Exposure Value (Ev) – what does Ev mean?

I get asked this question every now and again because I frequently use it in the description annotations of image shot data here on the blog.

And I have to say from the outset the Exposure Value comes in two flavours – relative and absolute – and here I’m only talking mainly about the former.

So, let’s start with basic exposure.

Exposure can be thought of as Intensity x Time.

Intensity is controlled by our aperture, and time is controlled by our shutter speed.

This image was shot at 0.5sec (time), f11 (intensity) and ISO 100.

We can think of the f11 intensity of light striking the sensor for 0.5sec as a ‘DOSAGE’ – and if that dosage results in the desired scene exposure then that dosage can be classed as the exposure value.

Let’s consider two exposure settings – 0.5sec at f11 ISO100 and 1sec at f16 ISO 100.

Technically speaking they are two different exposures, but BOTH result in the same light dosage at the sensor.  The second exposure is TWICE the length of time but HALF the intensity.

So both exposures have the same Exposure Value or Ev.

The following exposure of the same scene is 1sec at f11 ISO 100:

The image was shot at the same intensity (f11) but the shutter speed (time) was twice as long, and so the dosage was doubled.  Double the dose = +1Ev!

And in this version the exposure was 0.25sec at f11 ISO 100:

Here the light dosage at the sensor is HALF that of the correct/desired exposure because the time factor was halved while using the same intensity.

So half the dose = -1Ev!

Now some of you will be thinking that -1Ev is 1 stop under exposure – and you’d be right!

But Ev, or exposure value, is just a cleaner way of thinking about exposure because it doesn’t tie you to any specific camera setting – and it’s more easily transferable between cameras.

What Do I Mean by that?

Example – If I use say a 50mm prime lens on my Nikon D800E with the metering in matrix mode, ISO 100 and f14 I might get a metered exposure shutter speed of 1/10th of a second.

But if I replace the D800E with a D4 set at 100 ISO, matrix and f14 I’ll guarantee the metered shutter speed requirement will be either 1/13 or 1/15th of a second.

The D4 meters between -1/3Ev and -2/3Ev (in other words 1/2 stop) faster/brighter than the D800E when fitted with the same lens and set to the same aperture and ISO, and shooting exactly the same framing/composition.

Yet the ‘as metered’ shots from both cameras look pretty much the same with respect to light dosage – exposure value.

Exposure Settings Don’t Transfer between camera models very well, because the meter in a camera is calibrated to the response curve of the sensor.

A Canon 1DX Mk2 will usually generate a evaluative metered shutter speed 1/3rd of a stop faster than a matrix metered Nikon D4S for the same given focal length, aperture and ISO setting.

Both setups ‘as metered’ shots will look pretty much the same, but transposing the Canon settings to the Nikon will result in -1/3 stop under exposure – which on a digital camera is definitely NOT the way to go!

‘As Metered’ can be regarded as +/-0Ev for any camera (Note: this does NOT mean Ev=0!)

Any exposure compensation you use in order to achieve the ‘desired’ exposure on the other hand can be thought of as ‘metered + or – xEv’.

Shot with the D4 plus 70-200 f2.8@70mm in manual exposure mode, 1/2000th sec, f8 and ISO 400 using +2/3Ev compensation.

The matrix metered exposure indicated by the camera before the exposure value compensation was 1/3200th – this would have made the Parasitic Jaeger (posh name for an Arctic Skua!) too dark.

A 1DXMk2 using the corresponding lens and focal length, f8, ISO 400 and evaluative metering would have wanted to generate a shutter speed of at least 1/4000th sec without any exposure compensation, and 1/2500th with +2/3Ev exposure compensation.

And if shot at those settings the Canon image would look pretty much like the above.

But if the Nikon D4 settings had been fully replicated on the Canon then the shot would be between 1/3 and 1/2 stop over exposed, risking ‘blowing’ of some of the under-wing and tail highlights.

So the simple lesson here is don’t use other photographers settings – they never work unless you’re on identical gear! 

But if you are out with me and I tell you “matrix/evaluative plus 1Ev” then your exposure will have pretty much the same ‘light dosage’ as mine irrespective of you using the right shutter speed, aperture or ISO for the job or not!

I was brought up to think in terms of exposure value and Ev units, and to use light meters that had Ev scales on them – hell, the good ones still have ’em!

If you look up the ‘tech-specs’ for your camera you’ll find that metering sensitivity is normally quoted as an Ev range.  And that’s not all – your auto focus may well have a low light Ev limited quoted too!

To all intents and purposes Ev units and your more familiar ‘f-stops’ amount to one and the same thing.

