Using Gray Cards for Tricky Lighting Situations

[Jason Dunn]

<div class='os_post_top_link'><a href='http://www.acdsystems.com/English/Community/ColumnsArticles/TechTips/tech-2004-03-13.htm?CMP=EMC-DIBFY2004&CRA=20040313' target='_blank'>http://www.acdsystems.com/English/Community/ColumnsArticles/TechTips/tech-2004-03-13.htm?CMP=EMC-DIBFY2004&CRA=20040313</a><br /><br /></div>"What's the best exposure for a candle-lit couple sitting in the moonlight on a darkened patio? How about for the best sunset of your vacation? Or, for bright shafts of sunlight filtering through palm fonds onto your subject? In tricky lighting situations like these, using your camera's built-in light meter to take a reading off your subject will often not produce the results you want. Bracketing your shot will be a good idea, but is still no guarantee. Likewise, you could use your flash, but to really capture the mood or the subtle play of light, you may not want to."<br /><br />How many of you use grey cards when shooting? I haven't - I tend to rely heavily on post-processing the fix exposure issues, which of course isn't the best way to do it - it's better to get the photo you want at the source, because you'll have more to work with.

[Lee Yuan Sheng]

You should Jason.. even if you don't have a grey card, taking a white balance reading off say, a known white or grey object is better than nothing in tricky lighting conditions.

[possmann]

couldn't you just adjust the photo (assuming it is a digital image) post snapping on your computer by using some photo touch-up software? Adjust the lighting for 18% grey?

Interesting article though...

[Jason Dunn]

You should Jason.. even if you don't have a grey card, taking a white balance reading off say, a known white or grey object is better than nothing in tricky lighting conditions.

That's the thing - I've NEVER done that with my EOS 300D yet...I keep meaning to get around to learning how to do it. :-)

[Lee Yuan Sheng]

Hey, it's quite simple. Once I took a reading off a friend's white socks (no card handy). Worked pretty well, and went through minimum post processing afterwards.

You can adjust it afterwards, but if you can get it right in camera, why not? Faster too, in the post processing stage.

[Lee Yuan Sheng]

ok ok, I was sleepy, I must add, what I did works only if you have a spot meter! In a 300D you'll need to switch to partial meter (whatever Canon calls that sub-spot metering mode), and make sure whatever you're using to reference (card, known grey, etc) fills up the partial meter area! Also the ambient light falling onto the reference must match your subject's, else it's pointless.

[ppcsurfr]

Okay... Getting the correct exposure means that you should have an 18% Grey Card. This is how meters are calibrated to show which is the correct exposure.

Once you have this... you can disregard the meter reading in your camera.

There are two metering systems used... one is Incident Metering... and the other Reflective metering.

If you are using reflective metering... which is basically what your camera has... you can get wrong results when you use, 1. Center Weighted Metering, 2. Matrix or Average metering, or 3. When you use your spot meter and you point it in the wrong spot.

Incident metering is the most accurate but requires you to go up to the subject and measure the light falling on them.

The grey card helps by allowing you to get accurate readings while using your reflective metering system built into your camera.

A good equivalent of 18% gray is... green grass... although not when you get reflections of the sun bouncing of the blades of grass... or the face. Now a fair skinned person will average at close to 18% grey... Caucasians might be about a stop or so away... and on the other side... like Asians with brown skin... another stop in the other direction.

The best way to master light is to think black and white and go for the Zone System...

Oh... BTW, yes I use a gray card... when I copy books, paintings, other artwork and shoot them for presentations... You have to get the exposure right to get the colors right on too. The best way... meter off a grey card and shoot away with that particular setting.

Mabuhay!!!

Carlo

[pixelhead]

The article this refers to is talking about exposure, not white balance. Using a gray card for exposure metering can really help is this type of situation, but I would not use the same graycard to set the white balance of a digital camera. Doing so will remove what I would consider to be a desirable color cast (the warm glow of candle light, a fireplace, or sunset).

