Mobile website does not zoom on Safari in landscape mode

I have browsed through many topics here and on the web, trying to solve my issue. I'm very close to finding a solution but there is one thing that is still not working correctly.

I have a mobile website that is 640px wide. It should:

Show zoomed in on all browsers correctly, also when changing the orientation of your device
User should not be able to zoom in (input fields neither).

I have used this viewport meta tag to accomplish this:

< meta name="viewport" content="width=640,maximum-scale=0.5,initial-scale=0.5, minimum-scale=0.5" />

This works great on Android, both portrait and landscape. It works great on iOs on portrait too, but it shows the site zoomed out on landscape, so it's not adapting to the width of the phone, but keeps the original portrait size. I have tested this on iPhone 5 and iPhone 4S.

I have found mostly issues with responsive websites and not so many with fixed mobile websites. Is there a solution for my problem by changing the viewport meta tag? Or should I be looking into javascript to solve this?

Thanks in advance.

Edit:If anyone is interested, I used this bit of code in Coldfusion which is working perfectly fine now (iOs & Android):

<cfif findNoCase('iPhone', cgi.http_user_agent,1)><meta name="viewport" content="width=640" /><cfelse><meta name="viewport" content="width=640,maximum-scale=0.5,initial-scale=0.5, minimum-scale=0.5" /></cfif>

Solution

I tend to use those lines in my head to optimize responsive sites:

<meta name="HandheldFriendly" content="True">
<meta name="MobileOptimized" content="320">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta http-equiv="cleartype" content="on">

Maybe adding those will fix your problem?

I know from experience, that iOS Safari behaves strange in some cases. Here is a line that I use to fix some other issues I had with it before:

<!-- This script prevents links from opening in Mobile Safari. https://gist.github.com/1042026 -->
<!--
<script>(function(a,b,c){if(c in b&&b[c]){var d,e=a.location,f=/^(a|html)$/i;a.addEventListener("click",function(a){d=a.target;while(!f.test(d.nodeName))d=d.parentNode;"href"in d&&(d.href.indexOf("http")||~d.href.indexOf(e.host))&&(a.preventDefault(),e.href=d.href)},!1)}})(document,window.navigator,"standalone")</script>
-->

Alternatively you can try running your site as iOS Web-app:

<meta name="apple-mobile-web-app-capable" content="yes">
<meta name="apple-mobile-web-app-status-bar-style" content="black">
<meta name="apple-mobile-web-app-title" content="">

Hope that some of that will fix your problem:)