The easiest way to find out about the filter or cap size is to look at the back of your lens cap. You will find a 2 digit number common sizes are 49, 52, 55, 58, 62, 67, 72, or 77. These numbers might have mm or circle with a slash across it as well. If you have lost your lens cap -> you might find it written on the barrel of the lens or more commonly on the top of the len around the glass. If the above is not much of help -> search on google. Write in as much information as you can like "sigma 28-70mm ex" plus review. And look at the search results many internet websites have the filter size in specification. Hope this helps.