proper exposure than ISO 400.  You also may have guessed that ISO, just like aperture and shutter speed, is expressed in “stops”.

Stops

The reason Aperture, Shutter Speed, and ISO are all expressed in stops is because each stop affects the exposure equally.  This means that opening up the aperture by one stop effects the exposure the exact same as slowing down the shutter speed by one stop.  Below is a table of full stops for all three:

Aperture (f-stop)	Shutter Speed	ISO
f/2.8	1/1000	50
f/4	1/500	100
f/5.6	1/250	200
f/8	1/125	400
f/11	1/60	800
f/16	1/30	1600
f/22	1/15	3200

Moving down the table each f-stop allows exactly half as much light into the camera as the previous setting, each shutter stop allows twice as much light, and each ISO setting needs twice as much light for a given exposure.  For instance, if the correct exposure in a given situation is f/5.6 with a shutter speed at 1/125 and an ISO of 200 and you wanted to open up the aperture to f/4 (1 stop), what could you do to get the correct exposure?  If you only opened up the aperture, the photo would be one stop over-exposed so to correct it you could either speed up the shutter by 1-stop (1/250) or decrease the ISO by 1-stop (ISO 100).

f/5.6, 1/125, ISO 200 = f/4, 1/250, ISO 200

and

f/5.6, 1/125, ISO 200 = f/4, 1/125, ISO 100

Dave this is so confusing, why can’t I just leave my camera in auto mode and let it figure it out?  The camera’s auto modes stink because they doesn’t allow any changes to the camera's built in functions.  I compare it to forever driving with an instructor, sitting shotgun that has his or her own brake and tells you where to go.  I’d say get the f-stop outta my car!  Digital cameras have sophisticated built-in light meters built and they should be taken advantage of but not in the auto mode or any of the "creative" modes!  Those darn “creative modes” drive me just as crazy!  Most digital SLRs (DSLR), however, have three semi-auto modes that are great to use. 

Canon EOS shooting modes (wiki)

In Canon world, the "semi-auto" modes are P, Av, and Tv.  These are Program, Aperture priority and Shutter priority (T for time, I guess), respectively.  All three allow you to use all the built-in functions of your camera that you paid good money to have: White Balance adjustment, Picture Style, ISO adjustment, focus point selection, etc. and even whether or not you want the built-in flash to fire.

Program

In Program once you depress the shutter button half way it will select an aperture, a shutter speed and ISO (if ISO is set to auto) that will produce correct exposure. Then by turning the selector wheel you can scroll through aperture/shutter speed combinations that will produce a proper exposure.  The disadvantage to Program mode is that if you want to keep shooting in a specific aperture or shutter speed you need to crank on that wheel each time you take a shot.

Av
Aperture also affects a photo’s depth of field.  In laymen’s terms this is how much of the photo is in focus and how much of the photo is out of focus.  As a rule of thumb for portrait work you want to shoot with a large aperture (small number) to blur the background and make your subject stand out.  For landscape work you want the entire scene (foreground and background) in focus and by “stopping down” your aperture (larger number) you increase the depth of field to accomplish this.  To shoot with a consistent aperture, use Av mode.  In Av mode the camera will adjust the shutter speed as the light changes to give you correct exposure without changing the aperture.

Tv
Shutter priority (Tv) works much the same way as Av mode, except you set a shutter speed and the camera adjusts the aperture and ISO (if ISO is set to auto).  For instance, this mode is helpful when shooting sporting events.  In this situation you know you want a fairly fast shutter speed to freeze the motion but you don’t want to keep adjusting the aperture as the amount of light changes.

Helpful Tip:  As a rule of thumb the inverse of your focal length is the slowest shutter speed recommended if the camera is hand held.  In layman's terms, with a 50mm lens one can shoot at 1/50 without worrying too much about camera shake.  With a 250mm lens that shutter speed drops down to 1/250.  The more zoomed in the lens is the more susceptible it is to camera shake.  If you have a steady hand, or your lens has vibration reduction (VR-Nikon) or image stabilization (IS-Canon) built in, you can slow up that shutter speed slightly.  Shooting with shutter speeds largely slower than the inverse would require a tripod, or other means, to steady the camera.

Manual Mode (M)

DSLR’s also have a Manual mode.  In this mode the user manually sets the desired aperture and shutter speed.  All of my photos that I’ve taken for the Lighting 102 course have been shot in Manual (M) mode.  Shooting in Manual does not somehow make one photographer superior than another so I don’t say this to be egotistical.  I shoot in Manual for this class because some of these exercises require that photos be taken purposely under- or over-exposed.  Also while I am learning off-flash photography I need to be able to control everything and modify one thing at a time until I get my desired exposure.  However, I will take a test shot in Program priority to take advantage of the light meter and get me in the ballpark of where I need to be for an ambient light exposure.

ISO

Most DSLRs have the ability to set the ISO setting in either Auto or a specific ISO setting (200, 400, etc) in any of the semi-auto modes or manual mode.  In Auto, the camera will set the lowest ISO possible to achieve proper exposure.  As another rule of thumb, the lowest possible ISO should be used for any shot.  The lower the ISO the better quality the image will have.  Quality cameras will have great photo quality up to ISO 400 or even 800.  Beyond ISO 800 and image degradation will begin to be noticeable, but don't be afraid to crank up the ISO if you are shooting in very low light situations. A grainy photo (loss of image quality due to high ISO) is better than a fuzzy out-of-focus one. 

If this hasn’t confused you enough in the next post I’ll be discussing multiple exposures which complicates matters even further.  In flash photography not only do you have the ambient light exposure but you also have an exposure for each of the flashes used.  It’s madness but its so much fun!!!

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