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Making sense of MTF

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.

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.

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:

• 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 ) .

All of the MTF charts that accompany the lens descriptions in the latter part of this brochure follow these conventions.

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.

Green: Contrast value at maximum aperture

Red: Contrast value at F8

100

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.

Distance from optical center of lens (mm)

Max. aperture

F8 aperture

Spatial frequency

10 line pairs/mm

30 line pairs/mm

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 "micro-contrast" here, which is the ability of a lens to differentiate

between tiny details that have similar tonal values. Micro-contrast is different from

global contrast, the overall range of tones in an image that people usually associate

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 real-world applications.

Choosing the right lens

Portraits

For most portraits, the person

being photographed is the

most important element of

the photograph, so it can be

effective to de-emphasize

other non-essential elements.

The usual way of doing this is

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 E-mount 50mm F1.8

( SEL50F18 ) are excellent choices for this type of photography.

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 wide-angle zoom such as the Vario-Sonnar T* 16-35mm F2.8 ZA

SSM ( SAL1635Z ) would be an excellent choice because it covers a range

of focal lengths that are extremely useful for landscape photography with

outstanding resolution and contrast. Stopped down to F8 or F11 lenses in

this focal length range will give you sufficient depth of field to keep the entire

scene in sharp focus. Hint: include prominent foreground objects to give your

landscape images a greater sense of scale.

Snapshots

The term "snapshot" refers to

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"

street-shooting mode. The key is to capture the moment, and that requires

mobility and speed. Some photographers prefer to use a prime lens with a

focal length they're comfortable with for this type of shooting: a "simple is

faster and better" approach. Others choose a compact mid-range zoom like

the 28-75mm F2.8 SAM ( SAL2875 ) for maximum versatility. If you're going to

be shooting snaps indoors or in evening or early morning light you'll want to

choose a lens with a large maximum aperture.

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 E-mount 30mm F3.5

( SEL30M35 ) for stationary subjects that you can get very close to, or the

100mm F2.8 Macro ( SAL100M28 ) where a bit more working distance is

required. You can also shoot impressive close-ups such as flowers with any

lens that has a maximum magnification ratio of about 0.25x or more and a

sufficiently short minimum focusing distance. The 75-300mm F4.5-5.6 zoom

( SAL75300 ) is good for this type of close-up shooting, or you could use the

70-300mm F4.5-5.6 G SSM ( SAL70300G ) for truly stunning image quality.

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

action-freezing shutter speeds.

The 300mm F2.8 G telephoto

prime ( SAL300F28G ) is an outstanding choice for this genre, but if you want

the framing versatility of a zoom the 70-200mm F2.8 G ( SAL70200G ) is a great

alternative. You could even use the SAL14TC 1.4x Teleconverter or SAL20TC 2x

Teleconverter with either of these lenses to provide more reach for distance

subjects or to grab close-ups of the action. Of course there are always

exceptions: if you can get close to the action you might be able to use a fast

wide-angle prime or zoom to capture a more dynamic perspective.

Wildlife

Since you can rarely get close,

super-telephoto is the first focal

length choice for shooting

wildlife. Of course you won't

need that much magnification

if you're shooting pets at

home, but in the wild you'll

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 APS-C format body.

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.