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Photography Questions and Answers

a few technical questions answered in (mostly) non-technical terms

©2006-2007 Jess Isaiah Levin

This is a work in progress, and will cover various topics that seem of interest, or could use some clarification for many readers.  More questions and answers will be added as time permits.  Please feel free to email suggestions from the contact page.  Thank you.

1.  What is depth of field?

2.  What is selective focus?

3.  What is a pixel?  What is the difference between ppi and dpi?

4.  What is the focal length of a lens?

5.  What is an f/stop, and why do the numbers get larger as the opening gets smaller?

6. Why do digital camera descriptions talk about "35mm equivalent focal length"?

1. What is depth of field?

Depth of Field refers to the zone of "acceptable sharpness" in a photo.  Theoretically, a lens can only be focused on one distance at a time.  If you focus your camera on a distance of 8 feet, only objects 8 feet from the camera will be in perfect focus.  However, there are limitations in the ability of any camera lens (or our eyes) to resolve (perceive) fine details.  Depending upon the focal length of the lens, the size of the aperture, and the focusing distance selected, there will be some range extending both nearer and farther than the exact focused distance in which everything will appear to be perfectly sharp and clear.  By juggling the variables mentioned above, the photographer can set things up so that everything in the scene - from very close by all the way out to infinity - is sharp.  Conversely, a nearly paper-thin plane of sharpness can be arranged, so that for example a portrait subject's eyes are perfectly detailed, yet other parts of the face are softly blurred. 

The creative control and use of depth of field is one of the ways in which mastery of the craft of photography allows it to become an art form in the right hands.

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2. What is selective focus?

Selective Focus draws attention to the subject (and prevents other objects in the field of view from distracting the eye) by making the desired object the only one that is even close to sharp.  It could also be thought of as "creative blur", in this case achieved by using a long focal-length lens and a very wide aperture to minimize depth of field.

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3.  What is a pixel, why is resolution important, and what's the difference between ppi and dpi?

With the field of photography increasingly dominated by digital techniques, we think of photos in terms of pixels.  A pixel is simply the smallest unit of a digital image, one tiny area with a specified color and brightness.  With enough of those pixels, we can create an image with sufficient detail to satisfy the human visual system. 

The total amount of detail that can be contained in a digital image is limited by the total number of pixels.  An image that measures 3000 by 4000 pixels on a side (12,000,000 pixels, or 12 megapixels) can obviously show more detail than one that is 1500 x 2000 pixels (3 megapixels).  Note that this is the total resolution of the image. 

PPI, or pixels per inch, is an arbitrary way of correlating an image file to the way it is printed or displayed on a monitor.  Let's say that we are using a file that is 1200 x 1800 pixels, and we decide that we want to print it at a size of 4x6 inches.  That corresponds to a resolution of 300 pixels per inch, which is a good setting for high quallity prints through many processes.  On the other hand, if we display that file on a computer monitor, with each pixel of the image corresponding to an actual pixel on the screen (at "100%", i.e., no resizing), then the physical size of the displayed image will depend upon the monitor resolution in pixels per inch.  If the monitor happens to have a resolution of 100 ppi, then the image (which we printed at 4x6 inches) will show on the screen as 12x18 inches.  We haven't changed the resolution of the image, just the resolution per inch.

DPI stands for dots per inch, and really applies to printers, not monitors or digital images.  Many inkjet printers can apply well over 1000 dpi, but the printer software is using many "dots" of ink to create each pixel of the original digital image file.  This helps to create smooth tonal gradations, and an appearance of continuous tone.  Nonetheless, our eyes will not perceive much more than 300 distinct bits of information per linear inch, unless we are extremely nearsighted or have the aid of a magnifier.  True photographic digital printers, which create a chemically processed silver halide and dye print from digital input (the Fuji Frontier is a common example, used in many photo labs) generally are optimized for 300 pixels per inch input, which results in output that appears as continuous tones.  Photo quality inkjet printers often do best with input of 240-360 ppi, which is then put on the paper anywhere from 600 to 2880 dpi.

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4.  What is the focal length of a lens?  How does a lens work?

A lens focuses light so that we can obtain a bright image of an object or scene.  To gain a basic understanding of optics (how lenses work), it can help to imagine a pinhole camera.  Think of an ordinary rectilinear box, such as a shoe box.  Inside, you fasten a piece of photographic film (or even a fancy digital sensor) to one wall of the box.  In the middle of the opposite wall, you use a pin to make a small hole.  If you aim the hole in the box at a subject, and allow time for enough light to get through the small hole, you can get a reasonable, though somewhat fuzzy image on your film or sensor.  The advantage of a lens is that it gives you a much larger opening for the light to go through.  The curved surfaces of the lens bend the paths of light in such a way that all rays coming from a certain point (let's say, reflecting off the tip of someone's nose) will be focused to one point on the film/sensor, no matter what part of the lens they travel through.  Each point in the scene corresponds to one point on the film/sensor, so we have a clear, accurate image.  In theory!

