getting the exposure right

Getting The Exposure Right

Photography is nothing more than recording light and what it is doing. A painter uses paints to express their vision, and a composer uses music. As a photographer, your medium of expression is light.
Cameras, lenses, filters, lights and various other pieces of equipment are all only tools used to help you achieve your capture of light in the fashion you want to. So to that end, and understanding of how light works and how the different tools can be used to modify it to your vision is essential to the successful completion of your art. The following is an overview of different tools and how to use them.

Lenses and Light

The single most important piece of equipment after the camera itself is the lens. Without a lens, the camera is useless. So what exactly does a lens do? In the most basic terms a lens takes the random light flying around all over the place and directs it onto the film or sensor in a focused manor that can be then recorded. Light bounces off objects as it hits them in a very random fashion, think of a water balloon hitting something and breaking open with water splashing in all directions. That is how light behaves when it strikes an object. A lens will take those multi directional rays and bend them to be of a constant direction in relation to the curvature of the lens. Stacking more lenses in series will modify the direction of the light further and the end result is a sharp in focus image for your camera to record.

Components of a lens

The overall construction of a camera lens is rather basic. It consists of a barrel that is the housing for everything, the lenses, an iris and a mechanism to move the lenses forward and back to achieve focus.
The lenses can be rather simple groupings with as few as two lenses or as many as 15 (the most I’ve personally heard of). The main elements are the front and rear lenses. The front lens has the job of gathering in the light and directing it into the barrel and the other elements inside. The rear lens is the one that takes the light and actually projects it onto the film or sensor plane.
The brightness of a lens is measured in its “f” ratio. The closer the f ratio is to 1, the brighter the lens. The f- ratio of a lens is determined by two factors, those are the focal length (fl) and the diameter (d) of the lens. The f- ratio is figured out with the equation: f l/d=f -ratio. So a lens with a focal length of 50mm and a diameter of 10mm will have a f ratio of f5. The same lens focal length of 50mm but on a lens with a 20mm diameter will have a value of f2.5, So for a lens with a 10mm focal length to have the same f2.5 ratio as the 50mm lens, to only needs to have a diameter of 4mm. Both lenses even though they are different diameters will transmit the same level of brightness through them due to the difference in focal length. This is why different elements within a lens grouping do not have to all be the same diameter to achieve the same f -ratio.
The next piece of the equation is the iris. The iris consists of thin metal pieces that are interlocked to close down or open up to create a variable opening for light to pass through. What this effectively does is change the diameter of the lens in front of it. A lens with a now much smaller diameter with changing its focal length now has a lower f ratio. This is where the f stop numbers come from, the ratio that is now created from the new smaller or larger apparent lens diameter.

Fixed Focal vs Zoom

Long before we had zoom lenses, we were stuck with having to use a wide variety of different fixed focal length lenses. The early zooms were bulky, slow and lacked the sharpness of a fixed. Modern optics and construction materials have greatly lightened the burden of the zoom as well as increasing its ability to deliver a sharp image.
A zoom lens operates by using elements that effectively change the focal length of the lens within the barrel. By moving elements back and forth you are able to change the point where light interacts with certain elements thus shortening or lengthening the focal length of the overall lens (you can not change the focal length of an individual lens without altering it’s shape).
Benefits of a zoom: With a variable focal length you are able to utilize different aspects such as compression and composition without changing or needing multiple other lenses.
Disadvantage: Due to more elements needed, image clarity does suffer to an extent. Zoom lenses are also traditionally slower than their fixed counterparts. High end glass for a zoom does get very pricey.
Benefits of a Fixed: Faster and sharper than their zoom brethren. If you want razor sharp images and shallow DOF, these are the best and usually cheaper than a zoom with the same f ratio.
Disadvantages of a Fixed: You will need to have more lenses to cover the range usually needed for work. The price advantage is quickly eaten up by volume.

Best scenario: Have a good set of zooms that cover the ranges you use most. A 24-70 f2.8 and a 20-200 f2.8 is a great set. If you have a certain focal length you prefer for a higher percentage of images you shoot (say you like a 135mm for portraits) then get a fixed lens with a good speed to it, say f1.8, to use for this work.
The other consideration is the glass itself. Fluoride glass is a low dispersion glass that produces excellent image quality with lighter weight than its regular quartz glass cousins. It is a much higher priced commodity though. The Canon “L” series lenses utilize this glass. As a general rule it is not recommended to use the kit lens. The glass quality is usually low and the f- ratio is normally high.
The lenses you buy will be the one long-term investment in photography. Especially with digital, expect to replace your body every few years as technology makes better and better sensors. The lens will continue to work with new bodies, so a high end lens will still be in use 20 years from now if taken care of even if you’re on your 10th body.

