I'm interested in the light level that a shot was taken in so I can plan in future shoots what exposure combinations will work. You can use the embedded EXIF to calculate the LV (Light Value) required for that exposure setting.

EV (Exposure Value) is covered here. Of particular importance is Table 2: Exposure values (ISO 100 speed) for various lighting conditions. EV is the same as LV, except that EV is only for one ISO value, typically EV100, whereas LV considers the effect of varying the ISO as well. Another LV chart can be found here: http://www.photokaboom.com/photography/learn/tips/054b_exposure_light_and_exposure_values.htm

Here's my LV calculator for Excel 2003 (extended version)

I mainly used the indirect addressing & conditional formatting methods mentioned in a link heliphoto provided in response to varying items in a drop-down pick list based on another selection. This was critical for this program as I wanted to cover 1EV, 1/2EV, 1/3EV ISO & aperture steps, and 1/2EV & 1/3EV shutter speed steps.

Daily Dose of Excel Blog Archive Conditional Data Validation


The formula used in the LV Calculator is:

LV = log2 (f-number^2 / shutter speed / ISO/100)

An example: f/8, 1/125s, ISO100

= log2 (8^2 / 1/125 / 100/100)
= log2 (64 * 125)
= log2 (8000)
= LV 13 approx. (2^13 = 8192)

Variations in LV:

Changing just ISO100 to ISO200 = LV 12, meaning there was less light because, at the the same exposure, we had to use a larger ISO (more gain) than in the previous example to make the captured scene brighter.

Changing just aperture from f/8 to f/5.6 = LV 12 (approx.) meaning less light was available and we had to open the aperture more.

Changing just shutter speed from 1/125s to 1/250s = LV 14, means there was more light, so we were able to use a faster shutter speed.

Now some real world calculations. A blowing-out-the-birthday-candle-in-the-dark shot from a K100D Super:
f/5.6, 1/6s, ISO1600 = LV 3.6. The camera's EXIF has a embedded Effective LV of 3.1.

K20D, bright sunny day, tall office block shot:
f/8, 1/60s, ISO140 = LV 11.4, Embedded EXIF Effective LV is 11.8.

K20D, at the beach:
f/9.5, 1/1000s, ISO200 = LV 15. Embedded Effective LV is 15.6.

I don't know why there's a difference in the calculated and "Effective" LV recorded in the EXIF as there was no EV comp. Perhaps it is affected by the rounding in shutter speed selection or is a characteristic of matrix metering.

[dosdan]

I've updated the LV Calculator in the link in the first message to include EV compensation.

Dan.

[smilesnkisses]

LV, Light Value and EV, Exposure Value

Exposure Value, or EV, varies from LV, or Light Value, depending on your film speed.

EV = LV at ISO 100

LV is a measure of the luminence or brightness of a setting.
Often measured by a light meter as the ambient light falling on a subject.
Older Analog light meters measured LV
Newer Digital light meters have you put in the film ISO speed ( sensitivity ) and then give you an EV value.

Wikipedia, camerpedia.org and many online sources have this wrong or misquoted or jump back and forth between EV and LV, which confuses people, and only adds to the hoards of people ranting and quoting mis-information as is too often in wikipedia (etc).

Conventionally, you would measure LV with your meter, then using ISO, ( in a table or in camera ) convert to EV, then you could dial in which Aperture/Speed combination you desired to reach that EV, ( see a Hasselblad lens )

kind regards,
snk

[dosdan]

LV, Light Value and EV, Exposure Value

You page is all correct except, your formula is for EV ( the desired number )
not LV as indicated.
sometimes EV is written EV100 or EV400 but I can't find where that is technically correct.

Snk, thank you for your feedback. I'm sending off a few emails to cross-check. I actually did ask GordonBGood (an engineer involved in camera design) about this point before I wrote the article and his response led me to think I'd got it right, but Exposure Value eV and Light Value seems to support you.

EV100 & EV400 are probably incorrect usage. Since Tv, Av & ISO sensitivity are included in the formula I've used, the result is "normalised" to the ISO100 value, so just EV or maybe Ev (since "v" for value is used in Tv, Av, TAv, Sv) seems correct.

Dan.

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Last edited by dosdan; 07-25-2010 at 05:15 PM.

