In the previous blog, I discussed briefly what focal length is and how it affects your images in practice. Another important feature of a lens is the Aperture, an opening of variable size measured in f/stops. Aperture is the main focus of this post. By the end of this article, you should be able to answer the following questions:
- What is and does aperture?
- How does aperture affect the look of your images?
- What does tell you the f/stop on a lens barrel?
- What is a fast lens?
- What is the sweet spot(s) of a lens?
- What are the optical problems of a lens?
- What is Vibration Reduction?
- What are lenses that are made specially for cropped sensor cameras
Aperture is an opening of variable size inside an interchangeable lens. Opening up the aperture allows more light to pass, and closing it down will reduce the amount of light that reaches the camera sensor. Aperture together with the shutter speed defines the quantity of light that passes through the lens. Using a wider aperture allows you to work with a faster shutter speed.
As depicted in Figure 1, the aperture’s diameter is measured in f/stops. The lower the f/stop the wider the aperture and the higher the f/stop the narrower the aperture. Moving from one f/stop to the next one, will double or half the amount of light falling on the camera sensor. For instance, f/2 allows twice more light to reach the camera sensor in comparison to f/2.8. Similarly, f/8 halfs the amount of light, in comparison to f/5.6. This is called a change of “one stop”. However, you can change the aperture in increments of 1/3 of a stop. For instance, between f/4 and f5/6, you can select f/4.5, and f/5. By each click you change the aperture by 1/3 of a stop, hence 3 clicks is one stop.
Figure 1. Aperture and its relation with the amount of light that reaches the camera sensor and the Depth of Field.
The f/stop that is depicted on a lens barrel
The widest aperture of a lens is always depicted on the lens barrel. In figure 2, the first lens is a prime lens with a focal length of 35 mm, and the widest aperture of 1.8 (depicted here as 1:1.8). If you have a zoom lens, the widest aperture might remain constant for the entire focal length range. In this case you will find a single f/stop on the lens barrel. For instance, a 70-200 mm lens, with the maximum aperture of f/2.8. This means, whether you shoot at 70 mm or at 200 mm, you can still shoot at f/2.8. Such a zoom lens is, however, expensive. On the barrel of most zoom lenses, you will find a window of the widest aperture, like f/3.5 – f/5.6, like the second lens in figure 2. This means, when you shoot with a focal length of 18 mm, you can select for f/3.5. As you start zooming in, the aperture will get automatically smaller and when you shoot with a focal length of 140 mm (for this lens), the widest aperture you can select is only f/5.6. This is a manufacturing strategy to produce more affordable zoom lenses, while limiting lens’ optical problems.
Figure 2. On a lens barrel you can find useful information about the lens. For instance, whether it is a prime or zoom lens, what the diameter of the lens barrel and the widest aperture of the lens is. The first lens here is a 35 mm prime lens, with a maximum aperture of f/1.8, which makes it a fast lens. Also it is a DX lens; only to be used on a cropped sensor cameras (in this case of Nikon). You will learn more about this at the end of this article.
Aperture and depth of field (DoF)
In addition to the role aperture plays in creating a desired exposure, selecting the f/stops will affect the depth of field in your images. A lens at its widest aperture, e.g. f/1.4, creates the shallowest depth of field. By narrowing down the aperture the depth of field increases. For instance f/16 creates a much greater depth of field in comparison to f/5.6.
Definition of a Fast Lens
A prime lens with a maximum aperture below f/2 is considered as a fast lens. Because it allows substantial amount of light to pass through the lens. This, in turn, allows you to work with a faster shutter speed. A huge advantage when working under low light conditions.
