This is one of the most iconic images in horror film history. Our protagonist, who we have seen struggle through horrific circumstances, is bathed in blood, sitting in the back of a pickup truck, looking back at Leatherface as he chases her with his chainsaw.
This film came out during one of the most innovative decades for filmmaking (both in general and in terms of horror). Independent productions, like Tobe Hooper's The Texas Chain Saw Massacre and John Carpenter's Halloween, became hugely influential and popular. Horror, as a genre, was changing.
There are a few things that differentiate the horror of the 70s and modern horror (mainly the integration of PG-13 horror films and the reliance on jump-scares to sell tension) but there is one thing you may not have noticed.
If you watch horror movies nowadays they are crisp, polished, and slick. They have the "digital" look (which makes sense, since a large portion of Hollywood has switched from film to digital in the past two decades). But, except in certain circumstance with certain directors, the digital look doesn't lend itself to horror in quite the same way.
The grainy film look of the 1970s -- seen in films like The Exorcist, Alien, Carrie, Jaws, Suspiria, and so many other horror films -- was an aesthetic in and of itself. It took away the polish of Hollywood and put horror in the dirt of middle-class America. It became more real because of this.
The other difference between the film and digital look is that lighting these two mediums is very different. Just look at these shots from The Exorcist and The Conjuring.
Now, the difference here isn't necessarily in the lighting setups themselves (though I have tried to find stills that are similar to each other) but in how the light affects the image (both in terms of quality and clarity).
The lighting in The Exorcist feels dramatic -- the shadows are large and powerful; the light is harsh on the left side of the frame while the priests are bathed in darkness; the contrast between light and dark could not be harsher.
The lighting in The Conjuring still evokes a dramatic mood, but the contrast between the lights and the darks are much smaller. Its frame is more elegant, more refined, and more polished.
You may not care about this difference in the slightest and that's okay. However, I prefer the visual style of The Exorcist over The Conjuring. The film grain just as much of a tool for the horror genre as anything else. It adds this indescribable aura to the film that no other genre has. It grounds everything, whether that's a supernatural menace or a rogue killer.
You can see it in remakes especially. Just check out these two stills of Carrie from the 1976 original and the 2013 remake.
The first image is just more striking. The way the light hits Carrie, and the way the camera registers that image, is much more grounded than the polished, shiny Carrie in the 2013 film. Something feels fake about the latter image.
Now, this is not to say that shooting on film or digital is a defining feature in "good" and "bad" horror films. There were plenty of bad horror films made the in 70s that used this aesthetic and there have been a few good horror films (mainly indie projects) made in the past 20 years that have shot on digital.
However, there is something to be said about the differences these visual styles bring to the table. The aesthetic of 70s horror helped enhance the fear and terror in a particular film while digital doesn't necessarily do that.
Granted, there are reasons filmmakers shoot on digital over film nowadays -- film is expensive, digital is cheap. You have a specific amount of time per roll of film that you can shoot on, whereas you can shoot forever with digital. Plus, there are now filters where you can put grain onto your film to get that aesthetic. But it's not quite the same. Pasting film grain over a polished image does not make that image look like it was shot on film.
Nowadays the film vs. digital argument is largely up to personal preference. The industry is moving away from film, while some filmmakers (like Christopher Nolan) refuse to shoot on anything other than 35mm. When you go to a horror movie next, consider this small difference. Go back and watch those classic horror films from the 70s. I think you'll find that film grain enhances the horror in a small, very subtle way.