You can set the white balance based on a gray card, but I would only do so when you don't want the color of the light source to influence the overall color of your image. In the case of a desirable color cast (candle light, fireplace, sunset), I wouldn't use a custom white balance setting and if shooting RAW, it would experiment with the white balance in Photoshop's Camera Raw dialog box to see which setting will make the desirable color cast look it's best.

And as far as adjusting things in Photoshop to adjust the brightness of the image... keep an eye on the histogram in your digital camera... as long as there is not a tall spike on the absolute ends of the graph, then you can manipulate the image in Photoshop to compensate for the brightness of the scene, but if a spike appears, then you've either lost all detail in the highlights or shadow areas and you won't be able to get it back in Photoshop.

[ppcsurfr]

You should Jason.. even if you don't have a grey card, taking a white balance reading off say, a known white or grey object is better than nothing in tricky lighting conditions.

The 18% Grey card is for exposure... not white balance.

Mabuhay!!!

Carlo

[ppcsurfr]

Hey, it's quite simple. Once I took a reading off a friend's white socks (no card handy). Worked pretty well, and went through minimum post processing afterwards.

You can adjust it afterwards, but if you can get it right in camera, why not? Faster too, in the post processing stage.

If you are taking an exposure off of a white object such as your friend's socks... the rading would have been about 2.5 stops under right away...

True white will always appear overexposed when you meter it... that is why it is essential that you practice with a grey card until you get the hang of it in comparing other colors which can give you an 18% grey equivalent.

Ohhh... and red is really a wild color that can influence other colors...

Better stick with grey...

Mabuhay!!!

Carlo

[Lee Yuan Sheng]

If you are taking an exposure off of a white object such as your friend's socks... the rading would have been about 2.5 stops under right away...


And you think I'm stupid enough not to compensate for it? Pure unreflective white is easy to compensate for, and it's more like 2 stops.

[ppcsurfr]

If you are taking an exposure off of a white object such as your friend's socks... the reading would have been about 2.5 stops under right away...


And you think I'm stupid enough not to compensate for it? Pure unreflective white is easy to compensate for, and it's more like 2 stops.

Nope... didn't mean anything like that...

I'm sorry if it sounded that way... I must have missed you mentioning that somewhere.

And yes... opening up by 2 stops will bring you back to 18% grey reading... and... I also failed to mention... I normally overexpose by .5 stops for film hence the 2.5 comment... a carry over from film. :D


Mabuhay!!! ~ Carlo

[Neil Enns]

I've never used a grey card, and to be honest I don't think you really have to with today's digital cameras.

I have my 10D set to show a historgram after every shot and to leave it up until I press another button on the camera. I can easily check the histogram to accurately see at the time of the shot what my exposure was like. If I have to compensate I'll know right away. A grey card would be one more thing to carry in my camera bag!

Neil

[Jason Dunn]

I have my 10D set to show a historgram after every shot and to leave it up until I press another button on the camera. I can easily check the histogram to accurately see at the time of the shot what my exposure was like.

Care to tell us more about the exposure histogram and what it means? :-) I've used it myself, but I'll freely admit that I don't understand it as deeply as I like.

[Lee Yuan Sheng]

It's a graph of luminosity values present in the photo. It's a good way to gauge if you got the exposure right. Basically if most of the values are on the right, you might have an underexposed image, and too much to the right might mean an overexposed image.

However, like I said, it's only meant as a guide. Depending on what the scene is like, and what result you want, the histogram will be different.

[ppcsurfr]

I've never used a grey card, and to be honest I don't think you really have to with today's digital cameras.

I have my 10D set to show a historgram after every shot and to leave it up until I press another button on the camera. I can easily check the histogram to accurately see at the time of the shot what my exposure was like. If I have to compensate I'll know right away. A grey card would be one more thing to carry in my camera bag!