[dosdan]

Snk, looking at this again, I think LV or Bv is what I'm after. I want to determine the brightness of the scene that produced the resultant exposure. Using a combination of TV, Av and ISO Sensitivity, I want to know how bright was the scene that produced that image.

The formula for EV does not consider ISO sensitivity, at all,
Exposure Overview

So I'm not interested in the EV value as it's only applicable at ISO100.


Dan

[smilesnkisses]

LV is the lighting for the scene, it doesn't change.
EV is adapting LV by incorporating ISO, and thus gives you a set of pairs to choose from
for 'that' particular LV reading and that chosen ISO.

EV is a number, usually on a lens it will scale from 2 to 18
think of it as a set of equal pairs, for shutter speed and aperture opening
for instance EV = 12, has the following pairs f/22 and 1/8s, f/16 and 1/15s,
f/11 and 1/30s, f/5.6 and 1/125s, f/4 and 1/250s, f/2.8 and 1/500s.
You can just imagine these all give equal exposure.
you derive your EV from a light meter reading and then using the ISO you desire.
light meter + ISO, give you an EV to work with on your lens.

In taking a picture you normal go from the light meter's LV then add ISO and get EV and then choose which pair you wish of aperture and shutter, ( for reasons of coma(spherical aberration, distortion/sharpness diffraction, bokeh, depth of field frozen motion etc )

In the reverse, if you have already taken a picture, you can back calculate out LV with your formula.
putting in the shutter speed and aperture and ISO will give you that lightings LV value.

You can take the shutter speed, and the aperture,
and come up with an EV, but this is not the lighting of the scene,
the lighting is a hard number

effectively LV = EV / log(base2)(ISO/100) where 100 is the defining reference ISO of the LV formula.
a change of ISO to 200 -> 200/100 gives a change of 1 EV a change of ISO to 400 -> 400/100 gives a EV change of 2
2^1=2, 2^2=4

thus, EV will change with ISO, LV will not.
given a scene, a lighting, LV is the same, irregardless of ISO
if you change your ISO in your meter, and get a new reading from the same position and same angle, then you are actually looking at EV, if the reading doesnt change when you change the ISO then it is an LV reading.

I suspect you are back calculating LV,, to try learn the 'zone' system equivalent, in LV's
as in,, how bright is this scene,, it feels like a 5 to my eye.
this is a 12,, this is a 15, etc.
then your formula will work correctly.

I like to work with EV=12 mostly
so,, if I meter a scene and it is a LV of 9 I need to increase my ISO, 3 doubles from 100 iso
thus 200-> 400 -> 800
with an ISO of 800, the LV 9 scene can be photographed like it was a 12
if the scene is a 14, then I need to use ISO 25 to take the EV combination I like.
(5.6, 125s)

I think your formula is the one you want, since it sounds like you are back calculating from a photo, to find what was the lighting/brightness level in that scene.
I apologize if I added any confusion, I find your post is the BEST on the web to explain HOW to calculate and use and understand LV

kind regards
SnK

[dosdan]

I think your formula is the one you want, since it sounds like you are back calculating from a photo, to find what was the lighting/brightness level in that scene.

Yes, that's what I'm doing. I want to find out the LV that caused the resulting exposure. Setting different ISO sensitivities will change the EV that is used to expose correctly for that LV.

For example, a circus shot. 1/160s, F4, ISO1600.


EV1600 is 11.3 EV.
Normalising it to ISO100 (4 stops less sensitive), where it's the same as LV, EV100 is 7.3 EV.




Another shot. 1/125s (not fast enough), F4, ISO3200.


EV3200 is 11.0 EV
EV100 (5 stops less sensitive) is 6.0 EV/LV.

I remember pulling the exposure of this raw file back in PP because it was over-exposed. I no longer have the raw file, but I expect the actual LV was 7.0-7.3 LV

That's what my calculator is showing: the LV.

Let's assume that a decently exposed shot (at any sensitivity setting) is around 11-12 EV (since EV only considers Tv & Av). If you know the LV you will now be able to see what sensitivity also needs to be used, assuming the Tv & Av can't be changed much more due to max. aperture and worry about motion blurring. I'm interested in determining the LV that produced a scene, so the next time I go to the circus, for example, I can plan what Tv, Av & Sv (ISO Sensitivity) combinations will work under those lighting conditions.

