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Exposure (Histogram)

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delboysafa
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Exposure (Histogram)

Unread post by delboysafa » Fri Nov 10, 2006 3:03 pm

I am currently doing a photography class to help me with certain technical aspects of my photography.

One of the areas we covered was the Histogram i.e. Exposure. I always underexpose an image (in RAW) as I believed I would retain more details if I had to add exposure (so to speak i.e. overexpose it slightly). From my experience, I have found that if you overexpose an image, you lose the detail in it if you try and step down the exposure. The instructor said that try, where possible, to slightly expose the image to the right as it retains more detail.

What are everyone's thoughts on this?

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DuQues
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Unread post by DuQues » Fri Nov 10, 2006 3:10 pm

Completely correct! Expose to the right!

Why? Easy. The sensativity of the sensor is far higher in the white area than the black. Madach was going to write an article about it, but still has to do that. And as he's off into unmentionable parks located a little to the north of SA it will be at least a month till he does so.
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delboysafa
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Unread post by delboysafa » Fri Nov 10, 2006 3:14 pm


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Jay
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Unread post by Jay » Fri Nov 10, 2006 8:02 pm

thank you very much for that link delboysafa, finally some illumination on the histogram (very nicley explained btw) :wink:

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bucky
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Unread post by bucky » Fri Nov 10, 2006 9:25 pm

I have tried this technique , and it does work .

It all has to do with the electronics and the sensor as dq says , a little technical , but basically if you look at the histogram , there are 5 blocks representing the 5 stops from bright to dark the camera can capture , the right being the brights , and the left the darkest stops .
The first stop on the right takes in x amount of info , then the second from the right captures half the info of the first , then the third captures half of the 2nd etc etc .
Why this happens is very technical , and I am not sure how to explain it in laymans terms .

So if you push your exposures to the right , you capture more data , and hence your picture will be better .

The biggest danger of using this method , is of course pushing bits of the photo's info right into total overexposure , and then you have a blow out , in turn turning all your picture data into plain white .
The other draw back , is that you are going to shoot at a slower shutter speed to capture more light and push the exposure up , increasing the chances of blur .

I used this overexposure method here , and it caused a few bloopers , although the good ones are stunning as far as clarity and detail are concerned .
Canon 350d with 70-200 F4 L

If you look at this photo carefully , you will see that the area to the bottom right of the mouth are blown out , so the whiskers are gone .
Image

After taking the above and a few others, I noticed I had some overexposure by looking at the flashing white/black bits on the viewer, and reduced the settings by a third of a stop , so was really pushing to max exposure in this next shot .
This is a crop of the same leopard in the previous shot , taken at the same zoom level as the first shot .I think it has retained a lot of detail (especially in the dark bits)and clarity for the amount of crop , illustrating how much data is available to work with.
Image
If you look at the above photo in the raw browser It looks terribly bright , but by droping the exposure level by 1 and a third in the raw to jpeg conversion program , The pic comes out nicely .

Practice this method a lot before using it in the field , it is very easy to mess up your shots when you try use this , rather try push your histogram till about half way in the right hand stop , than trying to squeeze it right onto the edge .
Last edited by bucky on Sat Nov 11, 2006 1:06 pm, edited 3 times in total.

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Unread post by restio » Fri Nov 10, 2006 11:18 pm

bucky, I must admit I didn't understand all the technical details. :redface: But you obviously know your stuff - those are beautiful photos! :D
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Unread post by Nikon12 » Fri Nov 10, 2006 11:37 pm

The instructor is correct. There are risks in doing it the correct way as explained already. This is especially true as the camera histograms on the lower end DSLR's are not always accurate enough to "live on the edge". The best technical explanation I have read so far on this topic is available on the official Adobe website. Look for the article resource called Raw Capture, Linear Gamma and Exposure. It is a bit technical but as a technical student you will find it easy to follow. Compulsory reading in IMHO for any RAW shooter.

http://www.adobe.com/digitalimag/pdfs/linear_gamma.pdf
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Unread post by richardharris » Sun Nov 12, 2006 9:39 am

I love to be different!

I think you need to be very very careful with this sort of technique actually in the field, especially where the light can be extremely bright, and any number of things we want to photograph are white or at least bright light colours.

