Underexposed - is the Camera meter lying?

[DennisS]

I was testing my White Umbrella lights at home to check what kind of light output I was getting - 250W/6300 Lumen/3200K. Using my camera as the light meter, it appears I may be getting only around 5 lux (some ambient light was present and of course some loss due to lens absorption) with 2 of the above at equi distance on either side of the camera (@ 5ft).

Anyway, the real issue is that I appear to get an underexposed picture (2 stops) with my Nikon D5100/55-200mm DX lens set at around 180mm in Manual Mode/Spot/Single Point Auto Focus/Auto WB/No Flash/RAW. The below paper sheet (A4) was 5 feet from the camera (tripod) and the Meter was centered.. Setting to Matrix and moving the camera back 5 ft more also did not make a difference. Why does the camera meter show me centered when I need 2 more stops? Could it be due to the zoom lens set @180mm?

Underexposed
Aperture: F/6.3
Shutter Speed: 1/20s
Exposure Mode: Manual
Exposure Comp.: 0EV
Metering: Spot
ISO Sensitivity: ISO 100




I increased the ISO to 200 and added +1 EV:

Aperture: F/6.3
Shutter Speed: 1/20s
Exposure Mode: Manual
Exposure Comp.: +1.0EV
Metering: Spot
ISO Sensitivity: ISO 200



...and here adjusted for Contrast (Post Processing in Nikon View2) to be very close to the actual image. Why does Contrast also get impacted even after adjusting for Exposure?

[Shadowman]

Does your meter measure the aperture value of your lens?

[Manfred M]

Hard to say. Correct exposure is based on a grey card and your target is has a lot of white. I wonder if this is fooling the light meter. Reflective light meters in modern cameras are set to 12% - 13% gray.

[Graystar]

Why does the camera meter show me centered when I need 2 more stops?

Exposure is based on the light source...not the light being reflected from the scene. However, light from the scene is the only light your camera has to work with. So your camera will guesstimate the exposure from the scene luminance. It does this by making an assumption...a very big one. The camera assumes that the scene in front of it is gray. And not just any gray...about 12.7% gray (a 12.7% gray surface will reflect 12.7% of the light that falls on it.)

So if you meter a white wall and shoot, it comes out gray. If you meter a black wall and shoot, it comes out gray. The camera doesn't know about black or white...it only knows that it must adjust exposure so that the scene comes out gray. This is why your exposure is off...the spot meter likely metered the center target and the surrounding area, which is mostly white. That would cause underexposure.

The predictable behavior of the metering system is actually a good thing, as it allows us to set exposure correctly with the application of a little thought and experience. The easiest way to set exposure is to simply give the camera what it wants...it wants a gray surface, so use a gray card to set exposure. Note that the gray cards you buy in the store are usually 18% gray, and so you have to increase exposure by 1/2 stop to have technically correct exposure (better called Standard Exposure...because in some cases it may be technically correct...but not what you really want.) I think Lastolite has started producing a 12% gray target.

You can also achieve standard exposure by metering something that is brighter or darker than 12.7% gray if you also know how far off the exposure is. For example, you can meter snow and set your meter to +2.3 (+2.7 if you're really on top of things) and that will give you correct exposure for the snow. This works with most white objects. For example, you can meter a bride's white gown, adjust settings till the meter reads +2 (or set Exposure Compensation to +2 when using auto modes) and the exposure will come out very close to what it should be.

Remember that I said exposure is based on the light source...not the light from the scene. That means that if you set standard exposure for one object in your scene, you've set it for all objects that are in the same light. That's why using a gray card (or a known reference point) to set exposure works for all objects that are in the same light.

[thequacksoflife]

Denis

as an aside. A friend of mine (a canon user) was using a nikon camera for the first time yesterday. She was using spot metering (as she does on her canon) and found that pics of white flowers were underexposed. when I was taking the same photo mine were ok. I switched her to matrix metering and the shots were fine. as I felt that the bright white was fooling the meter.

which I hope ties in with Greystars more technical answer!

Pete

[DennisS]

hmmmm - appears the grey card solution did not make a difference - had to increase exposure by same amount...focused on the grey card using both Spot and Center.

After compensating with 2 stops:


I then created a PRE white balance (with grey card) and only got a cooler image - exposure remained the same...the colors are now exactly as they are in the subject.



FYI - I have not noticed this behavior in other shooting situations - only with this lighting scenario. I have made sure I do not have ADL turned on.

[Graystar]

hmmmm - appears the grey card solution did not make a difference - had to increase exposure by same amount...focused on the grey card using both Spot and Center.

Is that an 18% gray card? It doesn't look like an 18% gray card...it looks like a White Balance card.

Also, remember that even with an 18% gray card, you still need to increase exposure by 1/2 stop.

[DennisS]

I had come to believe this was an 18% grey card - got it from here - http://michaeltapesdesign.com/whibal.html

this is an excerpt - We designed and manufacture the WhiBal White Balance Reference Card to be the best digital Gray Card available anywhere.

Are you saying there is a difference between a White Balance Card and a 18% grey card?

[Manfred M]

Is that an 18% gray card? It doesn't look like an 18% gray card...it looks like a White Balance card.

Also, remember that even with an 18% gray card, you still need to increase exposure by 1/2 stop.

White balance cards are white... This shot is hardly white...

[DennisS]

The card has both - on the other side it has both white and black - all supposedly caliberated according to the web link.

