new (and newbie) 40D metering problem

[moldovamark]

Hi folks,


I've spent the past several days getting to know my new 40D, which I purchased as a refurb (now recertified?) from Adorama. Judging from the camera's histogram, and especially processing images in Lightroom, some, but not all of my images seem to be noticeably underexposed.


One example that was particularly noticeable, I was shooting some flowers in the shade of my house on an otherwise sunny morning. I don't have a macro lens (yet) so I was using the 55mm end of my 17-55 f/2.8. Camera was set on evaluative metering. White balance was set on shade. f-stop varied between 2.8 and 5.6, depending on how much depth of field I was aiming for. I was shooting in aperture priority. The histogram showed the images to be a underexposed, and when imported them into Lightroom, I had to bump up the exposure by a stop and a half.


Even brighter, more direclty lit shots have seem a skoosh underposed, but much less so.


My XS, the camera from which I am upgrading, also underexposed things, but less so, and a lot more uniformly--to the extent that I bumped up the exposure compensation a third of a stop and just left it there unless the histogram told me I was overexposing a particular shot.


So my newbie question--is this within the range of normal, or something I should be thinking of returning the camera for? Image quality and everything else about the camera--particularly with the 17-55 f/2.8 (yeah, I treated myself)--is outstanding.


Mark

[Colin]

I don't know what's going on, but I can say that the camera is trying to put the image exposure in the middle, whether your subject is bright or dark. So, if you're filling up the framewith something that's fairly bright, the camera will make it dark. If you're taking a picture of something dark, the exposure will be on the bright side.

[moldovamark]

That I understand. What I'm wondering is whether waht the camera has decided is "the middle," is off, i.e. the camera's "middle" is darker than the real "middle," at least as the histogram measures it.


After writing my initial post, I bopped back outside to take a few more shots. Everything the same except it's a uniformly cloudy day. No shade. I set my exposure compensation to add a stop, and the histogram was telling me everything was pretty much centered, maybe still a skoosh underexposed.

[Colin]

When you say that the histogram is telling you underexposed or overexposed, what do you mean by this? You certainly don't want information smashed up against either side, if you can help it, but it doesn't necessarily have to be in the middle to be what you want.


I just took twopictures with my 30d, in Av mode. One is of white kitchen cabinets, and the other of a black wood-burning stove (which if you use, you will die from CO poisoning). Both turned out rather grey, and the lumps of the histogram were averaging just a little below center. They're both 'wrong' in the sense that if you printed them straight off the camera JPEG, they would look inaccurate compared to how they are compared to their environments...


You will have to do some work, you know []

[moldovamark]

Oh, I don't mind doing a bit of work, Colin, and as much as I try to get things right in the camera, I'm used to having to tweak things a little bit. But I haven't been shooting long enough or with enough different gear to know what's a "normal" amount of tweaking. So that's what I'm trying to sort out. Is the meter on my camera properly calibrated? And is that even the appropriate way to describe it? In getting used to the controls might I have unwittingly altered a setting that's causing the camera to underexpose things a bit? Or does every camera have it's own idiosyncracies and I should just shut up and shoot? (I'd be perfectly happy if it's the latter.)

[Colin]

Well, you definitely want to get the exposure as right as you can when you take the shot, but you need to be aware that there are simply limitations to doing the metering while taking a picture. Lots of bright stuff in the picture will make the image meter darker, and vice versa.


You can try the different metering modes and see if that works better for you. You can try another 40D and see if it behaves differently. But, having to adjust the exposure compensation for a particular scene is not uncommon. My default is to go -1/3 of a stop just to avoid blown highlights, but I often have to adjust up and down to maintain my data. Capture the information is priority number one. Making it look right in post processing is always an option. I like to optimize the exposure to capture the image data, and then adjust for subjective brightness later, often by adjusting the curves more than the 'brightness/contrast' adjustments, though it's often useful and/or necessary.


I'd defer to somebody more familiar with the camera, but it sounds like it's likely 'normal' stuff you'll just get used to dealing with.

[Daniel Browning]

What I'm wondering is whether waht the camera has decided is "the middle," is off

It's possible that the meter really is malfunctioning, such as if it were obscurbed by a lot of really bad dust, but I don't think that's what's going on here.


The histogram on your camera does not represent the exposure, but the JPEG. The meter is tied to the exposure, not the JPEG, so the amount that the histogram differs from the meter will be the same as the amount of processing done to the JPEG.


Many raw photographers find it very frustrating, so we build special white balances, tone curves, and other settings to try and manipulate the JPEG so that the histogram will more closely represent the actual information recorded by the sensor. The downside is that it makes the jpeg preview almost useless. It's a technique mostly used by more advanced photographers. You can find more information on it here:


http://forums.dpreview.com/forums/re...ssage=30376790
http://forums.dpreview.com/forums/read.asp?forum=1032&message=30233555


Kind regards,

[Don Burkett]

Hi Mark: I'm not sure you can get a definitive answer to your question from the group. There are too many factors that can be at the source from the camera, the scene and the user. However, light meters from cameras can vary in sensitivity and settings. My suggestion, is go find an average daylight scene that is mostly midtones. Something like a large field of grass and keep the sky out of the shot. Shoot a series starting at 0 Exposure Compensation and go up in +1/3 increments. Repeat the same series of the same scene, under the same lighting a couple of times.


After processing the shots, find the exposure value that gave you the best shot. It should be the same value in each of the series. If it's not, you may have a malfunction.


Repeat the exercise with other midtone scenes. If you always find that the same "exposure value" gives the best result, it's just the way your meter operates. If it's within 1 stop either way, I wouldn't worry about it. Just leave it dialed in to that +/- EC value. On the other hand, while shooting these additional series of midtone shots you find different "exposure values" work best, it's not the meter because we proved that in the first series. In this case, you just need to learn to read the light more. Good Luck

[Daniel Browning]

So my newbie question--is this within the range of normal, or something I should be thinking of returning the camera for?

Returning the camera would not help.

[clemmb]

<span style="font-size: 10pt; color: black; font-family: Arial;"]Everyone is correct. You may be fooled by the overall scene. If you can get a grey card and shoot with it filling the entire frame, the histogram should show a narrow curve in the middle of the histogram. <o></o>


<span style="font-size: 10pt; color: black; font-family: Arial;"]I did this with my XTi for flash exposure. I adjusted the flash exposure till I got it in the middle. I leave it set there all the time and get good results. Mine is about 2/3 stop under. If it were more than that I would send it in for calibration.<o></o>


<span style="font-size: 10pt; color: black; font-family: Arial;"]Hope this helps<o></o>


<span style="font-size: 10pt; color: black; font-family: Arial;"]Mark C.