The performance of the lens is one of the most critical parts of producing an image. There can be large variations in the images produced by different lenses. This is not necessarily a notion of better or worse only different. Understanding the strengths and weaknesses of lenses helps photographers choose the lens with optical characteristics to suit the compositional objectives in their image.
Photographers who conduct their own testing gain an understanding of lens characteristics in general and as well as their lenses in particular. These are subjective tests relying on the photographer to interpret the images to achieve the results. However, this does have the additional benefit of helping to improve the photographer’s critical eye.
Another aspect of lens performance is comparing mage quality at various aperture settings. Some lenses are not at their best when used with a large aperture, signified by a low f-number. One of the objectives of the testing is to find if there is noticeable sweet spot in the aperture range and to find if the lens has a practical limit for the largest aperture setting.
What To Test
There are a number of lens characteristics easily seen in suitable images:
- Chromic Aberration (CA) is a colored fringing around the edges of objects particularly evident on straight-line edges.
- Resolution – the ability to resolve or differentiate fine lines and produce sharp edges
- Edge or corner sharpness- the resolution performance of a lens tends to fall off away from the centre of the image.
- Color and contrast of the image can change between lenses.
- Vignetting is the darkening in the corners of images
How to Test
The basic method is to take series of photographs of the same subject from the same position except for changing the lens or a lens setting then, compare the results. Use a close view on computer software such as Adobe’s Photoshop, or Lightroom that displays two files on the screen at the same time.
Selection of a good location may take some time and luck but it should have a number of attributes.
- Local convenient site with good access to allow for repeated shots.
- A number of objects with well-defined edges spread over a wide area, especially important for wide-angle lenses.
- A range of natural and artificial colors
The location in the example shots has a range of light poles that reveal tendencies for any Chromic Aberration. The white support framework on the white building is a good test of the lens to resolve fine lines, particularly if they are in the corners of the image. The advertising signs on the white fences provide another test of resolving sharp edges. Finally, there are the natural colors of the grass, mountains, and the river to illustrate how a lens reproduces colors.
The example location has a handy opening window and the frame provides a helpful support for the camera as well a physical reference point so all the photos are from the same spot. A pair of lights on one of the towers is the reference target point to help keep the images the same.
Once in position take a series of photos changing only one lens setting. For example, set the camera to Aperture Priority metering mode and go from the lowest f-number, or maximum aperture, thought to larger f-numbers, or smaller aperture to see how changing the aperture affects the image. View the images side by side on the computer monitor and zoom in the view to look for any differences. To compare lenses repeat the process with a series of images for each lens.
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