DSLR Photography lessons - Lesson 3. Depth of Field / Hyperfocal Distance.

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Hi Pentaxian friends.

Previous lessons: Lesson 1, Lesson 2

Lesson 3 - Depth of Field / Hyperfocal Distance

Depth of Field

The depth of field is the measurement at which subjects, located at various distances from the DSLR, are in focus. Depth of field changes each time you change the aperture. The lens' focal length also affect the depth of field. A telephoto has a shallower depth of field than a wide-angle lens. Assuming that you are using a 50mm lens with a maximum aperture of f/1.4 and a minimum aperture of f/22, using the larger aperture (f/1.4) would give you the minimum depth of field. In other words, the area before and after your focusing point would be shallow. Look at the photo below:

Using the small aperture such as f/22, would give you the maximum depth of field. In other words, the area before and after your focusing point would seem in focus from near the camera to near infinity. Look at the photo below:

On modern DSLR cameras, when you compose your picture looking through the viewfinder, the lens' aperture is always opened to the maximum. To get a peek at what the final picture will really look like, most DSLRs have a depth of field preview button. Some even have a choice of giving an optical preview (in the viewfinder) or previewing the image on the LCD monitor. When using the optical preview, the smaller the aperture (less light in) the darker the preview. The preview is primarily to check the depth of field. However, if your camera is equipped with previewing on the LCD monitor, the image will appear as it would if you press the shutter release button.

Does shooting at the smallest aperture render a sharp image from the foreground to infinity? The answer is: Not necessarily.

Hyperfocal distance

On most lenses, even modern auto focus lenses, a distance scale shows the distance at which the lens is focused. Using a lens focused at infinity and at a small aperture, f/16 for instance, will render a sharp image of far away subjects but the sharpness of the foreground might not be optimum. Depending on the lens' focal length, the closest focused distance of the lens may be 10 or 20 feet away. This is when you want to take advantage of the hyperfocal distance of your lens. The hyperfocal distance of a lens is defined as follow: The closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp; that is, the focus distance with the maximum depth of field. Take a look and download this free Hyperfocal distance chart from DOFMaster.com

http://www.dofmaster.com/charts.html

When the lens is focused at this distance, all objects from half of the hyperfocal distance out to near infinity will be acceptably sharp.

For example, using the downloaded chart above, the hyperfocal distance of the Pentax FA 50mm f/1.4 lens used on your K20D and set at f/16, would be 17 feet. To take advantage of the maximum depth of field available with this lens, manually focusing at a distance would assure a sharp image from 8.5 feet (half the hyperfocal distance) to near infinity. Try it, it really works. I'm not making this up!

Hyperfocal distance shooting is mostly used with wide-angle lenses to about 100mm lenses. Lenses with longer focal lengths have a depth of field too shallow to produce acceptable sharpness from the foreground to infinity.

It is commonly used for street photography, as you want to take your picture as quick as possible. You simply set your aperture (as small as light permits), focus your lens manually at the hyperfocal distance, and make sure your subject is within half the hyperfocal distance to infinity. Using a 50mm lens at f/16, your subject would have to be from 8.5 feet to infinity, to be sharp.

Download this depth of field chart from the same source

http://www.dofmaster.com/doftable.html

With it, you can check what the closest and farthest depth of field is for any lens at any aperture. If you look at the comments below, you can see that the Circle of Confusion (CoC) affects the depth of field. If you like formulas, you can immerse yourself in the calculations as shown on the following site.

http://www.largeformatphotography.info/articles/DoFinDepth.pdf

However, it is far more practical to find the hyperfocal distance from the chart above and then use the depth of field preview button to test the actual depth of field. Or you can always try a test shot as digital photos cost nothing and you can erase it from the card.

Thanks for reading,

Yvon Bourque

DSLR Photography lessons - Lesson 2.

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Hi Pentaxian friends.

Lesson 1 here: Lesson 1

Combining all three elements described in lesson 1.

