Monday, May 23, 2011

Lenses: Introduction to Focal Lengths

The focal length of a lens refers to the actual length of the lens from a point inside its front lens element to the point where it comes to focus -- in our case, that is on the camera sensor -- when the lens is focused at infinity.  Modern optical systems and zooms can make this calculation more exacting, but this definition is sufficient to understanding the elementary difference between lenses and how to use them creatively.  Focal length is typically expressed in millimeters for modern DSLR lenses. 

What we call a "normal" lens is one where the focal length is approximately equal to the diagonal of the sensor plane.  On a full-frame (35mm) sensor that would be a 50mm lens.  On the smaller APS sized sensor that would be a 35mm lens.  All other focal length categories are given relative to what is "normal".  For instance, a wide angle lens is one that is wider that a normal lens, while a telephoto lens is longer than a normal lens.  

Wide angle lenses take in a larger visual area that makes them most suitable for wide landscapes, while telephotos act more like telescopes for close-up viewing of distant subjects.  These are the obvious, practical reasons for selecting one focal length over another.  More importantly, however, are the creative characteristics each focal length has.  

This introduction will give a brief overview of the most common focal length categories and their characteristics.  Later posts will cover each focal length separately and in more detail.

These photos illustrate the difference between four common focal lengths. All were taken at the same distance from the subject.  Upper left is a 28mm wide angle (18mm on APS sensor).  Upper right is a 50mm normal lens (35mm on APS sensor). Bottom left is a 100mm portrait lens (65mm on APS sensor). Bottom right is a 200mm telephoto (135mm on APS sensor).  You can notice from the relationship of the lions head to the background columns that when different focal lengths are used from the exact same distance all they do is crop or take in more of the scene.  To gain a creative benefit from the different focal lengths you need to take alter the distance of the lens to the subject.  This is illustrated in the samples below.
The four broad categories of focal lengths are: wide angle, normal, portrait, and telephoto.  The main creative difference between focal length categories is the sense of perspective they give.  Wide angle lenses tend to expand space, while telephoto lenses compress space.

Take a look at the sample photos. The 400mm telephoto lens used for the photo on the left produces a compression of space between the jumps and the jumper.  The 24mm wide angle lens used on the right produces a more "rounding" effect on the jumper and puts the viewer right into the action.

The long 200mm telephoto lens on the left compresses the scene by bringing the Independence Hall building in Philadelphia right up to the statue in front.  A 35mm wide angle lens used on the right relates the two in a very different way.
For any given aperture a wide angle lens will have more depth of field, that is it shows more of the background in focus when the lens is focused on the foreground.  Depth of field will be covered on its own in a later blog entry.

Wide angle lenses:
A wide angle lens is one with a focal range from around 14mm to 35mm on a full frame sensor, or 10mm to 28mm on an APS sensor.  The smaller the focal length, the more area the lens covers.  A 14mm lens (10mm on APS) would cover a viewing angle of 114° (81° with APS).  On the other, longer end of the wide angle spectrum, a 35mm lens would have an angle of view of 63°  (44° with APS).

The most obvious use of a wide angle lens is to gain a wide view of the subject.
In this photo of a departing storm and rainbow over the Grand Canyon, a 24mm lens on a full frame camera provided plenty of surrounding detail by including the blue sky in contrast to the departing stormy sky.  Relating the central subject of the storm and rainbow to its larger environment told more of the story of what this scene was all about, and gave a better sense of the immense scale of the Grand Canyon.
Wide angle lenses could be used creatively by placing them very close to the foreground area of a scene.  This establishes a relationship of the foreground detail to the background scene.  When doing this it is often best to place the focus in the front, and by stopping the lens down to around f/8-f/16 to provide a very deep area of focus, called depth-of-field (more on this topic in a later blog).
In this photograph of the giant sequoias a 24mm lens on a full frame camera was place right next to the tree trunk on the left and the camera pointed upwards.  This enhanced the sense of scale and provided a better idea of how large the trees are.
In this cityscape a 35mm wide angle lens was used to create an interesting sculptural frame of the Chicago city skyline

Normal lens:

The normal lens of 50-60mm on a full frame camera (35mm on an APS sensor)  is most closely related to the natural perspective the human eye has of the environment.  It is best used when you want to recreate a scene that feels natural, as if the viewer is actually part of it.  In a creative sense, a normal lens places the viewer in the scene without drawing attention to the fact that the camera was even present.  It downplays the role of the photographer in the scene.

Use a normal lens when you want to achieve a realistic perspective between the foreground subject and the surrounding scene as in the samples below.

