thanks
Edited by Joshua Dannais, 21 April 2008  12:30 AM.
f4 rated at 1000 iso vs. f1.4 rated at 500
Started By Joshua Dannais, Apr 21 2008 12:28 AM
5 replies to this topic
#1Posted 21 April 2008  12:28 AM
I'm using a slow lens on a slr to take reference pictures of locations i'm planning to shoot on 16mm... since the lens is slow and I'm planning to use a zeiss prime for the 16mm shoot, would shooting the stills at f4 rated at 1000 iso give a similar result as shooting at f1.4 rated at 500 iso?
thanks Edited by Joshua Dannais, 21 April 2008  12:30 AM. #2Posted 21 April 2008  12:50 AM
1000 ISO is one stop faster than 500 ISO. f1.4 at 500 is f2.0 at 1000. F4 is three stops from f1.4; so you'd need to use an ISO that's three stops faster (4000), or a shutterspeed that's three stops slower than 1/48 (1/3 second).
#3Posted 21 April 2008  01:19 AM
thank you... On my light meter 640 was the next click up so I thought that was a stop for some reason. How did you come up with a shutter speed of 1/3? Is it half the speed three times: 1/48 > 1/24 > 1/12 > 1/6? #4Posted 21 April 2008  02:42 PM
Oops! It's 1/6th. You've got the right idea; I somehow counted the wrong fingers! ISO numbers have a geometric relationship; double the number, you double the sensitivity (i.e. one stop). The common ISO speeds are divided in 1/3's of a stop. 640 is 1/3 stop faster than 500. #5Posted 21 April 2008  11:55 PM
awesome, thats big help... i didn't know that. thanks #6Posted 22 April 2008  01:49 AM
Each stop of exposure is doubling or halving the actual exposure. But the various exposurerelated scales work in several different ways.
Shutter speeds work on a geometric scale, usually stepped in full stops: 1/1,000th; 1/500th; 1/250th; 1/60th; 1/30th etc. For a motion picture camera at 24fps, the shutter speed is normally 1/48th, (or 1/50th at 25fps) which is about 1/3rd stop slower than 1/60th. Shutter angles are related to shutter spped: halving the angle is the same as halving the speed (one stop). If 180deg is normal, then in one stop intervals you have 90deg; 45deg, 22deg. In one third stop intervals you have 180deg; 144deg; 113deg; 90deg; 72deg; 56deg; 45deg ISO ratings (aka ASA ratings aka EI ratings) are also geometric, and tend to progress in 1/3 stop increments: starting with the slowest (least sensitive) speed, you have: 50; 64; 80; 100; 125; 160; 200; 250; 320; 400; 500; 640; 800 etc. Note that every third number is double, which ever one you start from. Aperture (f or T stops) are different: they work on a square root of two scale, with a doubling or halving of the number representing two stops. Starting with the biggest aperture (the most light passing through the lens) you can go in onestop intervals: f/1: f/1.4; f/2; f/2.8; f/4; f/5.6; f/8; f/11; f/16; f/22. In one third stop intervals (for part of the range as an example) they go f/2.8; f/3.2; f/3.5; f/4; f/4.5 f/5; f/5.6; f/6.3; f/7; f/8; f/9; f/10; f/11 etc. As another clue, each one third of a stop is a number about 12% more than the previous number. ND filters are different again: they go in additive steps: every time you add an ND3 (which has density 0.30), you alter the exposure by one stop: so the typical one third of a stop intervals are each 0.10ND. Then 0.30ND is one stop (half the light): 0.60ND is two stops (quarter the light); 0.90ND is three stops (one eighth the light) and so on. 
