Part 6 - Photographic Lenses & Depth of Field

The subject of Depth of Field was explained earlier in relation to f/stop settings selected when deciding correct exposure. As you begin taking pictures and evaluating the results, you will soon discover that the optical effect created by Depth of Field is a critical component in your photography. While f/stop settings alter the amount of light entering the camera and define depth of field (the amount of the scene that is in focus from front to back), there is one more factor that adds or subtracts from the picture's total amount of depth of field.

Lens Focal Length

You selected one or more lenses when you purchased your camera. This decision was likely based on what you and a helpful store expert determined most appropriate for your photographic interests. The conversation probably went something like this: “I’m an avid outdoor and wildlife enthusiast. I want to be able to take close-up pictures of wildlife and also be able to capture the beauty of the landscapes I visit.” Given these parameters, the store's recommendations probably included either 1) a wide-angle and telephoto combination or 2) a zoom lens that could perform both duties.

It’s doubtful that the subject of depth of field was mentioned. However, because of the optical physics involved, focal length plays a major role in determining how much of your image is in focus. Focal length is expressed in millimeter numbers. You heard them in the store:

• A normal lens for a typical DSLR falls in the range of 50mm to 58mm. A normal lens "sees" what your eyes see - excluding your peripheral vision.
  
  
  What the 52mm “normal” lens sees.
  
  
• A wide-angle lens has a focal length of less than 50mm (the smaller the focal length number the wider the angle of view). In other words, the smaller the focal length number, the wider the lens and the more it “sees” of the original scene. Giving the impression that the camera is farther away from the subject. (FYI: a wide-angle lens with a focal length of approximately 21mm "sees" everything you would typically see in a scene -- including your peripheral vision.)
  
  
  What the 18mm “wide-angle” lens sees.
  
  
• A telephoto lens has a focal length greater than 58mm. The larger the focal length number, the "longer" the lens and the less it “sees” of the original scene. Giving the visual impression that the camera has moved closer to the subject.
  
  
  What the 200mm “telephoto” lens sees.
  
  
• A zoom lens has the ability to vary the position of glass elements that make up the lens to cover a range of focal lengths. In essence, a zoom lens acts as many lenses from wide, to normal, to telephoto.

Lens Focal Length's Impact on Depth of Field

These features and benefits ultimately lead to your lens purchase decision. Now consider the impact a change in focal length has on depth of field. The pictures below will illustrate this optical reality.


This picture was taken with a 52mm “normal” DSLR lens. Notice the rose in the foreground is very sharp. The background garden, however, is slightly out of focus. The aperture setting was f/8 and the shutter speed was 1/1000th.


Compare this 200mm telephoto picture taken of the same scene. The camera was physically moved away from the front rose until it appears to be about the same size as captured with the “normal” lens in the first picture. Notice the “shallow” depth of field. The background is very out of focus. The aperture setting was f/8 and the shutter speed was 1/1000th.


Finally, the picture above was taken with an 18mm “wide-angle” lens. In this case, the camera was moved closer until the foreground rose was approximately the same size as the rose in the “normal” lens photo. In this version of the photo, every object in the frame is in focus (extreme depth of field). The aperture setting was f/8 and the shutter speed was 1/1000th.

You can see clearly the optical phenomena that impact depth of field as the focal length of the lens changes. Although each picture was taken at exactly the same f/stop and shutter speed, the amount of depth of field in each case is obviously different. The rule is: As focal length increases, the depth of field at each f/stop decreases. And, conversely, as focal length decreases, the depth of field at each f/stop increases. So, wide-angle lenses inherently have more depth of field at each given f/stop while telephoto lenses have less depth of field at each given f/stop.

It’s no wonder that portrait photographers are likely to use telephoto lenses to ensure the subject is sharp and the background is out of focus. It’s also understandable that architectural and landscape photographers most commonly use “wide-angle” lenses to capture the breadth of their subject and maximize depth of field to ensure the entire image is sharp (in focus) from front to back.

The same depth of field and optical rules apply to zoom lenses. The longer the zoom length (focal length increases as you zoom "in"), the less depth of field at each given f/stop. Zoom "out" to your shorter focal lengths -- wider angle -- and the depth of field increases. Magic.

Now you have two major factors impacting how much of your scene is in focus. Photographers who master depth of field will produce the most compelling and memorable pictures. The two major factors are:

• The f/stop you select when determining the correct exposure. The smaller the aperture (larger f/stop numbers) the greater the depth of field.
  
