Here is a head to head comparison of the Nikon 18-35mm f/3.5-4.5D ED-IF AF, 18-70mm f/3.5-4.5G ED-IF AF-S DX and 20mm f2.8D AF prime lenses.
The D70 camera settings were:
JPG-Fine, aperture priority, Exposure Comp +1.0EV, ISO 200, WB Cloudy, AF-S, all images auto-focus, color mode Ia, Tone comp: Normal, Hue adjustment: 0 deg, Saturation: normal, sharpening: normal, shutter speed between 1/20th and 10 seconds depending on aperture. The tripod is a Bogen 3051 with a heavy pan/tilt head – a beast.
The 20mm was done first, then the 18-35mm then the 18-70mm were set to 20mm. I made sure the zoom lenses were at 20mm by shooting and adjusting the lens until the camera LCD indicated the lenses were shooting at 20mm. The light was becoming progressively more blue and only 2 minutes passed between lens tests. The test was done after the wall was in the shade. I used the histogram to try and make the exposures relatively equal. The 18-70mm required an higher exposure compensation of EV +1.3 because I’ve found my lens is off by -0.3EV.
f-stop 20mm f2.8 AF 18-35mm f/3.5-4.5D ED-IF AF 18-70mm f/3.5-4.5G ED-IF AF-S DX
f 2.8 N/A N/A
Image samples removed for website update
20mm Sharpness data
Using the JPG file size as an indicator of sharpness, several series of shots were taken with the 20mm lens on a heavy Bogen 3051 tripod with matching pan-tilt head. The favorite subject, a brick wall, was used to conduct these tests. Between every series, the camera was refocused using autofocus. The camera body was made parallel to the wall to help eliminate depth of field reductions in the sharpness measurement.
From graph 1, it can be seen that my particular 20mm Nikon lens sample performs best at f8. These numbers cannot be compared against other lenses, as I do not have a baseline standard for comparison. This graph only provide information about the sharpness of my Nikon 20mm lens sample at various apertures.
Oddly, f22 measured better than f16 in several sample runs. I cannot explain this, as one expects that the image quality will degrade more at f22 due to diffraction. If a suitable explanation comes up, it will be posted.
From 1.5 feet to 7 feet
Based on JPG file size, the samples showed the most sharpness at f8. There is a marked decrease in image sharpness from f16 to f22 on each lens, as is expected when the lens becomes diffraction limited. The 20mm and 18-35mm look good stopped down one from wide open. The 18-70mm isn’t decent until f5.6 and it is not nearly that good compared to the others at equivalent apertures. This is expected as the 18-70mm is known to be weak at the wide end. The 20mm beats the other two at equivalent apertures but the 18-35mm keeps up fairly well. Even though the 18-70mm isn’t nearly as good, the quality trade-off for versatility is worth it many time to me. If you would like to see the lens aperture affect in a progressive way, download each of the images above, put them into a folder and quickly flip through viewing them in Window Fax/Image viewer or Mac iPhoto. Using the flip-book technique is how I did my initial assessment of how each lens was performing.