Why use different focal lengths?

So why use a telephoto lens instead of just getting close up on something? With subjects like wild animals or sporting events it is easy to understand why you can’t go up 2 feet away from what you’re shooting to get a shot. But what about models or still life?
There is more to focal length than how close something looks to you. A telephoto lens does not magnify something that is far away to make it bigger; it gives you a smaller angle of coverage as the focal length increases. So using basic geometry you can see that by using a longer lens, you can effectively limit the amount of surrounding items seen in your image.

Another thing to consider when choosing the focal length to use is the compression factor of the lens. Whatever is closest to the lens will appear larger than objects further away. The perceived distortion is in direct relation to the percentage of the distance from the lens. So, using a wide angle lens and getting close up on the subject means that the 1 ft difference between the nose and the back of the head is a much bigger percentage at 3 feet than it is at 20 feet. The distortion will be greater thus making the nose (which is closer to the lens than the back of the head) appear much larger than it really is.
This can be used effectively to show vastness for a landscape, or separate an object from its far distant background. Using a longer lens though can be effective for compressing the distance between foreground and background. Using a longer lens with a more open aperture will also create a much more shallow depth of field effect.

Exposure Value

There is only one correct exposure value for any given image. That is not to say that you can only expose an image one way. Depending on what you want to accomplish there are multiple ways to get to the right exposure. The same subject with identical lighting conditions can even be given a different exposure value for a different look altogether. Both exposure values are correct, but only for each individual look you were trying to achieve.
That is a topic for multiple posts though. In this set of notes I will cover how to change your exposure value using the 3 variables available with a camera and the difference each one will give you for the same exact exposure value (EV).
The definition of Exposure is: The total amount of light allowed to fall onto the photographic medium during the process of taking a photograph.
Please note that there is no exact right amount named in the definition.

The three variables available to you on your camera to control the EV are Shutter Speed, Aperture and ISO. These three are on the points of a triangle that shifts in size, yet must remain equilateral in shape.





For each shift in the direction of favor of one of the variables, the other two must compensate in some fashion.
A smaller aperture for improved depth of field will result in a slower shutter speed thus having a greater possibility of motion blur. To increase the ISO to compensate for the shutter speed can introduce greater noise and less detail.

Here is a chart for one set of exposure values and how they change based off of changing the ISO. This is not a chart for any specific condition, just to show the relationship

F ratio Shutter speed at ISO
100 200 400 800 1600

2.8 4000
4 2000 4000
5.6 1000 2000 4000
8 500 1000 2000 4000
11 250 500 1000 2000 4000
16 125 250 500 1000 2000
22 60 125 250 500 1000
32 30 60 125 250 500
45 15 30 60 125 250

Most digital cameras now a days allow you to adjsut exposure values in third or quarter stop increments with options to fine tune it between those even. The chart is done in traditional 1 stop increments.

So the options give you a trade off of…

Higher shutter speed:
Freezes motion more readily. Great for sports or other action shots such as wildlife. Does require either a wider aperture or higher ISO to achieve so be prepared for their disadvantages.

Slower shutter speed:
Since it requires a smaller aperture, you will have agreater depth of field. This is at the cost of freezing motion. This can be useful with still life or landscapes one windless days. Another feature is that it can add a sense of motion from the blurring that might occur. Very long shutter speeds can be used to blend water together for waterfalls.

Opened up aperture:
Increases the amount of light in allowing for higher shutter speeds to freeze motion. It does come with a shallower DOF however. This can be used effectively to blur out background for portraits (called the bokah of an image). Careful focusing is often required.

Closed down aperture:
Increases your DOF to make focusing less critical. You will decrease your shutter speed to compensate thus loosing the ability to freeze motion or possibly hand hold a lens during the exposure.

Higher ISO value:
This is the “sensitivity” value of your medium to gather light. In film it is accomplished with larger pieces of silver, for digital it is through gain, or amplificaiton of the data collected. Depending on the camera being used, grain or pixilation can start to occur rather quickly as the ISO is upped. This is expecially evvident in shadow and darker color detail. The sensitivity of the digital sensor does not change when you alter the ISO, it only changes how the data is processed. At higher ISO’s the camera will fill in missing information which is what causes the grain.