MTF Charts

mercphoto

How does one read MTF charts, and do they really tell you anything useful? When I look at MTF charts for Canon lenses, I can't always draw a conclusion from them. Some highly regarded lenses have charts whose lines stay very near the top of the chart from side to side, while other highly regarded lenses have lines that drop radically on the right. Doesn't appear to be any pattern between the chart and a len's reputation.

Shay Stephens

This is probably going to be confusing also
The Canon 16-35mm f/2.8 lens



Sigma 17-30mm f/2.8 - f/4


MTF charts are great. For me it is more informative to compare charts of comparable lenses as opposed to a high power zoom and a wide angle prime.
When reading the charts, I like to look at the thin blue an black lines for canon, green for sigma (wide open). Those tell me the resolution at f/8 and wide open respectively. The higher up on the chart the sharper the results. As you look to the right of the chart you look further from the center of the lens.

The thick blue and black lines for canon and red for sigma tell me he contrast in the same way.

The closer the solid and dashed lines are to one another the nicer the bokeh will look.

Looking at the vertical direction, lines .8 and better will give excellent results. Lines .6 and above will be satisfactory. Below that and, well, things start to break down the further down you plunge.

The horizontal direction goes from the center of the lens "0" and out to the edges "20"

And not all lenses can have a high line all the way across. But compare a highly regarded lens' MTF chart to and equivalent lens that is not highly regarded and see how they compare. Comparing apples to apples in this way is very revealing.

It takes a little bit of time and effort too get used to MTF charts, but once you get the hang of them, they are worth their weight in gold. It is a shame more lenses don't have this data available.

In the full zoom chart of the two lenses shown above, you will notice that the Sigma looks to have better results than the Canon. You also have to realize that at full zoom, the Sigma is at f/4 which tends to help with resolution, but not necessarily exposure.

mercphoto

Great answer Shay!

Now, can someone explain to me the meaning of all those little symbols at the bottom of a page such as this: http://consumer.usa.canon.com/ir/controller?act=ModelDetailAct&fcategoryid=154&modelid=7317 Some are self-explanatory, such as the box with "USM" in it. Others aren't so self-explanatory, and I can't find a glossary on the Canon page.

Thanks!

Shay Stephens

Brief summary
U: The funky looking U with a circle around it represents the Ultrasonic motor technology which is fast and quiet compared to regular motors.

AL: The AL in a circle symbol represents an aspherical lens designed to lessen optical distortions in the corners of the frame.

CaF2: The CaF2 symbol in a circle represents a fluorite lens which helps keep colors focused on the same spot, cutting down on chromatic aberrations.

UD: (ultra low dispersion glass) when two elements are used the results are as good as one fluorite lens.

S-UD: (super ultra low dispersion glass) is almost as good a fluorite

I/R: Inner focus and rear focus is faster and less energy hungry than regular focusing methods

FT-M: Full time manual focusing allows you to manually focus while in autofocus mode.

FP: Focus preset allows a focus distance memory to be set so you can autofocus on stuff and then quickly change to the preset distance instantly.

Float: Floating system helps correct aberrations

IS: Image stabilization compensates for camera shake.

AF-S: Auto focus stop feature helps prevent unwanted focus changes (i.e people walking in front of the lens when you are focused far away.

A more detailed explanation can be found here:
http://ww2.canon.co.uk/products/eflenses_tech.html

mercphoto wrote:

Great answer Shay!

Now, can someone explain to me the meaning of all those little symbols at the bottom of a page such as this: http://consumer.usa.canon.com/ir/controller?act=ModelDetailAct&fcategoryid=154&modelid=7317 Some are self-explanatory, such as the box with "USM" in it. Others aren't so self-explanatory, and I can't find a glossary on the Canon page.

Thanks!