Test charts can transform teh general impressions of a lens into measurable facts. Test charts allow a person to decide whether a lens is worth keeping or whether it should be returned. While test charts will not replace use a lens in the field, test charts will allow a person to eliminate guesswork based solely on memory of the qualities of a lens.
There are several main chart families from which a person can select a chart to use with a lens. One of the most common is an ISO 12233 pattern, which most film printing labs use. The slanted edges of this chart allow for the lens’s contrast to be measured.
How to Test a Camera Lens with Charts
Another of the main test charts is the USAF 1951 three-bar chart, which is an older chart. With this chart, a person must determine the smallest group of bars that the human eye can separate. Another of the most common charts is the Siemens star.
This chart provide visual cues for a person to understand the lens’s resolution. If the lens features an oval shape with the Siemens star chart, the lens has astigmatism. Each of these charts provides an answer to a different question, and a person can choose the best chart based on whether they would like to use numbers or merely view a lens’s weaknesses.
After a person selects a chart, they must understand the meaning of the Modulation Transfer Function (MTF) curve. The horizontal axis of the MTF curve represent spatial frequency, or how fine the detail in the image is. The vertical axis represents the contrast of the lens at each spatial frequency.
A lens with a high and slow-falling MTF curve has both fine and overall sharpness. A lens with a steep drop in the MTF curve will produce an acceptable image at normal viewing sizes, but it will not be sharp when cropped or use for large prints. The MTF curve can also reveal whether a lens maintains its sharpness across the entire frame.
If the lens’s center and corner MTF curves remain close together, the lens is even across the image. However, a wide gap between these two curves indicates that a lens’s corners will be softer than the center; this may be acceptable for portrait lenses but not for lenses used to copy flat subjects. On a test chart, several aberrations will become apparent to the human eye.
Chromatic aberration is one of the more common aberrations, and it will show as color fringes around subjects. This aberration is most prominent in the corners of the image. Distortion will make straight lines curve within the image; wide angles will have barrel distortion, whereas longer focal lengths will have pincushion distortion.
Vignetting creates a darkening of the edges of the image; this is most prominent when the lens is set to its widest aperture. Coma makes point sources of light look like comets; this aberration ruins shooting star fields with lenses that suffer from this problem. Field curvature will make the plane of focus curved within the image; thus, the center and corners of the image cannot be in focus at the same distance.
All of these aberrations will be visible to the human eye, but knowing which aberrations are prominent in a lens allows a person to know when a lens will disappoint when shooting certain subjects. There are several mistake that a person can make when setting up a test chart. The distance that the camera should be from the test chart can be calculated to ensure that the test chart takes up the frame.
However, a person should not place the camera too close to the test chart; aberrations will appear if the camera is placed too close to the test chart. The image should be aligned to be parallel with the test chart; otherwise, one side of the image will be softer than the other. The lighting for the test should be even; otherwise, vignetting will be mistaken for even lighting.
The file type to use when taking the test chart should be RAW and with lens correction turned off. Using a RAW file with lens corrections off will provide the cleanest measurements for the lens. Finally, any tripod leg should be even and straight; otherwise, a good lens can appear more mediocre.
The aperture sharpness curve will exhibit the same features for almost every lens. When a lens is at its widest aperture, the spherical aberration and coma will be most prominent in the image. These problems will decrease when stopping down the aperture.
However, diffraction will become a problem when the lens is used at very small apertures. The “sweet spot” for a lens occurs two or three stops from the lens’s maximum aperture; thus, f/5.6 and f/8 are common apertures for lenses to be evaluated. The trade-off between lens aperture and sharpness is also dependent upon the sensor size of the camera body.
To print your own test chart, the print media should be able to resolve finer detail than the lens. Otherwise, you are measuring the print media instead of the lens. Matte stock should be used for printing the test charts to avoid glare issues within the image.
The resolution of the print should be high and sized at least to A2. Charts should be printed every year because contrast will fade over time. Furthermore, using a faded test chart will make it appear as if every lens is soft. The resolution of the lens and sensor can be calculated with the system resolution formula.
Using this equation, a lens that resolves beyond the sensor’s resolving power allow a person to upgrade their sensor in the future. A lens that does not resolve to the sensor’s resolving power creates a bottleneck in the image quality that is regardless of sensor size. A lens should be matched to the camera body that a person owns today rather than tomorrow.
Finally, while lens test charts will not provide an absolute answer as to the quality of a lens, the results of these tests will be most helpful in providing a map of the performance of a lens. A lens with soft corners is not broken, but it may not be appropriate for certain subjects. A lens that reaches its peak contrast at f/8 is not slow in its function, but it is optimized to perform best at those apertures.
Thus, while the test charts will not tell a person exactly what a lens will do, they will help to inform a person of their lens choices in the field.