Looks like blight. Will start at the bottom and move up. From my experience if it doesn't start too early you can still get a harvest but since it kills off the leaves your tomatoes will be "sunburned." I have never successfully treated this. Has anyone else found something that works?
The picture is rather blurry so I can't see the inner part of the spot, but if you see concentric rings, then it's Early Blight ( A. solani), and if you don't it's probably Septoria Leaf Spot.
Take of all affected leaves ASAP and dispose of them.
Since the two pathogens above are two of the most common fungal foliage pathogens what you need to do is to get on a regular spray schedule with a good anti-fungal and what I suggest is Daconil which works quite well.
What the molecules do is to attach to the specific sites on the upper leaf surface and prevent the spores of either of the above from attaching.
Two of the best Daconil preps are Ortho Garden Disease Control and Bonide and you MUST check the label to be sure that the active ingredient, chlorothalonil, is present at a concentration of 29.6% b'c there are products with different concentrations.
THe spores of both are spread by wind and embedded in rain drops. And you want to get rid of all the affected leaves b'c if you don't the spores will fall to the ground and then next year you can have what's called splashback infection which happens when rain or irrigation splashes those spores back onto the lower foliage and infection starts. Mulching can help prevent splashback infection.
So I assume from what you said that you've never seen this kind of infection before which means that your soil is not yet infested, if you will, with spores from last year.
Bacterial leaf spot. You can control it. But not cure it.