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# Stop calculating

What's the easy way to do it? Can you calculate them or is it logarithmic and you have to learn them off by heart?

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ISO & shutter speed are easy -just double / half to go up and down.

But to understand things in stop terms helps (like 4 mins compared to 30 seconds) - It really comes to life with a 10 stop filter, 1/30 sec becomes 30 seconds (or more dramatically a 1 second exposure becomes a 68 minute exposure).

A 256mm lens at f/16 has a theortical aperture of diameter 16mm and area about 200 sq mm.

A 256mm lens at f/8 has a theoretical aperture of diameter 32mm and area about 800 sq mm.

In order to allow settings equating to halving (or doubling) the exposure, we need to have f/ numbers in steps of the square root of 2, which is about 1.414.

Hope this helps.

U.S. 16 is the same aperture as f/16, but apertures that are larger or smaller by a full stop use doubling or halving of the U.S. number, for example f/11 is U.S. 8 and f/8 is U.S. 4.

lol

Quote:So if I've got this right you learn the apertures and it's a mistake to think that the relationship between f16 and f8 is double?

With the times and the ISO you just double or half depending on the direction?

Shutter speeds, ISO settings and apertures are all the same in that respect - yes, the simple way of putting it is that you half or double. Which is why they are not linear progressions.

It is also why the mysterious figure of 18% is used for a mid-tone grey card. It has nothing to do with the oft quoted myth about a survey undertaken by Kodak; it is purely because on that non-linear scale, 18% is exactly halfway between 0% and 100%; in other words midway between pure white and pure black.

Boils down to the radius of the aperture - if you double the aperture, the hole doubles in size... the are of a circle is "pi" * radius squared.... so one stop brighter would be 2 * pi * radius squared... somewhere along the line in the sums, that "2" gets rooted and that's where the ratio comes from ... I think

I find explaining what Aperture is one of the more interesting challenges in workshops/1-2-1 trainng sessions, using all sorts of metaphors and analogies, beer being my favorite.

Quote:that's where the ratio comes from ... I think

Yep - pretty logical once you get your head round it - 1.4 x the diameter of the aperture = twice as big an area letting light in = twice as much light hitting the film/sensor in a given time. So the f/stops go up in ratios of 1.4 whereas time and ISO sensitivity simply double for each stop.

Quote:the difference between 30 seconds and 4 mins isn't that great

This depends on the temperature. Time dilates as the temperature falls, so 3.5 minutes at room temperature is not long, but 3.5 minutes at -5 celsius is a hell of a long time, especially if there's a wind chill factor or you forgot your gloves.