Exposure 100

Film Speed Aperture Shutter Speed

Film Speed

In this article, we will learn about film speed, not different film types. Please see my Film Recommendation page for recommendations on specific film types.

Film speed refers to the sensitivity of a particular film. This is generally expressed as a film's ISO number. Sample ISO numbers are ISO 25, 50, 100, 200, 400, 800, 1600 with lower ISO films being less sensitive to light than higher ISO films. Note that a doubling or halving of a film's ISO number represents a doubling or halving of how much light is required to expose the film. For example, ISO 100 film is half as sensitive to light as ISO 200 film and requires twice as much exposure to yield an image. Likewise, ISO 100 film is twice as sensitive to light as ISO 50 film and requires only half as much exposure to yield an image. More sensitive films, such as ISO 400 or ISO 800 are said to be "faster" than films such as ISO 25 or ISO 50. Likewise, less sensitive films, such as ISO 25 or ISO 50 are said to be "slower" than films such as ISO 400 or ISO 800.

An important term for you to learn is the term "stops". A "stop" represents a halving or doubling of light intensity. Therefore, ISO 200 film is said to be one "stop" faster than ISO 100 film and two "stops" faster than ISO 50 film because if only requires half and one quarter the light intensity to yield an image as the ISO 100 or ISO 50 film, respectively. Likewise, ISO 200 film is one "stop" slower than ISO 400 film because ISO 200 film requires twice as much light as ISO 400 film to yield an image.

So why not just always use faster, higher ISO films?

Film chemistry dictates a tradeoff between film sensitivity and film grain. Slower films, such as ISO 25 or ISO 50, yield finer grain images than the faster films. This grain influences the apparent sharpness of the image.  Grain can start to show up as you make bigger and bigger prints from higher and higher ISO films.  Grain can become very noticeable, for example, on 8X10 prints made from ISO 400 film or on 5X7 prints made from 800 speed film.

Thus, for sharper images, you will want to use the slowest film you can get away with. This is why professional photographers usually use films with ISO ratings of 100 or less.

I shoot 50 speed film (Fuji Velvia) from a tripod-mounted camera for nearly all of my natural history work.  This film gives me very sharp and colorful images.  However, for travel, wildlife, or action shots, I will use 100 speed film (Kodak E100GX or E100VS) for a bit more speed.  (Sometimes I'll push this film to ISO 200 but the topic of pushing film is for another article).  If you shoot a lot without a tripod, you are often forced to use faster films so that you can get faster shutter speeds to help negate camera shake.

One final note on film speed.  Faster films extend the reach of your flash.  As a matter of fact, every doubling of the film speed doubles the reach of your flash (at the cost of high grain don't forget).  So if you are an indoor snapshooter, you will want faster films like ISO 400.

Aperture  

The aperture control on your camera controls how big the diaphragm opening will be in your lens at time of shutter release, which, in turn, controls how much light will pass through the lens to expose the film. This diaphragm opening, called the aperture, or f-stop, can generally be set to a value between f22 and f2.8, with f22 being a smaller opening and f2.8 being a larger opening (I know it seems a bit backwards). Generally, you will have choices of apertures such as f22, f16, f11, f8, f5.6, f4, f2.8. The aperture is usually selected by a dial on the camera body but the diaphragm is actually located in the camera lens. Thus, there is often an electrical or mechanical link between the camera body and the lens so that the camera body can control the lens diaphragm. Some older camera models use a ring on the lens to control the aperture.

It may not be obvious how the f-stop numbers relate to the diaphragm opening. The f-stop number is actually related to the reciprocal of the square of the diaphragm's diameter. Thus, in terms of the openings area, the inverse of f22 squared is about half as big as the inverse of f16 squared.

But the math is not important. What is important for you to realize is that each of these aperture settings represent a diaphragm opening generally half or double of the opening of the aperture before or after it. Thus, each step in the aperture scale results in a halving or doubling of the amount of light passing through the lens and exposing the film. Again, we call this halving or doubling of exposure a "stop". f16 is said to be one "stop" smaller than f11 and one "stop" larger than f22.

Not all lenses have all of the apertures listed above and some lenses have more apertures than those listed above. Lenses will often have apertures in between the apertures listed above, but the apertures listed above are considered the basic apertures while the apertures in between are usually considered secondary apertures. These secondary apertures are generally at one half or one third of a stop intervals between the basic apertures and allow even more fine tuning of the diaphragm opening than the basic apertures.

One thing to note here is that all lenses are designed such that an aperture on one lens passes the same amount of light as the same aperture on any other lens. Thus, f8 on a 28mm lens passes the same amount of light to the film as f8 on a 200mm lens.

