[esman7]

This might help clarify a bit as well. The inputs for ISO, shutter speed, and aperture are all set up so you can make equivalent & offsetting changes in EV steps (or stops).

ISO 1EV steps: 100-200-400-800-1600-3200.
Shutter speed 1EV steps: ...1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500...
Aperture 1EV steps: ...f/2, 2.8, 4, 5.6, 8, 11, 16, 22...

Intermediate steps may be available (usually either 1/2 or 1/3 EV) but the idea is that you can easily make equal and offsetting changes to other parameters to influence depth-of-field, freeze motion, etc.

Example: Meter tells you 1/125 @ f/5.6 with ISO 200. You know your subject demands extra depth-of-field so more subjects are in focus. Without even re-metering, you can "stop down" aperture to f/11 (two stops), increase ISO to 400 (1 stop) and decrease shutter speed to 1/60 (1 stop) and get the same level of exposure.

Awesome explanation, that makes a lot of sense - scales are designed for this and I didn't even notice. As for 0.3 and 0.5 EV steps, how are those acheived if shutter speed, ISO, and aperture and tuned to adjust in +/- 1.0EV step?

For example, if you were to use your 1/125 @ f/5.6 with ISO 200 example, how can you achieve +1.3 of -0.5 EV (just throwing numbers out there)

[Marc Sabatella]

As for 0.3 and 0.5 EV steps, how are those acheived if shutter speed, ISO, and aperture and tuned to adjust in +/- 1.0EV step?

They aren't. Shutter speed and aperture default to being adjustable in 1/2 EV steps. Most if not all Pentax DSLR's also provide a custom option to allow you to use 1/3 EV steps instead. And while some Pentax DSLR's are limited to full EV steps for ISO, some include a custom option to allow you to get 1/2 or 1/3 EV steps there too. i use 1/2 steps across the board, because my manual lenses have only 1/2 stops, not 1/3 stops, and I like to keep thing as consistent as possible. Although I have my ISO set in 1/2 stops, I rarely use them - mostly it's 100, 200, 400, 800, or 1600.

[esman7]

They aren't. Shutter speed and aperture default to being adjustable in 1/2 EV steps. Most if not all Pentax DSLR's also provide a custom option to allow you to use 1/3 EV steps instead. And while some Pentax DSLR's are limited to full EV steps for ISO, some include a custom option to allow you to get 1/2 or 1/3 EV steps there too. i use 1/2 steps across the board, because my manual lenses have only 1/2 stops, not 1/3 stops, and I like to keep thing as consistent as possible. Although I have my ISO set in 1/2 stops, I rarely use them - mostly it's 100, 200, 400, 800, or 1600.

...Marc, if shutter speeds follow 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500..., wouldn't those be considered +/- 1.0 incremental EV steps?

[Marc Sabatella]

...Marc, if shutter speeds follow 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500..., wouldn't those be considered +/- 1.0 incremental EV steps?

Are those the steps in your camera? They aren't in mine. Mine go 1/8, 1/10, 1/15, 1/20, 1/30, 1/45, 1/60 1/90, 1/125, 1/250, 1/350, 1/500 - those are 1/2 EV steps.

[nanok]

shutter speeds are purely conventional, they can be anything. in modern cameras (electronically controlled), the camera is able to control shutter speed steplessly i think (not that it trully matters). what andrew was refering to is the original conventions used for mechanical cameras, which could only do 1ev steps in most cases, so yes, you are correct, it's 1ev. the numbers chosen for the aperture are also a convention, and have a lot to do with flash usage, amongst other things. they may sound strange, but there's a point to them

somebody mentioned reflective versus incident metering. the point with reflective metering is that the camera doesn't know the "albedo" of the subject you are metering, so it does not know how you want it rendered in the final result, there are two ways around that: 1. reflective metering, assuming the scene across the frame should go to something close to middle gray (18%) when avergaed (that's mostly what centre weighted does, while also giving more influence to the centre when averaging) 2. incident light measurement. the first is convenient but inherently inaccurate, the second is almost scientifically accurate, but in most cases inpractical. with digital, (and, according to adams for instance, with anything), i tend to believe both are wrong aproaches to begin with, but that's a different storry. what you need to understand is that your camera does not know your black wall is black, or your white dog is white, it will make an assumption (very roughly, based on middle gray), once you understand that, you made a big step towards being able to use your ev compensation effectively. it's downhill from there .

[mattdm]

shutter speeds are purely conventional, they can be anything. in modern cameras (electronically controlled), the camera is able to control shutter speed steplessly i think (not that it trully matters). what andrew was refering to is the original conventions used for mechanical cameras, which could only do 1ev steps in most cases, so yes, you are correct, it's 1ev. the numbers chosen for the aperture are also a convention, and have a lot to do with flash usage, amongst other things. they may sound strange, but there's a point to them

Well, it's the same convention as shutter speeds: each stop is one EV, or doubling/halving of light. The strange series comes in because it works out that to let in double the amount of light, you increase the aperture by the square root of two. So:

Code:

sqrt(2)^0 = 1     = f/1
sqrt(2)^1 = 1.414 = f/1.4
sqrt(2)^2 = 2     = f/2
sqrt(2)^3 = 2.828 = f/2.8
sqrt(2)^4 = 4     = f/4
sqrt(2)^5 = 5.657 = f/5.6
sqrt(2)^6 = 8     = f/8
sqrt(2)^7 =11.314 = f/11
sqrt(2)^8 =16     = f/16

You can work out the partial stops with the same math — sqrt(2)^1.5 = f/1.7, sqrt(2)^2.5 = f/2.4 and so on.

