The Lens

As woodworkers we are interested in photographing our work for show; either in a printed portfolio or on a website. I believe the most important part of photographing a piece is getting the exposure right. This starts with understanding the functions of the lens. I will use two lens examples to highlight the important functions:

  • Canon Zoom Lens EF 28-135 1:3.5-5.6 IS USM / Ø72mm
  • Canon EF 24-70mm f/2.8L USM / Ø77mm

 

Canon Zoom Lens EF 28-135 1:3.5-5.6 IS USM / Ø72mm Let’s go through the above examples and decipher the specifications. Both of the above lenses are members of the Canon EF line, meaning they can be used on any Canon EOS camera. They are both variable focal length or zoom lenses. Whether they are telephoto, normal or wide angle depends on the camera to which they are mounted. More on that later. The first lens has a variable focal length of 28mm to 135mm and the second lens 24mm to 70mm. The first lens’s maximum aperture opening depends on what focal length is set, that is, the maximum aperture opening is 3.5 at 28mm and 5.6 at 135mm. Aperture openings are measured in f/stops which we will discuss soon. The second lens has a fixed maximum aperture of 2.8 independent of focal length. The first lens is an “Image Stabilized” lens. Both have a “Ultrasonic Motor” used for auto-focusing. The last two numbers (72 & 77) are the diameters of the lens cap and filter attachments used on that lens. The L following the f/2.8 aperture specification stands for “Luxury” meaning it is a professional lens (lots of very high quality glass). The first lens sells for about $400 while the second $1,050.

Viewing Angle

Canon EF 24-70mm f/2.8L USM / Ø77mm OK, that was a once over. Lets go into a little more detail. Imagine driving through a long tunnel through a mountain range which exits on the other side to trees, a lake and homes. When you are one mile from the exit you have virtually no view of the trees, lake and houses, simply a spot of light. When 100 feet from the exit you might see a home or two and some trees. When 50 feet from the exit you recognize the existence of water, a field of trees an a few homes. Finally, when you are 10 feet from exiting the tunnel you get the whole view of the lake, the trees and a small town. This is similar to the view provided by various types of lenses. The 1 mile view is analogous to a pin-hole camera, the 100 foot view a long zoom lens, the 50 foot view a normal lens and the 10 foot view a wide angle lens.

The difference in all these views is the viewing angle and the corresponding “focal length”. Neglecting the pin-whole camera which we are not interested in, the 100 foot view may have a viewing angle of less than 10 – 20 degrees, the 50 foot view 50 – 60 degrees and the 10 foot view a viewing angle over 100 degrees. It is the same with lenses. A long focal length lens, say 250mm, has a viewing angle of 10 degrees on a 35mm camera. A normal 50mm lens a viewing angle of 45 degrees on a 35mm camera. Finally a wide angle lens of 15mm will have a viewing angle of slightly over 90 degrees on a 35mm lens. This is why telephoto (large focal length) lenses appear to enlarge things while wide angle (short focal length) lenses tend to shrink things relative to a normal lens.

Aperture & f/stop

To Halve Exposure Multiply f/stop By One Over The Square Root Of TwoThe aperture opening, as mentioned previously, is measured in f/stops or f-number written as f/# or N. An f/stop, or N, is a ratio of the focal length to the aperture diameter or f/D. You will not find a lens with an f/# of f/1; this would require the maximum opening to be equal to the focal length. This is difficult and expensive to achieve for reasons well beyond this post. However, f/1 is theoretically possible and is what f/stops are based on. Suppose we expose a picture with a shutter speed of 1 second (the length of time we allow light to enter the lens) and f/1 and discover it is overexposed. We wish to cut the amount of light in half. We have two options; set the shutter for 1/2 second or choose an f/# of f/1.4. Where did the 1.4 come from? High school geometry. If we want an aperture that has 1/2 the area of f/1 we need an aperture opening 1/(sqrt 2) or 1/1.414…… times f/1 or f/1.4 (truncation typical). Standard f/stops are those that follow the 1/2 exposure (or aperture area) progression and look as follows:

f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128

Each f/# is 1/2 its left neighbor or 2 times its right neighbor in terms of the amount of exposure it permits. If you have a 135mm focal length lens set at f/5.6 the aperture diameter is 24mm. At f/8 this same lens has an aperture diameter of 17mm and permits 1/2 the exposure. Note that the larger the denominator the smaller the opening and the less the exposure. This is a little confusing, but with use you will get accustomed to it.

Shutter Speed

Shutter speeds have standard “stops” as well. They are defined also by the progression of halving (or doubling) exposure. These are the typical shutter “stops”:

1/4000, 1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1, 2 etc.

