This is not an in-depth article on Modulation Transfer Frequency (MTF) theory. To learn about MTF theory, take a look at Norman Koren’s excellent article here. Instead, this article only provides the basics of what you need to know regarding MTF charts. More importantly, it provides a method for choosing the best lens to fulfill a broad range of needs.
Why Lenses With Good Reviews Disappoint People
When choosing a lens, most photographers are not aware of all of the considerations to help them make a well informed decision. Most do not thoroughly consider what they will be using the lens for, at least not as thoroughly as this article proposes. Many simply want a general lens, a “sweeper”, that they can glue to their cameras and forget about. After looking at several subjective lens evaluations and perhaps even comparing MTF ratings, they finally purchase a lens, only to be surprised that their photos are not as sharp as they expected them to be despite the lens having good reviews or a decent MTF rating. How could this happen?
Subjective Lens Reviews Are Subjective
It is simply not enough to read subjective lens reviews. The reviewer may have a completely different use for the lens than you. For example, in Ken Rockwell’s website, he has much praise about the sharpness of the AF Nikkor 28-85mm f/3.5-4.5. If you look at the technical data of that lens, it is a lot sharper than you might expect for its price, especially at f/8. However, when the aperture is wide open, only the center remains adequately sharp. If you are a landscape photographer, you might consider the lens to be sharp because when shooting landscapes, you are more likely to use the lens at a wide angle, with the aperture stopped down to f/8 or smaller. But if you were an outdoor portrait photographer and wanted to place your subject somewhere other than the center of the frame against a creamy, out-of-focus background, the same lens would not appear to be sharp. So, depending upon your style of photography, you may come up with a completely different conclusion about the same lens.
MTF Ratings Are Misleading
To me, subjective evaluations are great for getting a general idea about build quality, how a lens handles, and its flaring characteristics, but for determining sharpness, there is nothing more objective than understanding the MTF data of a lens. But even MTF ratings (as opposed to MTF charts) can be deceiving. First of all, MTF data does not take into account sample variation. Although a lens may have a good MTF rating, the particular lens you purchase may not be as sharp due to quality assurance issues at the factory. Additionally, MTF ratings only provide an overall weighted average of lens performance. You need to look at the entire MTF chart, not just the overall rating. For example, the Nikkor AF 24mm f/2.8 has an overall MTF rating of 3.7. This is slightly higher than the 20mm Nikkor, which has a rating of 3.5. However, by looking at the entire chart, you will see that while the 24mm Nikkor is sharper in the center than the 20mm, at the corners, the 20mm Nikkor is slightly sharper. Depending on your photography style, you may prefer the more even sharpness of the 20mm to the center sharpness of the 24mm.
Figure 1: AF Nikkor 24mm f/2.8 vs. AF Nikkor 20mm f/2.8
The Bottom Line
The bottom line is that you need to first understand what you intend to use the lens for. Once you narrow down your choices, you need to then look at the entire MTF chart for those lenses, not just the overall MTF ratings. The steps outlined in the following posts will hopefully help you to make a more informed decision when purchasing a lens.