This is a question about how VPHOT works.
If I take a time series of 10 images and change the exposure time to 10 seconds for the 1st image, then add a second of exposure for each of the next nine images, does it affect the differential photometry? I am thinking that there should be no difference in the differential magnitude of any two stars in the field. Only SNR should be a different.
Also average stacking should not be a problem. The stacked image should show the same differential magnitude as any of the single images.
It is a late hour. 
Am I thinking correctly?
Ray:
Your logic makes sense to me. I assume VPhot is coded that way. I really don’t want to hunt through the code to confirm that.
You could run the experiment quickly on some bright comps and confirm that the time series light curve is a nice flat (zero slope) line with only random error.
Ken
Thanks for the reassurance Ken. I thought I had read something on one of the forums saying that we needed to make all the exposures the same else the photometry would not be correct. I am trying to get more consistent readings.
I often take maybe 14 images, average 14, then average the first 8 and the last 8. I do that to get a better SNR and to provide the user some idea what my real error bars are. I often (statisticaly often?) see the three averaged data points in a hi-lo-hi plot. Don’t know why that preponderance exists. Very seldom a lo-hi-lo plot. Sometimes the three filters all go their own way. Blue goes up, V goes down, I shows all three data points the same. These errors are of course more severe at 17th magnitude than they are at 9th magnitude. I often adjust exposures after the first exposures guess for BVI. A typical times series is 30 minutes to an hour so that I can get 3 B, 3V, 3 I data points. So I am scratching my head over that often (but not always) seen V-shape. The V is generally about 0.2% of the magnitude. My works shows many other boo-boos but I decided to spend a little effort on this one first.
I am using a well marbled old ST8XME having at least one hot pixel at ~80% of full well and many warm pixels while tracking can vary 5 seconds on 3 second stars. I don’t take flats every night, Anyway, a work in progress to chase down more of the error so I can make pretty light curves.