BTW, this circus situation was what first got me interested in LV. I had enormous problems getting any decent shots, particularly for aerial action, with a Pentax K100D Super & Pentax-A 70-200/F4. In desperation I even tried ISO3200. In Exposure value - Wikipedia, the free encyclopedia, Fig. 2, the EV100 for a floodlit circus is given as 8 EV, so the small circus I attended was probably under-lit - it certainly was when the performers were higher up in the air.

Dan

---

Last edited by dosdan; 07-24-2010 at 11:45 PM.

[dosdan]

Since I've modified the LV Calculator to include EV compensation in the calculation, I want to explain how it affects the camera settings.

Take an indoors shot, without flash. The settings my camera chose in Av mode with ISO sensitivity set to ISO100 (at which sensitivity EV and LV are the same) with a F4 lens fully open were:

0.6s, F4, ISO100 = 4.6 LV

The effect of using +1 and +2 EV comp was:

1.3s, F4, ISO100 = 3.7 EV + 1 EVcomp = 4.7 LV (small error due to camera Tv step that is not exactly double).

2.5s, F4, ISO100 = 2.7 EV + 2 EVcomp = 4.7 LV

So positive EV compensation is forcing the camera to expose longer i.e. to consider the scene is dimmer than the camera's metering determined. The light level of the scene does not change so the LV remains the same.

What about exposure adjustment applied in post processing? Raw converters have an exposure control marked in stops/EV.

Take the first shot again:

0.6s, F4, ISO100 = 4.6 LV

If we considered this shot too dim and had to apply +1 EV exposure boost in PP, should we add or subtract the EV boost to work out the LV? I think we should subtract it if it is added in PP since, in our option, the camera should have exposed longer, as if the scene was dimmer e.g. 1.3s, F4, ISO100 = 3.7 LV.

What has happened in reality is that some part of the scene was brighter and caused the camera's metering to be too conservative for our taste. The matrix-metering in our brain considers this scene to be more 3.7 LV rather than 4.6 LV (rounding errors excepted) and we're prepared to accept a bit of over-exposure in some parts of it.

Should we therefore add +1.0 EV comp in the camera if we shoot there again? I think this is too risky. We can fix it, to a certain extent in raw PP. At best, maybe +0.5 EV comp next time, if we're shooting in the same direction, with the sky in a similar condition and at the same time of day, or shooting under the same indoor lighting.

Dan

[smilesnkisses]

You are 100% correct Dan,
this is one of the few posts I can find that deal with this topic correctly.
I congratulate you on that, and hope it is used for many references.

[smilesnkisses]

If people wonder why they dont get 'exact' numbers with theoretical calculations, it is because time is actually based on a 64 sec photo minute.
with increments of time being 1 sec 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128, 1/256, 1/512, 1/1024
not 2,4,8,16,30*, 60*, 125*, 500*, 1000*
and f-stops similarly are
1, 1.4*, 2, 2.8*, 4, 5.6*, 8, 11, 16, 22
again the *'d values are rounded for simplicity not exactness
these are actually the square roots (of 2, 8, 32, 128, 512 ) so not exactly 1.4, 2.8, 5.6, 11, 22
true f-stops are
1, 2^0.5, 4^0.5, 8^0.5, 16^0.5, 32^0.5, 64^0.5, 128^0.5, 256^0.5, 512^0.5 etc
Knowing these will help you in calculating whole numbered LV values.

ex
1/128 sec and sqrt(32) ISO 1600 -> LV = 16
vs
1/125 sec and f/5.6 ISO 1600 -> LV = 15.93663794

[smilesnkisses]

Yes EV100 = LV and so your circus was probably a LV =7 by my guess, it depends on the zoom level.
If you were zoomed in on only the SPOTLIGHT lit subject it would be much higher than if you take a wide angle and intend to crop.
It is all quite wonderful with matrix metering and spot meters and so SO many variables.
I try to avoid the average meter reading of zone 5 medium gray.
I am usually taking a picture of something NOT that shade,, perhaps I want more vivid or more black or more white, but rarely do I find myself liking the avg meter readings.

I see you enjoy this experimentation as much as I do, it is such a joy to capture an image you intended or hoped for.
take care enjoy the summer, and I love your example pics,, it brings the blog forum into a website
cheers and kind regards.

[smilesnkisses]

I have a great respect for someone like dosdan,
He is a photographer first, but also technical like Ansel Adams was.
Mr Adams is the father of photography in my mind.
(not the camera, or the process, but the capture, negative and the print )
Composition, exposure, dark room.