Many normal lenses come with a very "fast", or wide open, aperture.  This enables them to be used in very dark situations without a flash.  The photo above was taken with available light in a dark barn using a 50mm lens set to f/1.4.  A normal lens is very good at relating a subject to its background without distorting the perspective.
In portraiture the normal lens provides story-telling detail by relating the subject to the surrounding area

Portrait Lens:

A "portrait" lens is so-named because it causes the most natural perspective on the human face.  In addition, it has a slight telephoto effect that throws the background gently out of focus.  This serves to put more attention on the main subject.
This illustration shows the difference in the perspective properties of a wide angle lens versus a portrait lens.  The photo on the left was taken with a 35mm lens on a full frame camera, while the one on the right was taken with a portrait lens of 85mm.  You can easily see how the wider 35mm lens distorts the face, making it unnaturally rounder, while the 85mm lens produces a flattering, natural look.
 Portrait focal lengths range from 70-110mm on a full frame camera (50-75mm with an APC sized sensor)

Here an 80mm lens kept the background scene of the mountains slightly out of focus so that they tell a story of where we are but do not compete with the main image of the cowboy.
Two examples of the 100mm lens delivering a natural perspective to the faces.  Placing the mother and baby on the same plane in the top photo kept both of them in focus and at the same time kept the background softly out of focus.
Using the same lens for the bottom photo but placing the mother and the baby on different planes allowed the mother's face to be sharp and the baby's to be soft -- a result of the longer focal length of the portrait lens.

Telephoto Lens:

Telephoto lenses range from 135mm on up.  The upper practical limit of the telephoto range is 200-400mm (135-300mm in APS), but even longer  lenses can be had at 600-800mm.  In addition to magnifying distant objects, telephoto lenses have the drawback of also magnifying camera movement.  A shutter speed that might be sufficient to stop camera motion with a 50mm lens needs to by three times greater to stop the same motion with a 200mm lens.  In other words, every time you double the focal length, you will need to increase the shutter speed the same amount.  For instance, 1/250 second with a 100mm lens is equivalent to 1/500 second with a 200mm lens.

Fortunately, most modern telephoto lenses are equipped with a vibration reduction technology that can effectively freeze the action for you and allow lower shutter speeds to be used hand-held with long lenses.  Still, it is best to use very long focal length lenses on a tripod to minimize motion blur by increasing steadiness.  With extremely long telephotos it is almost impossible to steady them without a tripod or monopod.

Compressed perspective and shallow depth of field are typical characteristics of the telephoto lens.

This photo of a football shot with a 200mm lens compresses the lines on the field and makes them very out of focus due to the shallow depth of field.
Notice how in this picture of Prague all the buildings and statues are compressed together in a "telephoto effect".

A telephoto lens is a great way to relate a foreground shape to a background as in this example of the pine trees echoing the shape of the Grand Teton mountain range.


The real reason for choosing one focal length over another is not necessarily the practical one of simply including more or less of a scene, or putting you closer to your subject.  A more important point to consider when choosing a focal length is how it can contribute creatively to your image.  When you want to bring the viewer right into the action, a wide angle lens used up close would be the best choice. The compression effect of a telephoto lens will relate a foreground to background in a very different way than the expanse created by a very wide angle lens. You might want to switch to a normal or portrait lens in situations where you want to illustrate the subject in a more natural way.

In later blogs I will be providing an explanation of the important creative role the lens aperture of each focal length plays in the area of focus, or the "depth of field".

Wednesday, May 18, 2011

Photographing cities at night

The moment of perfect harmony:

The modern DSLR has made night photography easier than ever.  There are a few tricks you need to know to improve your photos beyond the ordinary.  First of all, let's discuss the equipment you will need.

A tripod or some sort of steadying device is mandatory.  Night exposures are long and camera blur will occur if the camera is not rock steady.  A cable release, while not necessary, is helpful for keeping your images blur-free.  The simple act of touching the camera to press the shutter button is enough to add some motion blur to the shot.  That said, I will also discuss some alternative work around for dealing with situations where you do not have a cable release or tripod.

Timing is the most important aesthetic element in taking night photos.  Photographs look better when there is some detail and color in the sky.  The ideal time to take your night shots is 10 to 20 minutes after actual sunset.  I call this time slot the "moment of perfect harmony" where the exposure for the sky is perfectly balanced with the exposure of the city lights. At this moment you will be able to record some detail in the sky, which often goes a bluish color.  Don't be afraid of taking night photos on cloudy days.  The sky will still turn into a nice deep blue background.  This is due to reciprocity failure, a topic I will cover in a later post.

