This article discusses adding a fourth dimension to create an exposure triangle pyramid to help us maximize our camera settings, and related photographic performance.
As photographers we commonly consider three ‘exposure triangle’ factors (aperture, shutter speed, ISO) when deciding on camera settings. There is a fourth factor that is sometimes overlooked which can also impact exposure… lens focal length.
Unfortunately we can get into the habit of stopping our lens down beyond what is actually needed, which can negatively impact our shutter speed and/or ISO value. We can also fail to appreciate how using different focal lengths can directly impact our other exposure settings.
As we know, our choice of lens focal length is one of the factors that impacts depth-of-field. Aperture, distance to subject, and the subject’s distance to the background are other factors that we consider. The shorter the focal length, the more depth-of-field we will have assuming that the aperture used, and the distance to subject, remain the same.
NOTE: Click on images to enlarge.
Obviously when we’re shooting in good light we can stop our lens down to achieve the desired depth-of-field without having to even think about it that much. We can typically shoot at base ISO and use an appropriate shutter speed to deal with potential subject movement.
Where things get more interesting is when we are faced with low light conditions, and we have to work within the limitations that our camera gear may have. For example, the photograph above was captured handheld in poor light, at Belleek Castle in Ireland with Nikon 1 equipment.
I knew from experience that I could shoot the Nikon 1 J5 up to ISO-3200 and still get good, useable images by doing some work in post with my RAW files. Going past ISO-3200 had proven somewhat problematic for me. The 1 Nikkor 6.7-13 mm f/3.5-5.6 zoom had vibration reduction (VR) built into the lens. So… I could confidently shoot handheld down to about 1/15 of a second when necessary.
The challenge was getting the deep depth-of-field that I wanted for this photograph while staying within the practical limitations of my equipment, and dealing with the poor lighting conditions. This meant shooting wide open at f/3.5, keeping my shutter speed no slower than 1/15, and my ISO no higher than ISO-3200.
In this situation, the choice of lens focal length was the key success variable. By using a focal length of 6.7 mm I was able to get deep depth of field, even when using an aperture of f/3.5. To get the entire scene in focus I needed to move in fairly close to the table in the foreground, and set my focusing point about 1.2 metres (~4 feet) away from my shooting position.
Here’s another example of a low light interior image, where an aperture of f/3.5 could be successfully used to achieve deep depth-of-field because a short focal length of 6.7 mm was utilized. The same technique of moving in close to foreground elements and choosing a focusing point near to my shooting position was used.
As we can see with the image above, landscape photography is another genre where using a shorter focal length (e.g. 12 mm) can be key to help achieve deep depth-of-field while maintaining base ISO. Rather than having to stop my lens down to f/8 or f/11, I could use an aperture of f/5.6 to get the desired depth-of-field because of the shorter focal length used.
Above, is another example of achieving deep depth-of-field while using an aperture of f/4. This image was captured handheld at night from the deck of a cruise ship. This was made possible because a short focal length of 10 mm was utilized.
It can be interesting to shift our thinking about aperture and lens focal length from being independent factors, to experiencing them as being joined at the hip. With them jointly impacting exposure.
On a personal basis I like to think of the exposure triangle with an added dimension… and picture it in my mind as an exposure triangle pyramid. This helps remind me about the importance of focal length, and how it can impact the exposures that I use for my photography. It also helps me push the functionality of my camera gear. Using shorter focal lengths can sometimes make the difference in being able to shoot at base ISO value, or having to crank it up a few stops.
One of the reasons that I love using zoom lenses is the depth-of-field options that having variable focal lengths can provide. This is especially true with shorter focal length wide angle zoom lenses to achieve deep depth-of-field.
Regardless of the camera gear that we may own, it can be very useful to consider our choice of focal length from both composition and exposure perspectives. Focal length can be an important fourth dimension when determining exposure settings.
Before habitually stopping down a lens to get the deep depth-of-field we need for a specific composition… we can consider how changing the focal length to a shorter one, can impact depth-of-field. It may result in a more effective exposure for our image, given the lighting conditions we face.
Technical Note
Photographs were captured handheld with the camera equipment noted in the EXIF data. All images were created from RAW files using my standard process in post. This is the 1,399 article published on this website since its original inception in 2015.
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