Dosdan pre computers, the Zone system was so wonderful,
ten zones, black to white, etc.
With color and digital and computers, we are now easily more able to be precise and numerical.

The nice choice of using LV to learn a 'zone system' is,, it makes math to exposures easier. ( especially with digital or Hasselblad ( EV locking lenses )

Once dan learns to read a scene, LV 9 LV 12 LV 15, he will then be able to say,
I want to make this exposure setting,
and he can even go to saying it IS a LV 12 scene, but, I am capturing the shadows ( low lights ) which are a LV 9 so, I will shoot for that.
or,, I want the highlights at LV 15 so I will shoot for that.

One note I would make is, with transparencies ( my preferred ) or digital,
it is KEY to learn this one thing.
Meter ( expose for the highlights you desire ( the brightest thing you want to see in the picture ) and develop for low lights ( shadows ) (the darkest thing you want to see.
In High contrast scenarios, back light, dark shadows, landscapes, this is a KEY thing
in low contrast shots it is not so key.

The reason for doing this ( like anything 'should do' recommendation, is
once a object is 'blown out' bleached, washed burned out too much in a flim transparency or a digital image, it is gone.
If 1 is black, and 256 is white ( 8 bit ) or 512 or 1024
then once you make an object a 256,, if you 'brighten' the picture in the (digital photoshop) darkroom, it is still a white,,, you CAN make it a grey,, but not a blue or yellow or whatever shade it was.
however, if your whites (chlorox brights ) are 230, or 240,
and your darks are, 0.001, or 0.1 or 1.1
you can brighten those 'shadows' in a darkroom and enhance the shadow detail.
blown out details are LOST forever, dim details CAN be enhanced.

so,, what is the point?
1 you want to shoot an object, it is a LV 12, but backlite by LV 17,, what do you do,,
shoot LV 12
2 you have a landscapes with cool shadow detailed moss, as LV6 and landscape at LV 12 and a great sky at LV 14, what do you do?
shoot for the LV 14,, then develop and enhance your shadows, burn and dodge and white black adjust exposure to bring out shadows.
3 you have a night scene, with deep blue sky, it is LV 4, you want to make an artistic pic, and overexpose it to look daytime, not caring about highlights or shadows.
take your LV4 and shoot it as LV 2, you get a daytime scene from nighttime.

the first step is to learn the ZONE, (or in dan's case, the LV of the scene )
how bright is it.
then decide what part of it you want to capture,
do you want close detail with a blown out bright backlight sky?
or do you want to actually make the bright sky colorful, and lose the darkforeground into a shadow?
In these high contrast scenes, the average camera will give you a black foreground and a white sky so you go NOTHING,,, a blah picture.
you could of taken the LV 17 sky and shot it,, to get a brilliant sunset from a blah one, by exposing, at LV 17 r( a much shorter exposure ) ather than the avg the meter read of LV 14 for the whole scene,
or,, you could should it at LV 6 the shadowy foreground detail ( a much longer exposure ) to blow out the sky to white, but show the awesome rocks and moss the camera would of made black.

in low contrast, you learn the LV for the whole pic
in high contrast, you learn to see the LV of the highlight, the LV of the shadow, and the LV of the middle ( if present )
then,, you decide what you want to see.
to see the most possible,
you can, alter your lighting,, add fill flash to highlight the near shadows, and also underexpose to add color to your sky.

I am just rambling, for that beginner out there, who wants to learn,
and there is SO much material they dont know what to read.

Realize a meter is giving you the exposure for Zone 5,
medium grey
18%
tanned caucasian skin
deep blue mid day north sky
luscious green grass.
med beige paint,
light oak.

if you are shooting darker oak, ( red oak ) you will need to under expose by 1/3 a stop to get it right,, ( not bleached )
if you are shooting walnut ( dark brown ) you will need to adjust your exposure by 1.5 stops darker
if you are shooting light pine, you may need to over expose by 1 stop brighter
the meter assumes it is a medium dark/bright object,
you need to adjust it, to say, no,,, this IS a bright scene, ( snow, ice, beach )
or no,, this is a dark scene, night, romantic candles, forest shadows

once you learn the LV of the scene, or the item you want,
you can then shoot with no meter,

I wish more people were into learning LV like dan
it would be nice to have discussions like,
I am shooting indoor a scene with LV 10 on my subjects or key interest,
but I want to capture some of the bright background which is LV 14
also, this is an action shot, so I want shutter of 250 to 1/500s
how should I do this,, on film, and digitally?