You can understand that to take a properly exposed photograph, one has to balance the aperture and shutter speed according to a sensitivity value (ISO). In the 35mm film era, the sensitivity would remain constant for the duration of the roll of film. In digital, the sensitivity (ISO) can be adjusted on the fly, making this third element more important and flexible than ever. Luckily, technology spares us from long exposure calculations. We can adjust one of these three elements, and the camera will adjust the two other automatically. We can set the Aperture, for example, and let the camera select the appropriate shutter speed. That is called Aperture Priority, (Av or Aperture value) on your camera mode dial. If the exposure is impossible at the chosen aperture, the camera will either suggest using the flash or increasing the ISO rating. Both of these functions can be set to make the decision automatically or within some preset parameters. Alternatively, you can decide to set the shutter speed and the camera will choose the appropriate aperture. That is called Shutter Priority (Tv or Time value) on your mode dial

Your camera (K100D and most of the Pentax 6MP - The K10D and K20D don't have the small icons) has many preset and automatic modes, represented by small icons on the mode dial. This further simplifies the decision making for the photographer. You set the camera to portrait, and the camera is already programmed by Pentax to use the standard settings typical for a portrait shot. You set it to moving objects, and the camera leans toward a fast shutter speed to freeze the action. You set it to landscape and the camera leans toward a small aperture for maximum depth of field. These settings are fine for the casual photographers and at the beginning of your learning curve as photographer, but sooner or later, you will want to have more control over the camera. Although the technical progress made in just the last decade is nothing short of astonishing, the human element is still far superior in decision making. Once you fully understand the three essentials items explained in the preceding post, you will take the creativity control over the camera.

Adjustments to consider

Choosing a small aperture can make the scene in focus from just a few feet or inches in front of the camera to infinity.It is very useful in landscape photography. This effect is proportionally accentuated when using wide angle lenses. However it will require a slower speed or a faster ISO. Tripods are commonly used in landscape photography.

Choosing a large aperture limits the depth of field, making your subject stand out with a blurry background and foreground. (It's really called selective focusing) It will likely require a fast shutter speed or lower ISO value. (P.S. Always try to shoot at the lowest ISO value as possible. It will always produce the minimum amount of noise) This is very useful for portraits. This depth of field is proportionally accentuated with telephoto lenses. However, with telephotos, it will probably require a faster shutter speed (because a telephoto that brings the subject say, five times closer, will also register the movements as being five time faster) or a higher ISO value, as the maximum permitted aperture will be smaller

Using a fast shutter speed can freeze the action and is often used for sports and fast moving activities. However, it will require a wider aperture, a faster ISO value or the use of a flash.

Using a slow shutter speed will allow taking pictures at night or will render a feeling of movement by deliberately blurring a moving area of a scene such as water falls, etc. You will likely need a tripod and perhaps a wider aperture and faster ISO.

You can shoot with a very fast ISO and capture almost any image with little effort, but the image will contain noise.

You must have realized by now that photography is indeed all about light. Modern DSLRs have other tools to further refine the art of capturing the images with the optimum results. Built-in meters give you the choices between multi-segments metering, center-weighted or spot metering. Auto focus modes in either single or continuous focus make your photos crystal clear and sharp despite you. Shake reduction allows using slower speed than most humans could ever achieve by hand-holding the camera. White balance control assures, most of the times anyway, that the color of the scene will be the same on the saved image or file. Instant view of the capture images allows you to retake the shot(s) if not satisfied. The use of memory cards allows you to take hundreds or even thousands of shots, at practically no cost other than the purchasing cost of the card. I tell you, with digital cameras, especially with Pentax DSLRs, we’ve got it made. Remember what they say; “practice makes perfect” or close to it anyway as I don’t believe that perfection can ever be achieved.

I will continue to post Digital Photography lessons in the weeks and months to come. All of the advanced photographers out there, this may be over-simplified for you, but remember that there are many entry-level photographers that need such information. We all started by knowing nothing about photography, and learned Little by little. You can certainly and hopefully contribute to this post, but try not to confuse the "newbies" with too much technical information. Once the basic of photography is understood, the technical stuff will sink in better.

Thanks for reading.

Yvon Bourque

DSLR Photography lessons - Lesson 1.

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Hi Pentaxian friends.

I posted the article below in March of 2008. I am posting it again because this year I will post a complete series of DSLR lessons. I figured I should start here.

Too often, we forget that new Digital Single Lens Reflex (DSLR) cameras are sold every day to someone with no experience with DSLR cameras at all. Whoever we are and however good we are, we started somewhere. .

This post will not make you an expert in photography alone. Like any other skill we learn, practice is what makes us excel in any of our undertakings. There are many books on photo techniques on the market that you can get to further learn the craft or art of photography. For years, Kodak has regularly publish updated books targeted to beginners all the way on up to the professional photographers. Of course, lots of information is also available on the internet.

Photography, is about light. It’s about light reaching a media, film or digital sensor, and turning the results into an image that we can see, on a computer screen for example, or print on paper.