The other very important aspect of aperture is how it affects the "depth of field" of the image portrayed on film. "Depth of field" refers to the depth of the area that will be in focus measured from the plane of focus. For example, if you are focusing on a flower 10 feet away, "depth of field" refers to the amount of area in front of and behind this 10 foot distance that will appear in focus on the film.

Smaller apertures, such as f22 or f16, result in greater "depth of field" and larger apertures, such as f4 or f2.8, result in less "depth of field". For example, if you focus on that flower 10 feet away, a smaller aperture such as f22 may allow everything from 16 inches to infinity to be in focus while a larger apertures such as f4 may allow only things from 9 feet to 12 feet to be in focus.

Please note that the lens focal length also plays a big part in the depth of field as well, with shorter focal lengths, such as 35mm, providing greater depth of field, and longer focal lengths, such as 200mm, giving less depth of field.

So, what aperture should you use?

There are times when you will want to maximize depth of field. For example, if you want a whole field of wildflowers from 16 inches away to the horizon to be in focus. You are likely to use f16 or f22 on a wide angle lens.

But there are also times when you will want a small, or shallow, depth of field. Perhaps you want to show a pair of fern fronds razor sharp tulip against a background of soft-focused ferns. If you focused a 100mm lens on the fern fronds you want to emphasize and used an aperture of f4 from a couple inches away, you could achieve this effect.

You should experiment with different lens focal lengths and different apertures to learn how much depth of field various combinations give you. These values can be calculated as well but the actual depth of field can also be seen through the viewfinder for those who have cameras with "depth of field preview" capability.  Unfortunately, this feature is becoming rare on most modern less-expensive cameras.

One final note on aperture selection. Be aware that most lenses are designed to be sharpest when used at the more moderate aperture settings like f8 and f11. When used at the extremes of their aperture scale, some reduction of image quality can result. This reduction in sharpness varies from lens to lens and can be insignificant or moderate in magnitude (and is often correlated to the price of the lens of course).

Shutter Speed  

The shutter speed control on your camera controls how long the shutter curtain will be open to expose the film. This duration can be set to a value between 1/1000 second to several seconds, depending on the camera model. Generally, you will have choices of shutter speeds such as 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1, 2, and 4 seconds.

Notice that each of these shutter speeds are generally double or half of the shutter speed before or after it. This is important to note. Each step in the shutter speed scale results in a halving or doubling of the amount of time the shutter is open and allowing light to expose the film.  We call this halving or doubling of exposure a "stop". (Beginning to see a pattern here?)  1/125 second is said to be one stop slower than 1/250 second or one "stop" faster than 1/60 second or would be called "2 stops" faster than 1/30 second.

Cameras will often have shutter speeds in between the speeds listed above, but the speeds listed above are considered the basic shutter speeds while the shutter speeds in between are usually considered secondary shutter speeds. These secondary shutter speeds are generally at one half or one third of a stop increments between basic shutter speeds and allow for even more fine tuning of the shutter speed and exposure.

The other important aspect of shutter speed is how it affects the way motion is portrayed on film, both subject motion and camera motion. Obviously, shorter shutter speeds result in a shorter duration of the shutter curtain being open. This results in a shorter slice of time being recorded on the film. Since a moving object will travel less distance during this shorter amount of time, it appears less blurred on the film. So, shorter shutter speeds reduce subject motion better than longer shutter speeds. Likewise, shorter shutter speeds reduce the effects of camera motion better than longer shutter speeds.

So, what shutter speed should you use?

For stopping subject motion, it depends on how fast the subject is moving in the viewfinder. This will depend on how fast the subject is moving in the real world, the direction of the motion relative to the camera, how close you are to the subject, and what focal length lens you are using. Generally, you will use shutter speeds such as 1/125 to stop swaying flowers in a wide angle scenic. Or 1/500 to freeze the motion of a runner 50 feet away. Or, 1/1000 sec to freeze a birds wing from 20 feet away. There are so many scenarios that you will simply have to experiment for yourself to get a feel for judging shutter speeds.

Of course there are times when you may not want to stop subject motion. An example would be if you are shooting a waterfall or fast flowing stream. Using a 1 second shutter speed will allow the waters motion to appear as a soft blur on film, thus implying the water's motion to the viewer. Try it!

For stopping camera motion while hand holding the camera, you will generally want to follow the "Reciprocal of the Focal Length" Rule. This Rule states that you can get sharp images when hand holding a camera if you use a shutter speed equal to the reciprocal of the lens focal length, or faster. For example, if you are shooting with a 60mm lens, you should be able to hand hold the camera at 1/60 second shutter speed, or faster, and generally get sharp images.

However, for best results, there are some things about hand holding the camera that you should read about on my Hand Holding a Camera page.

Also, when using shutter speeds in the order of a second or more, you may have to take into account reciprocity failure for some films. To learn more about this, read my Reciprocity Failure article.