( answer to sample,,, shoot for the LV 14 highlight you want ,
not the LV 12 of the whole scene, then in developing,
bring exposure up ( brighten, to LV 9 on your subject )
by burning, or, digitally, by just raising you black level from 1 to 35,
for a quick dirty solution, or take the time to burn,the areas you want, just like ansel adams did.
there is SO much,, technical talk about lens fall off ( darkening at edge of photo )
and softness in corners of lenses,
and then, in the end,, in GREAT photo Developing,
those are the things you put INTO the print,
to highlight the subject.

We are becoming TOO technical yet not knowledgeable.
You don't need THAT lens,
you need to COMPOSE artistically, and intelligently
and you need to be technically smart
but artistically creative and productive in the capture and darkroom

This isnt advice for dan, he is way beyond this
it is for the beginner, that LOVES the capture and art of photography
but, would like some more technical knowledge.

I hope someone, out there reads this, and it helps them think and figure something out,
or learn a little more, takes MORE pictures, Experiments,
takes the SAME picture 10 times
but ends up getting THE picture they wanted.

I love photography.
snk

[dosdan]

I originally wrote this program for a K20D, and it only went up to 30s shutter speed. I've now extended most ranges:

Tv: 1/8000s - 160s

Av: f/1 - f/45

ISO: ISO80 - ISO102400


The link to the new version is the same as in the first email: LV Calculator for Excel (extended version)


Dan

[dosdan]

ExifTool (ExifTool by Phil Harvey), written by Phil Harvey, shows 2 LV entries in Pentax image files:

Light Value: computed by Exiftool using LV = 2 * log2(Aperture) - log2(ShutterSpeed) - log2(ISO/100)
Effective LV: in the MakerNotes section.

This is different from APEX, which uses ISO3.125 (Sv 0) as the Speed Value reference, whereas the formula above uses ISO100 (Sv 5).

Probably because LV = EV100 has gained some currency. Lv & Bv are being used interchangeably too.
See: Light value - Wikipedia

Doug Kerr mentions this trend on p.10 of http://dougkerr.net/Pumpkin/articles/APEX.pdf

Scene brightness in Ev?

We often see, especially in camera specifications, a factor that seems
to be scene luminance (brightness) described in terms of an Ev
number. Such a factor might be, for example, the lowest scene
luminance for which the exposure metering system of the camera is
able to function reliably.

This usage is unfortunate and technically inappropriate, as Ev is a
measure of exposure, not luminance. There is a perfectly good APEX
quantity for luminance: Bv. I suspect the motive for the practice is
that many photographic enthusiasts have heard of Ev but not Bv.

Of course, if we know the Ev that a camera’s metering system
recommends for a scene, we can in fact equate that to scene
luminance if we also know the ISO sensitivity (Sv) in effect.

It turns out that, when a manufacturer states some critical scene
luminance in terms of “Ev”, and nothing to the contrary is stated, it is
most often based on the assumption that the ISO sensitivity is
ISO 100 (Sv 5). (Canon, for example, so states explicitly.) In other
words, the luminance being described is that which, if the ISO
sensitivity of the camera were ISO 100, would lead to the camera
arranging for an exposure of the stated Ev.

The relationship between this irregular description of scene luminance
in “Ev” and the description of that luminance in the proper value, BV,
is as follows:

Bv=Ev’–5

where Ev’ is the so-called “Ev” used to describe the luminance.

I discourage this usage.

It looks like common usage is winning out.

The Exposure Value table (Table 2) "for various lighting conditions" below uses EV100 as an apparent surrogate for LV, presumably because ISO100 is of more relevance to normal photography than ISO3.125.

Exposure value - Wikipedia

Since the "Effective LV" tag in the MakerNotes section is showing maker-specific info, Pentax is also chosen to use ISO100 as their sensitivity reference value for LV.

My LV Calculator also uses ISO100 as the ISO stops reference, but compared to the formula in Exiftool, I do the log2 conversion once overall, rather then separately on each term. The one log2 conversion should reduce rounding errors.

Dan.

[pxt]

Camera EV - Exposure Value, EV chart, calculator