Making sense of MTF
Choosing the right lens
Those MTF (Modulation Transfer Function) graphs that often accompany lens specifications are really not as impenetrable as they look, and they can give you a good idea of how a lens will perform, so it might be worth taking a few minutes to learn what they mean.
MTF describes a lens’s ability to resolve finely spaced black and white lines printed on a test target.As the lines get closer together they start to blur and blend together as the limits of the lens’s resolving ability are reached. MTF is plotted for multiple levels of subject detail (Y axis) at a number of points from the optical center of the lens to its periphery (X axis).The more lines per millimeter the lens can resolve, the better the resolution and contrast of the lens.* This resolving power is expressed as line pairs per millimeter (lp/mm), and sometimes as the more scientific sounding “spatial frequency.”
*For more info about these closely related terms, refer to the “Resolution, contrast and sharpness” column below.
Take a look at the sample chart below to see how it all works to describe lens performance.The solid green line shows radial contrast values for 10 lp/mm detail with the lens wide open. The line is almost flat, indicating that resolution is constant at approximately 93% from the center to the periphery of the lens. Very good.The solid red line shows contrast with the same parameters except that the aperture has been stopped down to F8. The red line is higher than the green line, indicating that stopping down has improved resolution somewhat.
Basically, the higher and flatter the line, the better the performance for the corresponding set of parameters.The smaller the distance between the green and red lines, the more consistent the performance of the lens is over a range of aperture settings.The smaller the gap between the solid and dotted lines, the more attractive the defocusing is likely to be.
That’s really all you need to know to glean useful information from an MTF chart. Just remember that comparing MTF graphs of different lenses is really only meaningful if both lenses have similar focal lengths.
Portraits
For most portraits, the person being photographed is the most important element of the photograph, so it can be effective to
to defocus the background so the viewer gets a sense of location without being distracted from the main subject by too much surrounding detail. Choose a lens that has a large maximum aperture and a focal length between about 75mm and 150mm for flattering perspective, and so that you don’t have to get uncomfortably close to your subject.The Planar T* 85mm F1.4 ZA (SAL8514Z), DT 50mm F1.8 SAM (SAL50F18), 85mm F2.8 SAM (SAL85F28), 135mm F2.8 [T4.5] STF (SAL135F28) and
Macro and close-ups
“True” macro lenses that can be used to shoot extremely clear, detailed images of very tiny subjects have a maximum magnification ratio of 1:1 (1x), and that limits your choices. Use the DT 30mm F2.8 Macro
SAM (SAL30M28), 50mm F2.8 Macro (SAL50M28), or
The X (horizontal) and Y (vertical) axes of the chart correspond to the following values:
• X: Distance from the optical center of the lens to
a point near its periphery, measured in millimeters.
•Y: The degree of contrast measured at each point, expressed as a percentage.
Indicates excellent performance with high contrast and resolution at the center of the lens.
Indicates the level to which resolution and contrast are maintained at the periphery of the lens.
Green: Contrast value at maximum aperture
YRed: Contrast value at F8
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(%) | 40 |
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Contrast | 20 |
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| 0 | 4 | 8 | 12 | 16 | 20 | ||
Distance from optical center of lens (mm)
Landscapes
Although you can use anything from wide angle to telephoto lenses for landscape photography, you’ll probably get the most use out of wide lenses that can capture the grandeur and scale of nature
at its best. A
Sports
Since sports almost invariably involve fast action, usually at a distance, you’ll want to use a telephoto lens that’s “fast” enough to allow the use of
prime (SAL300F28G) is an outstanding choice for this genre, but if you want the framing versatility of a zoom the
A number of parameters are represented by different line types on the MTF chart, as defined by a legend that accompanies each chart.Those parameters are:
landscape images a greater sense of scale.
•Two lp/mm values: often 10 lines per millimeter and 30 lines per millimeter.
•Two different aperture settings: lens wide open and F8.
•Two orientations of line pairs in relation to the lens:“R” (radial = lines parallel to the radius of the lens), and “T” (tangential = lines perpendicular to the radius of the lens).
Spatial frequency | Max. aperture | F8 aperture | |||||
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10 line pairs/mm |
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30 line pairs/mm |
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Snapshots
The term “snapshot” refers to
Wildlife
Since you can rarely get close,
All of the MTF charts that accompany the lens descriptions in the latter part of this brochure follow these conventions.
Resolution, contrast and sharpness
Although it is possible to have high resolution and low contrast, or vice versa, in the context of MTF measurements these terms mean almost the same thing. Both good resolution and contrast are necessary for a lens to be perceived as “sharp.” We’re talking about
any photo opportunity that arises spontaneously.You’re shooting snapshots when you take your camera for a walk in the park, or on vacation, or even when you’re in “serious”
want to be as far away as possible, to avoid scaring off your subject and for safety.The 300mm F2.8 G telephoto prime (SAL300F28G) with the 1.4x or 2x Teleconverter (SAL14TC or SAL20TC) is probably the most suitable choice.
Not only does that combination give you the reach you’ll need, but the quiet, responsive operation of the SSM autofocus drive will be an advantage as well. Hint: the above lens/teleconverter combination will give you even more reach when used on an
with the term “contrast.” MTF measurements are useful because they show us the relationship between a lens’s resolution and contrast in graphic form that makes it easy to judge how the lens will perform in
In the product pages that follow, this star icon identifies lenses: prime lenses that offer outstanding value in compact, lightweight designs that are ideal for photographers at all levels. Each lens in the series is suited for a particular type of photography, such as portraiture or macro, for example.
14 | 15 |
