i went to funan this morning to buy a lens, am choosing between the tamron 18-270mm VC and the sigma 18-250... went to allan photo and ask the salesman if i could test both lenses. i tried the tamron first, focusing was not bad as i've read on the reviews, and sharpness is remarkable (tried several shots with it...), macro function is also ok, no lens creep either... then i tried the sigma 18-250 DC OS HSM, when the salesman mounted it on my D5K, F-- appeared on the LCD... i dunno what that means, but anyways, i tried to shoot with the lens, but it cant autofocus, so i tried doing manual, but still can't find the focus... the salesman got another copy and mounted it on my cam, same thing appeared on the LCD F-- (what does that mean anyway?), the salesman, quite embarassed, took out a D5K from their stocks and mounted the lens on it... still the same thing appear, and the lens can't work... i tried the sigma 18-200mm OS HSM, and it worked... but am not convinced in it's performance in terms of focusing speed and sharpness (am not anti sigma... in fact, am using the sigma 70-300mm). i end up getting the tamron. anybody experienced the same with the sigma 18-250mm DC OS HSM? any explanation about why this happened? what does F-- in the lcd mean? hope some experts here could enlighten me about this... would like to share this experience for bros contemplating about getting this lens as well... maybe this is an isolated case, but i hope this helps... thanks!