Canon 40D - OverExposure

Cavking

Cavking

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Name
Shane
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Hi all,

Ive taken delivery of my new Canon 40D and I must say, Im impressed..

Its a lovely camera! - I had a couple of problem regarding RAW files but that was user error and no fault of the camera... But I did come across one thing I wanted to ask you guys about...

I was trying to take a picture of an archway which is part of the entrance to a church, the arch had lovely inscriptions inside to commemorate the guys in the war. The problem occurred when I took the shot and realised that the sky in the shot was flashing black - this I understand as being over exposed, but what is the camera expecting me to do? At a guess, Id say knock it down a couple of stops or fitting a polariser on the camera but if I was to do that the inscriptions would not be visable in the shot...

Is this going to be a case of 'Thats the problem with digital' or can I do something in camera to cure/act on this problem..

Thanks in advance,

CK
 
shoot in a raw and HDR or get some filters ... its just cameras cant pick up a huge range of light and dark stuff 'cos thats the problem with digital '
 
Thats the problem with digital!!
You could try and hit a happy medium... or take 2 shots on a tripod and blend them in photoshop.
 
I thought that was going to be the case..

Thank you guys.. - Ill definately go back, it was going to be such a beautiful picture..

CK
 
You can take multiple exposures and blend, as others have said, or....

- shoot at a different time of day, with more light hitting the inscriptions, to reduce contrast between them and the sky;

- Shoot under different weather conditions - fluffy white clouds can be very bright, but a clear blue sky is much easier to deal with. Overcast skies should be equally easy.

- You may not have been over by 2 stops. It might have only been 1/3 stop or 2/3. Did you actually try other exposures?

- Apart from the blinking warning, what did the histogram look like? Was it heavily bunched up over on the right or just a thin tall spike? Your camera may have indicated that the shot was blown but was the raw file actually blown? You may find that in-camera processing is creating the impression of overexposure when actually your raw data is fine. In those circumstances you could use highlight recovery to bring back the detail, or simply lower the exposure (highlight tones only) in post.

- If you want an exposure that holds the details in the sky and lets everything else fall where it may, use spot metering to meter off the brightest part of the sky and set your exposure to +3 when metered off the brightest part. If you shoot at low ISO you will probably be able to pull out the mid tones and shadow a bit, to give you what you need, with little noise.

- Instead of HDR, just cut and paste in a sky from another image.

- Try the shot with HTP enabled.

It may not help with this specific challenge, but you may like to browse this thread - http://www.talkphotography.co.uk/forums/showthread.php?t=64540 - about using a grey card for metering. It wandered off a bit into alternative metering approaches - my fault I'm afraid.
 
HTP? whats that then?

I like it idea of taking it at a different time of day, I never thought of that! - sunrise would be brilliant!

CK
 
Highlight Tone Priority.... it just knocks those bright highlights down a tad!

its in your menu........somewhere!!
 
Highlight Tone Priority. It is explained in the DP Review review of the 40D - http://www.dpreview.com/reviews/canoneos40d/page20.asp. Basically it shoots with an ISO one stop below the one you set for the camera. This obviously gives you a whole extra stop of highlight headroom. It then applies a special tone curve to boost the mid and shadow tones and give the highlight tones a more gradual rolloff to help prevent blowouts. The downside is that it can reveal a little noise in the shadows because they are effectively underexposed and then boosted with the special tone curve.
 
Oh ok, ill check that out!

Thanks for your help guys, ill let you know of the results and hopefully post a nice photo for you to look at!

tdodd - thank you.

CK
 
The histogram you see in the camera is generated from a jpeg preview image that is embedded in the raw file. It will have been massaged (cocked up, if you prefer) by in-camera settings for WB, contrast, saturation, sharpness and picture styles. This means that the in camera histogram is no more than an approximation of what the sensor actually captured. The same thing is true of the blinking highlight warnings. Sometimes they can be tripped too early, sometimes too late. They are only a guide, not gospel.

If you want to check out the real histogram for your raw files, download the free software from here - http://www.cryptobola.com/PhotoBola/Rawnalyze.htm - and you can see the true histogram for the raw data. If none of the RGB channels are blown then you can fully recover supposedly blown highlights in your editing software. They are clearly not actually blown. It's just that the adjustments by the software have boosted one or more channels too much, so cut back on the boost (highlight recovery or exposure).
 
There is a very interesting article / tutorial on exposure in this months " Digital SLR Photograpy" magazine that may help you out here.

It teaches you all about the various different exposure situations and how to combat them with your cameras settings.
 
flick5848 said:
There is a very interesting article / tutorial on exposure in this months " Digital SLR Photograpy" magazine that may help you out here.

It teaches you all about the various different exposure situations and how to combat them with your cameras settings.
Click to expand...

I have got a copy of that, I must not have got to that part yet :D

CK
 
tdodd said:
The histogram you see in the camera is generated from a jpeg preview image that is embedded in the raw file. It will have been massaged (cocked up, if you prefer) by in-camera settings for WB, contrast, saturation, sharpness and picture styles.
Click to expand...

I never knew that! :eek:

CK
 
Cavking said:
I never knew that! :eek:

CK
Click to expand...

Oh yes!

