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2,3. From the data sheet of instax film, available here http://www.fujifilm.com/products/instant_photo/pdf/instax_mini_datasheet.pdf
:"Do not leave the photo under direct sunlight while the image is still emerging"
In the same data sheet, we find a density curve versus exposure (H, in lux.seconds) which shows that:
- density is almost flat up to H=2*10^-3 lux.s
- density decreases strongly from H = 3.1*10^-3 lux.s
- density is minimal, film completely overexposed at H = 3.1*10^-2 lux.s
How to understand this?
In full daylight (not direct sun), luminance is in the interval 10000 - 25000 lux.
1st example : f22 aperture is used. With a focal distance typically of 95 mm for instax wide cameras (from 92 to 99, most currently), this means that the diameter of aperture will be 95 / 22 = 4.32 mm, with a surface of 14.7 mm2.
The useful surface of the Lomo Instax Wide film is 6138 mm2 (62x99). So, the luminance at the film level will be comprised between 10000*14.7/6138 = 24 lux and 25000*14.7/6138 = 60 lux.
In order to avoid overexposure, exposure duration should be less than 3.1*10^-2/24 = 1/774 s for external luminance of 10000 lux --> this cannot be reached with usual instax wide cameras.
2nd example: f45 aperture is used; the aperture surface is then PI*(95/45)^2/4 = 3.5 mm2. Luminance at film level is comprised between 5.7 and 14.3 lux.
In order to avoid overexposure, exposure duration should be less than 3.1*10^-2/5.7 = 1/180 s for external luminance of 10000 lux --> this cannot be reached with usual instax wide cameras, but could be reached with, for example, a 110A/B converted to Instax Wide camera.Short answer:
Instax films have a high ISO sensitivity, 800 ISO: it is easier to shoot in shadow, more difficult to shoot in bright light!What to do?
- With simple cameras, to avoid overexposure, just use "darken" or "-1" compensation on the camera, or avoid bright light.
- With more sophisticated cameras, to avoid overexposure, when duration / aperture cannot match the expected results from a light meter, use a neutral Grey density filter, and takes into account the filter attenuation value to determine the exposure duration / aperture value.