18% Gray Card

[Lexi1016]

I am having trouble understanding what a Gray Card is used for and how to use it.

 

[Kerbouchard]

It's used to give you a reference later when post processing for white balance corrections. It can also be used prior to taking the shot by setting a custom white balance. It can also be used to help you fine tune exposure.

Basically, it's a calibrated reference point that will always be the same even in different lighting conditions.

 

[ann]

This is a film section and so the answer is going to be a bit different than the above. With an incident meter it was used (mostly studio work) to determine correct exposures.

Meters were built to read the world as a middle gray value, which in the film world and for many years was considered to be 18% gray. (in the digital world there is some feeling that this percentage is more to 12-13,).

 

[Kerbouchard]

Oops, saw it in the new posts and didn't notice the category. In that case, just ignore everything I said except fine tuning exposure.

This is a film section and so the answer is going to be a bit different than the above. With an incident meter it was used (mostly studio work) to determine correct exposures.

Meters were built to read the world as a middle gray value, which in the film world and for many years was considered to be 18% gray. (in the digital world there is some feeling that this percentage is more to 12-13,).

 

[Helen B]

Although not as common with film as with digital, grey cards are used as both a neutral and an exposure reference in film photography, particularly with colour negative.

 

[brian_f2.8]

They really help out a lot. When I shoot hockey I always use a grey card. Every rink is different and each game at the same rink can be different because lights do go out and rink maintenance is usually behind. For a while I would set my WB to 3500 Kelvin(told from a NHL shooter) and it worked fine. When I used a grey card the images were better.

 

[Helen B]

18% gray cards are actually 1/2 stop too bright, and the instructions that come with a Kodak gray card tell you to increase your exposure by 1/2 stop. That's because the meter is actually looking for 12.7% gray. As to why we don't use 12.7% gray cards...that's a whole 'nother discussion!

There is no fixed calibration factor for meters - it varies from manufacturer to manufacturer. The ISO for light meters only gives a suggested range of calibration factors, and manufacturers don't even stick to that.

There are 14% cards, such as the Smethurst, and they have existed since the Neutrowe Gray Card of 1940. It doesn't matter much what the exact value is, as long as it is consistent and your use of the grey card is consistent. Because of that, any consistent tone can be used in place of a grey card if the grey card's colour neutrality is not important to you. The palm of your hand is a good reference - you may have to open up half a stop or so, but it will be consistent.

Grey card metering and its variants are really incident metering methods in effect. It's probably better to use an incident meter if you have one, because of the directional nature of a grey card reading. If you are using the camera's TTL meter then a grey card or the palm of your hand is a convenient alternative to an incident meter.

 

[Judith57]

grey cards are used as both a neutral and an exposure reference in film photography, particularly with colour negative

 

[Damndirtyape]

You can also use it to shade the lens if shooting close to the sun to avoid flare... call it flagging.

 

[Helen B]

There is no fixed calibration factor for meters - it varies from manufacturer to manufacturer. The ISO for light meters only gives a suggested range of calibration factors, and manufacturers don't even stick to that.

It's my understanding that ISO 2720:1974 gives a range for K of 10.6 to 13.4, but that was for meters in general. ISO 2721:1982 defined exposure for in-camera meters, and supposedly doesn't even mention a K parameter. Exposure is described in terms of H. The definition can be used to work out an average reflectance, which works out to be 12.7%.

Here's the quick answer:

This is in the film section, so my remarks were about metering for film.

You are correct about the range of K given in 2720 - 10.6 to 13.4. I suggested that not all manufacturers used the same calibration constant, and that some went outside that range. Gossen uses 11.4*, Sekonic, Canon and Nikon use 12.5, Pentax and Minolta/successors use 14.

It's actually difficult to tie that K to a fixed reflectance value - there lies the long version of the answer - but because of the variation in K factor we can say for sure that one cannot fix 12.7% as the design reflectance, so there is no real need to go into it, but we can if you wish. There is a case to suggest that K=12.5 corresponds to 15% and K=14 corresponds to 18%, but it isn't rock solid - no correlation is.

There's a further complication in Kodak's use of Aim Density as a working film speed determination method. Kodak have given aim densities for an 18% grey card exposure for their colour negative films for a very long time. Here's the quick version of how it works:
You take a series of meter readings off a grey card, changing the EI over a two-stop range centred on the ISO speed, at 1/3 stop intervals and make a set of exposures of the grey card (in practice you take one reading and work out the set of exposures). Then you read the density of the negative using Status M filtration and select the neg which is the best match to Kodak's numbers. Other methods of determining working film speed also use an 18% card. They all have the same purpose - to calibrate your equipment and your method to enable you to get the results you desire. It's really the photographer's choice, and the type of film will also have an effect on what the optimum 'standard reflectance' or adjustment to an 18% reading should be.

***********************

There's another way of looking at the standard reflectance thing: how it all came about.

It had a lot to do with the invention and use of Kodachrome pre WW2. Kodachrome needed fairly accurate exposure for best results, and exposure meters were the answer. Simple averaging reflected light meters. The meter manufacturers did their best to set the meter up so that if you pointed it at a nice scenic landscape it showed you an exposure that would be a good one. Although photographers learned how to cope with different mixes of light and dark - where to point their meter - there was a desire to produce a standard reflectance that could be carried in the field. Various densities of grey were tried, and the one that was successful in practice with Kodachrome was a 14% reflectance card in the hands of an experienced Kodachrome user. Metering with a 14% card gave the same results as metering what was regarded as a 'standard Kodachrome scene for metering purposes'. This tied in with a lot of other work that was done separately to study the range of average reflectance of real scenes. Therefore it could be said that those meters were 'calibrated to 14% grey' even though they weren't designed that way originally - it just happened that that is what worked well.

The danger with Kodachrome tends to be overexposure rather than underexposure - a slightly underexposed picture generally looks better than a slightly overexposed picture. Therefore some meter manufacturers might want to build in a slight amount of underexposure. Imagine two meters that give identical readings when meter A is pointed at a 14% reflectance and Meter B is pointed at a 12.5% reflectance (in the same illumination). You could say that Meter A is calibrated for 14% and Meter B for 12.5%. What happens if Meter B is pointed at a standard Kodachrome scene of 14% average reflectance? It gives slight underexposure, so it lies on the safe side. It's 'calibrated for 12.5%' but it is meant to be pointed at 14% because we've decided that the pretty scenes have 14% average reflectance - it is really calibrated for 14% with slight underexposure built in deliberately to protect the highlights. It's not a card that sets the value, it is the best estimate for the real world.

This general practice remained, because reversal film is usually the most demanding in terms of accurate exposure, and it is best to protect the highlights. Experienced photographers could still set their own adjustments based on their experience and/or formal film testing.

The use of an 18% card as a reference had a lot to do with Ansel Adams - 18% is used as a guide to what we perceive as mid grey. It is perfectly valid to use with the Zone System, because the 18% reflectance is taken into account. It is perfectly valid to use it with any methodical approach to exposure that requires a standard reflectance. So blame St Ansel.

*Some Gossen meters may use a higher value - I am not certain. The practical differences I have noted between Sekonic and Gossen meters (I have five Sekonics and four Gossens) suggest that the Gossen K factor is higher than Sekonic's.

 

[Helen B]

grey cards are used as both a neutral and an exposure reference in film photography, particularly with colour negative

I couldn't have put it better myself. Oh, wait.

Although not as common with film as with digital, grey cards are used as both a neutral and an exposure reference in film photography, particularly with colour negative.