by John Gerlach

All photographers are familiar with the ISO settings on their cameras. ISO is an acronym for International Standards Organization which is also called the International Organization for Standardization. Film was assigned an ISO value depending on how quickly it responded to light. A slow speed film like Kodachrome 25 is assigned a low ISO value of 25. A film that is more sensitive to light such as Fujichrome 400 is rated at ISO 400.

ISO values could be written as a series. The slowest speed film I ever used was Kodachrome 25 which had an ISO of 25 so let’s start there. Here is the series:

25 32 40 50 64 80 100 125 160 200 250 320 400 500 640 800 1000 1250 1600

This series can actually be expanded on the lower and upper end, but the range shown here covers the ISO values you will most likely be using.

The purpose of assigning an ISO to any film is to be able to compare film speeds. In theory, any film that is ISO 100 would react the same way to light as any other ISO 100 film in terms of exposure. Obviously, the colors, contrast, and dynamic range captured by different films that all have the same ISO might be somewhat different, but the exposure should be similar. This system of comparing the light gathering ability of films by using the ISO system worked quite well for decades, though it wasn’t perfect.

Many photographers including myself often rated the film they used at an ISO that was different from the official one. While I always rated Kodachrome 25 at ISO 25, I often rated Kodachrome 64 at ISO 80, instead of ISO 64 to get a bit more color saturation. When I began using Velvia 50 when it first appeared, I soon learned to rate the film speed at ISO 40 to get the exposure I was expecting. When I tried Velvia 50 at the ISO 50 setting, I tended to get slightly underexposed slides, even when compensating for reflectance values. I did this throughout my Velvia 50 career which ended in 2004 when I switched to digital only. I never shot enough Velvia 100 to really get a feel for it in terms of film speed so don’t automatically adjust the speed for Velvia 100 to ISO 80 or anything else unless your results show an adjustment is necessary.

When more film speed was necessary, photographers commonly pushed a roll of film by one or even two stops of light. In dim light, at times I found it necessary when photographing wildlife to get a faster shutter speed to insure sharp images by pushing Provia 100 to ISO 200. To do this, load a roll of Provia 100 in the camera and set the ISO to 200. Shoot the entire roll of film at the ISO 200 setting and meter in the usual way. When you take the film out of the camera, mark it right away so you know is has been pushed to ISO 200. Essentially, if you metered correctly, the entire roll of film is now one stop underexposed. Tell the custom lab that processes your film to push the film one stop. They extend the first developer time a certain amount to make the image look properly exposed. Pushing film worked fairly well when it was necessary, but it wasn’t without costs. Pushing film usually cost more for developing, grain tended to increase, and the colors might shift a bit. Still, pushing film was an excellent solution when you needed more speed.

The ISO setting was a great way to calibrate your camera for middle tonality too. When we shot film in our field workshops, I always calibrated everyone’s camera the very first night. I did this by projecting a blank slide on the projector screen so I had a huge white area that was uniformly illuminated with a constant light source. I metered the screen with my camera to get an exposure value. Since I knew from shooting images all the time that my meter was accurate, I made sure every bodies meter agreed with mine so I had each participant meter the same white screen using the same ISO setting and similar lenses. Most camera meters were fine, but at least 10% were at least one stop off. If I found a camera that was underexposing by one stop, I had them divide the ISO value of whatever film they were shooting by 2 and set that. If they shot Velvia 100, setting the ISO to 50 would put one more stop of light on the film and the camera would no longer be underexposing, assuming the subject was metered correctly in the first place. If a camera was overexposing by two stops, setting ISO 400 when using ISO 100 film would darken the image by two stops and now the camera meter is properly calibrated for middle tone.

So far I have spoken of films so now it is time to switch this discussion over to digital cameras which use ISO too, but it is fundamentally different. When you put a faster film in your camera, the film is more sensitive to light. When you set a higher ISO on your digital camera, the sensitivity to light of the image sensor doesn’t change at all. The imaging sensors of digital cameras have a native speed which typically falls in the ISO 100 to ISO 200 range. Cameras have an ISO speed latitude range too. For example, my Canon EOS 20 offers a choice of ISO settings which include ISO 100, 200, 400, 800, 1600, and 3200. The ISO speed latitude ranges from ISO 100 to ISO 3200 in full stop intervals on a Canon 20D.

