I have what looks the same on my tomatoes except it is on the back of the leaf also. IE it go all the way through the leaf. My experience in the past if they got frost or freeze damage the leaf would turn dark, nearly black and it would be the whole leaf. It would look wilted. This doesn't look freeze damage I've saw in past. Here is phot of my leaf.

Looks like a little environmental damage, and they should out grow it quickly.

I mix a couple of tablespoons of epsom salt and ground up tums ( 3 or 4 ) in a gallon sprayer and staurate my plants thoroughly 2 to 3 times during the growing season. This helps stop blossom end rot as well.

Epsom salt is for magnesium deficiency, though it is often recommended as a cure-all for all manner of troubles. Magnesium deficiency in tomatoes looks very different than this. Ground up Tums? That would be a VERY inefficient way of dealing with calcium deficiency, which also looks very different in tomatoes than what we're seeing here. Blossom end rot is understood to be a calcium issue, but the problem is more how well the plant takes up calcium, and less how much is in the soil.

http://lmgtfy.com/?q=epsom+salt+myth+plants

We had a couple of cold mornings here a few weeks ago, and my sensitive new foliage got lightly burned. Nothing drastic so the leaves wilted and turned dark and crispy.....Just a little light brown in places. Your plants will recover naturally. Nothing to worry/panic about. Just my worthless (free) comment ;-)

I don't think it is frost damage but it looks environmental to me, not fungal. I wouldn't lose any sleep over it.