People often use sharpness and focus interchangeably when looking at an image. While they are related to one another, they are different. Sharpness is the degree of detail within an image while the focus is the area where detail is located in an image.
We define sharpness by two characteristics: resolution, and acutance.
Resolution is the level of detail within an image. It is determined by the number of pixels on your camera’s sensor. The more pixels your sensor has, the more light and detail your camera can capture. By increasing the number of pixels, you increase resolution and the potential sharpness of your images.
Acutance is defined as the degree of detail between objects in your image. The amount of contrast between each pixel determines Acutance.
The overall acutance in an image is determined by your sensor’s PPI (pixels per inch).
Higher acutance will result in higher levels of sharpness and greater detail. Conversely, a lower acutance will result in lower levels of sharpness and reduced detail.
Factors Affecting Sharpness
Several factors go into determining the sharpness of an image: the sensor, lens quality, and focus.
Two sensor qualities affect the sharpness of an image: number of pixels and PPI.
The number of pixels in your sensor determines your image resolution. Image resolution determines your image detail. It also determines how large your image can get before getting blurry. More pixels means higher resolution and greater potential for sharpness.
The pixels per inch (PPI) often referred to as pixel density tells you how compact the pixels are in your sensor. PPI influences sharpness by dictating how precisely pixels capture light.
For example, consider two sensors of different sizes with the same number of pixels. The larger sensor will have a smaller PPI, while the smaller sensor will have a larger PPI.
The larger sensor with a smaller PPI will produce sharper images because it can capture light more precisely (see image below).
2. Lens Quality
Lenses have a specific resolving power. The resolving power determines how well your lens can convert details into a digital image.
If your lens has less resolve power than your camera, it will limit how sharp your pictures can be. Meaning, your camera’s sensor won’t be used to its fullest ability.
It is essential that the lens you are using can handle your cameras sensor quality. Doing so will allow you to use your camera sensor to its fullest sharpness potential.
Focus affects sharpness in that only areas that are in focus can be sharp. Those that are out of focus will have little or no sharpness. While the level of sharpness can vary within the area of focus, the area of focus is where your sharpest elements will be.
Focus is the area in which details are located. It is the point where light rays converge on a digital sensor to create a clear image. Simply put, it is the location in your image that contains a clear, detailed depiction of the scene you were trying to capture.
Image focus is determined by your choice of aperture and shutter speed. Aperture influences sharpness by adjusting an image’s depth-of-field. Shutter speed influences sharpness by affecting motion blur.
Factors Affecting Focus
Aperture influences sharpness by adjusting an image’s depth-of-field. Depth-of-Field refers to the amount of space in your image that is sharp or contains detail. It can also be described as the distance between the farthest and nearest objects that are sharp and contain detail.
A small aperture will produce an image with a shallow depth-of-field. An image with a shallow depth-of-field will have a small area of focus (sharpness). Shallow depth-of-field is often used when taking portraits to place the subject in focus and blur out the background.
A wide aperture will produce an image with a deep depth-of-field. An image with a deep depth-of-field will have a large area of focus (sharpness). Landscape photographers often use deep depth-of-field so that the entire image is sharp and in focus.
That being said the depth of field dictates where the focus will be within an image. Only areas within the depth-of-field can be sharp and in focus. Areas outside of it will be out of focus and blurry.
If you want to learn more about depth-of-field and aperture you can read my article here.
2. Shutter Speed
There are two types of movement that impact focus: camera shake and subject movement.
Camera shake affects focus by creating blurry images. Blurry images occur when you move the camera while the shutter is still open. The result is light being spread across multiple pixels creating the blur in the picture. Camera shake can be avoided by using fast shutter speeds or a tripod.
Subject movement can also impact the focus of your image. If your subject is moving, you will need to use the appropriate shutter speed to freeze the motion.
For instance, if your subject is moving and your shutter speed is too slow, your subject will be captured at multiple positions in your frame. The shutter speed you use will determine if the movement in your photo will be in focus.
Sharpness vs. Focus in Photoshop
As we’ve seen, though focus and sharpness are closely linked to one another, they are quite different.
To put what we’ve talked about into perspective let us consider the image below as an example.
A shallow depth-of-field was used to place the fence in focus while leaving the rest of the image out-of-focus. In this case, the area that contains detail is the fence in the foreground.
Within this photo, we have varying levels of sharpness. The fence in the foreground has a high level of sharpness, while the background has little to no sharpness.
