I'm not a beginner but have some basic understanding of photography. I use a Canon 60D camera to take pictures with the 18-135mm lens that came with the camera. Recently I bought a sigma 30mm f/1.4 lens to take portrait photos. This lens is great, but I found a strange issue with it. If I take pictures indoors, with or without flash, the results are great. However when I take any pictures outdoors, on a sunny day, it works fine. The problem is, if I use flash (for filling), the pictures taken is over exposed (almost white). I tried playing with settings to take pictures but same result when using flash. For example I used Av mode with f/1.4 with flash. ISO was A, but the picture is too bright. After struggling for a while I found that when using Av or M mode, even though I set shutter speed to 5000 (because I used f/1.4), the moment I press button half way to focus, camera change shutter speed to 250 (max). After that, I can't increase the shutter speed. For testing purposes I tried the same with my zoom lens (18-135mm), and everything works great with f/3.5, and I can increase the shutter speed to the max possible.
So I'm wondering if there is some limitation when using a sigma prime lens with the 60D or am I doing something wrong?
Thanks for any suggestions or clues!