ISO variation with Instax film ?

There seem to a general tendency to varied ISO sensitivity in the INSTAX film, when exposed directly with a camera. (Which means hybrid cameras are excluded, since they are primary digital, and then you print out the Instax images).

In my case I use the Zinstax back, adapted to the the Mamiya RZ67 Pro camera - like you see in the main photo above. I have come to the conclusion that there were no major errors in the central shutter of my Mamiya lenses - but more like a general tendency of the Instax film itself - dependent in what light level you exposed the film.

It seems to:

1) overexpose in bright daylight

2) being accurate (ISO 800) in normal not too bright daylight

3) underexposing in indoor light / low light / even daylight indoor light

I also read some comment at dPreview (albeit there it about the black & White Instax film version, which nominally has the same ISO 800 sensitivity), the guy suggested that in low light, Instax had more like ISO 400-640 sensitivity, and ISO 1600 in bright daylight.

Even i myself have found this to me more accurate as well. Since then I have greater success with correctly exposed images. And yes, my Minolta Lightmeter works accurately.

Preliminaty, loose guidelines to Fujifilm INSTAX film

ISO 400
Indoor, warm light, low light, indoor daylight without direct sunshine (EV 10 and lower)

ISO 800
Not so bright outdoor light, overcast moderate daylight (around EV 10)

ISO 2000:
Outdoor, bright light, sunny days, sky towards sunrise / sunset

So. This might help

I will and i do follow this up continuously as I make more experiments with Fujifilm INSTAX film together with the Mamiya RZ67 camera.

Indeed it helps !

Every time I use ISO 1600-2000 in daylight with the Instax Square film - i get definitely better images !

How wonderful !!

Finally.

Now i also read about this approach on reddit users, which use low ISO indoors in lower light (ISO 400-500), and higher ISO in daylight (ISO 1600) giving generally off better results on Instax film with its nominal ISO 800.

It is really getting more interesting. But yeah - the quality is abysmal.

Even sharpness is... well, is there any really? Or am I too picky ?