Hi Nick
I know aphantasia - I enrolled myself as a test guinea pig for a research programme at Toronto University. Did a questionnaire and a bunch of online tests. It seems it was a condition that was not even recognised until recently.
I seem to exhibit with both elements of the condition:
- I find it very difficult, if not impossible to create internal mental imagery ("imagine a ginger cat - how fluffy is its tail?") - how the hell do people do this? Although dreaming seems to function just fine. My partner has no trouble with a task like "Think of an electric kettle. Turn it around in your mind. Where is the serial number". I find that fascinating, I have no real ability to imagine a picture of anything. When I question friends about their experience of reading a descriptive passage in a book, they sometimes say things like "Oh, as I read, I see it like watching a film". That is completely alien to me. I am a prolific reader but I kind of skip the descriptive bits as they serve no purpose.
- I also have trouble with episodic memory ("where were you on tuesday evening?", "tell me about that trip to the zoo". I often conflate two events that happened in different places at different times. I once spent ages searching the local park in Bromley where I live for a sunken rose garden that I later found out is actually located in a different park in Dumfries in Scotland. The two parks have some geographical similarity in shape and size and ridiculous episodic memory just stitched them together. It would be embarrassing if I haven;t had a life time to get used to it. i would make a very unreliable witness. I also get lost quite easily. I need dependable well remembered landmarks or to repeat the same journey a lot before I can find my way around reliably. I often turn the wrong way momentarily in my own house if I'm not paying close attention. It appears these things are linked to the ability to visualise in a mind's eye. They reckon between 1-5% of people are like this.
I don't think composing an image is affected much by this though. It seems to be a different process. I've always thought that the trick to composition is to be really sure what you like. Years ago, i always found finding subjects to photograph a struggle but not so much anymore. And that is simply because I have worked out what kind of photos I like and can see the opportunities better. However, if I tried a different genre, I'm back to square one and as helpless as a beginner.
I don't tend to have the episodic memory issues you do but it takes me some time to get to know a journey well enough that I don't end up lost

Going to a new place or taking a different route takes me much, much longer than others. I know other people who drive somewhere once and then remember the route, it takes me nearer 10 goes before I feel as though I know it.
I too skip past descriptive passages in books - also an avid reader. As you say there is little point reading them when all it conjures up is a blank space in my head ;-)
Is there a link between the condition and Sigma cameras ?
I doubt it. I have a strong visual imagination, and I've used all kinds of cameras.
A guy I knew in the 1960s had your problem, and had to make verbal lists of features so he could identify microscope slides for a biology course that he was studying. So I've been aware of the range here for a long time.
I think there's a continuous range from unusually vivid imagination, through average, to can't-see-nothing.
How about imagining smells, tastes and sounds ?