In this blog post we are going to compare sharpness of 9×7 digital cinema camera with 15-perf 70mm IMAX film, which is considered one of the sharpest mediums in motion picture.
Note: In imaging systems, one cycle (cy) is equivalent to one line pair (LP) or two line widths (LW). These nomenclatures are used interchangeably.
Most film MTF curves can be closely approximated by a function known as the Lorentzian,
MTF(f) = 1/(1+( f/f50)2)
f is spatial frequency;
f50 is the spatial frequency where MTF = 0.5 = 50%.
For Kodak E100VS reversal film green layer, f50 = 40 line pairs/mm. MTF( f ) is expressed in functional notation, which indicates that MTF is a function of spatial frequency, f.
Accordingly, f20 = 80 line pairs/mm, or 160 lines/mm.
In case of Kodak E100VS, the MTF 50 is approximately 40 line pairs/mm or 80 lines/mm. As IMAX 70mm film is 52mm high, this translates to
MTFIMAX 50 = 4,160 LW/PH (or 5,600 LW/PW)
MTFIMAX 20 = 8,320 LW/PH (or 11,200 LW/PW)
PH stands for Picture Height, PW stands for Picture Width.
The above data is for reversal film (slide). Film negative is usually sharper with Kodak color negative films measuring about 72 cycles/mm (142 LW/mm) and higher for Black and White film.
However, this sharpness of negative film doesn’t translate to distribution prints, let alone theatrical projection. Typical answer print typically measures about half the spatial and generation loss going from film negative to interpositive to internegative and to release print is significantly higher again.
All above figures relate to film medium only. The actual MTF of the image is a combination (a product of) MTF of the lens and MTF of the medium. The biggest limitation of 15-70mm imaging is the lens.
Most medium format lenses that cover 52mm x 70mm frame rarely show 50% contrast MTF50 at 30 LP/mm (60LW/mm) and MTF20 at 60 LP/mm (120LW/mm). This translates to:
MTFlens50 = 3,120 LW/PH
MTFlens20 = 6,240 LW/PH
While some medium format lenses can produce slightly higher numbers in the centre of the frame, for example MTF of Rodenstock HR Digaron-W 50mm f/4 lens, the above numbers should be considered rather optimistic and certainly significantly lower away from the centre of the image.
How does it compare to high-end digital cinema cameras?
Find out in our MTF Camera Comparison.