Focus Shift Feature: Which Cameras Have It?

Focus shift is an optical issue that occurs due to spherical aberration, resulting in blurry images and focus errors. It is particularly noticeable when using fast aperture lenses and focusing on close subjects. While it is impossible to completely eliminate focus shift, it can be minimised. Some camera models known to have focus shift issues include the Canon EF 50mm 1.2 L USM and the Panasonic Lumix G 25mm 1.7 ASPH. Additionally, certain camera brands, such as Olympus and Panasonic, offer built-in focus stacking capabilities to address this issue.

Focus shift is a result of spherical aberration

Focus shift is an optical issue that occurs as a result of spherical aberration. Spherical aberration is a common issue in lenses and curved mirrors, as these components are often shaped in a spherical manner for ease of manufacturing. When light rays strike a spherical surface off-centre, they are refracted or reflected more or less than those that strike close to the centre. This deviation can lead to a reduction in image quality.

In the context of focus shift, spherical aberration causes the outer portions of a lens to focus light rays differently from the inner portions. When the lens aperture is stopped down, the outer light rays are cut off, resulting in a shift of the best-focus plane. This shift can cause blurriness and erroneous conclusions about lens sharpness, particularly when using "`fast`" lenses with large apertures.

The impact of spherical aberration on focus shift can be mitigated by using aspherical lens elements in the lens design. Aspherical elements help to correct the aberration, resulting in improved resolution at large apertures. However, lenses with aspherical elements may have worse bokeh, with out-of-focus areas exhibiting concentric rings of light and dark patterns.

To address focus shift, photographers can employ various techniques, such as avoiding affected apertures, focusing while the lens is stopped down, or getting to know the behaviour of their specific lenses and compensating accordingly.

It occurs when an object is brought into focus at maximum aperture

Focus shift is an optical problem that occurs when an object is brought into focus at maximum aperture and captured with the lens stopped down. It is caused by spherical aberration, which is when incoming rays of light converge at different focal points due to spherical aberration along the optical axis. This results in a blurrier image than intended and can lead to focus errors, especially when working with close-distance subjects and using fast aperture lenses.

When the lens aperture is fully open or "wide open", light rays converge at different focal points due to spherical aberration. The light rays passing through the periphery of the lens focus closer to the centre. As a result, the point of best focus shifts, and if the focus is not readjusted after changing the aperture, the sharpest focus plane will move away from the lens, resulting in a blurry image.

Most lenses prone to focus shift problems have very fast maximum apertures of f/1.0, f/1.2, and f/1.4. This is because a large portion of the lens surface is used to transmit light, and fast prime lenses with uncorrected spherical aberration will always have focus shift issues. Slower lenses with maximum apertures of f/1.8 and smaller will have less noticeable focus shift.

Focus shift can be a frustrating issue for photographers, especially when using prime lenses with a maximum aperture wider than f/2. It is important to note that focus shift is not an issue with macro-capable lenses, as they are designed to work correctly at very shallow depths of field. Additionally, video-oriented lenses may exhibit focus shift when used in photography but not in video work, as the aperture used for focusing is the same as the one used for shooting in video.

While focus shift cannot be completely eliminated, there are ways to minimise its impact. One way is to use the maximum aperture for both focusing and shooting, as focus shift only occurs when the lens is stopped down. Another option is to use slower lenses with maximum apertures of f/1.8-f/2.8, which have fewer issues with focus shift. Additionally, learning the behaviour of your lenses and compensating accordingly can help maintain the speed of operation.

It can lead to blurry images and focus errors

Focus shift is an optical issue that occurs due to spherical aberration, resulting in blurry images and focus errors. When an object is brought into focus at maximum aperture and the lens is stopped down, the light rays converge at different focal points, causing the sharpest focus plane to shift. This can lead to the desired subject being out of focus, such as focusing on an eye only to realise the nose is in focus instead.

