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    A Comprehensive Method for Choosing the Best Lens for Your Needs Using MTF Charts
 

As beginning or amateur photographers, we do not have the luxury of going out and purchasing a lens, only to find out that it does not meet our specific needs in terms of sharpness. Also, we typically do not have the budget to simply throw our money around and buy the most expensive lenses, although I have seen many amateur photographers spend as much as they would on a car for their equipment. Personally, I would rather spend my money on a trip to the Caribbean or Hong Kong, where I would be able to actually use my cheap lenses to take real photographs and not just test photos of porcelain figurines on my desktop.

Luckily, there are many inexpensive lenses out there that can give lenses several times their price a run for the money. You may be surprised to find that the most expensive lenses are not always the sharpest lenses. In fact, one of the cheapest lenses that both Canon and Nikon make, the 50mm f/1.8, also happens to be one of the sharpest lenses they make. Don't get upset that it's not a zoom lens. With a 50mm prime lens, you only need to walk backward or foward several feet to get the same zoom range as the Nikkor 35-70mm f/2.8 lens that costs 6 times as much! The 35-70mm itself is less than half the price of the Nikkor 28-70mm f/2.8, yet, it actually outperforms the 28-70mm at f/8. The 28-70mm only surpasses the 35-70mm when it is wide open at the telephoto end. Depending upon your photography style, this may not even make a difference to you.

The most objective tool we have available to us in finding these "gems" is MTF charts. This article provides the basics of what you need to know regarding MTF charts and provides a comprehensive method for helping you choose a lens that will best suit your needs.

 

Introduction

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 simply provides the basics of what you need to know regarding MTF charts. More importantly, it provides a comprehensive method for choosing the best lens to suit your needs.

 

Why Subjective Lens Evaluations and MTF Ratings Can Be Inaccurate

Many people, when choosing a lens, do not seriously consider what they will be using the lens for, at least not as seriously as this article proposes. Most people simply want a general lens, a "sweeper", if you will, 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. However, after examining their photographs, they are suprised to find that they aren't as sharp as they would've expected despite the lens having a relatively good MTF rating. How did this happen? Aside from proper technique, one of the problem is that the lens they purchased may indeed be sharp, but not at the focal length and aperture they tend to use it at. In other words, for their style of photography, the lens is not sharp.

It is simply not enough to read subjective lens evaluations and reviews. First of all, as you might expect, they are very subjective. The reviewer may have a completely different style than you. For example, in Ken Rockwell's informative website, he has much praise about the sharpness of the AF Nikkor 28-85mm f/3.5-4.5. If you look at that lens, it is a lot sharper than you might expect for its price, especially at f/8. However, when it is wide open, only the center remains sharp. Well, it just so happens that Ken Rockwell is mainly a landscape photographer, and when you shoot landscapes, you're more likely to find yourself shooting at a wide angle with the lens stopped down to f/16. So, depending upon your style of photography, you may come up with a completely different conclusion about the same lens, although, in this particular case, I find Ken Rockwell's evaluation to be very accurate.

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 a lenses MTF. But even MTF ratings can be deceiving. This is so because they 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 want from a lens. To figure this out, you must understand your style of photography. Secondly, to get an accurate evaluation of a lens, you need to look at the entire MTF chart, not just the overall MTF rating. Armed with the knowledge of your own shooting style and a basic understanding of MTF charts, the following steps outlined below will hopefully help you to make a more informed decision when purchasing a lens.

 

Step 1: Determine the style(s) of photography you expect to use the lens for

The first step in purchasing the perfect lens for your needs is determining what your style of photography is. In the example above, we see that the 24mm lens is sharper as long as the subject is in the center. But what if you preferred placing your subject off center, in adherence to the rule of thirds? Then, the 24mm Nikkor may not be the best choice. Of course, for the beginning photographer, you may not know what your style is. When I say style, I mean, what combinations of aperture, focal length, and subject placement do you find yourself at most of the time? Although the following rules do not always apply, they may be helpful as a reference point for beginning and amateur photographers:

1. Landscapes - Most landscapes are shot at a wide angle with the aperture stopped down to get the most depth-of-field (DOF) and sharpness. If you expect to photograph mostly landscapes, you'll probably want a wide angle lens (36mm or less) that is evenly sharp throughout the frame when the aperture is stopped down (f/8 or smaller).

2. Portraits - Most portraits place the subject directly in the center (unless you're doing environmental portraits) and are shot with a moderate telephoto lens. The aperture is often fully open or almost fully open to blur the background. Therefore, if you plan on using your lens for portraits, you probably want a moderate telephoto lens (70mm-150mm) that is sharp in the center at its widest aperture (f/4 or larger).

3. Street/Candids - For street photography and candids, where you must capture spur of the moment action, you probably need a normal zoom lens (between 35-100mm) that is sharp throughout the frame when it is stopped down some (f/8). The reason why you want it sharp throughout the frame is because candids often include several people or objects interacting with one another, not just a singular subject. You usually want to have your lens stopped down because it will give you a greater DOF so that even if you did not have time to focus accurately due to the spontaneity of the moment, you are more likely to have the subject in focus. And, unless you are like Henri Cartier-Bresson, you'll need a zoom because you may not have time to compose your images by repositioning yourself.

