Quote:
Originally Posted by Alister Chapman
This is aliasing and most cameras normally have a filter to soften the image a little so that stair stepping etc doesn't occur (that's why an EX only resolves 1000 TVL despite having 1920 sensors).
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Point of clarification...TV lines are the lines that can be counted within a circle whose diameter is equal to the vertical frame dimension. So the nyquist limit for either horizontal or vertical is just 1080 TV lines. If Imatest is used to report the MTF50 transfer, the resolution number is measured at the point where the contrast between the black lines and the white is 50%. But if you extend the reading out to MTF40,30,20%...there is still measurable resolution all the way to the nyquist limit, it just happens at reduced contrast. For the EX1/EX3/PMW350 the high MTF resolution is not being mistaken for aliasing. The key is the smoothness and shape of the transfer curve itself. If it has squiggly amplitudes that are resonating, that's aliasing. But if it's smooth all the way to the nyquist frequency limit, it's resolving all the detail the format permits.
One of the keys of understanding thus, is that it is not the absolute number that matters most always, more informative is the shape and smoothness of the transfer curve, and the flatness which defines the rate at which contrast decays as frequency is increased.
Two cameras could share equal MTF50 numbers of 1000 TV lines, but the one whose contrast decays at a lower rate will be perceived to have the greater transparency, better reality.
Sharpening (edge enhancement) on the other hand, primarily creates the illusion of sharpness by drawing outlines around shapes. This generally occurs in the frequency range of 200 to 500 TV lines. It's not possible to draw a line less than one pixel wide, therefore details more than 520 lines cannot be enhanced. The real high frequency information cannot be improved, and often what is mistaken for real resolution is the edge enhancement outlines that are being applied at much lower frequencies, particularly needed if the camera has a steep rolloff in contrast ratio.
Regardless of the subjective preference for the 7D (or the 5DMkII that I own), they don't do full raster sampling with video, and the MTF50 resolution is no better than 550-650 lines. Heavy amounts of edge enhancement are applied to the lower frequencies to assist the perception of detail.
There is another type of aliasing (stairstepping) separate from edge enhancement related artifacts, caused by the temporal difference of motion between the frames. Sampling every 3rd line as happens with the Canon DSLR's only makes this worse. It cannot be eliminated by reducing applied sharpening.
What mitigates many of the ugly aliasing artifacts in the 5D/7D is the shallow depth of field permits a larger percentage of the frame's subject matter to remain out of focus, smoothing the jaggies, while drawing attention to the intended subject which is the unique benefit of DSLR video.
April 6th, 2010, 11:01 AM