Note, if an exposure meter determines the correct exposure is f/5.6 at 1/250, you can choose a number of equivalent exposures by doubling f/stop and halving shutter speed or vice versa. For example, f/8 and 1/125 is equivalent exposure to f/5.6 and 1/250 as is f/4 and 1/500.

Depth Of Field

Depth of field (DOF) is another factor we need to consider. If we focus a lens on a subject there is a range of image depth that is in focus. The exact definition of DOF is beyond this post but here are a few things to remember. The image will be in focus a distance d in front of the subject and roughly 2d behind the subject. The size of d is a function of film format, lens focal length, aperture (f/#) setting, and distance of the subject from the camera’s focal plane. For example, a subject 15 ft from a 35mm camera’s focal plane, shot with a 50mm lens set at f/8 will have a DOF of 16 feet; 4.5 feet in front of the subject to 11.5 feet behind.

Camera Format & Focal Length Multiplier Or Magnification Factor

One last factor we need to discuss to complete our discussion of lenses is actually a discussion of camera formats. 35mm cameras use 35mm film. The exposed area of this film is 36 x 24 mm. Unless you purchase a professional DSLR, whose sensors are the same dimensions as 35mm film, the sensor will be substantially smaller. My Canon EOS 40D has a sensor that is 22.2 x 14.8 mm. If you take the ratio of the diagonals of these two formats you get 1.6. That is, the diagonal of a 35mm camera is 1.6 times that of the Canon EOS 40D. Another way to look at this is to compare the images produced by the same 50mm lens used to take a picture on a 35mm camera and then a Canon EOS 40D. The Canon EOS 40D picture will be 1/1.6 that of the 35mm picture. When both are printed on equivalent paper, for example 8″ x 10″, the Canon EOS 40D print will appear as though it were taken with a 80mm lens on a 35mm camera. Therefore, since the lenses used on DSLRs are based on the 35mm format I must remember to multiply a lens’s focal length by 1.6 to understand how it will work on my Canon EOS 40D. This factor is generally referred to in the literature as Focal Length Multiplier or Magnification Factor.

Relative Sizes Of 35mm Format & APS Format Versus Lens DiameterMost DSLRs are either full frame format (35mm equivalent) sensors or APS format (Advanced Photo System) sensors; the latter requiring a Magnification Factor typically between 1.5 – 1.6. Check the specifications for you DSLR to determine the correct Magnification Factor. These smaller sensors have both advantages and disadvantages for the photographer. One disadvantage is that it is difficult to use a high quality wide angle lens since wide angle lenses tend to be shifted to normal lenses when multiplied by 1.6. For example, a medium wide angle lens on a 35mm format camera would be a normal 50mm lens on an APS camera.

There are a couple of advantages though. A 135mm medium telephoto lens is equivalent to a 216mm lens when used on an APS format camera, which is a decent long range telephoto. In addition, since the sensor is smaller, its image is the product of the inner portion of the lens. Most lens aberrations are caused by its outer edges and these will tend to affect the 35mm camera more than the APS camera making the quality of the lens seem higher than it actually is.

Summary Of The Lens

We can summarize all of these lens related factors as follows:

  • Larger focal length lenses result in a smaller viewing angle and more apparent magnification
  • Moving a full f/stop is either direction either halves or doubles exposure
  • Moving a full shutter speed in either direction either halves or doubles exposure
  • Decreasing aperture diameter increases DOF – e.g. going from f/2.8 to f/8 increases DOF
  • Larger focal lengths result in lower DOF – telephotos have shallower DOF than wide angle lenses.
  • The larger the distance of the focused subject from the camera’s focal plane the larger the DOF
  • Smaller format cameras have a shallower DOF
  • Many DSLRs require the use of a Magnification Factor to determine the focal length equivalent for any given lens – for example – a 32mm lens on my Canon EOS 40D is equivalent to a normal 50mm lens on a 35mm format camera

 

Often, what we want to photograph as woodworkers can be quite large, for example a country hutch. Further, we want the entire piece to be in focus so that grain and crafting detail are sharply displayed. We would like a large DOF to accomplish this. This typically means using an f/stop of f/8 or smaller (f/16, f/22 etc.). I almost always use my Canon EF 24-70mm f/2.8L USM lens, because even with the 1.6 factor, it is a slightly wide angle lens compared to a normal 50mm lens on a 35mm format camera. Hence it helps to get the entire piece in the image and also helps with DOF. In addition, the lens quality is substantially better.

This is a good place to break for now. We have covered nearly all the important features and functions of lenses. Next we will discuss the camera body itself, talking more about the sensor, shutter, and RAW images. It will take several posts before we can start to tie all this together, so be patient – it will all make sense soon. See you in Part 3.

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