Other than the lighting conditions of a scene, there are three major items that directly affect how the light reaches the recording media. The Aperture (lens opening), the shutter speed (the speed at which the shutter opens and closes) and the media sensitivity also known as ISO number (the amount of light recording on the media for a given exposure). Too much light reaching the media, and the image will be washed out. Too little light reaching the media, and the image will be too dark. This is to say that the balance between these three items must be just right. Technically speaking, a large aperture will let a lot of light in, and therefore the shutter speed and/or the recording media will need to be adjusted accordingly. Let’s assume for a moment that the sensitivity is set at ISO 200 and cannot be changed. We now have to balance the aperture and shutter speed to get the correct amount of light in. An aperture of f/4 at a shutter speed of 1/125th of a second would give the same results as an aperture of f/5.6 at a shutter speed of 1/60th of a second. In other words, if you increase the aperture by one F-stop and decrease the shutter speed by one shutter step, the same amount of light would would reach the film or sensor. The difference between the two is the composition. Since we are dealing with digital single lens reflex (DSLR) cameras, from now on lets use the word “sensor” for media.

Aperture scale explained
The aperture, being the lens diaghphram opening, lets more or less light pass through the lens. The f-number (aperture opening) is proportional to the ratio between the lens focal length and aperture diameter, which is proportional to the square root of the aperture area. Big analogy but what does all that means? Well, lenses are usually marked with the F-numbers ranging from the largest aperture to the smallest aperture. For example, a typical lens could have an aperture range of f/2.8 to f/16. The lens would be marked as follow : f/ 16, f/11, f/8, f/5.6, f/4, f/2.8. In this example, the largest aperture would be f/2.8 while the smallest would be f/16. Have you noticed something a little surprising here? The larger the number is, the smaller the aperture is. You need to remember that. Furthermore, each (f-stop) number to the right lets twice the amount of light in as the (f-stop) number to its left and each (f-stop) number to the left lets half the light in as the (f-stop) number to its right. For example, f/4 lets twice as much light in as f/5.6 but only one half the light of f/2.8, and so on. One unit of increment in aperture is called a stop.

Numbers to the right are letting twice as much light in.

Numbers to the left are letting 1/2 (or twice as less) light in.

Shutter Speed scale explained
In photography, shutter speed is the length of time the shutter takes to open and close. The total exposure is proportional to the duration of lighr reaching the imaage sensor. Similarly to the aperture, a standardized 2:1 scale was adopted for shutter speed so that opening one aperture f-stop and reducing the shutter speed by one step resulted in the identical exposure. The agreed standards for shutter speeds are typically 1 sec,1/2 sec, 1/4 sec, 1/8 sec, 1/15 sec, 1/30 sec, 1/60 sec, 1/125 sec, 1/250 sec, 1/500 ses, 1/1000 sec, etc. A shutter speed of 1/125 sec lets twice as much light in as a shutter speed of 1/250 sec, but half the light of a shutter speed of 1/60 sec. For example, combining aperture and shutter speed, a shutter speed of 1/125 sec with an aperture of f/16 is equivalent to a shutter speed of 1/250 sec and an aperture of f/8. Alternatively, a shutter speed of 1/125 sec with an aperture of f/16 is also equivalent to a shutter speed of 1/60 sec with an aperture of f/22. Remember that one unit of increment in shutter speed is called a step.

Numbers to the left are letting twice as much light in. Numbers to the right are letting 1/2 (or twice as less) light in.

ISO Value scale explained

Similarly to the aperture and shutter speed, the ISO linear scale, which corresponds to the older ASA scale, is 2:1. Doubling the speed of a film implies doubling the numeric value that designates the film speed. Here again, a film rated at 200 ASA or ISO 200 will absorb half of the light of a 400 ASA or ISO 400 film, but twice as much light of a 100 ASA or ISO 100 film. In the digital world, the sensitivity defines ISO speed in terms of the amount of light needed to achieve a certain quality in the sense of a per-pixel signal-to-noise ratio. The image sensors in digital cameras can be adjusted, or can have their outputs adjusted, in sensitivity to function with metering at any given comparative ISO setting. This is usually done by simply amplifying the output of the image sensor, which increases image noise, sometimes beyond the level that the ISO standard says is acceptable. Just as with photographic film, greater sensitivity comes with some loss of image quality, visible as image noise. What does that mean for you? The lower the ISO value on your digital camera, the less noise you will have and therefore, your image will appear clearer. ISO 200 is twice as sensitive to light as ISO 100 but half of ISO 400.

Next lesson, we will be Combining all three elements.

Thanks for reading,

Yvon Bourque