20D underexposure problem

[tim]

I just got the 20d today, and i'm having a bit of a play. I'm in a room that's reasonably well lit, while not massively bright I can easily read fine print. It's lit by 4 tungsten lamps on the roof. When I take a shot of a bright green towel hung over a chair, with that taking up half the frame and the cream wall in the background half of the frame, the camera seems to underexpose by quite a lot - the histogram shows a big clump in the middle. With exposure compensation 1 it's better, with EC 2 it's about where I think it should be. Nothing's blown at EC2. In daylight it was doing fine.

I'm using the 50mm F1.8 at F1.8 on Av mode, JPG parameter 1. The ISO I choose doesn't make any difference to it, neither does the mode - P works the same. My 300D does the same thing. Attached are the photo with no processing except for resizing, and the histogram, with custom white ballance and EC set to 0.

Is this normal and expected under these conditions, or am I doing something stupid? Either way I have something to learn, any advice or thoughts will be appreciated.

[jfred]

Hi Tim, (congrats on the new camera, btw!)

does the metering mode you choose affect the exposure chosen by the 20D?

(Just a thought.)

[mdr]

Have you tried partial and/or center weighted metering? Have you tried tungsten WB?

[tim]

Thanks Fred :)

Neither the metering mode or the white ballance make any difference. I tried custom white ballance too: it got white much closer but still the hump in the middle of the histogram.

[malum]

Surely it's underexposed because you have a large expanse of bright wall fooling the light meter, much the same as snow will fool the meter?
If you zoom in on the towel and get a reading just from that and then put those settings in manually what happens?

[tim]

Ok, this time I made the towel take up 80% or so of the frame. I tried all the metering modes. I used Av at F1.8, it suggested 1/125th. I went into manual and tried it, and got the same result. At 1/40th (5 click of the dial) it was properly exposed, at 1/30th it was slightly overexposed in one place.

What's up with the metering? Are there just some situations that it doesn't get things right?

[malum]

What's up with the metering? Are there just some situations that it doesn't get things right?

I would imagine so or there would be no need for dedicated light meters.

The other possibilty is that there is something wrong with your light meter

[jfred]

I guess the sort of contrast levels you're seeing might be behind the histogram you're getting. Were these tungsten lights studio lights, or domestic?

If you take the picture with flash, would the extra contrast come up in the histogram?
I'm guessing that the green towel and grey-ish background will all come up in the same chunk of the histogram.

I'm not an expert at historgrams, but I can imagine how taking that sort of picture in a room could produce that histogram and the overall result.

[jfred]

What's up with the metering? Are there just some situations that it doesn't get things right?
depends what you mean by "right" :) A light meter reading tells you what exposure will show a standard 18% (i think) gray reference card as a mid tone (or in the middle of your cameras dynamic range)

It looks like your wall and towel are being set to the middle of the camera's dynamic range... and you may want to adjust that upwards (or downwards) depending on your taste.

I've been playing around over the last couple of weeks, and trying to get to grips with histograms... http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml has a lot more on the subject.

[tim]

They're domestic tungsten bulbs, behind a frosted glass shade, and they look visibly yellow tinted to me. Using the flash, internal or 550EX in M mode, makes no difference to the histogram. I wonder if it's not as smart as I thought it was, or if i'm doing something wrong.

When I take a photo of my LCD displaying just white, the screen taking 1/4 of the frame, it comes out how you'd expect - the screen's overexposed, because it's on evaluative metering.

[jfred]

ok - I've run out of ideas! will try experimenting at home later with a similar scene and see what the results are :)

[tim]

Cheers fred, i'll be interested to see what you come up with, and what the experts around here have to say about it. If no-one can explain it I might try my old photography teacher.

[mdr]

Wouldn't this have been resolved when trying partial metering off the towel?

[Cadwell]

The cameras don't always get it right and the metering system can be fooled. That's why the manufacturers put helpful little tools like the histogram display and functions like exposure compensation in there - so you can see it and fix it. With practice you will come to recognise when the camera is likely to get it wrong.

It's in circumstances like this, where you the photographer, has to make some contribution to the picture taking process rather than just expecting the camera to do it all for you.

[tim]

Wouldn't this have been resolved when trying partial metering off the towel?

Nope. I go to partial metering, point it at the middle of the towel (which takes up 1/2 the frame or a bit more), recompose, still the same histogram.

The cameras don't always get it right and the metering system can be fooled. That's why the manufacturers put helpful little tools like the histogram display and functions like exposure compensation in there - so you can see it and fix it. With practice you will come to recognise when the camera is likely to get it wrong.

It's in circumstances like this, where you the photographer, has to make some contribution to the picture taking process rather than just expecting the camera to do it all for you.

No offense to anyone else, but that's probably the most useful post so far :) So long as I know the camera's meant to work like this, and it's expected that I need to correct it myself, I can learn what I need to do and when. Thanks :)

[mdr]

Tim, could you post the historgram for the EC 2 shot which you think is the right exposure? It would be interesting to see this.

[slejhamer]

Tim, depending on where you meter (using partial,) you should get very different results.

Attached is your image desaturated, showing the RGB values at various points. If you meter the top two points, even though they may be brighter than middle gray when you look at them, the camera will expose for middle gray and you should expect your exposure and histogram to come out as it did. That's just how the camera's meter works. It is not smart! If you point it at something black, it will overexpose; point it at something white, it will underexpose. It's always looking for middle gray. But it's up to you to tell the camera where middle gray is.

