What are some real life practical examples of how lens aperture affects Depth of Field (DOF) and various forms of apparent sharpness?
Changing apertures, sensors sizes or lens focal lengths have real world implications for several forms of distortion effects related to perceived visual sharpness.
For many years of shooting, I had just stopped down to increase DOF or opened up to increase Bokeh while throwing my backgrounds out of focus. These, it turns out, are not without significant repercussions.
As the technical aspects and math are covered in brain numbing detail elsewhere, let’s keep it simple and see what the factors are and how we can use them for our photography.
What Effects DOF?
- DOF Decreases as Apertures Increase
- DOF Decreases as Lens Focal Length Increases
- DOF Increases as the Center of Focus Distance from the film plain Increases
- DOF Decreases as Sensor or Film size Increases – for a given angular field of view
For those of you shooting film, film is just another sensor. Except for its flatness, as film is not perfectly flat, these discussions apply equally well to film and digital sensors.
These first four (4) bullets on DOF are just physics. Take them to the bank as is or read up on the whys and do all the math. Regardless, it just is. They just are.
The physics here apply to all lens and cameras.
What Effects Apparent Visual Sharpness as DOF Varies with Aperture?
- Diffusion Increases as a Blur type Sharpness Factor with Smaller Apertures
- Shorter Focal Lens are more effected by Diffusion at a given F Stop
- Small Glass Imperfections become dominant at Small Apertures
- Sperical Aberration Increases as a Sharpness Factor with Larger Apertures
- Coma Increases as Distortion Factor with Large Apertures
- Bokeh Increases at Large Apertures as DOF becomes paper-thin
- As Print or Viewing Size Varies, this will impact apparent distortions
For example, the sample shots here show different details of distortion at different scales. The shot at F 32 seems pretty clear seen small. It shows significant softening in the detail area when seen larger. The Red of the Chromatic Aberration, hard to see in the blow up detail shot, is a bright red spotty area on the tiny scale shot of the whole scene.
The interesting note, for those of us using aperture as a way to control DOF in our compositions, is the very real effect aperture has on various forms of Apparent Visual Distortions and Sharpness factors.
Just cranking down the F Stop to F 32.0 may not give us the sharpest flower picture we were looking for after all.
Not to make too fine a point of it, while Distortion and Sharpness are not exactly the same, they are used somewhat interchangeably here. Any base examples assume a 35mm format and usually apply closely to 35mm crop sensor format as well.
Visual Sharpness Factors with Aperture Explained (see 1-7 above):
1> At F 11.0 and beyond, significant Diffusion increasingly creates a softening of details. If you were stopping down to get fine focus on a wide range of depths around your center of focus point, you may actually be losing some detail you were hoping to gain.
Know your lens. Test and see.
2> The aspect ratio of depth to width of a pin hole effects diffusion distortion as light bends around its edge. A 10mm lens at F22.0 has a very fine narrow pinhole, where the depth of the diaphragm metal becomes a significant factor. The light here is kind of traveling through a tunnel, instead of a fine bladed hole. On a 1000mm Lens, the actual diaphragm at F22.0 is quite a bit larger in diameter than the 10mm’s and is more of a hole in a thin blade than a tunnel.
3> As we stop down, the amount of lens glass used to create the image shrinks from most of the lens, to a very tiny bit of glass in the center. Any imperfections in this small bit of glass get exaggerated in some form of distortion. Even a piece of dust on the lens, at F8.0 is unnoticeable, and at F32.0 can be quite the blurry eye sore.
4> Aberration, or more properly chromatic aberration, is the nature of light to refract differently based on its wave length and so focus at a different point. Red light, passing through a lens, will focus at a plain different from Blue light. As the focus point is narrowed by opening up your F Stop, the point of focus between the different colors of light become significant and create a blurring effect on your sensor.
5> Coma is an effect where points of light become big, and bigger masses of blurred lights. This effect occurs at large apertures. It can be disconcerting. Or you may like it in some shots.
6> Bokeh is an Anglicization of a Japanese word “boke” which means fuzzy. Many times we like to open a lens up, creating a thin DOF to isolate our subjects. Bokeh is the fuzziness in the places that are out of focus.
The number of blades in the diaphragm affect the shape of the blurred images. Many people prefer more blades as having a more perfect Bokeh.
Remember, when obtaining Bokeh, you are creating Chromatic Aberrations and Coma.
The in and out of focus area is itself a kind of visual distortion.
Also, your point of focus may be razor-thin.
As this DOF narrows, the actual area in focus may not be quite so flat either. The in focus area may be kind of more noticeably spherical.
This makes focusing often quite difficult at wide apertures.
7> How big must your final product be?
Distortion visible on an 8×12 print may be imperceptible on a 2×3. Scale counts.
Factors here include the ratio of your sensor size to final product, the absolute size of your final product and the intended range of viewing distances.
The long and the short of it is, if you require ultimate sharpness, most lenses must be shot within about an F 5.6 to F 11.0 range. Perhaps a bit narrower even. As a rule of thumb, you must stop down or open up a few stops from the maximums of any given lens.
Really though, you need to shoot and test your equipment in a controlled environment and learn what works how for you when.
Maybe a bit of softness makes your model’s skin or hair render a touch better. Maybe a touch of razor sharpness brings out her eyes?
Testing was done using:
Comment to discuss, so we may learn from your thoughts, experiences, opinions and questions.