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11-26-2010, 02:20 AM   #1
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Exposure

Hi,

I'm starting out with photography and I think I have a pretty good grasp on how ISO, shutter speed and aperture work together. One thing I don't quite understand is exposure compensation.

In my K-r it has a range of -3 to +3 with steps of 1/3 or 2/3 (I think). When I tried altering the setting (indoor shot) it didn't seem to make much difference in the picture. Can someone please explain to me what the setting is for and in what situations it's used?

Thanks and sorry for a probably dumb question to some.

11-26-2010, 02:54 AM   #2
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Well, if you have a beginning grasp of ISO, shutter speed and aperture, you will know that these factors combine to determine the exposure level. If you use a setting other than the fully automatic, you can pick a pair of these values depending on the result you want, and the camera will then adjust the third to achieve correct exposure.

So, for instance, if you want high depth of field for a landscape, you will go for a small aperture (high number f-value) using the aperture-priority setting, and if the light conditions allow you would go for a low ISO-setting too to get a high dynamic range, and then the camera would pick the shutter time for you to obtain correct exposure.

BUT in some situations, the exposure that the camera considers "correct" is not what you want to go for. Let's say your landscape contains a setting sun. Uncompensated exposure would ensure that the average element in the picture is not too dark - but then your setting sun would most likely burn out completely, and you would lose all the pretty colours. Here, your priority is getting the sky right, but the sky is the lightest part of the picture. This is one example where you might choose to stop down the exposure, leaving parts of the landscape dark, but getting more details from the sky. So, for instance -1 might be good here.

An example of the opposite might be shooting birds in a tree with a grey sky behind them. Here, if you let the camera determine exposure, it will attempt to avoid a burned out sky (which is most likely the biggest area in the picture), and as a result the birds will be very dark at the cost of detail. Here you want to prioritise the _darkest_ elements in your image frame, and so you increase the exposure with f.x. +1. This may cause some of the grey sky burn out, but you get more details in the foreground birds, which is what you really want.

In terms of the histogram, automatic exposure will attempt to center the histogram for the whole frame, but if your prioritised elements are in the dark part, you want to push the histogram to the right by adding to the exposure (+), if your prioritised elements are in the light part, you choose to underexpose (-) instead.

I hope that was helpful!
11-26-2010, 03:17 AM   #3
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EVcomp

BeerBelly, I wrote a LV (Light Value) calculator to work out what was the light level of a scene as determined from the EXIF exposure settings in a shot. Low LVs are very dark while a bright cloudless sunny day at the beach is about 15.5 LV.

https://www.pentaxforums.com/forums/miscellaneous-articles/88197-excel-2003-l...alculator.html

Anyway, as part of that article, I posted this about EVcomp so as I could get to grips with it myself:


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 f/4 lens fully open were:

0.6s, f/4, ISO100 = 4.6 LV

Now I applied EVcomp. The effects of using +1 and +2 EVcomp respectively were:

1.3s, f/4, ISO100 = 3.7 LV + 1 EVcomp = 4.7 LV (small error due to camera Tv step that is not exactly double).

2.5s, f/4, ISO100 = 2.7 LV + 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, f/4, 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, f/4, 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 EVcomp 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 EVcomp 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

Last edited by dosdan; 11-26-2010 at 03:56 AM.
11-26-2010, 03:57 AM   #4
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First of all, thanks to both of you for helping me with this.

I understand a lot more about the setting than I did before. I knew that the big 3 (ISO, shutter, aperture) basically work out to determine the right exposure and that by altering one you need to alter the other 2 (or just 1) to compensate.

I will try out the EvComp setting in some high contrasting shots since, if I understand correctly, that's where it's most noticable. I'm guessing AE-L is also recommended for use in this case?

Thanks Dan for the article. I'll be sure to check it out.