4. Action - The same as above, except you'll want a longer telephoto, something that is at least 200mm, but more likely in the 300m-600mm range. You may also want a lens that is sharp when it is wide open to blur the background, but it depends upon what type of action you are photographing and how you intend to photograph that action. For example, for motorsports, you may not need to blur the background because panning can give the same effect. However, if the vehicles are coming directly toward you, you cannot use panning to blur the background anymore, in which case, you'll need sharpness at wide apertures (f/5.6 or larger). I typically use f/5.6 for my surfing photos, which gives me a good balance between blurring the background and maintaining an adequate depth of field to allow me some room for play in case my focus was slightly off.

4. Abstract - Most abstract images trick the eye into judging depth incorrectly. To do this, you must eliminate as many clues about depth, one of which is focus. In photography, shallow focus is not only used to emphasize the subject, but also to add depth to an image, which you are trying to avoid. Therefore, if you are doing abstracts, you'll most likely want a lens that is evenly sharp thoughout the frame when it is stopped down (f/8 or smaller).

5. Nature. Most nature photographs are taken with extremely long telephoto lenses (300mm or more) with wide apertures (f/4 or larger). This allows you to blur the background to emphasize the subject. Thus, you want a telephoto lens that has a wide aperture and is sharp wide open. Coincidentally, this is usually the most expensive type of lens.

 

Step 2: Rank the various styles of photography you expect to use the lens for

In determining your photography style, you probably found that one or more categories apply to you. This step keeps you from expecting everything from a single lens, such as what people expect when they buy "sweeper" lenses, like the popular 28-300mm Hyperzoom. The problem with these types of lenses is that they are, in general, noticeably softer than the prime lenses in their range. However, in most cases, it is impractical to get a prime lens for every focal length you wish to cover because you may not have time to switch lenses due to the spontaneity of what you will be photographing. You must find a balance between the two. Ranking the styles of photography you expect to use the lens for in frequency of expected use will help you do this.

The very first lens I purchased using this methodology was my Nikkor AF 35-70mm f/2.8 D. When I purchased it, I expected to be shooting mostly portraits, followed by landscapes, followed by candids, and finally abstracts. Thus, I ranked the priorities of what I wanted the lens to do based on my expected shooting style:

Priority 1 - For portraits, sharp in the center at 70mm or greater with the aperture wide open (f/4 or larger).

Priority 2 - For landscapes, sharp throughout the frame at 35mm or less with the aperture stopped down (f/8 or smaller).

Priority 3 - For candids, sharp throughout the frame from wide angle to telephoto with the aperture stopped down some (f/8).

Priority 4 - For abstracts, sharp throughout the frame from wide angle to telephoto with the aperture stopped down (f/8 or smaller).

In summarizing my priorities, I was generally looking for a zoom lens that was sharp in the center when fully open at the telephoto end (for portraits), and sharp throughout the frame when stopped down from wide to telephoto (for landscapes, candids, and abstracts). As you know, I finally chose the Nikkor 35-70mm f/2.8, however, there were several other lenses I looked at with similar characteristics, as you will see in the next step.

 

Step 3: Choose possible lenses

The first step I took in choosing possible lenses was to look at all the lenses covering the focal lengths I required. I looked at every zoom lens with a focal length between 24mm and 150mm. I looked at the following AF Nikkors: 35-70mm f/2.8, 35-135mm f/3.5-4.5, 28-105mm f/3.5-4.5, 28-85mm f/3.5-4.5, and the 28-70mm f/2.8. I then eliminated several lenses based on subjective evaluations. What I looked for, in particular, was common complaints. The more common a complaint, the more likely it was valid. I also eliminated certain lenses based on my budget. For example, at around $1200, the Nikkor AF-S 28-70mm f/2.8 was definitely out of my league. Even the 35-70mm at $400 on E-Bay was beyond my budget, but I decided to keep it on my list because it was close enough. I also chose to keep third-party lenses off my list solely because of resale value. I have found original equipment manufacturer (OEM) items to hold their value much better. In fact, I recently sold a AF Nikkor 75-300mm f/4.5-5.6 lens for $65 more than I bought it for. However, you should not always limit yourself to Nikkors or Canons. Sigma, Tamron and Tokina have several excellent lenses that, in some cases, outperform their OEM counterparts.

 

Step 4: Rank possible lenses using MTF charts

This is where understanding the basics of MTF charts comes into play. In this step, I looked at the MTF chart of each lens that remained on my list. I ranked the lenses by comparing the relative sharpnesses at the apertures and focal lengths required for each type of photography I expected to use my lens for. I created a matrix with the results and finally scored each lens with the sum of its ranking for each style of photography multiplied by the importantce of that style for my needs. I will explicitly illustrate what I did, however, I must first explain the basics of understanding MTF charts.

What you need to know about MTF

To understand MTF and how it relates to sharpness, you must first understand what sharpness truly is, which is nothing more than localized contrast, or, in layman's terms, contrast between fine details. When evaluators create MTF charts, they are essentially measuring the contrast between lines on a test chart as seen through a lens and comparing it to the true contrast of the chart by percentage. A perfect lens would be able to transfer 100% of the original contrast of the the test chart. Of course, this lens does not exist.