If you meter the towel at the third point, currently 74-74-74, you might find better exposure there.

If you meter the towel at the lowest point, where it is currently 15-15-15, you should expect the scene to overexpose.

Try this exercise: put your camera in P mode, set it to partial metering, and then point the camera to various spots in the scene - top (highlight portion) of the towel, middle of it, bottom of it. Take note of the exposure readings at those points, then switch to manual mode and take a snap with each of those three settings. Exposure should be very different for each image, with the middle one closest to what you want.

Hope that is helpful.

[Ben]

This may sound really patronising, but I made this mistake and found I had underexposed images.

Have you checked the exposure compensation setting on the top info screen. I knocked the jog dial and ended up under exposing all my shots by 1/2 stop I think.

http://img146.exs.cx/img146/2627/lcdpanel013oa.jpg

Sorry if my answer is useless :)

[RJSorensen]

Congrats on the new 20D Tim . . .

[Monito]

Congratulations on the 20D. You've made a fine choice. I say so because I got mine three weeks ago and am very satisfied.
I made the same mistake as Ben for a few exposures until I clued into it.
Read the manual completely and carefully and read key parts several times again a few days later as often as necessary. You have not had time to do this yet.
The scene you chose is stocked with a very limited range of tones, and the meter is going to try to make an average picture out of it. It has no way of knowing that the cream wall is not gray or knowing that the bright green towel is not a mid-tone green towel.
If you try to light a room "by 4 tungsten lamps on the roof", not much light will penetrate through the ceiling. ;)

[rssfhs]

Are your photos only under-exposed using f1.8? I read somewhere that some wide angle lenses can give dark images when the aperture is at it's widest for some reason. It could be your lens, especially if you are having the same problem with your 300D.

[PacAce]

Just curious. What happens when you take the same shot with a lot of lighting in the room. In other words, gather all the lamps you have in the house, remove the shades and put them in the room where your "test subject" is and turn them all on so that the room is really brightly lit. Then take another shot of the subject. What do you get this time?

[Jon]

I think slejhammer's got it - the towel is not too different from the wall in overall tonal value, even though the colour may be quite different. The histogram supports this, with a big spike, then the low mound from the shadows. Try a set-up with a broader tonal range, and one that's distributed over more of the subject.

[jfred]

at a friends house - took a couple of pics that were the nearest i could manage. Tim - I'll PM you the url, because I can't do to much cropping etc to get them on here today.

[tim]

Thanks for all the help guys. I'll try and reply to everyone here. I'll start by saying i've deleted the underexposed shots, and since it's now daytime I won't be able to do any more until tomorrow night, as i'll not be around this evening (for mdr). Baiscally the same histogram moves further to the right the more I increase the EC.

Mitch - interesting, and very helpful, thanks. I'll try what you suggested.

Ben - yep i've been playing with exposure compensation, it's only on EC+2 that things come out right.

Monito - yep have read the manual now, hadn't before last night, and it's a bit different to the 300D. More control, more stuff to play with. I'll try with more tonal ranges, but my quick snaps last night of different areas of the room came out quite similar.

rssfhs/pacace/jon - i'll try on a smaller aperture and with more lighting and in a different scene too.

Fred - cheers, no PM yet but i'll take a look later.

Thanks everyone, i'll bump this in a day or two once i've had a chance to play some more :)

[tim]

Two and a half months later, after more than a little reading i'm a little wiser, I believe I now have the answer to this question.

The camera assumes that every scene it's taking a photo of is around 18% grey. This is a reasonable assumption, because most scenes will have some black, some white, and lots of shades in between. I believe this is one of the basics that camera metering systems are built on.

In the picture I posted on the first post of this thread you can see that 75% of the frame is filled by a white wall. 25% is filled by a green towel, which by itself would be towards the right of the histogram. To make the scene fit the 18% grey model the camera makes the white wall grey. If I wanted to have the white wall white i'd have had to set the exposure compensation to +2, give or take. This same thing happens with wedding dresses.

The opposite will happen with scenes that are mostly black - the camera tries to make the scene 18% grey, and will turn the black into grey. To compensate you need to dial down the EC, perhaps to -1 or -2.

For mixed scenes (eg bride and groom), watch your histogram, and practice.

To find out more from someone who understands this better than I do, have a read of this (http://www.rickrosen.com/digital/histogram/h1/) and this (http://www.fredmiranda.com/forum/topic/210041).

[quangdotca]

that last post..very useful

[mr.photoguy]

You guys should research the Zone System.
Can be very helpful with metering..

Good luck.

[Confalone]

There are many threads on this subject of flash underexposure. Just so a search "flash underexposure" on the home page and you'll get more info than you probably want. I went nuts for weeks with this problem, getting great exposures with my baby elph SD10 and 1-2 stops UNDER with the 20D and the 580EX ! The bottom line seems to be that the default settings [green rectangle] are designed to do this to avoid blowing out any highlights. So I have to use creative mode settings [P or M] and use a flash exposure compensation of plus + 1 - 1 2/3 to get spot on exposures, as evidenced by the histograms. The 20D is definitely NOT a point and shoot flash camera !!

[tim]

Actaully the answer to this particular question was around how camera metering works. Read my post about 4 posts up from this one if you're interested, it's information everyone who uses a DSLR should fully understand.