If the character has half a red heart or no red hearts after taking damage, it triggers the Book of Shadows effect, creating a shield that nullifies all types of damage for 5 seconds.
Allows entrance into Chest after beating Isaac in the Cathedral by touching the chest.
Unlike with the Chest entrance, The Polaroid gets consumed by the door and vanishes from Isaac's inventory.
Notes
This item is a guaranteed drop from Mom, if unlocked. Will drop alongside The Negative, provided they are both unlocked. However, they are mutually exclusive: Once one of them is picked up, the other one will disappear.
With the use of either a teleportation item and 5 pip Dice Room, a Forget Me Now, a Diplopia, a Crooked Penny, or R Key, both the Polaroid and the Negative or multiple copies of the Polaroid or Negative can be obtained.
With Jacob and Esau, proper alignment of both characters can enable picking up both The Polaroid and The Negative.
With frame-perfect timing and a bomb, it is possible to pick up both The Polaroid and the Negative.
The shield effect provided by The Polaroid can be stacked if the character takes damage multiple times. For example, using IV Bag 2 times provides 10 seconds of shield.
Although The Lost and Tainted Lost don’t have health, The Polaroid can trigger if they take non-fatal damage (e. g. take damage while Berserk! is active or imitate damage via Dull Razor).
The Polaroid will be the leftmost item after defeating Mom, in case Curse of the Blind is active.
It is possible to obtain two Polaroids by obtaining one as an Eden starting item.
Obtaining multiple copies of The Polaroid does not cause its effect to stack.
In the original The Binding of Isaac flash game, The Polaroidused to be a trinket that Mom would drop in addition to a regular collectible from the boss pool. When it was originally implemented, it was only available in the normal Trinket drop pool and was implemented secretively. Players were frustrated, so, with an update, Edmund made it so it would also only drop if the player took no red heart damage against Mom, similar to the conditions for the Devil Room to open its path. Since players would not always receive the Polaroid and might switch it out with another trinket, Edmund wanted to make the discovery of Chest (and its at-the-time new ending) something special.
As soon as The Polaroid or The Negative become available, they will drop in place of the regular Boss Room item Mom would otherwise provide, essentially providing a difficulty increase for Womb and beyond after either has been unlocked.
They can, however, be re-rolled (or converted into stat increases with Void) at the cost of closing off access to Chapter 6.
Book of Shadows effect, creating a shield that nullifies all types of damage for 5 seconds.
Chest after beating 
Isaac in the 
Cathedral by touching the chest.
Opens 
A Strange Door in 
Depths II and its variants that leads to 
Mausoleum/
Gehenna II.
Mom, if unlocked. Will drop alongside 
The Negative, provided they are both unlocked. However, they are mutually exclusive: Once one of them is picked up, the other one will disappear.
Dice Room, a 
Forget Me Now, a 
Diplopia, a 
Crooked Penny, or 
R Key, both the Polaroid and the Negative or multiple copies of the Polaroid or Negative can be obtained.
Jacob and Esau, proper alignment of both characters can enable picking up both The Polaroid and The Negative.
??? and 
Tainted ??? 
Dark Judas and 
Tainted Judas, 
The Forgotten (only as the soul), 
Tainted Forgotten , 
Tainted Bethany or any other character who has chosen to trade away their heart containers in deals with the Devil, 
Black Markets,
Guppy's Paw or the 
Potato Peeler. It will also trigger if the character has only half of a red heart left.
IV Bag 2 times provides 10 seconds of shield.
The Lost and 
Tainted Lost don’t have health, The Polaroid can trigger if they take non-fatal damage (e. g. take damage while 
Berserk! is active or imitate damage via 
Dull Razor).
Curse of the Blind is active.
It is possible to obtain two Polaroids by obtaining one as an 
Eden starting item.
Key Piece 1 and 
Key Piece 2, 
Knife Piece 1 and 
Knife Piece 2, 
Dad's Note, and 
Dogma), this item has protection from full-run rerolls. It does not, however, share the immunity 
Void.
Womb and beyond after either has been unlocked.
Isaac (Boss) Unlocks 
The Binding of Isaac: Rebirth