Photographing into the west 10-20 minutes after the sun has set can add nice color and detail to the sky.  Notice how the lights of the city and the light in the sky are perfectly balanced.  In this case, proper timing of the shot gave plenty of detail everywhere with no heavy black areas. The exposure was f/5.6 at 4 seconds.
Here the photo of Washington DC was taken facing east so the setting sun was behind me.  The sky had no color or detail that was visible to the eye.  Nonetheless the camera recorded a very pleasing overall blue balanced against the lights of the Capital Building. Both situations had water foregrounds to reflect and amplify the colors of the scene. This also avoided a foreground that might have gone dead black.  Exposure of f/4 at 1/2 second.

Proper exposure is very easy with modern DLSR cameras.  For the most part, I find it easiest to put the camera in its "A" for "Aperture Priority" setting.  This allows you to select the lens aperture you want and have the camera choose the shutter speed.  While almost any aperture will do, f/5.6 or f/8 is often the most optimum optical setting for a lens.  You can check your results on the camera display and by looking at the histogram to see if it is balanced. You want the high parts of the histogram graph to be in the middle.  But be careful because your histogram can be skewed to one side or the other if there are large dark areas of very bright lights in the scene.  Best bet is to bracket your shot.  Do this by setting your camera to over expose by plus one stop and under expose by minus one full stop.  You can set most DSLR cameras to do this automatically for you, but that might be more trouble than it's worth for the few exposures you are going to take of a scene such as this.  My preference is to use the over/under (+/-) exposure button and dial on your camera.  With your camera still on the "A" setting, change the +/- setting to +1.  Take a photo. Change it again to -1 and take another photo. That should do it, but you could play safe and go for a +2 and -2 also.  You should be shooting in RAW because this will provide the most options for adjusting the exposure afterwards.  The JPG format will be too limiting if you do need to make exposure corrections on the image later on in post processing.

For this scene of the El Morro Castle in Old San Juan, Puerto Rico, I took the same photo several times over the 10-20 minute time frame.  My camera was pointing north.  The sun had just set to the left so the sky was brighter on that side.  In addition the lights on the castle were very dim and I wanted to be certain the exposure was perfectly balanced in both the sky and on the castle.  Later I had to dodge some of the light off the right side of the photo to bring it into further balance with the left.  To complicate matters even more, this photo was done in two shots that were later put together to make a panoramic image.  A steady tripod is absolutely essential for a shot like this done at a 1/2 second exposure.

Working without a tripod or cable release:

There are times when you do not have a tripod or cable release with you.  To steady the camera you will need to find a firm place to rest it.  Then you can prop something under the lens to gain elevation and angle it in the right direction.  I usually use my wallet or keys.  You can use the camera's self-timer feature instead of a cable release.  I usually set the time for a delay of 2 seconds.  Press the shutter and remove your hand from the camera and 2 seconds later it will take the exposure.  For extreme steadiness I often use a feature most DSLR cameras have to lock up the mirror.  When the mirror slaps up and down it causes a vibration that can blur your shot, particularly if you are using one of these emergency techniques for propping up the camera. That is the method I used to take the photo of the Coliseum below.

By the time I took this photo of the Coliseum in Rome the sky was already past its prime in the light envelope and came in dark.  To offset this I did a time exposure that included the light streaks from moving autos in the foreground.  The camera angle was very low.  In fact I placed the camera directly on the sidewalk and propped my wallet under the lens to angle it up.  In this case I used the camera in Manual mode and read the exposure for the scene without the car lights.  I wanted to keep the shutter open long enough to blur the lights of passing cars as streaks in the scene.  The exposure was f/8 at 5 seconds. I waited until I saw a large group of cars coming by and opened the shutter.  It took several tries to get it right, but the lights added interest to an otherwise dull foreground.
This scene of the Flatiron Building in New York was taken 10 minutes after sunset facing south.  There is still plenty of light in the sky.  An exposure of f/4 at 3 seconds was enough to record the passing cars as blurred lights.

The important thing to remember is that you want your pictures to have plenty of detail.  Try to avoid dead spaces of deep black.  Choosing the right time of day to shoot is critical, but proper placement of elements is just as important.
This dusk shot of the arch in Washington Square framing the Empire State building in New York is the result of carefully balancing the time of night with the artificial lights in the scene.  Photographed facing north the sky is a deep blue.  Any later and it would have been totally black and the shot would not be as interesting.  Adding to the interest is the light post on the right.  All of this is a result of achieving a perfect lighting harmony in the scene.