I use Pano Tuner app on an iPad Pro to duplicate an aural tune that I am very happy with (as in "Grand Obsession"). I either just used the iPad internal mic or a unidirectional inexpensive external mic pointed directly at the string I am tuning. It works but there has always been some flighty false readings in the treble.
I got a Shure MV88 omnidirectional condenser mic that uses a lightening connection to record playing, and it's sound is far superior than the iPad mic. An added surprise plus is when using the Pano Tuner app with the Shure 88 no more false readings, the readings are now all rock solid.
Why Pano Tuner? And if something is more serious?
All I need is a reading of the chromatic pitch of each string, not a program that also then suggests temperament and stretch. The inexpensive Pano Tuner provided the consistently most reliable, simple and duplicatable results of reading just string pitch on my piano that I tested. All I am doing is duplicating a previous tuning.
Makes sense using a cheaper app that might not have control over note switching and targeting of higher partials and higher-level filtering that a better mic might allow for a more stable display.
I got the same rate of occasional reading fluctuations with the trial version of TuneLab as with Pano Tuner. I wrote it off to false beats (messaging the string terminations didn't help) or the shape of the capo bar (new piano). Since their frequency of occurance was the same I just went with the cheaper, simpler program. I wonder if users of other high and low end ETDs have similar occasional fluctuations while using their iPhones/iPads with ETDs if the Shure88 mic ($149 Amazon, free returns) would stabilize the readings.. Maybe with even true false beats?
Mic quality might be important when you want to measure the level of each partial accurately. But for frequency measurements iPhone mic is okay. And an expensive mic won't do much better.