I would not say correctness , yet it gives a measure typically say in terms of resolution (USAF 1951 target example). 

MTF is lines per mm frequency with +ve and -ve patterns

https://www.edmundoptics.com/optics/defense-optics/cots-testing/1951-usaf-glass-slide-resolution-targets/

Here object is Fourier transformed through the system resulting in a spatio-frequency representation by the delta function. The image modulation at the spatial frequency, 𝛏0 is given as 

Mi( 𝛏0 ) = Imax(x)-Imin (x)/Imaxx+Imin(x)   Imax and Imin are intensities

object modulation is given as,

Mo (𝛏0 ) = (H(0))/(H ( ξ0 )  * Mi ( 𝛏0 )

OTF = (H(0))/(H ( ξ0 ) )

OTF = MTF (𝛏)  * (e^(-iPTF(ξ))) is the product of MTF and phase (PTF)

Thanks Aparna. I guess my issue is more about the frequency values that apply - I have a measured lens MTF with a cut-off at 20 lp/mm. The object I am viewing is 600x600 mm - so do I need to sample (i.e. use nr of pixels) on the object to get the same order of max frequency before I perform the convolution?

Thank you Neels ,  I hope I am not complicating the discussion .

object -say gratings of sine (like USAF 1951) or something else can be contrast based . MTF is frequency related rather contrast is one measure of that .

Contrast in the image can be prepared by threshold so that non-uniformity changes !!!!!

You do get the variation in terms of # of pixels or coordinates .