Do not convert. You want to use the home currency and prevent the local institution from doing the currency conversion. See this:
https://lifehacker.com/5957566/alwa...youre-in-at-atms-to-not-get-swindled-for-cash
My experience traveling:
In general, if a big company like Visa manages your ATM card, you get a very favorable exchange rate. The big credit card processors use what's known as the "interbank rate",
although even they can differ slightly from company to company.
When you use an ATM and don't convert, the exchange is handled by the credit card processing company that services your ATM card, not by the local bank that operates the ATM. The local bank is likely to provide a much poorer exchange rate.
The exchange rate can work as a "hidden fee" much the way "expense ratios" do for mutual and/or index funds. It doesn't show up as a fee but comes out of your pocket just the same (in this case, in fewer Euros for your dollars/pounds/etc than you would otherwise receive).
Another note for US citizens (and maybe others?): Charles Schwab offers a bank account that provides reimbursement for ATM fees anywhere in the world. Schwab does not require you to trade with them to have this account. (There are no other fees, either, for regular banking. Presumably the "catch" is that the convenience of the bank account makes you more likely to invest with them, so the checking account serves as a loss leader.) Visa processes the payments for Schwab and uses the interbank exchange rate.
When traveling abroad and making many ATM withdrawals, both an unfavorable exchange rate and ATM per-transaction fees can add up. For the Camino, using an ATM card with these benefits (reimbursed ATM fees and a favorable exchange rate) can reduce your expenses and increase security - you can reduce the amount of cash you carry because you're not taking out a ton at a time to mitigate the per-transaction ATM fees.
I don't have any reason to promote this particular provider. I opened an account for this purpose after researching best options for international travel. There may be other institutions that offer similar benefits.
I also asked Schwab to prevent the ATM card's use for debit purchases. Now it can only be used for ATM withdrawals by someone who knows the PIN, which provides some security if it is lost or stolen. (The former being far more likely - I'm not paranoid about crime.) I will use a credit card with no fees for foreign transactions, an excellent exchange rate, and fraud coverage ($0 liability) for non-cash transactions.