Sweet Spot(s) of a Lens
If your main goal is having the sharpest image possible, then it is advised not to shoot at the widest or smallest aperture of your lenses. Because each lens has some optical problems that are more obvious at the widest and smallest aperture. These optical problems reduce the sharpness and clarity of your images. While photographing at mid-range f/stops create sharper images. Each lens may have one or more f/stops – in the mid range- that create the sharpest image. These spots are called “sweet spot(s)”, which has been depicted clearly in Figure 3. Notice how the details on the edges will become softer and blurrier when shooting at widest or smallest aperture. While f/8 and f/11 produce very sharp images. Also note that you need to magnify your images, or print them in large formats to notice these changes. When reviewing images in smaller format, no difference can be recognised, Figure 4. Photographing solely at the sweet spot(s) of a lens might not be practical at all time. In addition, if your images are not viewed / printed in large format, it would be really hard to notice changes we see in figure 3. Nevertheless, if you want the crispiest image, avoid the widest and smallest aperture of your lens.
Figure 3-4. Images in figure 3, depict only fractions of those presented in figure 4. They are enlarged to 200% (twice larger than their original size). In particular, notice changes on the edges.
Optical Shortcomings of a Lens
Lenses may have different optical problems some are related to manufacturing imperfections, some are related to the physics of light. The main issues you need to be aware of, are; Aberrations, diffraction and distortions.
Aberration is loss of sharpness, and clarity of details or appearance of unrealistic color lines at the edges of subjects in the frame. This might be the result of the lens’ inability to focus light at precise points on the camera sensor. Aberrations are more common when photographing at widest aperture of a lens. Working with a smaller aperture will resolve this problem. Alternatively, some kind of aberrations can be reduced by higher quality lenses or in the post processing.
Diffraction is happening when light hits edges (of the aperture) and bends. It is more obvious when shooting with the smallest aperture of your lens (e.g. in figure 2 bottom right corner). This causes loss of edge sharpness and loss of clarity. Diffraction cab be resolved by avoiding to shoot at the smallest aperture of your lens.
When vertical (and sometimes horizontal) lines in your scene are altered, we are talking about distortion (another distortion is the distance distortion we discussed in the previous article, Figure 3). When using wide angle lenses vertical (and sometimes horizontal lines) seems to be bent outward, which is called barrel distortion. Alternatively, when using telephoto lens, these lines might be bent inward, which is called pincushion distortion.
Distortion can be minimised by some precautions when photographing, e.g. a building, by aligning the camera’s view correctly with horizontal and vertical lines in the scene. When shooting with a wide angle lens, avoid placing your main subject in the periphery of the image. Distortion can also be rectified in postprocessing or by using specialty lenses.
Vibration reduction (VR) or Image Stabilisation (IS) is a feature of some lenses that helps you to shoot hand-held with slower shutter speed and yet having a sharp image. This feature rely on the ability of lens elements to move gyroscopically when there is camera shake. In general, when shooting hand-held, it is advised to have a shutter speed that is faster than the inverse of the lens’ focal length. For instance, if you work with a 200 mm lens, then the shutter speed should be faster than 1/200 of a second. If your lens supports vibration reduction, you might be able to capture sharp images with a shutter speed of 1/50th of a second or even slower. This is of paramount importance when shooting under low light conditions and not being able to use a tripod.
Note: Different brands may call vibration reduction differently, for instance: Nikon calls it VR, Sony “Optical Steadyshot (OSS)”, Canon, Image Stabilizer (IS) and Tamaron “Vibration Compensation (VC)”. For more information, check the manual of your lens.
Lenses specially made for cropped sensor cameras
In a previous article, I discussed differences between full frame and cropped sensor cameras. There, in figure 1 and 2, you see a phenomenon that is useful to understand this paragraph better. A normal lens that is meant for a full frame camera, creates a large circle of light, from which only a part is captured by a cropped sensor camera (smaller sensor). Hence, some camera manufacturers have designed lenses for cropped sensor cameras. These lenses create smaller circle of light, fit for cropped sensors. Canon calls these lenses, EF-S, Nikon and some other manufacturers mark them with DX.
Note: if compatible, you can mount a lens manufactured for full frame cameras without any problem on a cropped sensor camera (the angle of view will become smaller). However, mounting a DX or EF-S lens on a full frame camera, should be avoided. Firstly, this may damage your gear, in particular if you have a DSLR. Secondly, because the circle of light, the lens creates, might be smaller than a full frame sensor, you will have a dark circle around your image.
How to care for your lens
I have discussed this in a previous blog. To learn more about this please click here.
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