F2.0, F1.7 or even F1.4: In recent years, the aperture number or aperture ratio on smartphones has become a popular marketing tool. But what does aperture mean in mobile phone cameras? And how does the F number affect the quality of the image?
What are aperture and aperture ratio?
Strictly speaking, the terms aperture, f-number, and aperture ratio are mostly used incorrectly. The term “aperture ratio” is physically correct and describes the relationship between the entrance pupil and the focal length of the optical system. This ratio is normally given as a fraction normalized to the numerator one in the notation “f/1.8” or “1:1.8”.
The number f, on the other hand, is the reciprocal of this fraction and so is written as F1.8. The oft-quoted “aperture” is colloquial and is used in a mixed bag to indicate both the f-number and the aperture ratio. Since we are not a scientific publication, we at AndroidPIT also use the “Aperture” language.
Note: When calculating the aperture ratio, the entrance pupil and focal length are recorded in millimeters. Therefore, the result is always dimensionless.
Why is the aperture important for a cell phone camera?
Aperture plays an important role in smartphone photography, especially with these two points:
- The lower the f-number, the more light falls on the image sensor. This is logical, because the diameter of the entrance pupil is in the denominator of the aperture ratio. A double diameter halves the f-number, for example from F4 to F2. Since the amount of light is a measure of area and the diameter is a measure of length, a quadratically proportional relationship exists here. It means: Halving the aperture means four times more light. On the other hand, the amount of light doubles when the aperture is divided by the square root of two, for example from F2 to F1.4.
- The smaller the f-number, the shallower the depth of field. Since smartphones generally generate depth of field using an algorithm, this aspect remains a side note here. If you’re interested in how blur effects are created in optical systems, I recommend the following article, though it’s mostly about software-generated blur effects.
What does aperture mean on a smartphone?
If you look at your smartphone camera and roughly calculate the aperture, you’ll notice one thing: the focal length and (roughly) lens diameter don’t even begin to match. After all, aperture 2 would mean a 12.5-millimeter entrance pupil with a 25-millimeter focal length. However, you won’t find a multi-centimeter lens on any current smartphone.
The reason for this is that manufacturers always specify the focal length converted to the 35mm equivalent. Compared to a 35mm camera, the actual optical focal length of the lens system in smartphones is much shorter due to the small sensors. For example, the diagonal of a 1/1.7-inch sensor is 4.55 times smaller than that of a full-frame or 35mm sensor. Consequently, a smartphone camera with a 1/1.7-inch sensor needs a 4.55 times shorter focal length to achieve the same angle of view.
This relationship between the diagonal of a 35mm sensor and the sensor to be compared is called the crop factor or format factor. The actual focal length multiplied by the crop factor gives the 35mm equivalent focal length.
Since a camera’s depth of field depends on focal length and aperture, it’s now also clear why you can’t get as close to beautiful blur with F1.8 and a 50-millimeter equivalent focal length as you can with your DSLR at F1. 8. The actual focal length is decisive for the depth of field, and is usually between 5 and 15 millimeters. And that’s what bokeh software usually is.
But why is F1.8 better than F2.4?
While aperture has a significant effect on bokeh on full cameras, this effect is negligible on smartphones. Especially since smartphone cameras generally have no way to adjust the aperture and use it as a creative design option. But we’ll come back to that later.
Instead, the focus is on the intensity of the light. For example, an enhancement from F2.4 to F1.7 means that the smartphone has double the light available for the photo. This, in turn, allows:
- shooting at half ISO sensitivity. Half sensitivity means less image signal amplification, means less image noise.
- photography at half shutter speed. This reduces the risk of camera shake when moving quickly or in low light conditions.
So what is the difference between say F1.8 and F2.0? Realistically, he’s irrelevant. In the age of computational photography, image processing algorithms play a much larger role in resulting image quality.
Digression: why telephoto lenses are a disaster on smartphones
Incidentally, the details above also explain why telephoto lenses on smartphones mostly produce hair-raising results. After the focal lengths are comparatively high, the light intensities are mostly underground compared to wide angle optics. The Samsung Galaxy S20 Ultra, for example, only manages F3.5 with its telephoto lens. At the same time, telephoto lenses are much more prone to camera shake.
As a general rule of thumb, the S20 Ultra’s 103mm telephoto lens requires a shutter speed about four times faster than the main 26mm sensor (OIS should work just as well). At the same time, the difference between F3.5 and F1.8 also reduces the amount of light by a quarter. To compensate for identical lighting conditions, an increase in ISO sensitivity from ISO 100 to ISO 1600, for example, is required. Considering telephoto sensors, which are usually much smaller, it’s clear: that won’t work.
Aperture and image quality: high light, low sharpness
Before your brain takes a breather, I want to address one last aspect of the aperture: optical image quality. Building a fast lens is much more complicated than just placing a huge piece of glass in front of the sensor. Although light is practically not refracted in the middle of a lens, the curvature in the beam path is stronger towards the edge.
Unfortunately, light has the unpleasant property that the index of refraction depends on the wavelength. You know what sounds complicated by the reflected sunlight through the window that evokes a rainbow in your living room. This phenomenon becomes more and more pronounced with stronger light refraction, i.e. larger apertures, and has to be corrected more and more laboriously.
In technical jargon, fringes of color formed in this way are called “chromatic aberrations.” As a rule, they are stronger at the edge of the image than in the middle and occur mainly at high-contrast transitions, e.g. B. on branches in front of the bright sky. In order not to have the overzealousness of the datasheet quartet recognized for lousy test reports, Samsung has temporarily given some flagship smartphones a mechanical opening. In good lighting conditions, this covers the edge of the lens to minimize image errors.
Summary: A lot of fuss over nothing
So do I have to prick up my ears when Samsung, Honor & Co. tell me about record light levels? no Because the differences between F1.7 and F1.8 are marginal, other camera properties play a much bigger role here, such as the image sensor and algorithms.
Did you find this article useful? And what aspects of smartphones and especially their cameras still interest you? I await comments!