  
• The lens you are using. The longer the focal length (increasingly more telephoto) of the lens, the less depth of field it has at each f/stop setting. The shorter the focal length (wide-angle), the greater the depth of field at each f/stop setting.
  

There are many ways of checking the specific amount of depth of field any f/stop will produce with a given focal length lens. Some lenses are engraved with markings that allow you to calculate the depth of field distance. Very accurate tables are available in photo books and on the Internet that will give you the precise distance(s) for each focal length and f/stop combination.

I suggest you pull out your camera manual and become intimately familiar with your camera's depth of field preview button. It takes a while to become comfortable using this camera feature. Here's what is happening inside the camera.

When you look through your viewfinder, you are actually seeing the scene through the lens' widest aperture setting. This is to make it easier for you to compose and focus your image.

As you manually depress the "depth of field preview button", the lens physically "stops down" to the f/stop you selected (the metal leaves that make up the aperture mechanism in your lens are being closed to the selected f/stop position). Unless you selected the widest aperture on your camera, the viewfinder image WILL get darker. Learn to look through the darkness and you will visually see what WILL and WILL NOT be in focus when you take the picture. So, take the time to use your depth of field preview button. You'll thank me later.

In my next post, I'll discuss the shooting modes available on your camera, and how they influence these discussions on depth of field and motion stopping.

If you have questions, comments or suggestions don't hesitate to drop me a line.

Part 5 - Putting It All Together - Step by Step

Let’s end this exposure discussion by walking through the process I used for taking the landscape picture shown below. I'll base my exposure on histogram results and the second question photographers must address to determine the correct exposure (depth of field and subject motion considerations).

This picture taken at Zion National Park utilizes depth of field that is measured in miles -- from the blades of grass in the foreground to the farthest mountain top. In terms of densities, the full range of deep blacks to the whitest parts of the clouds require a careful exposure to ensure that both extremes retain the detail of the original scene.

Here's my typical sequence of photographic considerations and decisions:

1. The camera is mounted on a tripod. (See my footnote at the end of this walk through.)
   
2. I composed the image through my viewfinder. That was easy enough to say. Composition is a learned skill that will be the topic of later posts.
   
3. In the case of this image, stopping action is not a major concern. Mountains move very slowly. However, depth of field is of critical importantance. I wanted all the objects, from those closest to those farthest away, to be in focus. This requires an extreme amount of “depth of field” – a very small aperture opening. Remember to think backwards at this point. Great depth of field distances are achieved by using f/stops with high numbers (i.e. 8, 11, 16 and 22). The larger the number, the smaller the physical aperture opening.
4. Realizing the importance of the aperture setting to this scene, I selected the “aperture priority” mode and dialed in an f/stop of 16. (Refer to your camera's manual and learn to love "aperture" and "shutter" priority modes.) Now the camera will make exposure adjustments using only the camera’s shutter speed controls. (If the camera determines more light is needed, a slower shutter speed will automatically be selected. If the camera decides it needs less light, a faster shutter speed will be automatically used. The f/stop selection I make will NOT be touched.)
5. I decided to use the histogram display on my camera instead of a gray card. Knowing that I’m taking a general landscape scene, my meter should provide an exposure that’s close.
6. I took a test shot using the auto mode function and immediately evaluated the histogram display shown below:
   
   
   
   
   The histogram from the first test exposure indicates that the exposure settings will produce an image that’s too dark and lacking significant detail in the darker parts of the picture. A new exposure that allows more light to enter the camera must be determined. Increasing the exposure will move all the densities toward the right.
   
   
7. The exposure is close, but too much of the density range is pushed together at the dark (left) end of the graph. This will probably result in dark areas of the picture that lack any detail.
8. I adjusted the exposure compensation control on my camera to allow more light to enter the camera and move the histogram toward the right. Remember, because I fixed the f/stop by using the “aperture priority mode”, the camera adjusted only the shutter speed control to increase the light entering the camera.
9. Another picture is taken and a new histogram is presented on the LCD display. (see below) The densities have moved position on the graph, away from the left border. I know that my highlights and shadows will contain all the detail I want in my final print. Don’t be lulled into thinking that your image editing software will save a bad exposure. Light and dark areas that are totally devoid of detail data in the digital file cannot be fixed. There is no information in the file to fix! Correct exposure is everything. Get the detail in the original exposure.
   
   
   
   The second exposure resulted in this histogram. Notice that all of the densities have moved to the right to cause a separation in the darker densities. This density shift was accomplished by using the camera’s exposure compensation control to temporarily override the auto exposure and increase the amount of light that enters the camera. The print from this digital file will contain a full range of densities and detail.
   