Now let’s look at what will happen to the appearance of the details in the image if we enhance sharpness in Photoshop. Below, I created a side-by-side before and after example image. On the left, we have the original image, and on the right, we have an enhanced version of the same image.
Notice that the sharpness slider had a significant impact on the part of the image where we placed our focus (the foreground fence and leaves). That is, the color and veins of the leaves have become more pronounced. However, if we shift our attention to the background, we don’t see a similar result. That is, the out of focus background remained unsharp despite the increase in the sharpness slider.
The main take away here is that because sharpness and focus are not the same, adjustments in sharpness will not affect focus.
Trade-Off: Sharpness vs Focus
Now that we’ve discussed how sharpness and focus relate to one another you may be thinking that to get the sharpest image you can simply choose to use a deep depth of field. Unfortunately, this is not always the case.
The choice between depth-of-field and sharpness is not straightforward because of something we call diffraction. Diffraction causes a trade-off between sharpness and focus. Let me explain.
Diffraction is the bending or shifting of light around solid objects. When light hits a solid object, the light rays split and change direction.
In photography, diffraction happens when light hits portions of the lens aperture blades as it enters the camera lens. This phenomenon is known as Airy disk diffraction. Airy disk diffraction causes the reflected light from an object to be spread out over a larger space on the sensor.
The size of your aperture will affect how much diffraction occurs in your image. Small apertures (deep depth-of-field) will produce more diffraction than wider apertures (narrow depth-of-field). Diffraction is amplified with smaller apertures because smaller opening causes more light to hit the aperture blades.
So, though smaller apertures produce deeper depth-of-field and larger areas of focus, it also creates more diffraction. This means that when you decide to increase the area of focus in an image by using a deep depth-of-field you also inadvertently decrease the sharpness of your image. That is the more of the image you want in focus; the less sharp the image will be overall.
Let us look at the image for a clear understanding. You can see that when diffraction occurs, the light bleeds into multiple pixels. This causes the light from different areas of a scene to be captured by multiple pixels, reducing the detail and sharpness of the photo. Ideally, the pixel should hold the light without bleeding. Minimal to no bleeding produces an image with maximum sharpness.
So how do you decide on the best aperture? This will depend heavily on your photography needs and your lens.
Aperture Sweet Spot
You may feel a bit helpless after reading the tread-offs between aperture and sharpness. While there isn’t a perfect solution to this problem, understanding your lens can help you solve parts of it.
Lenses are said to have an aperture sweet spot. At this aperture setting, your lens will produce its sharpest images with its widest depth-of-field.
Note that the sweet spot is not universal; it depends on the camera and lens used. There are two ways to determine the sweet spot on your lens: self-testing or rule-of-thumb.
Aperture Sweet Spot: Self-Testing
You can test for your aperture sweet spot in a straightforward and time-efficient way. All you have to do is take a photo at every aperture setting holding all other variables constant.
When self-testing, choose a scene were differences in detail and focus will be easily noticeable. I like to use a newspaper because the color and detail make it easy to distinguish the differences in each aperture.
Once you have a photo for each aperture, download your images and compare the difference in sharpness in each image. From here you can figure out which aperture has the best depth-of-field to sharpness ratio.
Aperture Sweet Spot: Rule-of-Thumb
If you don’t have time to test your camera or prefer not to test your lens you can use common rule-of-thumb. The rule-of-thumb is the sweet spot of a lens will usually be around 2-3 stops away from the maximum f-stop.
If your lens has a maximum aperture of f/2.8 then the sweet spot will be between f/5.8 and f/8. On the other hand, if your lens has a maximum aperture of f/4 the sweet spot will be between f/8 and f/11.
Using this information you can decide what aperture works best given the scene you are trying to capture.
When deciding on which aperture setting you want to use you must always consider what your photography needs are.
When shooting you must decide what your photo benefits from the most: sharpness or focus. Is your goal to maximize sharpness or maximize focus or somewhere in the middle?
If you are a portrait photographer, this issue is unlikely to affect you because you will rarely need to use a deep-depth-of field that will reduce your image’s sharpness.
On the other hand, if you’re a landscape photographer, this problem affects you more often. Landscape photographers often need to take into consideration the amount of sharpness and focus that best suits their needs.
There is a clear distinction between focus and sharpness. Sharpness is the degree or level of detail present in a photo while the focus is the area of an image that contains detail.
It is important to remember that there is a trade-off between the two when taking photos: the more focus you want in your picture the more sharpness you must give up in return.