Pre-authorizations are used to hold funds on a customers credit card for a period of 3-7 days depending on the institution.
The pre-authorization must be completed using a pre-auth capture. If you don't use the pre-auth capture, you will either reserve the funds twice (upsetting the customer) or the transaction will be declined.
The pre-authorization system was designed for the restaurant industry and tips, not the rental industry and deposits. This causes a few issues.
A pre-auth can expire without the merchant knowing. If the merchant then attempts to complete the pre-auth transaction, the transaction may appear to initially succeed, but may be declined in the final settlement if the customer's credit limit is exceeded.
Customers using Visa / Mastercard debit cards will see the pre-auth amount debited immediately from their bank account, which may upset them if they are not notified of the amount you are pre-authorizing for.
An alternative to pre-authorization is that rental deposits should be taken as a full deposit and then refund the difference when the equipment is returned.