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Exposure Theory

While working on a blog entry on graduated neutral density (Grad-ND) filters, I wrote enough on exposure that I decided to split that out into this separate blog. Typically, a middle exposure is described as one that is 18% gray, which means 18% of the way from pure white to pure black. This middle exposure is good for some subjects but not all because some are predominantly dark and others are predominantly light. Using an 18% gray exposure on a dark subject will overexpose it, and using that 18% gray exposure on a light subject will underexpose it. Additionally, the majority of subjects are a combination of light, middle, and dark tones and, since a single image can only use a single exposure, you have to choose the best for the overall scene.


HISTORIC NOTE: Before digital cameras existed some photographers would carry an 18% gray card with them, put the card in the same light as the final subject, aim the camera at it, dial in a middle exposure for that gray card, then use that same exposure on the subject to ensure correct exposure. Since every single photograph cost money to make due to the purchase of film and later processing fees, we wanted to ensure we had the exposure right before pressing the shutter button! With digital, my workflow is different because I set what I believe will be the right exposure, look at the resultant histogram, adjust as necessary, and shoot again.  As with gray cards, incident meters held in the same light as the final subject also indicate the proper exposure for that subject. On the topic of meters, the meter in your camera is a reflective meter, because it measures the light reflecting off the subject. In contrast, an incident meter measures the light falling on the subject.

Getting back to exposure, it is measured in stops, and stops indicate the amount of light reaching a piece of film or digital sensor. You can adjust exposure in full stops, in half stops, and even in third stops on many cameras today. Increasing exposure by one stop doubles the amount of light reaching the sensor, and decreasing by one stop cuts the amount of light in half. Thus, exposing three stops to the right/up would result in eight times the amount of light on the sensor more than before (2 x 2 x 2). Exposing four stops to the left/down would result in 1/16th (0.5 x 0.5 x 0.5 x 0.5) the amount of light on the sensor compared with before.


There are three factors which affect the exposure you get and, although each has a different base unit, all three can be measured in stops: shutter speed with a base unit seconds, aperture with a base unit of the fraction of lens focal length in mm to lens entrance pupil diameter in mm, and film/sensor sensitivity with a base unit of ISO/ASA speed rating.


Shutter speed identifies how long the shutter is open allowing light to reach the film/sensor. Faster shutter speeds like 1/500th second allow less total light on the sensor (decrease in stops), while slower shutter speeds like 2 seconds allow more total light on the sensor (increase in stops). How many stops difference are there between 1/500th second and 2 seconds? You can count by starting at either end and halving or doubling the amount. If we start at 1/500th, we'd next go to 1/250th, then 1/125th, then 1/60th, then 1/30th, 1/15th, 1/8th, 1/4th, 1/2, 1, 2. That was 10 steps, so the difference between exposures of 1/500th second and 2 seconds is 10 stops. Put another way, 2 seconds is 10 stops higher than 1/500th second in shutter speeds. If you noticed that 60 isn't half of 125 and 8 isn't half of 15 you are of course correct. Shutter speeds are expressed on a standard scale which is how I ended up with 60 instead of 62.5, and 8 instead of 7.5.

Aperture, which is also called f-stop, works the same way as shutter speed with full stop moves either halving or doubling the amount of light on the sensor. Smaller numbers (e.g. 1.2) represent larger circles (the lens' entrance pupil) which let more light in, while larger numbers (e.g. 16) represent smaller circles which let less light in. You can actually calculate the aperture scale since it is powers of the square root of 2 but, for simplicity, the most used portion of the scale at full stops is: 1, 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, 32. If you change aperture from 4 to 11, you have stopped down (decreased the amount of light) by 3 stops. If you change aperture from 5.6 to 4, you have opened up (increased the amount of light) by 1 stop. Say you had a shutter speed of 1/125th set at aperture f/2.8 and wanted the same exposure at f/8. What shutter speed would you need to set? Knowing you decreased the exposure 3 stops by stopping the aperture down to f/8 (from 2.8 to 4 to 5.6 to 8), you will have to increase the exposure 3 stops via shutter speed to get the same exposure. That 3-stop progression would be 1/125th to 1/60th, to 1/30th, to 1/15th. Thus, f/4 at 1/125th second is the same exposure as f/8 at 1/15th second. That relationship between aperture and shutter speed where a one stop increase in one requires a one stop decrease in the other is called reciprocity.