Since the native speed of the digital cameras sensor doesn’t change, how is it possible to have different ISO speeds in the camera? Digital cameras are marvelous electronic computers now. If you need to use a higher ISO such as ISO 400 to get more shutter speed, setting the camera on ISO 400 doesn’t change the native speed of the sensor. Instead, the camera is able to amplify the signal before digitizing the image. However, amplifying the signal increases the problem of digital noise which often appears as random blue or red dots in the dark areas of the image. The effect resembles grain that is found in higher speed slide films like Provia 400 and makes the image appear less sharp. The higher the ISO setting, the more digital noise you’ll have. Fortunately, which each generation of digital cameras, the noise problem is being reduced with in-camera controls. Plus, many software programs can also reduce the harmful effect of digital noise.

Some cameras also offer ISO speeds below the native sensor speed. For example, my Canon 1Ds Mark II camera offers an extended range option of ISO 50. When I shot slide film, I always used the slowest film I could get by with to avoid the grain of faster emulsions and to get the sharpest image possible. Is it a good idea to use the lowest ISO speed on a digital camera too? No! Using ISO speeds lower than the native ISO speed of your sensor tends to decrease the dynamic range which may blow out the highlights. The native ISO speed on your camera will give you the best quality and that is the one to use if there is no other reason to use a different speed. Both of my digital cameras have a sensor with a native ISO 100 speed. That is what I set for most subjects such as closeups and landscapes where shutter speed isn’t critical.

There are times when it is most advantageous to use other ISO speeds, even if digital noise or reduced dynamic range problems might be encountered. Let’s start setting the camera (not all cameras can do this) at an ISO less than the native sensor speed. I love photographing waterfalls and like the effect of using long shutter times in the one second plus range to make the water appear silky smooth. It can be a bit difficult to get to a four second exposure with the cameras ISO set on ISO 200. To get to longer shutter times, I set the ISO on my Canon 1Ds Mark II body to ISO 50, stop down the lens to f/22, and use a polarizer to knock out another two stops and to remove glare from the water and wet rocks or vegetation. I often wish I had a choice of ISO 25 and even ISO 12 for waterfalls and even waves crashing on a beach. If the lowest ISO setting on your camera is ISO 200, another way to get to long shutter times so you can make a silky shot of flowing water is be there at dusk. As the sun disappear below the horizon, eventually it will become naturally dark enough to shoot whatever long shutter speed your heart desires. Just be sure to take a flashlight with you so you can get out. I use the small flashlights that strap to your head (freeing up your hands) for this purpose all of the time. Another way to achieve long shutter speeds is to use a strong neutral density filter. These filters come in various strengths and are gray in color. I commonly use a B&W .9 neutral density filter to take out three stops of light which permits me to go from a one second exposure to an eight second exposure.

Photographing wildlife with telephoto lenses such as Barbara’s Nikon 200-400 zoom or my Canon 500/4 telephoto requires the use of fairly fast shutter speeds, even when used on a tripod, to ensure sharp images. When using long lenses, I normally set ISO 200 to make it easier to reach shutter speeds in the 1/ 250 second range. While the camera does amplify the signal a bit, the increase of digital noise is very minimal and in my judgement, not a problem at all. On cloudy days when the amount of natural light is much less, I routinely kick up the ISO to ISO 400 and even ISO 800 if absolutely necessary to maintain fast shutter speeds with surprisingly good results. Indeed, the quality produced by my Canon 1Ds Mark II at ISO 800 with noise reduction turned on is shockingly good. One thing is clear, digital cameras deliver much better results in the ISO 200-800 range than slide film. I never found an ISO 400 slide film that I thought was truly acceptable, but ISO 400 on my digital camera is completely acceptable.

