EV and "stops"

[Ellie]

I hope it isn't too dumb a question.

The exposure compensation (EV) on my E400 is marked +/- 0.3; 0.7; 1.0, etc

Does this bear any relation to a "stop", or is it something completely different?

[beardedwombat]

Re: EV and "stops"

Hi Ellie
You are only likely to find dumb questions on Canikon forums, our inhabitants always ask sensible questions! The markings mean 0.3 of a stop, 0.7 of a stop and 1.0 stop.
Cheers
Chris

[Scapula Memory]

Re: EV and "stops"

No not dumb in the slightest.

It refers to increments of a stop. Usually you can pre set this somewhere in the cameras menus for auto bracketing or when you manually adjust the EV.

regards

John

[Ellie]

Re: EV and "stops"

Ah, thanks

So if the camera's set at f16 and I reduce the EV by -0.3 what's the equivalent f number? Am I still being dim?

[Nick Temple-Fry]

Re: EV and "stops"

hmm - ah, oh dear, well

EV means exposure value - the same EV on the same scene will give an image of the same brightness.

EV is related to the shutter speed and the aperture.

Exposure compensation comes from measuring exposure and then saying I want this picture a bit brighter/darker, and is measured in the same relative units.

Now in a film camera what altered was whatever you had not set as priority

- so if you set aperture priority then the shutter speed would alter
- if you set shutter priority then the aperture value (f-number) would alter.

However digital cameras are not that simple, first of all they have this very odd thing known as Program mode ( and somewhere in your camera manual there will be a funny attempt to draw a 3-dimensional graph in 2 dimensions showing the effect of shutter speed/exposure value/f-number on each other for a specific lens)

Secondly they can do things to sensor sensitivity (which is how different iso values are emulated) which a film camera could not do to the film.

What does this mean as far as your original question is concerned

1) Well I'm as confused as everyone else is, even though I've got a degree in Physics from a University that was rather keen on optics

2) In practise it does not matter - the maths are too hard to do in the field except with a slide rule

3) Come up with your own rule of thumb for each lens based on experience and unless you need to pass an exam don't worry too much.

4) If you are really keen read

http://en.wikipedia.org/wiki/Exposure_value

http://en.wikipedia.org/wiki/Exposure_compensation

and

http://en.wikipedia.org/wiki/Exposure_compensation

And a google will find a few thousand articles, all with that photographers individual interpretation of the truth.

Knowing this probably hasn't helped at all

Nick

[Ellie]

Re: EV and "stops"

*gulp* I was of the generation that didn't have to do all three sciences, and abandoned physics, but I did do maths. Even so what you say is nicely reassuring.
I'm as confused as everyone else is, even though I've got a degree in Physics from a University that was rather keen on optics

So, it's a case of suck it and see, and don't worry about what it's doing as long as it works?

I will read those links, but not right away.

[shenstone]

Re: EV and "stops"

However digital cameras are not that simple, first of all they have this very odd thing known as Program mode ( and somewhere in your camera manual there will be a funny attempt to draw a 3-dimensional graph in 2 dimensions showing the effect of shutter speed/exposure value/f-number on each other for a specific lens)

Nick

Nick

A good response and some useful links to follow-up next time I'm asked a similar question

I'd like to ask the wider audience re the statement you made above

Have you ever used any off these modes? I have to admit being rather luddite and of the film generation it took be about the 1st 3k images to remember I could change ISO and the thought of remembering what all those modes do terrifies me so I just stick to AP/SP and change the various settings as I think I need them.

Regards
Andy

[ukanalyst]

Re: EV and "stops"

Some really good questions here Ellie and a brilliant reply Nick IMHO ... It really encourages me to see you type this:

1) Well I'm as confused as everyone else is, even though I've got a degree in Physics from a University that was rather keen on optics

2) In practise it does not matter - the maths are too hard to do in the field except with a slide rule

3) Come up with your own rule of thumb for each lens based on experience and unless you need to pass an exam don't worry too much.

I do from time to time from various things I've read to date imagine everyone except me measuring x,y and z to get technical perfection for each shot and memorising complex graphs. So happy to hear I'm not doing it 'that wrong'!

Dave

[vicb981]

Re: EV and "stops"

Ah, thanks

So if the camera's set at f16 and I reduce the EV by -0.3 what's the equivalent f number? Am I still being dim?

Very good question.

The aperture numbers are in a series of half brightnesses; ie going from f 8 to f 11 halves the exposure. To calculate this one multiplies by square root (2) that is 1.414. So 8 x sqrt (2) = approx 11.

The values in the series f 2, 4, 8, 16 are exact ones; the intermediate ones aren't quite exact so please make allowance for this!

Now EV 0.3 is more difficult. Presumably this means 1/3 but I'm guessing. Just to make life hard you've asked about -0.3 EV. I think it means one-third of the exposure. If we said one half from f16 we'd be back at f11. So I'm assuming that -0.3 EV means of third of one half the exposure; that is, reduced by 17% approx.

So I'm suggesting that the value for f16 less 0.3 EV is f14.8 - that is, 16/sqrt (1.17). In the same way f16 less 0.5 EV would be f13.1.

This assumes a linear scale and I'm not sure that is correct. The error is seen if you calc f11 plus 0.7 EV which presumably is the same numerically as f16 minus 0.3 EV; on the linear scale they are not identical.

Essentially the EV scale enables you to vary the exposure either as shutter speed or as aperture and it's a measure of light intensity. 'Stop' just refers to aperture and I've always considered to be incorrectly interchanged with EV.

If you want to work out exact values you'll have to use logs.

I've made a few assumptions here and it would be interesting to hear from someone who actually makes exposure meters or cameras. I often dial in -0.3 EV and I have to admit that I've no idea what I'm doing! All I know is that it makes a small difference to slide film.

[Wreckdiver]

Re: EV and "stops"

In Ellie's example, -0.3EV from f/16 is f/18, one third of a stop (or one third of an EV) underexposed - for given a shutter speed.

The f/stop range, in one third stop, values are:

f/22, f/20, f/18, f/16, f/14, f/13, f/11, f/10, f/9, f/8, f/7.1, f/6.3, f/5.6, f/5, f/4.5, f/4, f/3.5, f3.2, f/2.8 (to the nearest calculated value)

The emboldened values are the traditional whole f stops.

f/stop can only refer to the aperture setting, whereas EV can refer to either the aperture setting or the shutter speed.

Steve

[Ellie]

Re: EV and "stops"

... you'll have to use logs.

I remember those with almost as much horror as slide rules!

Very good question.

It looks as if it was a better question than I'd thought at the time

Have you ever used any off these modes?

Yes, I have used some of the scene modes on the camera, if that's what you mean - when I first bought it. I also used the Programme Mode for a while, and checked the EXIF data to see what I was changing most often and ended up preferring to use Aperture Priority rather than Shutter Priority. At the moment I see no need to go fully manual, not with the E-400 anyway.

Is Olympus unusual in having +/- 0.3, 0.7 and 1.0 etc as options for EV. I'm sure one of my friends said that her camera only adjusts in whole numbers.

[Wreckdiver]

Re: EV and "stops"

Is Olympus unusual in having +/- 0.3, 0.7 and 1.0 etc as options for EV. I'm sure one of my friends said that her camera only adjusts in whole numbers.

1/3 stops is the norm these days, even on compacts.

Steve