Have you ever wondered how they measure camera lens quality in magazine and online reviews? Why is one lens considered to be better than another and why is there such a huge price difference between some lenses?
Well the standard lens test is based on what’s know as ‘optical metric scores’ which are standard measurements used to rate the performance of a camera lens, such as its sharpness, distortion, vignetting, transmission and chromatic aberration (see image above).
Sharpness while subjective, is considered to be the quality attribute of an image. Sharpness indicates the visually perceived quality of detail in an image and is associated with both resolution and contrast.
The score for Sharpness is based on something called the Modulation Transfer Function (MTF) of the lens, which is frankly only understood by geeks and lens heads.
For those of us who just want a single comparable number, then camera testing service DxOMark has announced a new metric : the Perceptual Megapixel. (P-Mpix) which uses megapixels in context to describe sharpness.
Perceptual Megapixels, (P-MPix), can be described as the “equivalent” number of megapixels when using a particular lens. Just as lenses offer different equivalent focal lengths when paired with different sensor sizes, sensors can have different equivalent megapixels when paired with lenses of various optical qualities.
If you stick a horrible lens onto a high-megapixel camera, your photographs may pop out with a large megapixel “size”, but the sharpness of the photo may be equivalent to the sharpness of a photo taken with a much smaller sensor and a “perfect” lens.
For example, say you’re shooting with a 24-megapixel camera, but are using a lens rated at 18 P-MPix. This means that the resulting photos are equal in sharpness to an 18-megapixel camera shooting with a optically perfect lens.
A perfect lens would render straight lines as straight, no matter where they occur. Most practical lenses aren’t that good, though, and instead bend lines outwards (barrel distortion) or inwards (pincushion distortion). Wide-angle lenses and wide-range zoom lenses often suffer particularly badly from this.
Camera lens distortion is the degree to which a lens cannot render straight lines in a scene as straight lines in the final image.
Vignetting (or light shading) consists of the attenuation of light as you move away from the image centre. It’s caused by the fact that not all rays from an off-axis element that go through the stop of the lens (the hole) will reach the sensor. Some of them will be blocked by mechanical elements inside the lens.
Transmission or Light Transmission
Transmission of a photographic lens describes its ability to transmit light from the photographic scene to the sensor surface. Lenses consists of many glass elements, each reflecting or absorbing a portion of the light; therefore the more elements, the lower the transmission.
Photographic lenses are made of several elements of very pure glass, each element being carefully coated to limit reflection. However, a portion of the light is lost in the optical system due to residual reflection and absorption.
The transmission factor (the proportion of light that actually makes its way to the sensor) depends on the optical formula (the number of lenses, the glass and coating formulas) and to a lesser degree on the shooting parameters.
Lateral Chromatic Aberration (LCA)?
Lens refraction depends on the wavelength. This means that magnification not only depends on the image field position but also on the light wavelength. A perfectly sharp edge going from white to black yields a smooth transition showing the decomposition of the light spectrum.
Light dispersion through a lens.
This has two major effects on the sensor. Each channel (usually R, G, B) is blurry because it integrates a certain range of wavelengths that are imaged at different positions on the sensor. Moreover, the different channels are shifted with respect to one another. This usually yields colored fringes along edges with a high level of contrast. LCA is expressed in µm on 24×36 mm. 10 µm is a typical level of acceptability.