Modulation Transfer Function (MTF) charts are a very commonly used tool in the photo industry for measuring and describing how sharp a particular lens is. However, it’s a system that is largely enigmatic to those outside the realm of optic experts and camera gearheads.
For those of you who don’t want to learn how to read MTF charts, camera gear testing service DxOMark has announced a new metric that boils a lens’ sharpness down to a single easy-to-understand-and-compare number: the Perceptual Megapixel. Read more…
Last week camera testing service DxOMark announced that the Nikon D800 had earned the highest sensor quality score ever awarded. Roger Cicala of LensRentals wanted to see for himself how much of an advantage the D800′s 36.3MP sensor had over its competition, so he did some sensor resolution tests on the camera, comparing it to the Canon 5D Mark III, 5D Mark II, and Nikon D700. His conclusion?
[...] there’s no question the D800 can actually get those pixels to show up in the final product (assuming your final product is a big print – they’re going to be wasted posting on your Facebook page). But you’d better have some really good glass in front of it. I don’t think the 28-300 superzooms are going to cut it with this camera.
In the real world, highest possible resolution is nice to know about and talk about, but usually not of critical importance compared to other factors. You’ll be able to make superb images with any decent lens for an 8 X 10 or even 11 X 16 print. But if you’re getting the camera because of the resolution, it makes sense to know which lenses will allow all of that resolution to be utilized. Just in case you get that job that needs billboard sized prints.
You probably know that stopping down (i.e. increasing your f-stop number) can increase the sharpness of your subject, but how much should you stop down to boost resolution without losing that nice, creamy bokeh? Roger Cicala did some research on this question and writes:
For those lenses that do benefit, stopping down just to f/2.0 provides the majority of resolution improvement. The difference between wide open and f/2.0 is generally much greater than the difference between f/2.0 and the maximum resolution.
Getting the edges and corners sharp requires stopping down to at least f/4 for most wide-aperture primes, and some really need f/5.6. Stopping down to f/2.8 may maximize center sharpness but often makes only a slight difference in the corners, at least on a full-frame camera.
None of the lenses performed any better after f/5.6 (for the center) or f/8 for the corners. Most were clearly getting softer at f/11.
If you’re using a wide-aperture lens, stopping down to just f/2.0 will reap big gains in sharpness while still keeping the depth-of-field narrow. Furthermore, for some lenses you don’t really even need to worry about stopping down for sharpness, since it hasn’t a relatively negligible effect on the outcome.
One of the things you’ve likely seen when looking at product or review pages for lenses is an MTF chart, used by manufacturers to give consumers an idea of how sharp a particular lens is. If you’ve never gotten around to learning how to interpret these charts, here’s a helpful 10-minute video tutorial on the subject. Luminous Landscape and Cambridge in Colour have great tutorials on this as well if you’re more comfortable with text-based tutorials.