If you’re familiar with film photography, the ISO of the film was effectively the speed of the film. The higher the ISO, the faster the film. ISO (pronounced Eye Ess Ohh) is actually an acronym for International Standardization Organization or more accurately, International Organization for Standardization. For film, ISO is an arithmetic and logarithmic scale, yet for simplicity the arithmetic number is used most commonly. As the arithmetic scale value doubles, the film sensitivity doubles. In digital photography it works very similar in theory, the lower the ISO value, for the same aperture or shutter speed, the brighter the picture. For fixed aperture and variable shutter speed, the higher the ISO, the faster the shutter speed. For a fixed shutter speed and variable aperture, the higher the ISO, the more closed the aperture is.
Sounds simple, right? Just jack the ISO up. Not so fast my friend. The higher the ISO, the more noise in the picture. For this post, I took a series of photographs in a controlled situation. I moved the ISO up from 100 to 6400 going up by double each time. I could have done logarithmic steps and gone 100-125-160-200-250, but I didn’t want too many photos, I just wanted a few to display some differences. I kept the camera in the same place each time and I tried to keep the light in the room the same. I let the camera automatically expose the shots, so the scale may shift as the shots get darker or lighter, but point remains the same.
Click on any of the images to see larger examples as these are constrained for blog viewing to 320 pixels wide. These first shots were taken with the aperture fixed at f/2 on my 24mm f/2 lens on my A77. I’ve also cycled the ISO 100 to the bottom for reference.
So at 320×213, there is really no huge difference. The real difference starts to show at higher resolutions. Here are closer representations at 100, 400, 1600, and 6400.
At ISO 800 I may have moved my body changing the light a bit as the shutter slowed down from the expected 1/200s to 1/160s, either that or the camera wanted something like 1/180s and it chose the lower value, which would be consistent with the shutter speed increases from 800 to 6400. ISO 800 is where the photo really starts to go south, noise wise. These were taken straight from RAW with zero post-processing. The ISO 6400 shot is just plain nasty. But I get the 1/1250s shutter speed, right?
So this brings up a good point. Sometimes ISO is associated with sports. High ISO means sports, right? For me, I look at ISO as a tool to figure out how wide I can be at a shutter speed I am comfortable with or a shutter speed for an aperture/Depth of Field I am comfortable with. My minimum level of comfort for hand-holding shots is around 1/60s for slow moving subjects and 1/120 for my kids, 1/250 for my son playing soccer. I like 1/400 for Hockey as it is fast enough to capture most detail, but leaves a streak if they’re playing fast.
The Sony A77 gets a lot of slack from high-end enthusiasts and professionals for its high ISO quality. As you can see in the samples, the smaller 320 pixel shots all look similar, however the close-ups reveal the weaknesses. The maximum resolution on the A77 is 6000×4000. To give you an idea of what that means, my monitor resolution on this laptop is 1440×900. Most HDTV screens are 1920×1080. The new “quad HD” screens are 3840×2160 at a reasonable $25,000 a pop. The maximum resolution width on Facebook seems to be 1024 pixels wide. So sometimes you’ll read or hear someone saying “ISO 6400 is usable” or “ISO 1600 is usable”. In my short experience and opinion, I have shot exactly three or four usable shots in ISO 3200 or 6400. Ideally I stay between 50-400, and sometimes I will go up to 800 if I really need to. 400 is where I start to get nervous. The first set look nearly identical at 1/5 width, however it gets really noticeable at 1600 and beyond.
So what about post-processing? I took the examples above and used the Noise Reduction Luminance slider, setting both to 50, on the ISO 1600 and 6400 shots. For baseline reference, I have included the ISO 100 unedited original.
The noise reduction feature is also a detail reduction feature, however if you’re worried about noise and are willing to sacrifice detail, the edited ISO 1600 doesn’t look bad, and the ISO 6400 is probably usable for the web and smaller prints.
So you’ve seen the difference in ISO from the f/2 shots. Next time I will keep the shutter speed constant and we’ll see the aperture difference. But at what cost?