HISTORIC NOTE: Back in slide film days, this stop halving and doubling relationship between aperture and shutter speed didn't always hold true due to the chemical reactions happening in the film as it was exposed to light. This was called reciprocity failure, because a halving of shutter speed didn't always result in a doubling of exposure, nor did a doubling of shutter speed always result in a halving of exposure. For very slow shutter speeds (typically 1 second or longer), the chemicals didn't react as well to the low amounts of light reaching the film and the photographer would have to compensate for the reciprocity failure caused by those reduced reactions by intentionally dialing in one or more stops of additional exposure. That was typically done by setting even longer shutter speeds than indicated by the middle gray meter reading (i.e. intentional overexposure). Additionally, some colors would react better than others causing color shifts in the final slide. We're fortunate today to have digital sensors that always uphold the reciprocal relationship between aperture and shutter speed so we don't have to worry about reciprocity failure.

The final factor affecting exposure is ISO/ASA sensitivity. ISO/ASA is measured on a scale with very slow/in-sensitive films/settings being in the 25-50 range, average settings in the 100-400 range, and fast/sensitive films/settings above 400. ISO/ASA is just like shutter speed in that a doubling or halving of the ISO/ASA value results in an equal doubling or halving of the amount of light recorded. If you go from ISO 100 to 200, you have moved one stop up and doubled the exposure. If you go from ISO 800 to 100, you have moved down three stops and are recording only 1/8th the light you were at ISO 800. If it helps, the typical full stop ISO scale is: 25, 50, 100, 200, 400, 800, 1600. As stated previously, most digital cameras now allow adjustment of shutter speed, aperture, and ISO by half stops and some even by third stops.


How do you put these three factors together? Say you want to photograph a moving subject and are using a lens with a maximum aperture of f/5.6 (meaning the barrel wasn't designed wide enough to physically open up wider than f/5.6 to let in more light). You have the camera set to ISO 100 and the lens to the maximum aperture of f/5.6. The meter tells you the correct shutter speed is 1/60th second but the resultant photograph is blurry due to the subject movement. You need a faster shutter speed to stop that motion! I see six potential options.


The first option and one I'd try first would be to change the ISO to 200, 400, or even 800. At ISO 200 you could get a shutter speed of 1/125th second. At ISO 400 you'd get 1/250th second and, at ISO 800, you'd get 1/500th second. Hopefully one of those would be quick enough to freeze the subject's motion. If not, you could set the ISO even higher, although the chances for digital noise would also be increased.


The second option would be to swap to a lens with a faster maximum aperture of f/4 or even f/2.8. At f/4 you'd get 1/125th second and at f/2.8 you'd get 1/250th second both still at ISO 100 due to reciprocity.

The third option would be to use flash. By putting more light on your subject, you could get a faster shutter speed, at least up to the camera's maximum flash synch speed. You could likely get to at least 1/125th second this way with most cameras. Some advanced cameras support sync speeds of 1/250th second or even slightly higher.


The fourth option would be to leave everything as is but intentionally underexpose a few stops to get the higher shutter speed you'd need. You could change from 1/60th to 1/250th and intentionally underexpose 2 stops, then try to pull that detail back in the computer by adjusting the exposure back up there. The downside to this is that you'd likely have more noise in the resultant digital files caused by the underexposure.


The fifth option would be to use a combination of the above. Typically, I try to keep my ISO at 320 or less. I know I'll get grainy junk if I have to go all the way to 3200, so would likely set a lower ISO and intentionally underexpose than use ISO 3200. As long as I had time, I'd try the subject with both the higher ISO and lower ISO coupled with intentional underexposure, then keep the digital file with the best result afterward.


The final option would be to give up and walk away.


Sound advice for both slide and digital photographers is to expose for the highlights. By doing this, a photographer ensures the exposure of the highlight regions is sufficient to still capture detail and color and not get exposed beyond the sensor's range leaving detail-less pure white areas. Digital cameras can often be configured to warn you when this happens by flashing the pixels that are pure white during image review, and I always set my cameras to do this. If you have those so-called blinkies, simply decrease the exposure a bit and take additional photos until they go away.


Related, for digital photographers, I recommend you always expose to the right unless you don't want to do any post-processing at all. If your images are all exposed right to the edge of getting blinkies, but not over, you will have the most detail available later when you prepare them for sharing. I set up my cameras to show the histogram during image review, and I strive to put some data in the far-right with every subject, provided that results in no blinkies. If there are blinkies I keep decreasing my exposure 1/3 stop (typically via shutter speed) until they go away.

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