As we’ve seen before, different exposures in terms of intensity and time can have the same exposure value, and all Ev is concerned with is the cumulative outcome of our shutter speed, aperture and ISO choices.

Most of you will take exposures at ‘what the camera meter says’ settings, or you will use the meter indicated exposure as a baseline and modify the exposure settings with either positive or negative ‘weighting’ via your exposure compensation dial.

That’s Ev compensation relative to your meters baseline.

But have you ever asked yourself just how accurate your camera meter is?

So I’ve just stepped outside my front door and taken these two frames:

EV=15/Sunny 16 Rule 1/100th sec, f16, 100 ISO – click to view large.

Matrix Metering, no exposure compensation 1/200th sec, f16, ISO 100 – click to view large

These two raw files have been brought into Lightroom and THE ONLY adjustment has been to change the profile from Adobe Color to Camera Neutral.

Members of my subscription site can download the raw files and see for themselves.

Look at the histogram in both images!

The exposure for xxx164.NEF (the top image) is perfection personified while xxx162.NEF is under exposed by ONE WHOLE STOP – why?

Because the bottom image has been shot at the camera-specified matrix metered exposure, while the top image has been shot using the good old ‘Sunny 16 Rule’ that’s been around since God knows when!

“Yeah, but I could just use the shadow recovery slider on the bottom shot Andy….”  Yes, you could, if you wanted to be an idle tit, and even then the top image would still be better because there’s no ‘recovery’ being used on it in the first place.  Remember, more work at the camera means less work in processing!

Recovery of either shadows or highlights is ‘poor form’ and no substitute for correct exposure in the first place. Digital photography is just like shooting colour transparency film – you need to ‘peg the highlights’ as highlights BUT without over exposing them and causing them to ‘blow’.

In other words – ETTR, expose to the right!

And seeing as your camera meter wants to turn everything into midtone grey shite it’s the very last thing you should ever allow to dictate your final exposure settings – as the two images above prove beyond argument.

And herein lies the problem.

Even if you use the spot metering function the meter will read the brightness of what is covered by the ‘spot’ and then calculate the exposure required to expose that tonal brightness AS A MID TONE GREY.

That’s all fine ‘n dandy – if the metered area is actually an exact mid tone.  But what if you were metering a highlight?

Then the metered exposure would want to expose said highlight as a midtone and the overall highlight exposure would be far too dark.  And you can guess what would happen if you trusted your meter to spot-read a shadow.

A proper hand-held spot meter has an angle of view or AoV of 1 degree.

Your camera spot meter angle of view is dictated by the focal length of the lens you have fitted.

On my D800E for example, I need to have a lens AoV of around 130mm focal length equivalent for my spot to cover 1 degree, because the ‘spot’ is 4mm in diameter – total stupidity.

But it does function fairly well with wider angle lenses and exposure calculations when used in conjunction with the live view histogram.  And that will be subject of my next blog post – or perhaps I’ll do a video for YouTube!

So I doubt this blog post about relative exposure compensation is going to light your world on fire – it began as an explanation to a recurring question about my exif annotation habits and snowballed somewhat from there!

But I’ll leave you with this little guide to the aforementioned Sunny 16 Rule, which has been around since Noah took up boat-building:

To use this table just set your ISO to 100.

Your shutter speed needs to be the reciprocal of your ISO – in other words 1/100 sec for use with the stated aperture values:

* – I would not shoot at f22 because of diffraction – try 1/200th f16

** – let’s try some cumulative Ev thinking here and go for more depth of field using f11 and sticking with 100 ISO. -2Ev intensity (f5.6 to f11) requires +2Ev on time, so 1/100th sec becomes 1/25th sec.

Over the years I’ve taken many people out on photo training days, and a lot of them seem to think I’m some sort of magician when I turn their camera on, switch it manual, dial in a couple of settings and produce a half decent image without ever looking at the meter on their camera.

It ain’t magic – I just had this table burnt into the back of my eyeballs years ago.

Works a charm – if you can do the mental calculations in your head, and that’s easy with practice.  The skill is in evaluating your shooting conditions and relating them to the lighting and shadow descriptions.

And here’s a question for you; we know our camera meter wants to ‘peg’ what it’s measuring as a midtone irrespective of whether it’s measuring a midtone or not.  But what do you think the Sunny 16 Rule is ‘pegging’ and where is it pegging it on the exposure curve?

If you can answer that question correctly then the other flavour of exposure value – absolute – might well be of distinct interest to you!

Give it a try, and if you use it correctly you’ll never be more than 1/3rd of a stop out, if that.  Then you can go and unsubscribe from all those twats on YouTube who told you it was out-dated and defunct or never told you about it in the first place!

I hope you’ve found the information in this post useful.

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Many thanks and best light to you all.