Editing Back to the Future (section)

===Special effects===
[[Industrial Light & Magic]] (ILM) developed the film's special effects under the supervision of [[Ken Ralston]] and [[Kevin Pike]].<ref name="ASCILM" /><ref name="BTTFWebsite" /> It contains approximately 27–32 special effects shots, compared to the 300 such shots typical in contemporaneous higher-budget films.<ref name="ASCILM" /><ref name="ShortlistDudley" />{{sfn|Pourroy|1985|p=40}} Despite working simultaneously on ''[[The Goonies]]'' and ''[[Cocoon (film)|Cocoon]]'', Ralston took on the additional project because it required relatively few effects, and he wanted to realize the planned ending of Marty driving the time machine into a nuclear explosion.<ref name="ASCILM" />{{sfn|Pourroy|1985|p=40}} The team had a nine-and-a-half-week schedule, reduced to less than nine once Universal Pictures moved up the release date. ILM was working on ''Back to the Future'' up to the moment it had to be handed over to print the theatrical film reels.<ref name="ASCILM"/>{{sfn|Pourroy|1985|pp=40, 59}}

The tight schedule affected the special effects' quality. Ralston was disappointed by the scene where Marty's hand fades away as his future is altered. Fox was filmed separately from his hand and the two were [[Compositing|composited]] together; the hand was filmed with a wide-angle lens, making it appear too large, and it had to be scaled down. Zemeckis wanted a subtle fade, but it resulted in a small circle of the hand fading away and there was no time to fix it.<ref name="ASCILM" />{{sfn|Pourroy|1985|p=63}} In the same scene, Marty and his siblings fade away from a photo. ILM found it difficult to fade the photo's individual aspects, especially as it was moving on the neck of a guitar. A replica of the guitar neck was constructed at four times the normal size; the guitar strings were made of cable up to a quarter-inch thick. An 11-by-14 aluminum plate was attached to hold the enlarged photograph. ILM used a version of the photo without Marty or his siblings and individually pasted each character into the photo. When this failed, four different photos were used: one containing the background, and one for each McFly sibling. A mechanical camera cycled through each photo and printed it to the film.{{sfn|Pourroy|1985|p=63}} The enlarged guitar was moved around to add to the realism.<ref name="ASCILM"/>

The original nuclear explosion ending was considered too complicated and expensive, with an estimated cost of $1{{spaces}}million.<ref name="ASCILM"/><ref name="SlashFilmFridge"/><ref name="ColliderDifferences2020"/> Art director [[Andrew Probert]] storyboarded the scene, which would have been created using sets and [[Scale model|miniatures]].<ref name="SlashFilmFridge"/>{{sfn|Pourroy|1985|p=63}} With the ending moved to the clock tower, ILM researched storms to achieve the right aesthetic.{{sfn|Pourroy|1985|p=63}} Clouds were constructed from [[Polyester Fiberfill|polyester fiberfill]], suspended in a net, and filmed from above while Ralston shone a powerful light from below.{{sfn|Pourroy|1985|p=64}} He used a [[Potentiometer#Rheostat|rheostat]] to rapidly change the lights' intensity to imitate lightning.{{sfn|Pourroy|1985|p=64}}

Developed by [[Wes Takahashi]]'s animation department, the lightning bolt that strikes the clock tower was described as "the largest bolt of lightning in cinematic history". It was intended to originate in the distance and move closer, but the footage was filmed too close to the tower and there was insufficient space between it and the top of the frame.<ref name="FXGuide" /> There was also an issue with showing the bolt onscreen for too long as it made it more obviously animated.{{sfn|Pourroy|1985|p=64}}<ref name="FXGuide" /> The frame count was reduced, but the bolt did not look chaotic enough.{{sfn|Pourroy|1985|p=64}} Zemeckis picked out a single frame of the bolt in an "S"{{spaces}}formation and asked that the effect focus on that shape and be reduced to twenty frames.{{sfn|Pourroy|1985|p=64}}<ref name="FXGuide" /> The bolt was drawn in black ink on white paper; diffusion effects and a glow were added by the optical department.<ref name="FXGuide" />