The lines on these test charts usually fall under two categories. The first type of line is oriented so that it extends like the spokes of a wheel from the center of the lens to be tested. These lines are called radial or sagital lines. The second type of line is perpendicular to the radial lines. These are called tangential or meridonial lines. Each of these lines are typically arranged in test patches, with increasingly finer thicknesses and distances, which evaluators describe in line pairs per millimeter (lp/mm). The greater the lp/mm, the finer and closer the lines are. Figure 2, below, shows a sample test chart used by the USAF that many other test charts are based off of.

Figure 2: USAF 1951 Test Target

 

When reading MTF graphs (see Figures 3 and 4 below), you will see two axes. The vertical axis represents the percentage of contrast a lens is able to retain compared to the actual test chart. The horizontal axis represents the distance from the center of the lens. In addition, there are usually several of these graphs, each representing lens performance at various focal lengths and apertures. For every graph, you will see that there are there are 3 sets of solid lines. These lines represent the ability of the lens to resolve the radial lines of the test chart. Likewise, the dotted lines represent the ability of the lens to resolve the tangential lines of the test chart. The topmost lines represent the lens performance at 10 lp/mm, followed by the second lines, which represent the lens performance at 20 lp/mm, followed by the lowest lines, which represent the lens performance at 40 lp/mm. In general, the topmost line (performance at 10 lp/mm) describes how much overall contrast the lens will have. The middle and lower lines, (performance at 20 lp/mm and 40 lp/mm, respectively), describe what we would typically perceive as sharpness. Strangely enough, however, a lens with more contrast may sometimes appear sharper than a lens with less contrast, regardless of its actual performance at the finer lp/mm.

Figure 3: AF Nikkor 35-70mm f/2.8 D

 

Figure 4: AF Nikkor 35-105mm f/3.5-4.5

 

The Digital Bonus

The MTF charts for most lenses are measured with the dimensions of film in mind, which, in the case of 35mm film, is 24mm x 36mm. The radius of the diagonal of 35mm film is approximately 21mm, which is how far from the center the MTF charts are measured out to. With the smaller sensors of digital SLR cameras, which, in the case of Nikon's DX standard, have the dimensions 23.7mm x 15.5mm, the radius of the diagonal is only about 14mm. This means that you can pretty much ignore everything to the right of 14mm on the MTF charts. As a result, you have essentially cropped out what is usually the most unsharp region of a lens. This is another reason why overall MTF ratings can be especially misleading for digital photographers.

 

Now that we understand the basics of MTF charts, we see that all we need to do in order to objectively compare lenses is compare performance at similar apertures and focal lengths by seeing which lens has a higher graph for a given lp/mm. To illustrate how I compared the lenses above, I created an overlay of the MTF characteristics of both lenses at around 70mm with the lens open as wide as possible (see Figure 5, below), which is what the style of shooting I gave the highest priority required.

Figure 5: Comparison between 35-105mm at 60mm (red) and 35-70mm at 70mm (green)

 

Surprisingly, the 35-105mm seems to do better when it is wide open as compared to the 35-70mm, however, keep in mind that the 35-70mm has a wider aperture of f/2.8 compared to f/4. I took this into account and in this situation, I ended up giving both lenses the same score. Thus my ultimate decision rested on lens performances in the other types of photography I expected to use the lens for. Figure 6 shows the final matrix comparing both lenses.

Figure 6: Matrix used to rank lens performance based on MTF characteristics

The lower the total score, the better the lens could be expected to perform for my specific needs. As you can see, this is the reason why I chose the 35-70mm f/2.8 over any of the other zooms covering a similar range.

 

Conclusion

When I initially did this comparison, I was very surprised at how well the 35-105mm lens did compared to the 35-70mm. Although you don't necessarily have to go into as much detail as I did when picking a lens, you should, at a minimum, consider the type of photography you will expect to be doing. The benefit of doing the comparison outlined in this article, however, is that it can result in insight that would otherwise be overlooked. In statistics, we learn that the most important insights are the ones that surprise you. Imagine how surprised your friend who purchased a 28-70mm f/2.8 will be when the photographs you show him from a lens half as cheap end up being just as sharp or sharper!

Hopefully, with this article, you have learned a little more about how to understand MTF charts and also learned of a way to objectively compare lenses based on your specific needs. Although this method may seem overly methodical, the crisp images you are rewarded with are well worth the extra effort.

 

Where to Find MTF Data

Unfortunately, Nikon only publishes the MTF charts for a few select Nikkors on their Japanese website.

Also, www.photodo.com had some MTF charts for the older Nikkors, but they seem to have removed the actual charts and left only the MTF ratings. Niklas Nikitin has several MTF charts on his website, but he switched to Canon and mentioned that he may remove them from the web. I have downloaded all the charts and posted them on my PBase account, here, in anticipation of them being removed.

For another good read about how to properly use MTF data when purchasing a lens, take a look at this article by Dante Stella.