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MTF Charts Explained

by Roger N. Clark

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Introduction

The Modulation Transfer Function, MTF, described a lens' ability to bring all the light to focus. A perfect lens would focus all the light to a point in the focal plane. Aberrations usually prevent a lens from reaching the ideal goal. Aberrations spread the light out reducing contrast with adjacent objects in the scene. The MTF chart tells a lot about the lens performance (Figure 1).

The MTF chart describes the lens performance in how much contrast two close lines would have as a function of distance from the center. Ideally, one wants the MTF chart to have all the lines near the top of the plot, meaning high contrast from the center to the corner of the frame. Dropping of the lines means lower contrast and small objects, like stars will be a larger disk. If the Meridional and Sagittal lines separate, that means detail will be worse in one direction than the other. For example, stars will be oblong. The greater the separation and the lower on the plot, the worse the problem is. Depending on your application, a lens may not be up to the job if the MTF is too low. This is particularly important in night sky photography where one needs wide open light gathering and pinpoint star images. Few lenses from any manufacturer have stellar performance in fast wide angle lenses when wide open, a reflection of the extreme difficulty in making these lenses.

Figure 1. Description of the characteristics of an MTF chart.

Figure 2. Probably Canon's top MTF performing lens: the Canon 500 mm L IS II lens. An ideal lens would have all the lines at the top with an MTF of 1 to the corner of the frame. The MTF for this lens is outstanding.

Horizontal axis is from center of the image to the edge/corner in millimeters. The chart is for the full 35 mm frame and the right edge is for the corner of a 35 mm frame. At about 13 (mm) on the horizontal axis is the corner of a 1.6x crop sensor.

The vertical axis is contrast 0 to 1 (0 to 100%). MTF is modulation transfer function which is just contrast seen on closely spaced lines. The higher the contrast the better the lens records detail.

There are multiple sets of lines. Think of it this way: thin lines are finer detail, thicker lines are for coarser detail. The thin lines represent 30 line pairs per mm, so pixel spacing of 33 microns. If you have a camera with 6 micron pixels, that represents the contrast over about 5 pixels. Since 33 microns is large in today's digital cameras, only look at the thin lines.

Black lines are for wide open.

Blue lines are for stopped down to f/8.

The dashed and solid lines are for detail radial and perpendicular to the radial direction.

So you want the thin dashed and solid lines close together and as high on the plot as possible. The new Canon 500 mm f/4 L IS II lens is near perfect: http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_500mm_f_4l_is_ii_usm (be sure to look at the plot with no TCs added, and with TCs is still better than most lenses without any TCs).

Now look at the Canon 100-400 at 400mm: http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_100_400mm_f_4_5_5_6l_is_usm Note that the thin black lines are well separated, and that means star images and small things will distorted into comma shapes features. The lower the overall level, the larger stars and other small detail would be smeared into a larger blob). Note too that at f/8, things don't improve much.

Shorter focal length lenses have larger angles off the lens axis, and it is very difficult to make a great lens, especially wide angle and fast.

For example, look at the expensive 24 mm f/1.4 L II: http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_24mm_f_1_4l_ii_usm The thin black lines are at least close together, giving round instead of comma shaped blobs, but the contrast is low all across the frame when wide open. Thins improve at f/8, but stars, while smaller, would be less round. But this lens is better than lower cost wide angles.

Other Resources:

Canon's lens lineup and MTF charts: http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup

Reading and understanding lens MTF charts http://www.learn.usa.canon.com/resources/articles/2013/reading_MTF_charts.shtml

Nikon lens lineup and MTF charts: http://imaging.nikon.com/lineup/lens/index.htm

What is a Lens MTF Chart & How Do I Read It? (Nikon)

http://www.lenstip.com/lenses_reviews.htm
Select a lens for a review. The Coma and Astigmatism page shows artificial star image tests--very helpful.

http://www.the-digital-picture.com/Reviews/
Click on lens reviews, select a lens, then image quality, and when it comes up, select a second lens fo compare.

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First Published October 12, 2011
Last updated July 26, 2013.