   
10. The image preview on my camera’s LCD display confirmed that my composition was just what I was expecting.
11. Done. Well, not quite. Just to make sure that I didn’t accidentally shake the camera or that mountain didn’t unexpectedly move during my exposure, I took one more picture as a precaution. This practice has saved me on more than one occasion and is especially true when taking people pictures. Everyone tries not to blink. But someone always does. Take extra pictures - including pictures taken with the surrounding shutter speeds if you have any doubt. Better safe than sorry. You can always throw them in the virtual trash.
12. Make notes on the conditions, date, time, location, etc. for future reference. Your digital camera will automatically log the technical details of the picture with your picture file. (Metadata - a subject for a later post.)
13. When you return home, download and review ALL your files. Make special note of everything that worked and determine what might have caused disappointing images. Each successive photographic shooting session will result in fewer poor images. You may well decide you don’t like an image after you see it on the large monitor, but these will be rejected for artistic reasons – not as a result of incorrect exposure.

A note about tripods: Tripods make interesting fuel for camera club and Internet blog debates. When I hear these discussions, I’m reminded of a study I read over 20 years ago in the heyday of conventional film and 35mm cameras. The study was conducted by one of the popular camera magazines. The objective was to determine if there was a visual difference in the sharpness between pictures taken with and without a tripod. The researchers took hundreds of pictures at virtually all shutter speeds with and without a tripod. The initial results were logical and expected. At slow shutter speeds, the pictures taken with cameras mounted on tripods were noticeably sharper.

What was surprising was that when all of the pictures were magnified and examined, the same was true at all shutter speeds. Even at 1,000th of a second, the pictures taken with a tripod-mounted camera were sharper than the images taken with a hand-held camera. Since then, my camera can always be found on top of my tripod. (I occasionally use my tripod's smaller brother - a monopod. Some camera stability is better than hand holding.) If you're just beginning, I suggest that a tripod be your second purchase. As Adrian Monk would say: "You'll thank me later."

A note about depth of field previewing on DSLR cameras: Modern cameras show you the image in the viewfinder with the aperture open to its maximum setting. This is done to make the image as bright as possible for composing the scene. Check your camera's manual. You should find that your camera is equipped with a "depth of field preview button". The depth of field preview button mechanically closes the camera’s iris to the f/stop you have selected as long as your finger remains on the button. This is a great feature but takes some getting used to. By actually seeing the depth of field through your lens, you’ll know exactly which f/stop to use for your situation. However, when the camera’s iris is stopped down to its smaller openings, the viewfinder becomes very dim and it can become difficult to see which objects are in and out of focus. Learn to "look through the darkness" and see which portions of your image will be in focus. If you need more depth of field, select a smaller aperture opening (a larger f/stop number).

This is certainly not everything there is to learn about taking pictures. But by following these basic techniques, you will return home with pictures that are well exposed and very printable.

As always, if you have questions or comments, you know where to find me.

Part 4 - So How Do I Determine a Correct Setting?

Considering the number of variables that come into play when taking a picture, this is a logical and very good question.

First, here are the answers to the exercise from my Part 3 post.

Question 1: What is the exact equivalent exposure to: f/11 at 1/60th of a second if you wanted to change the shutter speed to 1/500th of a second to stop motion?
Answer 1: f/4 at 1/500th of a second. This exposure will allow exactly the same amount of light to strike your camera's sensor while greatly increasing the camera's ability to stop motion.

Question 2: What is the exact equivalent exposure to: f/4 at 1/1,000th of a second if you wanted to change the f/stop to f/16 to expand the depth of field?
Answer 2: f/16 at 1/60th of a second will allow exactly the same amount of light to strike your camera's sensor but result in greater depth of field than f/4.

If you answered these two examples correctly, congratulations. That's as complex as photographic math needs to be for first-time DSLR owners. The rest is an exercise in logic and the infusion of your personal artistic taste.

The CORRECT Exposure

There are two parts to determining the correct exposure setting for any picture.

1. Which exposure setting(s) will give me the best detail in the highlight and shadow areas of my picture?
2. Which exposure setting gives me the proper amount of depth of field and motion-stopping capability required for the scene I'm shooting?

This post will cover the first of these critical questions.

In the first two posts of this series, Exposure is Everything, it was evident that the camera will faithfully provide you with a suggested exposure based on its sole mission of producing a color that has a density of middle gray.

Realize first that this middle gray is not an arbitrary shade of gray. It is a measurable shade of gray. That density is 18% -- a photographic given!