When I first began photographing hummingbirds with high speed flash (in 2000), I used Velvia 50 to get vivid colors and maximum sharpness. The problem with using such a slow speed film though was my flashes had to be very close to the hummingbird, about 15 inches away. While hummingbirds aren’t afraid of the flashes (they perch on the flash units and cords), it was difficult photographing the birds without getting part of a flash in the image too. If I moved the flashes further away, then I had to change f/stops from f/16 to perhaps f/8. I had less depth of field to cover the hummingbird so this wasn’t a good option either. More powerful flashes could be purchased, but they cost thousands of dollars more than the ones I use today which are Sunpak 544s on manual.

Digital easily solved all of these problems. Since ISO 200 is so good on my cameras, I typically use it for hummingbird photography. Where I once shot ISO 50 Velvia, I now use ISO 200 on my digital camera. In effect, this makes my flashes two stops more powerful so I can move them back to a much more comfortable distance of about 22 inches. Now the flashes are easy to avoid so they don’t appear in the image and I also get to stop down a bit more too.

Being able to change ISO values has helped me in another way when using flash. While photographing a nesting woodpecker with two flash units, I guessed the distance from the flash to the nest cavity and set two identical flash units up. One flash was about four feet from the woodpecker nest and about 30 degrees to the left. This was the main light. The second flash (identical to the first) was set twice as far away from the nest and as close as possible without getting in the image to a line connecting the nest to the lens. This was the fill light that opens up the harsh shadows produced by the main flash. By doubling the distance, the fill flash is set to be two stops weaker than the main flash. By firing some shots and checking the histogram, I determined the best exposure happened when the aperture was set to f/32. I did not want to use f/32 due to diffraction which causes unsharp images at such tiny apertures. Since the camera was set on ISO 200, I quickly reset the camera to ISO 100 and now the best aperture was f/22 which provides plenty of depth of field and diffraction isn’t a huge problem, especially with a 500mm lens.

Depending on the camera model, you may have a choice of intermediate ISO values or not. My Canon 20D offers ISO settings in full stop intervals only such as ISO 100, 200, 400, etc. My top of the line Canon EOS 1Ds Mark II offers ISO values in 1/3 stop intervals. Does having ISO values in 1/3 stop intervals offer many advantages? I tend to like having fine-tuned controls on my camera as much as anyone. However, I haven’t yet discovered any real advantage in having 1/3 stop ISO intervals so at this point in time, I don’t feel anyone shooting a camera with ISO values in full stop intervals is operating with a significant disadvantage. While I may at times set odd ISO values of ISO 500 or 640, I could easily get by with ISO 400 or ISO 800.

Be aware that the quality from camera to camera at the same ISO may be different. The pixels that make up the image sensor are not all equal. For example, many small point and shoot digital cameras boast 6 or 8 megapixels. This sounds impressive, both don’t expect to get the quality of a digital SLR that also has 6 or 8 megapixels. Eight megapixels on a point and shoot are crammed into a smaller sensor than the SLR camera. This means the pixels must be smaller to accommodate all 8 million pixels. The smaller the physical size of the pixel, the greater noise problems it will have. If you boost the ISO on cameras with small sensors, the quality will definitely go down due to noise. With small cameras, try to stay at the native sensor speed whenever possible to reduce the noise problem. High end cameras with larger pixels tend to deliver much better quality because the noise is greatly reduced due to larger pixel size.

Probably the best way to decide what ISO is acceptable to you is to run a test. Try photographing a landscape that contains some black areas so digital noise readily shows up and shoot the same scene at ISO 100, 200, 400, 800, and perhaps 1600. Now examine the digital image carefully to look for noise artifacts. Perhaps you’ll find that there is little quality differences with your equipment between ISO 100 and ISO 400. Now you know when you need more shutter speed or depth of field, you can boost the ISO to 400 without suffering a significant loss of quality.

You will certainly notice some noise issues at ISO 800 and 1600, but if there is no other way to get the image, using software or in-camera controls to reduce the problem of digital noise may certainly make these images acceptable. When you do need to use high ISO values, be sure to avoid underexposing the digital image because this increases the noise problem. I prefer to shoot RAW images and tend to bias the exposure to the right side of the histogram to let each pixel get as much light as possible which improves color, minimizes noise, and provides for a better image when I process the RAW file with the Raw converter in PhotoShop.