Let's say you are shooting to raw under tungsten light. The predominant colour of the scene will be yellowy red. The sensor will capture a lot of light at the red end of the spectrum and a fair bit in the green. The blue sub-pixels will not get much stimulus. The chances of blowing the red channel are high. The chances of blowing the blue channel are low. Green is about even.

So, in your camera you have to have some sort of WB setting configured - you can't shoot without one. If we assume you are shooting with tungsten WB, what will that do? Well it will give a very heavy boost to the blue end of the scale and downplay the reds/yellows. This means that the embedded jpeg preview you see will be reversing the true balance of light captured by the sensor. Your jpeg will be a bunch of lies. It'll make white look white, but it will not reflect what the sensor captured. The histogram displayed from that embedded jpeg will be a bunch of lies too. You will not be warned of blown reds, but you may be warned of blown blues when they are nothing of the sort.

At the other end of the temperature scale, you'll get the same problem in reverse. If you shoot in shade under a clear blue sky there will be a ton of blue hitting the sensor. Reds will be relatively weak, probably. The likelihood is that you may well blow the blue channel but the red will be safe as houses. What will Shady WB do? It will bump the reds, to make the whites look white, and downplay the blues. So the reality is that you might have blown your blues but the camera tells you you've blown your reds.

Lunacy!

Now, everything I've said above holds true when you are shooting raw, and especially when exposing to the right to capture the maximum data you can. However, if you are shooting straight to jpeg then the preview image is fully representative of the final end result you will get from the camera. So is the histogram. If you see something blown on the back of the camera then your full jpeg will indeed be blown. Reshoot it now.

It's these quirks of data analysis and representation that mean that often you can recover "blown" data in a raw file. Well, you can't if it actually is blown in the raw file. But, what if it only looks blown, but isn't really? That's where raw highlight recovery can be so effective. The very things you think are blown are the very things that are not.

So, WB is a curse for raw shooters who want to judge exposure from the in-camera histogram. The solution that has been invented to overcome this difficulty is a special custom WB setting called Uni-WB. AFAIK this first appeared in the Nikon camp but has since been picked up and cobbled together by some Canon folk, including me. There are some threads about all this over on the POTN forums. Look here....

http://photography-on-the.net/forum/showthread.php?t=470141&highlight=uni-wb

and here....

http://photography-on-the.net/forum/showthread.php?t=485349&highlight=uni-wb

Happy reading :)
 
Thanks again, Tim... for taking the time to explain all this so fully, Im sure it has helped a number of members, including me!

Can I just ask then, I recently found that to counteract my rear lcd screen on the 40d being a bit soft........i realised that it was in fact not the raw showing there but a small jpg. So I set my jpg picture-style to sharper and more contrasty and it looked great.

However im assuming that my histograms will be a bit wrong now?
 
Anything that changes the pixels you see is also going to change the histogram. The more processing you add to the file the more you deviate from the raw file. Whether the changes are significant I couldn't really say for sure. Certainly WB is the biggest villain, but some picture styles are pretty naughty too. Portrait picture style makes things look an odd colour to me, so it is certainly doing something to alter the RGB mix, a bit like a subtle WB adjustment. Landscape picture style champions the fact that it boosts the blues and greens - more lies there, and possibly the risk of causing clipping in the GB channels even if the raw was safe. Even Faithful tweaks the colours a bit. Obviously Monochrome picture style is messing with the RGB values big time.

The picture style that remains most true to the raw data is the Neutral picture style. Raw shooters will advocate its use for relative purity. That's the one I use. As for fiddling with contrast and sharpening, they will mess with the tone curve at a global and local level, but I'm not sure they will cause the appearance of clipping where there was none to begin with. Personaly I leave all parameters at default for the Neutral style. I wouldn't worry too much if you want something a bit more "poppy". However, if you are shooting in Standard picture style, that has already livened things up, so I wouldn't add further enhancements to that one. Anything that changes colours is clearly fiddling with RGB levels though. So if you boost saturation in any picture style you will surely be messing with something.

Regarding the apparent sharpness of the jpeg preview file, I have read that if you press the zoom+ button up to ten times you remain within the resolution of the file. But you can actually press the zoom+ up to fifteen times in all. Once you go beyond ten presses you are actually upsizing the preview file and that really is no good. Like I say, that's something I've read I'm not sure whether it's true or not, but I confine myself to ten presses when I zoom in.
 
you could try exposing for the sky and using fill in flash on the writing.:shrug:
 
fletch5 said:
you could try exposing for the sky and using fill in flash on the writing.:shrug:
Click to expand...

A nice idea, but I think impractical here. I do not see a Speedlite in the kit list. Max shutter speed with the popup flash would be 1/250 on the 40D. On a bright day, to hold a bright sky (not clear blue sky), at 100 ISO and 1/250 you are probably looking at around f/5.6-f/11, most likely near f/8.

The guide number of the builtin flash is 13 meters. Divide 13 meters by f/8 and you've got a usable flash range of under 2 meters. That is not going to light a whole archway very well, and certainly not evenly. At a range of under 2m you might also struggle to fit a whole archway into the frame. A 17mm lens on a 40D will give you around 2.4m of height in portrait orientation, at a subject distance of 2m.

Maybe with a 580EX, at a distance of 5-6m, you could make the shot reasonably well, but with the popup?.
 
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