So how do you and the camera’s light meter become photographic partners? And why 18% gray?

If the world contained colors that were all 18% in density, then every picture you metered would be correctly exposed in auto mode. Everything in the scene would be reproduced at exactly 18% gray density. The result would be a perfect picture but a very boring world.

As it turns out, if you merge all the elements that make up a “typical” outdoor, sunlit, landscape scene and calculate the average density for the total scene, the resulting density number is approximately 18%. That’s why the average family outdoor pictures result in acceptable prints. The exposure set automatically by even the simplest camera with a built-in light meter determines the correct amount of light to produce an 18% gray density image. The exposure is at least close enough that it can be easily adjusted by a photo lab or on a home computer.

But what if the scene you are shooting isn’t “typical”? You might have guessed there’s a strong probability that the camera’s automatic choice will be incorrect. The resulting picture will likely be overexposed (very light due to too much light entering the camera) or underexposed (too dark because not enough light was allowed to enter the camera).

The real challenge is to select an exposure that’s “just right” under any given light condition -- an exposure that will produce a print that matches your vision and yet has detail in both the white areas of the picture (highlights) and in the dark areas of the picture (shadows).

There are several ways of determining the correct f/stop and shutter speed combination. Professionals use these techniques while employing our most well-guarded secret. Once you determine the correct exposure of any single element in a typical scene, then all other elements that make up the image will be correctly exposed.

Try working through and understanding these popular professional techniques for determining exposure.

Method 1: Give the meter what it is looking for!

18% gray cards can be purchased in any camera shop. They are inexpensive. Want to make an impression at your local store? Just ask for a gray card. They will recognize that you know photography’s most well-kept secret and that you are a force to be reckoned with. Imagine all that respect for less than $15.

Now you own photography’s most valuable accessory. Take a few minutes and read the section of your camera’s instruction booklet that covers “manually setting your exposure”. That’s right, you are taking control of the exposure away from the camera.

• Now, when you are ready to take your picture, grab the gray card.
• Put your camera into auto, aperture or shutter priority mode.
• Place the gray card in front of your camera under the same lighting conditions as the scene you are shooting.
• Move your camera close enough to the gray card to fill the entire viewfinder. (Don’t let your body or the camera cast a shadow on the gray card.)
• Make note of the exposure setting your camera is recommending (let’s say, f/5.6 at 250th of a second).
• Set your camera to manual mode.
• Dial in those same recommendations. (In this case, f/5.6 at 250th of a second.)
• Set the gray card aside and compose your picture.
• Then “click”.

The reason you entered the f/stop and shutter speed manually into your camera is because, when left in auto mode, the meter will re-evaluate the total scene once you take the gray card away. Manually entering the correct exposure ensures that the camera is forced to take the picture based solely on the gray card reading.

Why does this work? Because... once you determine the correct exposure for one component of your picture, everything else in the picture will also be correctly exposed.

You can prove this by making one more picture of the white, gray and black cards. This time only one picture will be taken. First, take a meter reading from the gray card and manually dial in the recommended f/stop and shutter speed into the camera. Now place all three cards next to each other. Arrange your camera so that all three cards can be seen in the viewfinder. Take the picture. Now when you process this digital file, you will see the picture below.

This single picture containing the white, gray and black cards was possible because the meter was only allowed to meter the gray card. Now that the gray card will be exposed correctly, everything else in the scene (the white and black cards) will be rendered in the correct shade/density.

Method 2: Nature’s gray card

Using the same procedure described in Method 1, substitute grass or the green leaves of trees for the gray card. The photographic gray density of average grass and trees is approximately 18%. So, filling your viewfinder with only grass that’s in your scene and using it like the gray card in Method 1 will result in approximately the same exposure. Close enough. Again, get one thing right and everything else falls into place.

I’m recommending that you train your eye to recognize elements of approximately 18% density in your scene. It does not matter what color the object is – just how light or dark the object is. Once you identify something that’s near 18% in density, meter that object and manually enter those results into your camera.

Both methods produce good images in nearly every shooting condition. I consider the time required by these manual methods to be well spent because it requires me to slow down and evaluate what I’m doing as I compose and expose each image.

Method 3: The Histogram: Digital Photography’s Magic Bullet

A little melodramatic, but the histogram is an important new camera tool. If you could measure the light densities of all the elements in the scene you are shooting and plot their number of occurrences from darkest to lightest on a piece of graph paper, you would have created a histogram for that specific scene. As a manual exercise and considering the millions of densities that make up a photographic scene, this task is nearly impossible. Enter the wizardry of modern digital cameras.

