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I’ve put my Fujifilm Superia X-TRA 400 into my Minolta Hi-Matic 7 and I forgot to change the ISO setting for the new film. It’s set for the last roll at 200.
So what setting should I put for a normal sunny day, shaded afternoon, and night? When I develop it, what should I tell the lab?
Do not worry! Film has latitude -- it can tolerated a 2X error in exposure. Just continue as if nothing went wrong. Next time pay more attention to detail. Good Luck!
Your pictures will probably be overexposed by 1 stop. The effect depends on the type of film or sensor.
For negative films, overexposing is mostly OK and you may not notice. It will result in denser negatives, perhaps with more grain than you would like, and with more shadow detail. You can "pull process" the film to try and compensate--ask the processor to "pull one stop", but not all processors will support this, and the ones that do support it will probably charge you extra. If you are using black and white film you can just develop for a slightly shorter time, there are tables for your film and developer that will explain this. Pull processing is not necessary if you only overexposed by one stop, it is not a big deal. Some film/developer combinations notably recommend the same development time for normal development and pulling 1 stop!
For slide films, overexposing will result in a washed-out look with less vibrant colors. You generally want to avoid this, but it is not a disaster, and you can ask for pull process.
For digital, overexposure is usually a bit of a disaster, because it will blow out the highlights.
TL;DR: Since you are shooting a negative film, and only overexposing by one stop, you are probably fine and don't need to do anything.
Pull processing: Ask the lab to develop less, to compensate for overexposed film. Not really necessary with only 1 stop of overexposure and negative film.
Push processing: The opposite of pull processing, when you ask the lab to develop extra to compensate for underexposed film.
Ultimately the ISO speed on the box is not actually a rule or even a recommendation for how much to expose the film! It is just a property of the film calculated in a certain way, designed to be a pretty good starting point for exposure.
If you are shooting film often, you may end up with your own preferences for how much to expose the film you are using, and that exposure might not match the speed on the box. I know a number of people who like to shoot "T-Max 100" at 50 ("overexpose" 1 stop) or shoot slide films like "Kodachrome 64" at 80, or "Velvia 50" at 64 ("underexpose" 1/3 stop). I put "overexpose" and "underexpose" in quotes because you are not really overexposing or underexposing, you are really exposing correctly just not using the exposure that it says on the box.
I'm confused: have you just loaded the roll or have you exposed it? If you have just loaded it but not yet taken any pictures then just change the ISO setting on the camera to be right (ie to be 400). If you have exposed frames on it then I'd just use it at that speed: the negs will be overexposed by about a stop but that's mostly fine. You can get (some) labs to push or (in this case) pull-process film but this will probably cost you more than just buying another roll.
The absolute worst thing you can do with color negative film is underexpose it. Here's an example test using a different color-neg film (https://petapixel.com/2018/02/05/test-reveals-exposure-limits-kodak-portra-400-film/). All color-neg film is similar in this property.
So, you have loaded your 400 speed film into a camera that thinks it's metering for 200 speed film. This mean that you have, depending on your metering...
In all of the cases above, your images will turn out just fine if you do absolutely nothing. Have the film developed normally and call it a day.
The reason for this is the film's latitude. As you can see in the linked article above, color-neg film will tolerate exposures from 0 (perfect exposure) to 4 stops over and still look good. It'll tolerate from -1 to +5 and still be usable.
ALL of your exposure leeway is on the positive side. I know some people who downrate their color-neg film (for example, shoot a 400 speed film as if it were 320) simply to force a slight overexposure in their shots - as this drastically limits the probability of an underexposed frame.
