I am growing Batavian 'Arianna' lettuce in a container on my balcony. I'm in San Francisco, so it gets some sun, but it's not all day, and it gets a lot of shade too. I noticed that the tips of some of the leaves appear dark and feel brittle. Is this a lack of water thing or too much sun?
Ok, a few questions... did you grow these from seed, or from transplant, how windy has it been, and how hot has it been there lately? These appear to be older leaves that have been distressed at some point in the past, perhaps by being exposed to lower humidity from drying winds or heat or perhaps by having just been transplanted from, say, a greenhouse environment where the humidity was higher. Lettuce likes sun when in full growth, so I doubt it is getting too much. But if it was sunburn, I'd expect to see damage on the inner, more tender leaves as well as the outer, more mature leaves, and sunburn looks different - more like the leaves have semi-transparent "windows" burned into them. I don't think it's salt spray damage, either, because again, the younger leaves look fine.
If I am correct about the moisture stress, and the plants have recently put on several new leaves and don't show any signs of continued damage, you may be over the worst. I would perhaps give it a bit more water, but be sure not to soak it because you don't want to encourage rot. I'd also look for a way to temporarily increase the humidity around the plants, perhaps by misting them lightly a couple of times a day (not while the sun is on it, but at other times when it is warm and has time to dry out before dark) until you are sure they have been able to adjust.
Well, your lettuce looks very good for the middle of summer, especially for somewhere as hot and dry as San Francisco. The dry tips on the older leaves look like they were caused by heat and/or a lack of humidity. Giving it lots of shade is good, and it looks like it's been watered well.
What you can do about heat is limited, but you could screen it during times of direct sun. Even when it's in shade most of the day, direct summer sun in California is hard on lettuce. On the humidity, I'd spray the leaves with mist fairly often, and keep the plants from drying out. If there is any wind, protecting the plants from that will help also.