The problem is more prevalent in lenses with fast maximum apertures of f/1.0, f/1.2, and f/1.4. This is because a large portion of the lens surface is used to transmit light, and these lenses often have uncorrected spherical aberration. As a result, light rays don't converge at a single point, leading to a blurrier image and a different sharpest focus plane.

Focus shift can be a frustrating issue for photographers, especially when investing in expensive lenses. It can result in missed focus, soft images, and a loss of critical detail. The issue is not limited to a specific brand or model of camera, but rather depends on the lens being used and its maximum aperture.

To counteract focus shift, photographers can employ various techniques. One method is to use slower lenses with maximum apertures of f/1.8 to f/2.8, as these lenses have fewer issues with focus shift. Another approach is to shoot at the maximum aperture, as focus shift occurs when the lens is stopped down. Additionally, getting to know the behaviour of specific lenses and compensating accordingly can help mitigate the impact of focus shift.

While focus shift can be a challenge, it is important to note that it may not have a drastic effect on all photographs. With the right techniques and a good understanding of lens behaviour, photographers can minimise the impact of focus shift and capture sharper, more focused images.

It is an issue mostly known to rangefinder camera users

Focus shift is an optical problem that occurs due to spherical aberration. It is when an object is brought into focus at a maximum aperture and captured with the lens stopped down. This can lead to blurry images and focus errors, especially when working with subjects at close distances and using fast aperture lenses.

Focus shift is an issue that is mostly known to rangefinder camera users. Rangefinder cameras are known for their ability to produce sharp images with a shallow depth of field. However, due to the nature of their design, they can be prone to focus shift issues. The issue arises because rangefinder cameras use a separate optical path for focusing, which can result in slight discrepancies between what is seen through the viewfinder and what is captured by the lens. This discrepancy can become more pronounced when shooting at wider apertures, such as f/1.4 or faster, as the depth of field is extremely shallow, and even a slight shift in focus can result in blurry images.

Additionally, some rangefinder cameras may not have autofocus capabilities, requiring manual focus. This can make it more challenging to achieve precise focus, especially when dealing with moving subjects or shooting in low-light conditions.

To mitigate focus shift issues, some rangefinder camera users may employ techniques such as focusing at a narrower aperture, using manual focus lenses with aperture rings, or utilising focus peaking features if available. These features allow users to more accurately determine the areas of sharp focus, helping to ensure that the intended subject is in focus.

It is worth noting that not all rangefinder cameras will exhibit focus shift issues to the same degree. Some models may have built-in corrections or design features that minimise the impact of spherical aberration, resulting in improved focusing accuracy. However, for photographers who frequently shoot at wide apertures or require critical focus, understanding the focus shift characteristics of their camera and lenses is essential for achieving consistent results.

While focus shift can be a frustrating issue for rangefinder camera users, it is not insurmountable. With careful technique, an understanding of one's equipment, and the use of appropriate tools, it is possible to capture sharp images that take full advantage of the unique capabilities of rangefinder cameras.

Some lenses are more prone to focus shift

Lenses with very fast maximum apertures of f/1.0, f/1.2, and f/1.4 are more prone to focus shift because a large portion of the lens surface is used to transmit light. These fast prime lenses with uncorrected spherical aberration will always have focus shift issues. Any fast lens with spherical aberration problems will exhibit focus shift at different apertures.

It's important to note that slower lenses with maximum apertures of f/1.8 and smaller that suffer from spherical aberration will have a less noticeable focus shift. This is because the depth of field is larger, and the intended plane of focus will fall within this range.

Additionally, mirrorless lenses are less likely to exhibit focus shift because they are newer designs and are not constrained by size limitations.

To address focus shift, camera manufacturers can design lenses with aspherical elements to reduce spherical aberration. Alternatively, they can incorporate focus shift data into camera firmware, allowing for automatic focus adjustments based on the lens and its aperture.

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