Effective Aperture and Macro

There are few genres of photography as technical as macro photography, and it seems like the closer to a subject you get, the trickier it can be to work out all of the variables of exposure, focus, and sharpness. While figures like magnification, aperture, and depth of field are still relevant in all arenas of photography, they take on a new level of importance when working close up. The reward to solving a puzzle like this, though, is dynamic and amazing close-up views that simply aren’t achievable with your own eyes.

To begin, it’s worthwhile reviewing some basic concepts of macro photography. Magnification, a key concept in macro, describes the size of an object on your sensor or film as compared to its size in real life. If you photograph a 1-cm-long object and it physically takes up 1 cm of space on your sensor, then you are working at a life-size, 1:1 (or 1.0x) magnification. If you photograph something that is 4cm long and it takes up 1 cm of space, you are working at a 1:4 (0.25x) magnification and, if you photograph something 1 cm long and it takes up 2 cm of space, you are working greater than life-size at a 2:1 (2.0x) magnification.

Magnification is solely controlled by focal length and focusing distance, with different combinations yielding different results (for example, a 50mm lens might focus closer than a 100mm lens, but a 100mm lens might have a greater maximum magnification due to its longer focal length). It’s also worth noting that the distinction of a “true” macro lens only belongs to lenses with 1:1 or greater maximum magnification ratios; conversationally, most include 1:2 and even 1:3 lenses within the macro category, but they’re a stretch, in technical terms.

When working at higher magnifications, a unique set of challenges becomes apparent, too. Achieving focus can be more difficult, depth of field is reduced, and there is the unique challenge of having a changed effective aperture. Put simply, when working at greater magnifications, roughly 1:2 or more, the displayed aperture on your lens or in your camera will be slightly different from what the true f-stop is, and this number will continue to change as the magnification of your shot increases. This is due to the focal length of the lens beginning to change as focus extension changes; since the lens is physically further away from the sensor or film, there is a change in exposure and f-stop. This increase in effective f-stop comes with a set of consequences, including a change in depth of field, greater potential for diffraction, and the requirement for increased exposure time to yield the same exposure as if working at a lower magnification.

The good thing about effective aperture is that your camera can meter for this new figure if working with through-the-lens metering and, in certain cases, the effective aperture may even be used in the metadata. On the other hand, if you’re working with a handheld meter, there’s a useful equation to help figure out the effective aperture at a given magnification:

f-stop x (1 + Magnification) = Effective f-stop

So, using this equation, we can figure that if working at a 1:1 (1.0x) magnification at f/4, your effective aperture will then be about f/8 (or 2 stops slower). This equation is just an estimation, and if even more specificity is needed then the equation can be expanded further to:

f-stop x (1 + Magnification / Pupil Magnification) = Effective f-stop

This addendum will mainly benefit specialized macro lenses that have a pupil magnification different from 1 but will be one of those instances where you’ll need to know this information beforehand to make use of the more specific equation.

With this concept of an effective aperture in mind, it’s important to consider the ramifications of an effectively smaller aperture beyond just needing to increase your shutter speed or ISO. If you’re working with an SLR, for instance, this smaller effective aperture may impact your ability to focus or successfully work with a viewfinder. For either SLR or mirrorless systems, the effective aperture will also impact diffraction, which is an optical consequence that can affect resolution. Landscape photographers are familiar with diffraction and how it can reduce apparent sharpness when working at smaller apertures, like f/22, for the sake of achieving a greater depth of field. With high-magnification shooting, the same is true, but diffraction can creep up sooner when working at higher magnifications.

Effective aperture is the determinate for diffraction with a lens, but it’s important to also keep in mind that this is theoretical and dependent on the actual quality of the lens in use, as well as the output size of the image and viewing distance of the final print or image. Nonetheless, it’s worth considering and weighing the differences between shooting at f/4 versus f/5.6 when the effective aperture difference then becomes, say, f/12 versus f/16.8 (if working at a 2:1 magnification). Just like other aspects of exposure in photography, this becomes a series of tradeoffs, and in this case, it’s weighing the differences between depth of field and diffraction.

Takeaways:

• The higher the magnification, the slower your effective aperture
• The higher the magnification, the more exposure needed
• Consider diffraction and depth of field with respect to the effective aperture versus the actual f-stop number on your lens
• Double-check or even triple-check focus when working at high magnifications, because depth of field is incredibly shallow, and diffraction is already working against total resolution
• Test your lens beforehand to become familiar with where diffraction becomes overwhelming; have an ideal working aperture in mind that you can settle on and try to hit that when calculating your effective aperture (adjust camera or subject position if needed to achieve the desired magnification)

Let us know if you have any questions about effective apertures or just macro shooting in general. It’s an interesting genre filled with unique challenges, but with the goal of producing spectacular close-up imagery. Let us know your thoughts in the Comments section, below.