Neil

Hi Neil...

It's true with today's digital cameras... you can edit them on Photoshop or some editing software... but that would also be similar to shooting film by guessing and having the lab correct the exposure on printing.

The Grey Card can be used for practicing... and at least for looking for 18% grey equivalent objects in your area...

I don't think it's a bigger issue now with digital and film... unless you are shooting trannies.

I know someone who had an 18% grey card plastered onto his camera bag's stiffener... All he had to do was pull it out of its slit...

Mabuhay!!!

Carlo

[Neil Enns]

Care to tell us more about the exposure histogram and what it means? :-) I've used it myself, but I'll freely admit that I don't understand it as deeply as I like.

Check out the article at Luminous Landscape: http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml . It gives a good overview of how it works and why it is valuable. It also shows sample histograms and the images that created them. Also check out http://luminous-landscape.com/tutorials/expose-right.shtml for a guide to using the histogram to get the correct exposure for your image.

With regards to Carlo's comment, the histogram can be used in the field to immediately tell whether your image is exposed accurately You don't have to wait until you get back to a PC to edit them in Photoshop. With a grey card you have to be careful that it is positioned in the same place your subject is, under the same lighting, to get correct exposure. If I metered a grey card I'd still review the histogram to make sure that I'd exposed the way I want.

Neil

[Neil Enns]

So I did a quick google search on grey cards and digital cameras and came up with a cool thread on Photo.net about using them. Apparently they're still valuable, even when using a histogram. The full thread is at http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=006vLF, but here's the post by someone named Darrell that caught my eye:

First off, you can' always judge a correct exsposure using your histogram on your camera. Since your meter will try to expose the snow as a midtone it will also display it in the histogram as a midtone therefore it will be closer to the middle. Try taking a photo of both a dark blue and light grey solid surface. If both are exposed according to your camera meter they both should yeild a steep bell curve centered in your histogram. If you want either the dark blue or light grey to photograph as your eyes see them then you would need to compensate the exposure for both. I read about stopping up 1 or 2 stops when shooting on snow but did not because my histogram looked good. I was using my matrix meter mode and not my spot. 80% of my scene was snow and the rest was the skier which in turn represented only 20% of the pixels in the histogram. When I downloaded them later most were underexposed. Your best bet is to understanding how your various meters work, pay close attention to what is being metered in the scene or on the subject and make apropriate compensations based on your experience and desired outcome. BTW, I use a Kodak grey card for both exposure measurements and white balance.

[Doug Johnson]

I have to admit that I use a gray card for white balance more than exposure. I have a Canon 10D, and while it is very good about getting the right exposure and white balance, there are times when it just can't, and having that gray card with me has saved my bacon more than once. I generally only use it in situations where I know the camera can't expose properly (predominantly bright or dark scenes, or images where there isn't much white to balance off of).

As for exposure, the easiest way to make sure you have the proper exposure is to stick a gray card into your scene (or have a subject hold it), aim the camera at it, press the AE lock button on the camera, quickly hide the card (before the AE lock timeout expires), then take the picture. Or, since you're working with digital, take one picture with the card, then another (same exposure) without it. The exposure with the card can be used in Photoshop to determine if the exposure is right and to adjust white balance. Just don't adjust your zoom once the AE lock is set; adjusting zoom affects how much light enters the camera.

Since I use the gray card for setting white balance, I also keep some off-white homemade cards as well. A lot of the time when shooting people, a "correct" white balance is too cool, so I have created some cards to correct that. One that is slightly blue, and one that is slightly yellow. (Easy to produce in MS-Word: just draw a big rectangle and change the fill color, glue the printout to a piece of heavy card stock) Setting the white balance on the camera using the blue card gives a warmer color cast to the resulting images, and using the yellow gives a cooler cast. It all depends on the mood I'm trying to capture.

[Jason Dunn]

djdj - very interesting stuff! Thanks so much for sharing!