Tell me what I'm looking at.

[Crashoran]

Ive always breezed by MTF charts, but now I'm interested. This is the Sigma 10-20mm f/4-5.6
Explain like you would to a little kid :lol:
http://www.sigmaphoto.com/images/LensesMtf/114_big.gifhttp://www.sigmaphoto.com/images/LensesMtf/117_big.gif

[ron chappel]

It's been a while since i was heavily into these so i'll try to remember...
This one is abit tricky -it looks like they've shown the performance wide open only(?).Most graphs have eight lines rather than four so abit of guesswork will be needed.

One thing you should know is that these aren't measurements of lines per millimeter like most people measure lenses at home.These are actually a measure of contrast at very specific lkine spacings.
Why on earth do they do that?
Because they've found this is an excellent way of predicting both sharpness and contrast of lenses.
I can't for sure remember which is which but i think the lower (green) lines represent overall lens contrast,thus the upper (red) lines would represent overall sharpness.
Don't worry that the green lines are further down the chart than the red lines,it's not relevant .
The real point is how the respective lines compare to those given by OTHER lenses :)

Some simple stuff -
Obviously the left side of each line shows how good the lens is in the middle of the frame,while the right side shows how good it is towards the edges.
You've probably heard many times that lenses are worse at the edges than the middle -these graphs show that very well.

So what's with the dotted verses solid lines?
Those are for comparing resolution in the meridional direction verses tangental direction.Yeah-it's about as usefull as it sounds ;)
Basically,if there is a big gap between the solid and dotted lines than that's a bad thing.If they are close together then that's a good thing.
I've heard theories from people that claim they can predict bokeh character from variances in these lines but those theories don't quite match up with practice.There are obviously other factors involved.

Now after explaining all that i'm going to tell you not to worry too much about MTF graphs ;);)
It's extremely difficult to pick differences between lenses based on the graphs.Sometimes they are just point blank missleading!
The canon EF75-300usm verses 100-300usm is a great example.The graphs show the 75-300 to be better in both sharpness and contrast at the long end ,but everyone who's used them both knows it's the other way round (if not by alot)
Unless the graphs show really appalling or way above average performance,then you are FAR better off comparing test images i say.
And besides all that they can't show distortion,vignetting or other performance factors :(

Anyway if you want to learn more-
www.photodo.com has a good explanation of exactly how to decipher the standard MTF chart.
The canon USA site have a tutorial also.You really should search the sigma sites for an explanation on what their charts are saying exactly because theirs aren't standard-they should show tests at both f8 and wide open

And one final thing -you might be amused and amazed seeing some extreme examples of charts
The photodo site has tons of them listed by brand.Search in the sigma lenses and pick out a zoom with a rather low score (like 2.3 or so),it's quite funny how far down the chart some of those lines go:D
And at the other end of the scale go to the canon USA site and check out the EF300/2.8IS lens!

[Lester Wareham]

I guess from the number of lines they are only showing the stopped down performance, perhaps at f8. You need to read their web stuff on what the traces are. I could tell you for Canon but not Sigma.

I have some stuff on MTF here (http://www.zen20934.zen.co.uk/photography/mtf.htm) from when I was researching it.

[LightRules]

I guess from the number of lines they are only showing the stopped down performance, perhaps at f8.

Sigma MTF's show performance wide open; they don't show f8 (as Canon does), FWIW.

[Lester Wareham]

Sigma MTF's show performance wide open; they don't show f8 (as Canon does), FWIW.

Cheers fStopJojo

OK, not sure what's less useful - guess it depends on the application. Can you tell the OP what lp/mm the red and green trace pairs are at? (I know Canon's are 10 and 30).

[LightRules]

Can you tell the OP what lp/mm the red and green trace pairs are at? (I know Canon's are 10 and 30).

Same lp/mm as Canon's. Click on "MTF Chart Guide" at this link http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3301&navigator=6

[BigBlueDodge]

This will tell you all you want to know

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-mtf.shtml

and

http://www.luminous-landscape.com/tutorials/understanding-series/lens-contrast.shtml

and

http://www.luminous-landscape.com/pdf/UR2.pdf