11-26-2010, 04:10 AM   #5
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QuoteOriginally posted by dosdan Quote
BeerBelly, I wrote a LV (Light Value) calculator to work out what was the light level of a scene as determined from the EXIF exposure settings in a shot. Low LVs are very dark while a bright cloudless sunny day at the beach is about 15.5 LV.
Dan,

Have you ever cooked up a flavour of your spreadsheet utility that doesn't require all the malware propagating functionality enabled? If so I'd like to try it.

Fyi: I don't install/enable that in my Excel 2k3 due to it's legendary notoriety of competence in that calibre, so subsequently cop the predictable popup rejection as payment for my recalcitrant passion for avoiding trouble.
(yeah I've never used MS Doubtlook * as an email client either :ugh: )

.R.
11-26-2010, 04:34 AM   #6
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QuoteOriginally posted by Hypocorism Quote
Have you ever cooked up a flavour of your spreadsheet utility that doesn't require all the malware propagating functionality enabled? If so I'd like to try it.
I don't use spreadsheets very often at all. The spreadsheet itself contains no macros, so if you have Excel, you can get it to load this spreadsheet with macro execution disabled. As I understand it, malware in spreadsheets resides in the embedded macros.

However, it seems you don't want to use Excel at all. I don't have the interest now to investigate other spreadsheet programs (24 years ago I did use non-Microsoft spreadsheets: Lotus 123 & As Easy As). So I don't know how well the indirect addressing & conditional formatting methods used in this spreadsheet could be implemented in another spreadsheet program.

Dan

Last edited by dosdan; 11-26-2010 at 04:47 AM.
11-26-2010, 06:48 AM   #7
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QuoteOriginally posted by dosdan Quote
I don't use spreadsheets very often at all. The spreadsheet itself contains no macros, so if you have Excel, you can get it to load this spreadsheet with macro execution disabled. As I understand it, malware in spreadsheets resides in the embedded macros.

However, it seems you don't want to use Excel at all. I don't have the interest now to investigate other spreadsheet programs (24 years ago I did use non-Microsoft spreadsheets: Lotus 123 & As Easy As). So I don't know how well the indirect addressing & conditional formatting methods used in this spreadsheet could be implemented in another spreadsheet program.

Dan
I certainly do use Excel 2003 (by choice), but like you am way past the 1.2.3 etc SS phase.
To describe the problem more clarly, I find upon attempting to load your "LV Calculator.xls" file I get the following message:
"This workbook has lost its VBA project, ActiveX controls or any other programmability-related features."
And on dismissing that, what exists of the Sheet itself, a small, colourful but limp non-resizable sub-window, can do jack.

Fwiw; There's a bit more than basic macros that surrounds the curse I'm afraid, and this is not just my opinions or beliefs, it's long and well documented; thus while I do use the core product (it's the best) for general needs, that usage is strictly conditional - to remain part of the solution, not part of the problem.

Thanks for your troubles anyway. I should and will whip up a sheet if need arises, have intention to do one for other common calc/table pic taking needs as well.

.R.

Last edited by Hypocorism; 11-26-2010 at 05:28 PM.
11-26-2010, 09:13 AM   #8
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While MetteHHH has outlined one use of exposure compensation, i.e. a setting sun, you should note the following aslo.

if you use center weighted metering, his description makes perfect sence, because the bright areas will dominate in the metering, BUT, multi segment matrix metering has gone a long way to resolve this issue and take better first guess exposures. Regardless of what these metering selections are capable of, the best way to deal with really high contrast subjects is to use spot metering, and manual exposure. Set the exposure to meter correctly for what you want in the middle of the histogram, and then shoot with those settings.

EV compensation has other uses, that I find much more important.
- First and foremost, some lenses (especially when used with teleconverters) have a tendancy to always over or under expose. As a result, you should set your EV compensation for those lenses or lens / TC combos so that exposure is always correct. Many times screw mount lenses do this as well
- second in this area is whether you are deliberately shooting low key or high key shots. High Key Photography has the histogram pushed all the way to the right, low key to the left. using EV compensation will get you this consistently.
- Night photography, where there are bright point source lights in a very dark background Setting EV compensation can help you get the balance you want.