The picture and graph below contain the histogram for a well-exposed landscape image. The far left border represents the darkest components of the scene (black) while the far right border of the graph indicates the lightest densities of your picture (pure white).

The histogram and picture below show that the majority of the densities are pushed to the left of the graph. A photograph containing this histogram is underexposed and lacks ANY detail in the darkest parts of the picture. The selected exposure (f/stop and shutter speed combination) did not allow enough light into the camera to record detail in the dark areas of the picture.

If there is NO detail contained in the image data, then no matter what magic you perform in a computer image editing program NO detail will be reproduced in the final print!

The histogram and picture below show that all of the densities have been pushed to the right of the graph. The resulting print is overexposed and will have little or no detail in the lightest areas of the image. With NO detail in the lighter parts of the image, no detail will be achievable in the final print!

The goal then is to select an exposure where all or most of the densities fall within the borders of the graph – like the first example. Once this exposure is identified, all the dark and light values -- as well as everything in between -- will be recorded to produce a print that’s well exposed and contains acceptable highlight and shadow detail. It’s printable.

In most of today’s moderate to high-priced DSLRs, you can set the camera to display this histogram visually on the LCD monitor. (Quick, get your camera manual and look up histogram display. Set up your camera to display the histogram after you take each picture.)

In reality, viewing the histogram immediately after you take the picture is more important than looking at a tiny preview of your picture. You saw what you were shooting through the viewfinder. What's important is the EXPOSURE! Did the densities fall within the center portion of the histogram? Will it make a good print? Do I need to make an adjustment and shoot it again? Now you're talking professional.

In practice, you can now take a picture at your camera’s automatic exposure recommendation and immediately see the histogram for that image. If the distribution is incorrect, you can override your camera to produce an exposure that brings the densities back to a printable range. You guess it. Camera manufacturers have included a way for you to temporarily override the meter’s recommendations and “tweak” them to adjust for your shooting situation. (Back to the camera manual. Look up “exposure compensation”.)

Now take the picture, evaluate the histogram, and apply “exposure compensation” to adjust the exposure. (Letting more or less light into the camera.) When you move the “exposure compensation” control and take a second picture, you will see the histogram densities move in tandem to the left or right. Digital film is cheap. So take as many pictures as necessary to get the histogram density distribution correct. Eventually, this shooting routine will save hours of frustration in front of your computer.

Some thoughts:

1. This probably seems like a lot of work. The truth is that creative and technically accurate photography is work. Taking the time to consider the composition and exposure components of each photograph will result in the most gratifying images.

In reality, most of this process becomes reflex action over time and as second nature as brushing your teeth. There is also a silver lining to the process. When you are taking many pictures of similar scenes in the same vicinity and the lighting conditions stay relatively constant for a period of time, you’ll find that the same exposure settings remain constant. So, shoot away. You don’t have to go through the process again until the location, scene or lighting conditions change significantly.

3. Hopefully you are approaching an “I get it” moment. There are other considerations in setting the final exposure, but the steps we have already taken together should provide that “warm and fuzzy” feeling that you’ve left your shooting session knowing the pictures you envisioned are those on your data card. I will touch on those other considerations throughout the remainder of these posts.

4. Making these radical changes in the way you look at photography and take your pictures demands one critical discipline. Repetition. Shoot lots of pictures. Commit these actions to muscle memory by taking pictures every day. Take copious notes when you shoot. Download the camera files into your computer and review each image on your large monitor. You don’t have to print every picture to learn from all your hard work. But do take the time to, at least, digitally review your work. What worked? What didn’t work and why? Each digital frame has a lesson to teach.

5. Not all things are possible in photography. There are traditional as well as new digital “tricks” that can be applied to make almost every picture possible. But there will be times when the answer is: “It can’t be done”. Technology always has limits. Some of these situations will be discussed later.

Bottom line: I highly recommend you take advantage of technology and make your exposures based on the histogram results (Method 3). Take it from an old photographer, an understanding of Methods 1 and 2 is important to understanding the logic of exposure, but in today's evolving digital technology environment you will quickly learn to appreciate Method 3 and your camera's histogram.

Above is a histogram as superimposed on the Nikon preview screen after the picture was taken. The distribution of the densities on this graphic indicates that the darkest and the lightest portions of this picture will contain detail that will appear in the final print.

My next post will walk you step-by-step through my normal shooting routine using the histogram method of determining proper exposure. As always, let me know your thoughts or questions.