Let's go back to the pinhole camera for a moment, to help explain focal length.  Imagine that our shoe box can be compressed like an accordion, so the distance from pinhole to film/sensor can be varied.  Let's start with it compressed to a short distance, say 4".  We'll assume that we have a certain sized piece of film in there, say 4"x5".  If you project the paths that light can take through the hole and hit the film, you'll find that the film can "see" a fairly wide view.  Now let's stretch the accordion box out to 8".  The light that goes through the pinhole and strikes the film is now confined to a narrower angle.  The film "sees" a narrower view, and objects within the view are reproduced at twice the size they were in the first example.  What we have done is essentially to switch from a wide angle "lens" to a telephoto, but without any lens at all.

Now back to lenses.  The curved glass surfaces allow a lens to focus light (because light travels more slowly through glass than through air).  Because we are dealing with a relatively large curved surface, rather than the simple geometry of a pinhole, we find that a given lens will only focus all those rays coming from a certain point in our subject to one point on the film if the film is held a specific distance from the lens.  If the object is so far away that it can be considered to be at infinity (such as a star), then the distance is the focal length of that lens.  In other words, move a lens back and forth until you have a clear image of far away objects, and you've established the focal length of that lens.  To make a lens of a focal length that we want to have, we need to design the right curvature of the glass.

Remember what the pinhole taught us about focal length:  Do you want a lens that shows a wide angle view?  You have to arrange the curvature, or power of the glass so that it is in focus when it is quite close to the film/sensor.  Want a telephoto lens, to show a very narrow angle and seem to bring things "closer" by making them larger?  Design the lens so that it is far from the sensor when focused.  A wide angle/short focus lens will have strongly curved surfaces, while a telephoto/long focus lens will have gently curved surfaces.

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5.  What is an f/stop, and why do lens aperture numbers get larger as the opening gets smaller?

The reason that we write it as "f/stop" is that it is a ratio of "f" (focal length) to "stop" (named for one of the early methods of reducing the size of a lens opening).  Modern lenses use an iris diaphragm to vary the opening.  It is placed roughly in the middle (from front to back) of the lens, between elements.  Camera lenses invariably have multiple elements, actually separate lenses that are combined to reduce optical aberrations (a subject for later...).  This diaphragm/aperture can be varied in diameter to control the amount of light getting through the lens (as well as depth of field).  To identify the size of the lens opening, we use the focal length divided by the diameter of the aperture.  For example, a lens of 100mm focal length with the aperture diameter at 25mm would be set at f/4.  If we open the diaphragm to 50mm, we have f/2.  If we close the iris to a 12.5mm diameter, we have f/8 (100/12.5). 

So, when we have a lens with a maximum aperture of f/1.4, it is a lens with a very wide aperture in relation to its focal length, and that is what allows a lot of light into the camera.  When we use the iris diaphragm to close down to f/22, the focal length hasn't changed - the number has gotten larger because the aperture diameter has gotten smaller.  Aren't ratios fun?

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6.  Why do digital camera descriptions talk about "35mm focal length equivalent"?

This is a convention that developed for the convenience of all the photographers who are accustomed to using 35mm film cameras with interchangeable lenses. The focal length of a lens is not changed by mounting it on a different camera. [Focus a lens on a very distant object, like a star, so that it produces a sharp image on a surface, whether film, digital sensor, or a piece of paper. The distance between that image and the primary node of the lens - essentially its optical center - is the focal length of the lens.]

Now we can explain the "equivalence" idea. The size of the image that a lens produces depends upon its focal length. So for example, if you point a 50mm lens at a person standing 10 feet away from you, the image of the person will be (very roughly) an inch high. Here's where the "format" of camera comes inito play. If it is a 35mm film camera, the exposed area of the film is a bit less than an inch by an inch and a half (24x36mm). The whole person might just "fit" in a horizontal orientation (though a natural orienation for a standing portrait would be vertical). Some professional digital cameras based on the body design of 35mm film cameras use a sensor of the same area as the film. However, most use smaller sensors. So, if we take that same 50mm lens and set up the same situation, except that we put it on a camera with a digital sensor that is about 18x27mm, we now find that only part of our subject, still a person standing ten feet away, will be recorded by the sensor. If we used a 50mm lens on a "pocket" digital camera, with an 8x11mm sensor, only about a third of the height of our originally frame-filling subject shows up. The area covered by the sensor is now about one ninth of what the original "full frame" (i.e., film sized) sensor recorded. This is why a tiny digital camera may be touted as having a zoom lens " 35mm equivalent" of 35-140mm, but say on the lens itself "6-24mm". The latter is the actual focal length range, while the former tells people experienced in the use of 35mm cameras that it offers a range of angles of view that covers moderately wide angle (35mm) to moderately long telephoto (140mm).

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