[nanok]

i really didn't want to open that can of worms..

here is what i meant about flash: the flash guide number is distance * f-number. convenient, eh? . let's see why sqrt (as mentioned by matt above) comes in:

the f numbers are meant to designate 1ev step distances between them, as matt pointed out. they are also meant to have the same meaning regarding how much light goes in across focal lengths (so f2.8 means as much light for a 50mm and a 200mm), so the term of relative aperture was born: focal length/aperture diameter. to get 1ev between them, you need to have double/half the light going through when moving up/down one stop, this means, physically, that the aperture _area_ needs to be twice as big/small, but the area is a function of the _square_ of the diameter (pi*(d/2)^2), which means that you end up with these strange square root "leftovers" as f-numbers. they look strange, but are surprisingly convenient.

now back to the guide number: the light falling on the subject coming from the flash will depend on (the inverse of) the square of the distance, the light going back to the sensor will depend on the area of the open aperture, which is described by the square of the f-number, so for one aperture step up, we will have twice the light going through to the sensor/film, and thus we will be able to have sqrt(2)*distnace by using the same flash, when using aperture f-numbers, we already have numbers dimensionally suitable to put against the simple distances (instead of needing to work with squared distances), because, though adimensional, f-numbers are, like distance to subject itself, linear in nature (not areal, so to speak, so, you could say, already sqrt'ed)

2.8*3m=8.4=4*2.1m

and this is why the guidenumber formula works, it might not sound like a big deal these days, with pttl and all this crap, but "back in the days", it was very usefull, and even these days, strobists i think still use this formula to understand and ballance their more complex lighting schemes. so, while the convention seems terribly silly and meaningless, it's actually quite brilliant and well thought over, and has the only downfall of the funky-looking numbers.

[newarts]

I think the background of all this is based on old-fashioned machining practice and an ancient scheme to help with mental math.

When a dial is turned it is nice if it moves in "clicks"; that is, you turn it a bit and it "stops" as a spring snaps into a groove machined on the edge of the dial. Fishing reels, volume knobs, etc.. are routinely set up to move this way.

Photographic exposure for a given scene & film is proportional to (lens open area) x (time).

Shutter speed dials were arranged so that each stop (ie. click) represented a factor of two change in speed (time). The aperture opening dial was also set up so that each stop (ie. click) corresponded to a factor of two of open lens area.

So an change of a stop on one dial could be compensated by a change of one stop on the other dial.

This greatly simplfies the mental math involved in making adjustments; instead of multiplying (or, heaven forbid, dividing) numbers on the dials, you need only count clicks! It changes multiplication and division into addition & subtraction.

To freeze motion, increase the shutter speed by a couple clicks (stops)- to compensate for the reduced light, open the aperture the same number of clicks (stops.) Easy.

Later on, film manufacturers adjusted film characteristics and development instructions to follow the same factor of two pattern... That's why ISO (nee ASA) designations are in facrors of two. 100, 200, 400, 800, .....

Its all a matter of convenience and tradition.

Iowa Dave

[esman7]

I think the background of all this is based on old-fashioned machining practice and an ancient scheme to help with mental math.

When a dial is turned it is nice if it moves in "clicks"; that is, you turn it a bit and it "stops" as a spring snaps into a groove machined on the edge of the dial. Fishing reels, volume knobs, etc.. are routinely set up to move this way.

Photographic exposure for a given scene & film is proportional to (lens open area) x (time).

Shutter speed dials were arranged so that each stop (ie. click) represented a factor of two change in speed (time). The aperture opening dial was also set up so that each stop (ie. click) corresponded to a factor of two of open lens area.

So an change of a stop on one dial could be compensated by a change of one stop on the other dial.

This greatly simplfies the mental math involved in making adjustments; instead of multiplying (or, heaven forbid, dividing) numbers on the dials, you need only count clicks! It changes multiplication and division into addition & subtraction.

To freeze motion, increase the shutter speed by a couple clicks (stops)- to compensate for the reduced light, open the aperture the same number of clicks (stops.) Easy.

Later on, film manufacturers adjusted film characteristics and development instructions to follow the same factor of two pattern... That's why ISO (nee ASA) designations are in facrors of two. 100, 200, 400, 800, .....

Its all a matter of convenience and tradition.

Iowa Dave

True, it's funny how we take these types of things for granted when it's always been laid out in front of you in convenient little "steps". Imagin if we were dealing with f2.345 and 1/87 shutter speeds.... phew.