Understanding Depth of Field: How Focal Distance Affects DOF
This post is part two of an Understanding Depth of Field series. If you have not read part one you can view it here. Much of the material covered in part one is a crucial foundation of knowledge that will be helpful in understanding part two.
This video is seen from a third person point-of-view, perpendicular to that of what the camera user would see. Depth of field is a phenomenon of near and far, forward and backward from the point of focus. Changing our point of view rotates the axis of the depth of the field 90˚ so that we may view it laterally across the X axis. This helps us better understand the optic principles at play. The overlays in this video visually quantify the changing depth of field at the given lens setting.
Depending on skill level, each of you will likely see this video a bit differently. The goal of this video is to take all of the instruments of depth of field you've already read about, and orchestrate them in a way that is beautiful and advantageous to your personal composition.
The overlays in this video calculated based of the exact variables used in the experiment. Any changes in sensor size or focal length, would change the results.
How Focal Distance Affects Depth of Field
Interesting Characteristics of Depth of Field
You will notice that as the depth of field increases, it does not do so equilaterally from the focal point. Meaning if you were to focus 10 feet away, and your total depth of field was 2ft at a given aperture, it is not true that the focus range is exactly 9 - 11ft away.
The majority of your total depth of field exists beyond the focal point. This is because the increase in DOF is an exponential growth with larger apertures and further focusing distances. As you increase the viewable range of an exponential pattern from a given point, you will have much larger integers towards the positive end of the scale. This is why more is in focus beyond your focal point than in front of it.
In fact much of the depth of field illustrated in this video ranges from 49% in front of the focal point & 51% behind; to 25% in front & 75% behind (behind being away from your camera). The closer you focus or the more telephoto field of view you have, the more balanced this ratio becomes. The DOF Calculator I used shows results to the second decimal place, with many instances DOF equaling 50% in front & 50% behind. Though because the DOF growth is exponential, I suspect if one were to calculate to enough decimal places, a true 50/50 spread could never be achieved - Not important but quite interesting.
Hyperfocal distance is the distance at which to focus to achieve the maximum possible depth of field for your given lens and sensor. It can be defined as:
"The shortest focal distance at which the depth of field expands from half the focal distance to infinity"
The hyperfocal distance is a key tool for landscape photographers and street photographers. Part three of this article will address hyperfocal distance in more detail. Until then here is the formula to calculate it, though if you are using manual focusing lenses such as the one in this video, you can use the marks on the lens where the math has already been done for you.
Hyperfocal Distance Formula [pmath size=24] H=f^2/Nc+f [/pmath]
[pmath size=18]H[/pmath] is hyperfocal distance [pmath size=18]f[/pmath] is focal length [pmath size=18]N[/pmath] is f-number [pmath size=18]c[/pmath] is the circle of confusion limit
How to Apply these Principles to your Work
It's commonly accepted that for portraiture (for example) you should not be using small aperture numbers such as f/1.8 or f/2.8. The subjects eyes will likely be in focus but the ears and nose soft. In typical portraiture focal distances, small apertures do not provide a depth of field large enough to fully encompass your subject's face. You can understand from the video why f/4 and f/8 could be better aperture choices as they provide a depth of field large enough to contain your subject.
You should also be also be able to see that if standing 15ft away with an aperture of f/8, it's easy to get a sharp picture of a large group of people without having to arrange them in a drum line.
Improving Your Skills as a Photographer
It is important to understand depth of field to ultimately be in control of the images you capture. In the two examples above your knowledge of how focal distance and aperture affects depth of field would help you get the job done more efficiently. Here at Cameraplex, we believe:
"If you can learn the difference between what camera settings are needed to get the shot, and what settings can be seesawed to enhance the story you are trying to tell - it will make you a better photographer"
Specific Equipment and Variables
Lens: Canon FD 50mm f/1.8 Manual Focus Lens ( Shot on FD 28mm & FD 135mm)
Distance measurements calculated in post by grid overlay using distances measures on camera on site.
Depth of Field calculated using DOF calculator and aligning field overlay to corresponding feet and inches along grid. Likely margin or error + or – 2″.
Animation of overlays done by key framing 6 different preset published variables in the form of a final cut pro template Made in Motion 5 using rigs and sliders with multiple snapshots based on DOF calculations.
BONUS: 'Making of' Statistics!
- 467 keyframes across Motion 5 & FCPX
- 66 Video Layers
- 12 Compound Clips
- 6 Coffees
- 101 Ducks