Part 9 - Those ISO Settings

If you think about the film-based pictures that you have seen, you may have noticed that some pictures were sharp, very detailed, and the surfaces in the pictures "smooth" in appearance. On the other hand, some pictures you've encountered may have been sharp, but the surfaces and dark areas of the picture seem to be "peppered" with a noticeable sand-like grain. This usually unwanted effect was caused by the film the photographer selected. Traditional film is categorized by its sensitivity to light. This light sensitivity is expressed in a numeric ISO value (determined and assigned by the International Organization for Standardization).

Photographic Jargon Alert: Films with high ISO values (usually ISO 400 and above) are often referred to as FAST films. On the other hand, films with low ISO values (usually ISO ratings below ISO 100) are called SLOW films. Each term refers to a film's sensitivity to light.

This "light sensitivity" concept has carried over to digital imaging, and has a significant impact on the flexibility and quality of your photography.

• The first rule is: The higher the ISO value, the more sensitive the film (or digital camera) is to light. This is literally true in the case of traditional film. However, in terms of digital photography, the camera does not literally become more sensitive to light as the ISO values increase. Instead, the signal generated by your light sensitive sensor is electronically boosted to give the illusion of more sensitivity to light. This digital reality is more appropriately called "noise" than grain. But the term "grain" has been allowed to carry over to digital photography. In the end, the visual appearance of film grain and digital noise is nearly identical.
• The second rule is: As the ISO value increases the visual appearance of "grain" becomes more noticeable. So, pictures taken with DSLR cameras at LOWER ISO values have less visible digital grain. This results in images that appear sharper with the least amount of distracting grain.

ISO settings are located in the camera's menu. These choices typically range between ISO 100 and ISO 1600. That's right, at ISO 1600 your camera amplifies the light signal from your sensor by the equivalent of four f/stops. Today's DSLRs are pushing ISO values to new heights. Some cameras provide ISO values of 12,000+. (I can only assume that we are heading toward a capability of photographing action shots of football games by moonlight.)


The Nikon D200 displays ISO choices on a scrollable screen. Here, the range of user-selectable ISO values ranges from 100 to 1600.

If the sharpest (least amount of digital grain) pictures that a camera can take are at an ISO setting of 100, then why would I ever use another value?

Good question.

It all has to do with your shooting conditions. If you shoot only outdoors in bright sunlight, you could use an ISO value of 100 and always be a happy camper. However, if you shoot under low light conditions -- like late in the day, indoors without a flash, at night sporting events, etc -- then having the ability to change the camera's ISO value is crucial.

An example should help to illustrate this point. I frequently shoot pictures of my daughter's softball games. In this shooting environment, I need a reasonable amount of depth of field, and I definitely need a fast shutter speed to stop the action of the players. For daylight games, an ISO value of 100 works fine. I can typically shoot in the range of f/8 at 500th of a second. These settings satisfy my depth of field and action stopping requirements.

Shooting night games with the only illumination being supplied by the field lights is another matter. That same aperture setting of f/8 might require that I shoot at a shutter speed of 1/60th of a second. This shutter speed just isn't fast enough to stop the action of the game.

If I increase the ISO value to 200, my camera's apparent light sensitivity doubles. Now I can shoot at f/8 at 1/125th of a second (increasing my shutter speed by one full setting). If I increase the ISO setting to 400, I have again doubled the light sensitivity (actually amplifying the digital signal) to allow me to use a shutter speed of 1/250th of a second. And by going one step further to ISO 800, I am now back at the f/8 and 1/500th of a second my shots require. (The math here is exactly the same as the examples used in my Part 3 post on "Doing f/stop & shutter speed math")

With each increase of my ISO value (which boosted my light by a factor of 2), I was able to increase one full shutter speed (to reduce the total amount light entering the camera by 50%). The net result is the same total amount of light striking my DSLR's sensor to give me the correct exposure because the light signal was amplified by my camera to provide the sensitivity that my shooting conditions demanded.

But always remember that in photography whenever you gain some capability, you usually sacrifice something else. In the case of higher ISO values, you gain the ability to take pictures under lower lighting conditions. However, you INCREASE the visible "grain" or electronic noise in your pictures.


Here's a picture of some of my high-tech photographic equipment. The picture above was taken with an ISO setting of 100, and the lens area of the picture was enlarged in the picture below to show the detail in the image.

This enlarged section of the first image allows you to see the detail and
general appearance of a picture taken with an ISO of 100. Actual settings: f/8 at 1/15th of a second.

The picture below was taken with an ISO value of 1,600:

Enlargement of ISO 1600 image. Both pictures are shown "as shot" with no image enhancement. Actual settings: f/8 at 1/250th of a second.

Arrange this page on your monitor so that both images can be compared. Even at this small size, you should see the increase in "grain" in the ISO 1600 picture (look at the glass in the camera lens, and the smooth black area below the f/stop dial in the lower portion of the picture). Obviously the more this picture is enlarged, the more noticeable the "grain" will be.

This is the trade off.

The correct ISO setting is the lowest value that will allow you to use the f/stop and shutter speed settings required by your shooting environment.


PART 9 UPDATE (December, 2008):

I recently received an excellent question from a student of this blog, prompted by the ISO settings that appeared in the pictures below:



These images show the ISO selections available on a Nikon D200. The question concerns the ISO steps BETWEEN the full stops that I used in my explanation above. Specifically, how does the math work for ISOs like 125, 160, 250, 320, etc. that are less than full f/stops apart?

Understand firstly that there is a huge carry over here from traditional film ISO ratings to those available on digital cameras. So, there is a traditional film that's rated at 160 and 320. Digital camera manufacturers (man of whom were also the leading traditional camera makers) have provided these ISO settings as a convenience for traditional film-based photographers who have honed their photographic skills on these ISO films.

From a math point of view, nothing changes. For example, an ISO rating of 125 is 25%, or one-quarter of an f/stop, more sensitive to light than a setting of 100. An ISO rating of 160 is 60%, or about one-half of an f/stop, more sensitive to light than ISO 100. An ISO setting of 250 is twice as sensitive as ISO 125

Now for the professional reality of ISO settings. Most professionals will choose between ISOs of 100, 200, 400, 800, 1600, etc. These f/stops are all multiples of 2, and each is twice as sensitive to light as the previous ISO setting. In practice, these choices of ISO settings are as precise as needed. The reason is that the "in-between" ISO settings don't provide any detectable technical advantage.

For example, the difference between ISO 1000 and ISO 1250 is so small that leaving the camera set at ISO 1000 and simply using a larger aperture setting or slower shutter speed to achieve the same exposure RESULT is easier and quicker than changing ISO settings. Remember: In digital photography there is a quality reason for wanting to use the lowest ISO rating possible. The lower the ISO setting the less "noise/grain" will be visible in the final image. So why use a "noisier" ISO of 1250 when you could just as easily increase your exposure by one-quarter of an f/stop at ISO 1000?

I hope this ISO discussion has been helpful. If you have any questions or comments, you know where to find me.