Digital Camera Exposure Question

[brian_ellis19]

I hope this is not off-topic here. If it is the moderator hopefully will delete it or move to to a more

appropriate forum.

 

I used 4x5 and 8x10 cameras for years, taught a zone system course, processed and printed everything thing myself

in my own darkroom, and had a thorough understanding of film exposure. However, I'm unclear about making a

"correct" exposure with a digital camera. I understand that the best practice with a digital camera is to make an

exposure that results in as much information as possible being as far to the right of the histogram as possible

without blowing out the highlights. However, when that's done most photographs will be overexposed and it becomes

necessary to reduce the exposure in Camera Raw or to change things in Levels or in Curves or in some other manner

or fashion to bring the exposure down so that the print doesn't look like the image was overexposed. Is anything

really accomplished by having everything as far to the right of the histogram as possible when that results in an

overexposed photograph that has to be changed in Camera Raw or Photoshop by getting more of information towards

the center of the histogram? In other words, why is it better to use this technique (assuming it is in fact

"better") rather than just making the "correct" exposure in the first place (i.e. one in which much of the

information is near the center of the histogram as opposed to being way over on the right)?

[geoffs1]

The basic premise is that the signal/noise ratio increases with the recorded signal. Therefore, it's better to boost the exposure and shift the histogram to the right (lower noise region). Then, when you process the image intensities back to the middle, you end up with a lower noise (i.e. "better") final product.

 

IMHO, it's really the digital analog to the zone-system, where you adjust the exposure and development to yield the desired result.

 

http://www.luminous-landscape.com/tutorials/expose-right.shtml

[g dan mitchell]

There is an advantage to moving the exposure to the right side of the histogram in terms of better signal to noise ratio

and, so say, move bits devoted to a given portion of the dynamic range in the image. However, you can go to far with

this - and you are going to far if your photographs are overexposed to the point that you cannot recover details in the

highlight areas.

 

If that is the case, the simple answer is "not quite so far to the right."

 

By the way, don't worry too much about what the image looks like in the LCD display or even whether an "expose to the

right" histogram results in an image the seems too bright (though without blown highlights). You'll bring the overall

brightness back down to where you want in in post processing, and at that point you'll also reduce the level of noise, etc.

 

Dan

[link]

I could be wrong, but....

 

#1. Let's say there are 100 levels of brightness recorded by your camera for the purposes of example. (there are actually much

more). They are not evenly distributed from black to white. Most of the levels are in the brightest portion of the image with many

fewer in the darker portion of the image. So to record the most information, one would want the most possible exposure without

loosing desired information above the clip point where no information is recorded.

 

#2. The less exposure, the more noise to signal is recorded as well so this is also a reason to "record to the right of the

histogram".

 

That said, I think one should not get carried away with this approach. The histograms on the lcd are not the highest resolution

and might miss some fine detail in the highlights. Also, be sure to use the RGB histogram. If you just look at the luminance

histogram, it's easy to clip one of the color without realizing it. If this happens in a skin tone, it will look false and a bit "electronic"

looking. IOW, there will be a color shift at the clip point.

 

If you don't plan to make darker portions of the image lighter in post processing, then you might find little gain in the expose to

the right approach. Of course you can easily experiment with the technique and see how much IQ you gain. Of course you will

need to be careful with the approach in the field as clipped highlights are lost forever. I find when I shoot landscapes that most

often the issue is underexposing to hold the highlights vs over exposing to better record the shadows. Usually, I just let the in

camera meter do it's thing unless I anticipate a situation that will "fool" the meter. If its something I've lit myself, then the

handheld meter and manual exposure work just fine and gives consistent results.

[swilson]

As long as you don't have any levels bunched up against the right side of the histogram you are not over exposed. If the image looks over exposed then you might want to check your monitor.

 

If your histogram does not have some values going right up to the right side your print will not have any full whites in it, of course this is easy to fix in either the raw converter or an editing program.

 

It is better to be under exposed rather then over, you can correct for under but are pretty limited in what you can do for over exposed. If you are using a DSLR being under exposed by a stop or so is not big deal, a P&S needs to be better controled. If you shoot at higher ISOs you need to be much more careful about getting the histogram to the right.

 

The best thing to do is go out and take a whole lot of photos at different exposures and see what works best for you and your camera.