I have calibrated a number of digital cameras and found them to be most reliable and accurate. Perhaps this is because nearly all of the cameras I have calibrated were virtually brand new, unlike the days of film cameras where I saw many film camera bodies that were 20 years old so they had a greater chance of being banged and knock out of proper calibration. I have only seen one digital camera so far that needed an adjustment and it was overexposing by five stops because it had been dropped. We knew the camera was way off because the histogram was clipping huge areas on the right side and most of the image was blinking at us on the LCD viewer. Using the exposure adjustment control didn’t offer a minus five stop compensation so we had our client meter on manual and then underexpose everything by five stops to get a great exposure. Since this was the beginning of the tour, this solution worked for the duration of the tour, but the camera still needed to be sent in for repair when returning home.

While we commonly adjusted ISO values to calibrate film camera bodies, this doesn’t work with digital. If your digital camera body is overexposing by plus two stops, setting the ISO from ISO 100 to ISO 400 will not darken the image by two stops of light because the camera will amplify the signal to produce an exposure that is two stops overexposed so the image will likely be clipped on the right side and the highlight alert will be flashing. To reduce the exposure by two stops, you could meter manually and set the exposure to be two stops underexposed. Or you could set the natural light exposure compensation dial to minus two. Most digital camera have a plus and minus compensation range of plus and minus two stops so any exposure corrections that fall in that range can be handled in this way. If you need more compensation, going to manual exposure may be the best option.

The huge advantage of digital ISO is you can change the ISO setting on the camera from shot to shot to accommodate changing amounts of natural light. While ISO 100 might work perfectly for a cheetah in bright sun, having a thick cloud block the sun for several minutes while the peak of the action was occurring could be a huge problem for film cameras because you might need to switch to a faster speed film to maintain shutter speed or depth of field. With the digital camera, it is a simple matter to boost the ISO to perhaps ISO 400 during the cloudy period and move it back to ISO 100 when the sun returns.

This ability to change the digital ISO value from shot to shot is a huge advantage over film and one reason I would never want to go back to film. The second enormous advantage is ISO 800 for example delivers much higher quality images than any ISO 800 film whether it is pushed to ISO 800 or not. Being able to shoot wonderful images using higher ISO values has opened up a world of possibilities. Just in the past week, I used higher ISO settings to make images that I would not have even attempted using film. While in British Columbia, I developed some feeder strategies (which will be a future article) so I could photograph hummingbirds in flight with natural light. By setting ISO 400 on my digital camera, I had enough shutter speed and depth of field to sharply record the heads and throats of the hummingbirds while allowing the wings to blur, just as you normally see. While you could do this with ISO 400 film, the grain in the fast film would greatly reduce image sharpness which isn’t a problem for digital cameras. Just yesterday Barbara was using her Nikon D2X at ISO 400 to photograph birds at our feeder on a cloudy day. Since she could use ISO 400, she still had enough shutter speed to make sharp images. With Velvia 100, the shutter speed would be in the 1/30 to 1/60 range, making it difficult to achieve sharp images with a 500mm lens even on a tripod. Using ISO 400 on the digital camera got her back to the 1/125 to ½50 second range that she needed.

Being able to use ISO 800 proved to be quite useful this past winter when a Northern Pygmy Owl perched near our bird feeder late one winter evening. While the light was fading rapidly, I was still able to make totally acceptable images of this unusual visitor with my digital camera. By setting ISO 800 and shooting my 500mm lens wide open at f/4.5, I was able to get enough shutter speed to make sharp images of this seldom seen bird who was completely unafraid of me. Due to the very low light levels, I wouldn’t have even attempted to photograph this bird with a film camera. The owl perched near the bird feeder for about thirty minutes before flying to a thick conifer in my yard to roost for the night. Northern Pygmy owls are quite active during the day and commonly hunt small birds. While it didn’t make a kill that night, I noticed freshly picked tiny feathers drifting across the yard in the morning. I suspect one of the hundreds of chickadees that frequent our feeders became a meal for this fascinating predator.