Friday, October 17, 2008


Photographer, Marc Holden, gave a great explanation today on calculating exposure times.

When an f stop is halved, the area of aperture is reduced but the value increases (counter intuitively). The diagram demonstrates this and the relationship with exposure time.

diagram reference

Using a light metre on his Hasselblad, we determined that in the light conditions prevailing we needed an exposure of - 100 ASA film for 0.25 sec at 4.5fstop

but we were using an old Meagher camera like the one pictured below

To extrapolate the values taking into account we were using a paper negative of 6 ASA.

Adding 25% for reciprocity failure the total time required is 5 seconds.

Reciprocity failure is a characteristic of 'slow' film and the following table gives an indication of the compensation required.

No comments: