Aperture and distance
Even after understanding the correct way to interpret MTF curves, other factors need to be considered. These include the working distance of the lens and its aperture (f/#) since they will also affect the MTF of a lens and thus its performance. While MTF curves can show how a lens performs over a range of working distances and aperture settings, a new graph must be plotted for each distance and aperture.
This necessitates reviewing a large number of graphs to properly compare different products. However, by understanding how both working distance and aperture affect the achievable resolution of an imaging system, system integrators will be more effective at choosing specific lenses to meet their needs. To demonstrate the effects of varying aperture and working distance, two different 5-Mpixel lenses from two separate manufacturers were examined at different apertures and working distances. Each lens has an MTF associated with it that imply it is capable of resolving images from cameras based on 5-Mpixel sensors. However, by examining images taken at the center and corners of images captured with these lenses, differences in resolution can be visualized.
By using a Ronchi rule target and an 8-in. field of view (FOV), the imaging system is first set to capture lines on every other row of pixels on the sensor. This allows each of the lenses to resolve images at the pixel limited resolution of the sensor. At this resolution, contrast levels greater than 25% are difficult to obtain, resulting in gray-level images. Camera noise is more easily visualized at these lower contrast levels. The impact of this effect is seen most dramatically in images with lower levels of contrast. Using an aperture of f/2, both lenses A and B resolve 5-Mpixel images in the center sections of the image with lens B having a slightly higher level of contrast (see Fig. 2a and 2b). However, in the corner of the image, lens A no longer can resolve 5-Mpixel images at this particular aperture setting even though the lens’ MTF specification indicates that it can achieve 5-Mpixel performance (see Fig. 2c and 2d).
When the aperture of both lenses is set to f/8, both lenses still resolve 5-Mpixel images at the center sections of the image. The lower levels of contrast that are seen at the center of the image with the f/2 aperture set- ting show that diffraction is coming into play. At the corner of the images, 5-Mpixel resolution from both lenses is barely achieved (see Fig. 2e and 2f). Although this higher aperture setting overcomes certain aberrations and manufacturing issues (seen in the corner of lens A), this is at a reduced overall capability of the lens due to diffraction and reduced light levels. By using an even larger aperture (f/11) and examining the center of the image, the diffraction limit of the system has been exceeded, and no resolution is obtainable with either lens (see Fig. 2g and 2h). At this setting neither lens can achieve any contrast at the corner of the images due to the diffraction limits of the systems (see Fig. 2i and 2j).
Working distance
Working distance also affects the performance of an imaging system. If the working distance of the lens varies, so does the FOV. Thus, different targets must be used to maintain pixel-limited image space resolution requirements. By examining the center and corners of each image, the different achievable resolutions of the imaging system can be visualized.
Using an aperture setting of f/2, a FOV of 8 in., and a working distance of 24 in., lens B resolves 5-Mpixel images (see Fig. 3a and 3b). With an aperture setting of f/2, FOV of 4 in., and a working distance of 12 in., this same lens is still capable of resolving 5-Mpixel images. However, at this working distance, the contrast level in the corner of the image is reduced (see Fig. 3c and 3d). With an aperture of f/2, FOV of 1.5 in., and a working distance of 4 in., lens B shows no contrast at the limited resolution in the corner of the image. Even at higher apertures, no difference can be observed in the corner of the image (see Fig. 3e and 3f). Despite the fact that this is a high-performance 5-Mpixel lens when compared to other 5-Mpixel lenses, it still has its limitations.
The type of MTF data (geometrical or diffraction) being presented by the lens manufacturers must be carefully considered and questioned at the time of lens selection. Second, the fabrication tolerances should be stated in the MTF data of the manufacturer, and if not, should be questioned before final selection. Additionally, factors like working distance, aperture, and wavelength range should be considered to obtain the highest level of performance of the imaging system. Although on paper a lens may appear sufficient for the required camera/sensor selected, closer examination may show otherwise. Lenses sold as megapixel lenses frequently vary in performance once the system is assembled and tested. Often, the performance of the lens is not commensurate with the sensor performance, and if not, should not be classified as megapixel lenses.
Figure 3. Using an aperture setting of f/2, an FOV of 8 in., and a working distance of 24 in., lens B resolves 5-Mpixel images at this working distance and FOV position (a and b). With an aperture setting of f/2, FOV of 4 in., and a working distance of 12 in., this same lens is still capable of resolving 5-Mpixel images. However at this working distance, the contrast level in the corner of the image is reduced (c and d). With an aperture of f/2, FOV of 1.5 in., and a working distance of 4 in., lens B shows no contrast at the limited resolution in the corner of the image (e and f).