How do I save a zoomed PDF as TIFF/PNG of equal quality?

I have a photographic resolution chart in PDF format (vector graphics). Zooming in Acrobat results in a flawless upsampled image, as you'd expect from vector graphics. However, I cannot save the upscaled PDF as a upsampled TIFF or PNG--I get horrible artifacts. How can I get Acrobat to save the zoomed PDF file as a TIFF/PNG?

Thanks a lot...