[garyk]

I just looked at the MTF chart for the sigma 500mm lens and the canon 500mm lens.
I read on how to use the chart. The Sigma Is almost as good.

But when compared to the 800 canon lens the Sigma it is very much lower. As is the 300mm f2.8 lens.
Is the MTF chart I should take into conderation when buying a lens?????

[GatorPentax]

I dont know who's MTF charts you're looking at (manufacture's?), but I would say that it depends if it's from a reliable source. I personally look at MTF charts from reviews to get a general idea of what to expect from a lens. If the MTF charts rate a lens to be very good to excellent throughout all aperture ranges, then you generally have a winner there. (look up the MTF chart for the FA 31mm limited, its scary good).

[rkt]

you could look at the lens reviews at photozone.de where they include the MTF charts in the review, along with other aspects ... but all said and done, MTF charts should be only one of the things to consider ...

[pcarfan]

MTF charts are only a guide. It is not always accurate, and certainly doesn't reveal the whole picture. I would not base my decision solely based on MTF charts.

P.S: Sigma 500/4.5 is an excellent lens, it's good to know the MTF figures are excellent as well.

[bdery]

A modulation tranfer function (MTF) is only ONe measure of quality of a lens. What it represents is the capability of a lens to preserve contrast between black and white.

Say you have a perfect object, where there is a perfect switch between black and white along an infinitely thin line. Now you look at that object through a lens, and the transition between black and white will not be perfect anymore (except if you find me a perfect lens... which does not exist). The resulting image of the black/white transition will be flawed. the MTF measurement measures the importance of the "flaw".

We say that MTF is a good approximation of sharpness. That's true to some extend. It's a good measurement of the capability of a lens to resolve a line, if you will.

However, it does not take into account the capability of a lens to focus colours correctly (which is called chromatic aberration, or CA, and will influence the sharpness of an image). It does not take into account coma, which can be simply explained as the capability of a lens to focus light on the same spot, independant of the area of the lens being illuminated. It does not take into account resistance to flare, which will lowerr contrast and perceived sharpness. It will not take into account colour rendering, which again lowers contrast and perceived sharpness.

In addition, many elements define the quality of a lens, apart from pure sharpness.

So, MTF is useful, but does not tell the whole story.