Originally posted by jspi I don't consider myself a beginner, more of an intermediate but I feel this is a question I should have grasped (memorized?) long ago. If I'm wrong on any of my information, feel free to educate me.
1 stop of adjustment is either halving or doubling the amount of light hitting the sensor. This can be done by adjusting the shutter speed or aperture. Increasing the sensitivity of the sensor (ISO) doesn't increase the amount of light but has the same effect (although it does increase noise).
Let's say I'm taking a photo at f8, 1/640s, and ISO 6400 but it's "1 stop" too dark.
I could increase ISO to 12,800 (1 stop) but it'll have more noise than I like.
Or I could slow the shutter speed to 1/320s (1 stop) but the action will be blurry.
So I must adjust the aperture. Going with halving/doubling to adjust one stop, the aperture would be f4 but I believe that would be closer to 2 stops.
I understand the math behind 1 stop of aperture but I can't calculate that on the fly. My assumption is I need to memorize which f stops are one full stop from the one below/above (I've been refusing to do that). But I though I'd ask if there's a quick way to figure it out without having to memorize a chart. For now, I just assume f8 to f4 is 2 stops so f6.3 is probably close to one stop? f4 to f2 is likely 2 stops so f4 to f3.2 is probably 1 stop. Assume... Probably... Just set to M, look at the expose meter, and hope it's close. Yeah, there's a bit of guessing and I'm most likely wrong but I have a big SD card so I'm faking my way to descent photos... Although a bit of memorization would be a small price to pay for throwing away less photos. Not to mention the extra time I'd have to be more creative while framing shots.
If you were to actually calculate the areas of the apertures of the different numbered stops (f8, f11, etc.) you would find out that the area from one "full stop" to the next is either 1/2 or double (f11 is 1/2 the area of an opening as f8, which is 1/2 the area of 5.6, etc.). This is all based on the focal length of a lens, so an f1 50mm lens would have an aperture opening of 50mm. Given that each one stop change means the opening has been cut in half or doubled, then that also means that the amount of light has also been cut in half or doubled. SO f8 lets in twice as much light as f11, etc.
f stops work in conjunction with the shutter speeds, which will also cut in half or double the amount of light at the standard shutter speeds (1/60/sec lets in about twice the light as 1/125/sec). ISO works the same way, with ISO 100 needing twice the light as ISO 200 to give the same exposure. The DIN system, common in Europe, uses log changes, so DIN 21 (ISO 100) needs twice the light as DIN 24 (ISO 200). 3 is a natural log change. Doubling or halving the light is a logarithmic change, so these systems all work nicely.
In review then, f11 @ 1/30 sec gives the same amount of light to the sensor/film as f8 @ 1/60 sec, and 1/15sec @ f16, and so on (all for a given ISO, say ISO 100). f11@ 1/30 sec at ISO 100 is the same exposure as f16 @ 1/30 sec at ISO 200 OR f11 @ 1/60 sec at ISO 200. Depth of field does not play into the exposure calculations, only the compositional preferences of the photographer.
Last edited by BigDave; 05-17-2022 at 08:09 AM.