[DennisS]

I came across this so explains a part of the issue:

https://mtd.tenderapp.com/kb/whibal/...n-18-gray-card

Excerpt: The WhiBal is not 12% or 18% gray by design. It is light gray because we can get a better White Balance this way. An 18% gray card does not imply that it is neutral for white balance use. The neutrality of the gray (lack of any color), is the primary criteria for a proper white balance reference. The second is being a light gray to maximize the resolution of the white balance adjustment that the software or camera is capable of making.
The WhiBal card is Certified Neutral. By that we mean that every card that we ship is individually measured on a precision spectrophotometer, to certify that the card that you purchase meets our stringent specifications.

For exposure we would suggest that the histogram on the back of the camera is the best tool for exposure (short of an exposure meter). And to get a more precise exposure you can use the spot exposure setting on your camera to hone in on the proper exposure for the skin tones, for example (by spot metering on the subjects face).

[Graystar]

White balance cards are white... This shot is hardly white...

White balance cards do not have to be white, and typically aren't. Neither the Whibal, Digital Gray Card, nor Digital Grey Kard targets are white. What's important is that the cards are spectrally neutral.

A card that is white, such as the Xrite ColorChecker White Balance card, can be used for setting a custom white balance. However, you don't want to use it as a reference in your images for post-processing. If your exposure is off and you overexpose a small amount, the image may be fine, but the target may be clipped, giving an incorrect white balance. Because of this, a gray WB card is a more universal card, as it can be used for custom WB settings or for acting as a reference image in post-processing.

[Graystar]

Excerpt: The WhiBal is not 12% or 18% gray by design.

Okay. Yeah, it looked like a Whibal but it didn't have all the markings so I couldn't know for sure.

As you have learned, the Whibal is a light gray. The reason for this is to increase the signal that's recorded on the sensor for the card, and this leads to a more accurate white balance. A couple of comments on this. The shade of gray doesn't really matter when setting a custom white balance on the camera because cameras set exposure based on the card, so exposure is always "gray", even with a white card (with that said, please note that Nikon cameras will overexpose a WB target by one stop for a more accurate custom white balance.) A light gray card only matters when you place the Whibal in the scene for later processing. In post-processing, you use the WB tool on the Whibal card. The lighter gray should provide a more accurate white balance. This is the method of setting WB that is preferred by the Whibal people. In fact, before Photoshop started to provide this ability, Whibal used to produce a plugin that allowed you to select the white balance setting from one image and apply it to other images. This allowed you to use one image as a reference for all the other images shot under the same lighting.

So that's why your exposure is off when using the Whibal...the gray is too light.

[Inkanyezi]

I am amazed by the notion that people may think that the camera would know what is in front of it and from this knowledge evaluate a correct exposure.

The light meter is a dead, inanimate object. It has no way to understand what is in front of it (even though Nikon marketing dept claims so). Moreover, it is more influenced by light than darkness, so "white" will carry more impact on its reaction than black.

Your target consists to a large part of white, and the meter will register about half or more of the white, and place it in the middle gray for which it is calibrated. It is simple as that. The meter gives a correct reading according to its design, which is not correct exposure, but an average value for what is in front of it. Nothing less, nothing more.

Ansel Adams outlined the basics of exposure in his book The Negative in 1948. It is in fact very simple. White shall get more exposure than black, it is the very foundation for creating a photographic image.

So your light meter does not lie to you, but it shows you an average value of what is in front of it, and when the target is about half or more white, it gives a reading that will underexpose the image if used directly without correcting for the relative brightness of the scene.

You have a setting in the camera to compensate for the relative brightness of your target. It is called exposure compensation. When you understand to use this feature, the image will come out correctly exposed.

It may however take some time to understand how much to correct, and that is why we have histograms that we can chimp, which facilitates setting the correct exposure. You have those histograms included in the pictures you posted. After reading the histogram, it should be evident that a plus setting of correction will provide a better exposure.

A side track is the marketing blurb that leads us astray regarding the camera's ability to "evaluate a matrix". Nikon includes in its viewfinder a little camera, simpler than a cellphone camera, but it provides an image to the processor for evaluating according to a built-in program. This system often works reasonably well, but the spots measured are rather large, and with a target like this, all of the spots evaluated will show an average reading of largely white areas, so even though it is a technically complex solution intended to average contrast in the scene, it cannot do so for a target like this one.

There is in fact only one measuring method that is predictable, spot metering. All other measuring methods are predictable only if you fill the frame with the area you want to measure. There is no way to understand or know, how "center weighed" or "evaluative" metering will average its output. But the spot meter will give a reading for average gray at the spot you aim it to. So if you aim at white, you must adjust to a brighter exposure, and if you aim at black, you must adjust to a darker exposure. Often a reasonable exposure may be found by aiming at the brightest highlight in the scene and adjust to about two stops plus, but the exact value has to be tried out for the actual camera.

[Mike Langdon]

I agree with Inkanyezi, even my handheld spot meter puts the highlight or shadow reading at middle grey, so I have to put in +3 exposure compensation in to the meter to move the white from mid point to the highlight area. The amount of exposure compensation will depend on your cameras dynamic range from the mid point to the highlight clipping point.
My hand held meter comes with software to measure my cameras dynamic range so I can save my cameras profile into the meter.

[Mike Buckley]

Just a quick note to remind everyone that prior today's posts the thread's posts are dated June 2012.