There's often a great deal of excitement whenever a new camera comes out. The specifications are pored over in great detail and judgement passed on forums and review sites. For some reason this happen less with new lenses. And yet, arguably, the lens is the most important part of a camera system. (Click here to read ‘How Do They Measure Camera Lens Quality’) A bad lens will drag a good camera down, a good lens will make even a ho-hum camera sing. So, in celebration of the humble lens here are four facts about lenses that you may not have known (you get extra points if you did).
They produce a circular image
The image circle of a lens designed specifically for APS-C cameras is too small for a full-frame camera. This is what happens when you try this...
Because photos are rectangular it's easy to forget that lenses produce circular images. The edges of the circle aren't used, which in many ways is a good thing (see below). Lenses designed specifically for APS-C sensor cameras (Nikon DX, Canon EF-S, Pentax DA and Sony DT lenses) produce a smaller image circle than those designed to work on full-frame cameras. The image circle can be smaller, because the APS-C sensor is smaller than a full-frame sensor. This means that if a APS-C lens is used on a full-frame camera the image circle won't be big enough to cover the larger sensor. The result? An ugly black circle around the edge of the image. If you hanker after a full-frame camera at some point it's not a good idea to only ever buy APS-C lenses for this very reason.
The centre of a lens is better than the edge
This image was shot at f/22. Despite depth of field being at maximum the shot has an overall softness due to diffraction.
Optically, the centre of a lens is better than the extremities. This is particularly true when a lens is used at maximum aperture. As the lens is stopped down (the aperture made smaller), the edges do typically improve and often get close to the centre in terms of optical quality but they never exceed it.
Lenses have an optimum aperture
Another common problem when a lens is used at maximum aperture is light fall-off at the edges of the image. Commonly known as vignetting, this is seen as a darkening of the edge of image relative to the centre.
Used at maximum aperture a lens generally isn't performing at its best. However, the same is true when a lens is used at its smallest aperture too. Although it's tempting to use minimum aperture to increase depth of field, the image will be soft because of an effect called diffraction. A lens is at its optical peak roughly in the mid range of apertures (the peak aperture depends on the lens and camera combination but f/8 is generally a good bet).
There are two types of chromatic aberration
Transverse CA is typically seen as red/green fringing on high-contrast edges...
Chromatic aberration is seen as colour fringing around high contrast edges. It's caused by the inability of a lens to focus the different wavelengths of light to the same spot. Axial CA is seen only when a lens is used at maximum aperture (particularly on lenses with large maximum apertures such as prime lenses) but disappears the smaller the aperture used. Transverse CA is seen more at the edges of images than the centre and isn't reduced by using smaller apertures.
...however, with a bit of postproduction magic transverse CA can be substantially reduced, even eliminated.
The solution? Unfortunately, this is one of those occasions when you get what you pay for. Expensive lens elements such as fluorite do substantially reduce Transverse CA, but these elements tend to be found only in the top-of-the-range lenses in a manufacturer's line-up. However (for a slight reduction in image quality) many cameras can now fix Transverse CA digitally when you use Jpeg, or software such as Adobe Lightroom can do a similar job in postproduction.
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