Birds in particular are very easy to overexpose anyway - I have no end of (for instance) stork photos with the white areas completely blown - but an otherwise perfect shot! And yet it is the white areas where I wanted the detail. A full stop, or even more, of underexposure can be needed compared to the cameras 'autoexposure' setting, and the histogram can look dark or 'left biased'. Yet this is exactly what is needed, and you can use your RAW convertor or Photoshop to brighten the darker areas, leaving the whites perfectly exposed - show the detail in the feathers etc.

I have just returned from a holiday to Bangkok and had the same problem when photographing temples. The sun is so intense there (even early on in the day), and much of the temple decoration is white or gold, that frequently some underexposure was needed.

I also know the downside; when you brighten the now underexposed areas you risk noise - but this is more easily corrected than trying to do the impossible of recovering burnt-out areas. Fortunately when this problem is at its worst (ie intense sunlight) you can use low ISO to minimize noise problems. I used ISO 100 mostly in Bangkok - though thats impossible in the Kruger a lot of the time, as it interfers with the need for a fast shutter speed.

I think this is an area where theoretical concepts, or techniques which work well in studios, don't always work well in the field!

Richard

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Unread post by Boulder » Sun Nov 12, 2006 2:06 pm

I find it so easy to over expose in this new for me digital photography. I went taking pics the other day and I was underexposing .7 stop by 9am and by 11 am 1.7 stops. I cant see the histogram anyway in bright sunlight but I set my camera up for 10 am shooting as follows.
ISO 100
Aperture Priority
Vivid
under expose overide 1.0 stops
White Balance Auto

That seems to cover 95% eventualities on the D200 which seems to like a bit of overexposure given half the chance. This is so different to Velvia 50 when I just shot as the F5 meter read it on the legendary Matrix metering
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Jay
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Unread post by Jay » Sun Nov 12, 2006 4:23 pm

must admit to going through a phase of bracketing virtualy every shot for just this reason (overexposure) but the downside is batteries get used up far quicker and so does the memory card :roll: ....then discovered, once I started paying attention to the info display :redface: that Auto ISO tends to like ISO 80, which ofcourse gives my lotsa slightly soft pics :roll:, anyways, it's a learning curve :lol:

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Unread post by hari ari » Sun Nov 19, 2006 3:28 pm

The system I follow when there is little time to think about the picture (when taking moving animals/birds) is take the picture, check the histogram that there no peaks on either sides and the overall bulk of the histogram is slightly to the right. The beauty of digital is the ability to recover. Took a picture of a leopard stalking at night. The flash with a guide nr of 58 was not sufficient. Previewing the RAW capture, one could just make out the leopard, histogram was blocked to the left and clipped. Managed to recover a half decent picture of the cat.

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Unread post by bert » Mon Nov 20, 2006 10:28 am

Agreed with the above, but found if you want to play if save
underexpose 1/3. In Photoshop you keep more detail when you must make the picture a bit lighter. A overexposed picture will loose detail if you make it darker.

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delboysafa
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Unread post by delboysafa » Mon Nov 20, 2006 11:25 am

Agree with Bert. I always slightly underexpose for the detail element.

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Unread post by bucky » Mon Nov 20, 2006 2:07 pm

Lol , ok keep the histogram in the middle .

As with everything , the theory and practical dont always tie up , do they :lol:

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Re: 2x Converter or Telephoto Lens

Unread post by DuQues » Mon Dec 28, 2009 12:42 pm

Going offtopic a bit here, but this I don't agree to:
gmlsmit wrote:Also another simple rule rather over expose a shot slightly as this can be corrected when editing the shot without risk of increasing noise, should you underexpose you will always increase noise white brightening up the shot.

Instead of overexposing you are better off underexposing. White = white, and all the other colour information is gone. The underexposed bits can be (at the cost of grain) revealed as the information is still there.

Just to show you, I put up an example to play with on one of my sites. Grab this (7MB!) original photo and play with it. Up the exposure by 1.5 or more stops, and look at the colours. The ellies become brown, and you can see what happens to the background and such.
(It's not the best of photos, but keep it on your own pc only for playing please...)
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