Choosing Which Lens to Use

How does my Lens Choice Affect the Picture?

After realizing it's great light that makes a good picture, the next question is - how can I best capture this light?  This is the job of the camera lens.

When it comes to lens selection two questions come to mind.  First - "what focal length should I use?" and secondly - "what maximum aperture should the lens have?".  First, we'll look at what these two questions really mean, and then look at 5 lenses with differing focal lengths and maximum apertures and the images they produce. 

Selecting a focal length 

Choosing a lens focal length is more that just considering how close or far away you are likely to be from your subject.  In many cases, you can arbitrary decide how close you will to stand to your subject.  We call that zooming with your feet.  The selection of a focal length determines not only how wide the field of view will be, but the relationship between a your subject and the rest of the scene.

If you use a wide angle lens (typically 18 - 24mm) or a moderately wide angle lens (typically 28 - 35mm) then you will naturally bring more background into the picture.  Pictures taken with wide angle lenses give more context to the image.  Some of the greatest news and documentary photographers shoot almost exclusively with a 35mm focal length lens.

If you are a portrait photographer and wish to focus on individuals and couples, then you may want more attention to be placed on the individuals with less attention on the background.  For such images, a 75 - 100mm moderate telephoto lens is often used.  The narrower field of view brings less background into the picture.  It also has two other effects. First, is referred to as compression, meaning the background is also "pulled in closer" to the subject.  Secondly, for a given aperture, longer focal length lenses have less depth-of-field, meaning the background is more strongly blurred or out of focus.

If you wish to have your picture combine both context from the scene and focus on the subject, this calls for a "normal" focal length close to 50mm.  

Selecting a maximum aperture

We often refer to a lens that has a very large aperture (small f-stop number) as a "fast" lens.  One reason for this has to do with the fact that when we open up the aperture, we need to increase the shutter speed to maintain the same exposure.  Historically, photographers have used fast lenses so they could shoot under conditions with very low light.  With high sensitivity sensors, it's now possible to shoot with ISO values much higher that what we did with film.  This has reduced some of the need for fast lens for low light photography.  

Today, the desire to shoot with a lens that can open up to f/2, or f/1.4 or even f/0.95 has to do with the lens' ability to focus very narrowly.  The larger the physical lens aperture (small f-stop value), the more narrow the distance is over which it can be focused and the more out-of-focus the background (and foreground) are rendered.

The f-stop value is a ratio - that's why it's written f "divided by" a value.  F is the focal length of the lens. Let's say it's 50mm  If the f-stop is f/2, then the opening in the lens to let light through is 50mm / 2 or 25mm.  This means each time you decrease the f-stop rating for a lens, the larger the lens diameter needs to be.  Bigger lenses mean more glass, weight and cost. It is also far more challenging to re-focus light on to the sensor when the aperture admitting the light becomes larger.

Should I Use a Zoom or Prime Lens?

Many entry-level cameras come with a zoom lens.  It's less expensive and less cumbersome to simply have one lens.  A zoom lens can give you in one lens the ability to shoot with a focal length from possibly 28 to over 100mm, and longer zoom lenses are available too.  Sounds great!  As with most things, there's always a catch. 

First, to accommodate a wide range of focal lengths in one lens, the lens design has to reduce the maximum aperture.  Entry level zoom lenses often have a maximum aperture of f/4.0.  As a reminder, F-stop values increase by a factor of 1.4 for each stop  (mathematically this is the square root of 2).  If you wish to double the light through the lens and increase your ability to control the degree of focus / out-of-focus areas in your image then the next F-stop down is f/2.8, followed by f/2.0, f/1.4 and finally f/1.0 - where the aperture is equal to the focal length.  When you think about it in these terms, a zoom lens gives a lot of focal length flexibility, but with less depth of field control.

The second reason to consider prime lenses has to do with increasing our awareness of field of view properties mentioned above. With fixed focal length lenses we become more cognitive of the relationships of subject and background, context and compression, narrow and deep depth of field.  Most photographers will say that prime lenses make better teachers.  It does, however, come at a cost.

Filling the frame 

Regardless of whether you decide to work with zoom or prime lenses, there is one key point that many photographers struggle with and that is learning how to properly fill the frame.  This is particularly true with today's standard '2 by 3' ratio format (i.e. the height by width ratio).  Often the long dimension has irrelevant space - many times filled with "miles of blue sky" above the person's head, while the bottom of the image ends at the subject's waist.  There's a wealth of information about composition - things like the rule of thirds and removing vertical distortion, etc. that will improve your results very quickly.  

How about things like ASPH, APO or MTF Charts?

Each lens must do the complicated job of focusing light from the incoming scene to the micron size pixels on the camera sensor.  This is not an easy task.  Each lens is made up of many internal lens which must refocus light across the color spectrum.  Remember, when light passes through glass, the effect is that of a prism, with each wavelength having its own direction. 

ASPH stands for aspherical, meaning that the lens design includes a lens element that has a non-spherically shaped surface.  Most elements in lenses have a spherical curvature, which is easier to make.  An aspherical curvature can improve the refocusing properties in the lens design.  APO stands for apochromatic, and that it is corrected to properly refocus, light not just at the red and blue ends of the color spectrum, but in middle at green as well.  You can generally expect that an apochromatic corrected lens which show less purple or green fringing coming off of bright high contrast points in a picture.  An example of this can be seen in purple fringing on tree branches with bright skies behind them.

MTF stands for modulation transfer function and is a way to describe the resolution that a lens can achieve.  MTF charts generally show performance at multiple resolutions and from the center of the image outward towards the edge. Often a manufactuer will include MTF charts for the lens wide open and at other f-stop settings.  

Can a lens have a unique character?

In addition to all of the above, with the complexity that comes with any lens design, it also brings with it unique properties such as color cast and the ability preserve micro-contrast and unique out-of-focus rendering referred to as bokeh.  In addition, lenses have their own unique signature which can affect the qualities that we look for in images that speak to us. These aspects, though difficult to quantify, can make an important contribution to the artistic elements we try to bring to photography.