[obakesan]

Brian

<P>

I think you actually probably already know the answer but its just eluding your conscious grasp due to some extra

variables. You say:

<P>

<BLOCKQUOTE>

In other words, why is it better to use this technique (assuming it is in fact "better") rather than just making

the "correct" exposure in the first place (i.e. one in which much of the information is near the center of the

histogram as opposed to being way over on the right)?

</BLOCKQUOTE>

<P>

remember that like a negative (assuming you're not a chrome kinda guy) will naturally be altered in gamma by

things like contrast of papers (enlarger head, enlarger height ...) and you're doing this in photoshop now. So

just like your exposure goal was in negative <i>to capture the range on the negative</i>, the goal in digital is

also to capture the range which you'll want to print. As always there will be light and dark which will fall

outside of the capacity of any media to record, its just that this range is different for digital than it is for

film. Combined with digital's

advantages in being able to slide the dark areas up (and its inability to slide the bright areas down) it will

just take a little practice for you to get used to it.

<P>

I'm not sure if it will help, but perhaps <A

HREF="http://cjeastwd.blogspot.com/2008/02/while-hrd-is-all-rage-ive-taken.html" target="_blank"><b>this

read</b></A> or

<A HREF="http://home.people.net.au/~cjeastwd/photography/film/digiExposure.html" target="_blank"><b>this read</b></A>

might be helpful in understanding how to visualize this ?

[petemillis]

I think though that you also need to consider this.... It isn't always practical to expose all the way to the right. For instance, to expose right to the right may force you to use too wide an aperture and hence small DOF, or too slow a shutter speed and hence possibility of blur from camera or subject movement, if you don't have an ISO setting high enough to keep aperture and shutter speed in check. Perhaps it should be a case of exposing "as far to the right as is practical"

[brian_ellis19]

Thanks very much to all who responded, that helps a lot. I'll check out the links.

[mendel_leisk]

Just a thought:

 

It's always a given that the camera is metering so that the area being considered will be medium-grey, middle of the

brightness spectrum; that the scene is "average", mid-day brightness. Then if you're shooting something like dark

shadowy dusk scene, or sun-drenched beach or snow scene, it's up to you to make adjustments.

 

Maybe (for example) it would be good to be able to spot-meter the scene's hot spot and tell the camera "I don't want

that to blow-out, keep it right on the edge, but no brighter".

[paul_norman_dicaire]

Not a response, rather an exposure-related question...

In my years of shooting on file, I'vew always found that using a gray (grey???) card always provided me with great exposure. Is all that out the window with digital?

[roger_smith4]

"Is all that out the window with digital?"

 

No- it's the same. Gray cards are useful if you want a "normal" exposure for your scene- you may decide to override that recommendation however for aesthetic reasons.

 

The difference with digital is unlike with slides you can "expose to the right" and make the data look too bright and not contrasty and then easily fix it in post and end up with a noiseless file.

[robert_k1]

If you know how to properly expose slide film, digital ETTR is not that much different. To preserve highlight details in a slide, I manually spot meter the brightest area and set the exposure so that the camera's analog exposure display is stretched almost all the way to the highlight end. With a digital camera, I would do the same and watch the histogram stretching almost all the way to the right. Setting the right exposure makes post processing much less of a headache, film or digital.

[david_henderson]

Here's a slightly different perspective. For me the first task in exposing on a dslr is to make sure I can fit the brightness range of the scene into the histogram. Only when I'm sure I can do that will I even begin to think about the distribution of the data on the histogram, and I could, but don't, compress the brightness range with grads to allow me a greater flexibility to position the data more to the right. If the scene brightness gives me the choice of where to put the data, then I'll try to keep away from the left hand end of the histogram.

 

I have a hand-held meter and use it extensively for medium format film work. But I haven't found it necessary to use it with my dslr- frankly its easier and faster to take an Av sighting exposure and then assess how you want to change it by looking at the resultant histogram.

[obakesan]

Mendel

 

if you don't mind taking 2 identical shots and you have a good grey card then you can leave the grey card in one of the shots for making colour balance easier. Be careful with grey cards though as some may have slightly off colour 'grey' ... but I'm told this is rare

[jbs]

I don't know all the science like some, all I can tell you is to think of digital as you would slide film. Have you ever shot slide film? Hmm, Chris E. sounds like he is on the right track for you... And I usually do what David does.