mtf charts

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matt
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Does anyone know how to read these? And if they do can they explain it to me..:thinking:

ef24-105mtf_tele.gif


I love the reviews at digital camera review they are visually interactive and very clear I feel they are a little lacking though in the amount of lens reviews they have done, but still a wonderful site. :clap:
 
The horizontal scale (0-20) represents the centre of the image circle out to the peripheral edge (in mm's). The vertical scale (0-100% or 0-1) shows the amount of light transmission (contrast). Other charts will show the resolving power in lines/mm...all equate to the same thing ultimately.

The solid and dotted lines represent the measurements during testing. The solid line is usually at f/8 whilst the dotted line is with the lens wide open.

If you're using the chart for assessment then a "perfect" lens would have both lines horizontal at the very top of the chart. In reality lenses are usually a little softer when wide open so it's quite normal for the dotted line to start a little lower and fall off as it moves across the chart (away from the lens axis).

Some charts will have two sets of both lines and these represent both ends of a zoom's range.

Using a full frame body then 0-18mm would be the interesting part of the scale whilst a crop body user needn't be too concerned once the plot moves beyond the 12-13mm region.

More detailed info here http://www.luminous-landscape.com/tutorials/understanding-series/understanding-mtf.shtml

Bob

(Maybe this should be in the "equipment section")
 
MTF addresses a fundamental fact of optical life in that when resolution (the fineness of detail) increases, contast (how clearly those details are shown) reduces, and vice versa. MTF graphs can be presented in all sorts of ways, but that's the theory behind it. Canon, Sigma and Nikon (probably others) all present their lens tests in a similar, comparable, way.

It is generally reckoned that when assessing sharpness, contrast is usually more important, which is why those Canon graphs pick two resolution figures, 10 cycles and 30 cycles, and then show how contrast drops.

Where you saw that graph, there will be a key explaining which curves are what. All lenses start high in the centre, and it is how they stand up towards the edges, at 30 cycles, that sorts out the good ones. As Bob says, whatever happens to crop format lenses after about 13mm from the centre is obviously irrelevant, which is why Canon MTF graphs cut off at that point for EF-S lenses.

Here's a link to a PDF of a very good book, Canon Lens Work III, and the section on MTF, starting page 209. It also explains the graphs there. http://software.canon-europe.com/files/documents/EF_Lens_Work_Book_10_EN.pdf
 
I got that chart from canon usa! and there was no Key!!! that's why i got confused.

But thanks guys I now have a basic understanding of how they work so I won't be quite as confused next time I'm browsing for my next piece of kit.
 
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