11-26-2010, 04:38 PM   #9
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You also need to appreciate that high ISO produces digital noise, that lenses have an aperture they are sharpest at (often around f11) amd aperture affects depth of focus. The result is you can have many combinations which produce a correctly exposed image for a given scene, but some will work better artistically speaking than others.
11-26-2010, 06:06 PM   #10
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QuoteOriginally posted by Hypocorism Quote
"This workbook has lost its VBA project, ActiveX controls or any other programmability-related features." And on dismissing that, what exists of the Sheet itself, a small, colourful but limp non-resizable sub-window, can do jack.
I don't get that message on either Excel 2003 at home or the one at work with the ribbon (don't use it much so I can't remember what version it is - I downloaded that spreadsheet at work on Friday to use in an LV calculation before replying to someone online).

I also sent the first version of this (before I added EVcomp) to GordonBGood to see if I was doing things correctly and he operated it successfully.

I've taken out 2 junk macros. They weren't needed and were recorded accidentally. I don't use ActiveX or VBA. A full sweep of my PC with both the Windows Software Malicious Removal Tool Nov 2010 version & Malwarebytes (latest updates) shows nothing.

See if the drop-down menus operate in this version:
http://users.on.net/~dosdan/LV%20Calculator%20Mk4%20with%20tabs.xls

If not, you may still be able to copy the formulas, lookup lists, named ranges & conditional formatting in the input drop-down fields (to indicate when the current selection is invalidated e.g. you have ISO140 selected and then change to ISO_.33 steps - this field is then highlighted because ISO140 isn't in the 0.3 EV steps sequence so you know you have to reselect a valid value).

Dan

Last edited by dosdan; 11-26-2010 at 06:14 PM.
11-26-2010, 08:27 PM   #11
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QuoteOriginally posted by dosdan Quote
I've taken out 2 junk macros. They weren't needed and were recorded accidentally. I don't use ActiveX or VBA.

See if the drop-down menus operate in this version:
http://users.on.net/~dosdan/LV%20Calculator%20Mk4%20with%20tabs.xls

If not, you may still be able to copy the formulas, lookup lists, named ranges & conditional formatting in the input drop-down fields (to indicate when the current selection is invalidated e.g. you have ISO140 selected and then change to ISO_.33 steps - this field is then highlighted because ISO140 isn't in the 0.3 EV steps sequence so you know you have to reselect a valid value).
I suspect that you probably run a mostly default options installation of Excel, whereas I always wield the ruthless razor to unwanted components, esp VBA/Ax etc. support that are known traps and vandals so that it can't install or even exist on the system.

No, I still don't get normal usability, or even proper window display functionality with that new version either.
The only difference (improvement) being in that you've now exposed its Sheet Tabs in expected place at bottom of pane, although the navigate arrows are inoperative. Wonder what effect if any those two "redundant" macros might have imposed.

And I still get the exact same popup window warning on initial load of the new sheet Btw. No change there, it's demanding VBA/Ax functionality from my hobbled Excel that it can't give.

Yes I can get at the one formula and at least part of your lookup tables now, so will take a sniff around and with those cell-ref details you provided it should provide enough clues to the concept and math to get me going.

Anyway thanks but don't bother about it anymore Dan, I don't doubt your word that it would be quite safe to run but I won't re-configure my Excel or MS Word to permit its automation self-control for anything.
Gave up being a servant to PC hardware and software foibles a few years ago and prefer just to use them now as a tool, not a toy to continually waste time experimenting and routing out issues. Fun and useful to learn and it's amazing how easy 100% uptime is to achieve when you don't invite trouble.

Cameras and all their hanger-on gear is enough bloody trouble to tinker with at this stage of life.

Late addendum: I think it's near working now Dan, am getting response to inputs anyway, so what you doctored must have been on the right track.
Yep, can even get to edge of the dropdown picklist, although is cramped and partly off the screen pane. NP, should be able to sort it all out from here (will tend later), thanks.

.R.

Last edited by Hypocorism; 11-26-2010 at 09:34 PM.
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