I've been told that 昼ご飯を食べた時、散歩する isn't right because you can't have lunch while going for a walk. But in my textbook, there's the following sentence: 日本に行った時、写真をとります. The book explains that taking the photographs occurs after going to Japan (while in Japan) and that the overall tense is future, as the first clause's tense only indicates it's time relation with the second clause. So, why is that sentence correct, and 昼ご飯を食べた時、散歩する incorrect?
「日本｛にほん｝に行った｛いった｝時｛とき｝、写真｛しゃしん｝を取ります｛とります｝」 from my interpretation reads as "In the time after I have gone to Japan I will take pictures".
「昼｛ひる｝ご飯｛はん｝を食べた｛たべた｝時｛とき｝、散歩｛さんぽ｝する」 reads to me as "In the time after I have eaten lunch I will take a walk".
While that seems fine in English there's a quirk, where the first implies that when you are taking pictures you have gone to Japan, implying that you are still there -- and you haven't yet to return prior to taking the pictures. In this case it's being used for travelling, and the "round trip" so to speak has yet to complete when the pictures are being taken.
If you use that as the rule for the grammar point the latter now essentially means that you're in the middle of lunch when you take the walk, since the "round trip" lunch hasn't completed (if this isn't entirely correct please correct me).
That being said, I don't see why you can't take a walk while eating lunch. I mean, it is hard/ridiculous but it isn't impossible, so I'm guessing that it's not wrong wrong, it's just unnatural wrong, which is the case of the equivalent sentence in English as well.
I think that ～た時 translates better as "when" or "while" than "after."
When I go to Japan, I (will) take pictures.
When I eat lunch, I (will) take a walk.
A literal translation of 昼ご飯を食べた時 might be "At the time of eating lunch."
A quick sentence search on jisho looks like they usually use "when" as well: jisho.org た時 sentences