I am shooting wide shots of storefronts. I am standing across a street and using a zoom. I am confused about how to use the QP 101 gray card.
How big does the gray card have to be in the frame?
Do I maintain my camera position and try to zoom in as much as possible to the card being held in front of the store (which means it is very small in the frame)?
Or do I have the person holding the gray card come close to me, as long as the lighting on the card is the same as on the store?
No matter what, you need the card to be in the lighting you want to adjust for — you know that already. The rest depends on how you are using the result. If the goal is to have a reference for post processing, you need the card to be big enough in the frame to distinguish, but most software will give you a cursor where you can pick quite precisely.
On the other hand if you are trying to set white balance in the camera, you may need to fill the frame. Some models let you pick small portion of the image, or at least a reduced square, but many (particularly lower end models) only allow you to fill the frame. In that case, you'll need some combination of zooming in and moving close.
Depends on several things.
1) The easiest one. Do you have an assistant that can be standing in front of the store? Sure. Use him.
2) Is the light really the same across the street where you are?
This is the most critical part. If it is not, because you are in the shade and the store is not; if there is a big red wall next to you and far from the store...
If you are using artificial light, this will mostly be directional and fall off very fast, so no, you need to measure in front of the subject.
But if you are certain that the light is the same where you are standing vs where you are shooting and you are using the card just for white balance, yes, you can.
3) Are you using a methodology for applying the same settings on a program like Lightroom?
You take one shot with the gray card on a specific light condition and save the settings, white balance, and exposure adjustments, and then you apply this recipe to several shots with the same conditions.
Of course, you can zoom in to the card to be easier. Depends on the size of the card.
But if you need to change a lens, from another brand, no. You do not change the lens to take that zoomed in the photo.
4) Do you have time?
If you do not have an assistant, and you have time, you could stand in front of the store, take a photo of your card extending your arm, and move away to take the photo.
But I would only do this if the image has no clear white balance reference on the subject.
In the end, you need to use logic.
Are the settings and light conditions you try to measure the ones that you need to measure?
The gray card represents that shade of gray that the photo community decides is the center of the imaging grayscale. This card has a semi glossy surface that reflects away 18% of the incident light (incident from old French about to happen).
If you image a gray card using a camera and the exposure that is spot-on, the resulting image of gray card will replicate the density of the gray card. Further, if a print is made from this image the ray card is replicated. Thus the gray card is unique in that it is the only tone that when imaged replicates the gray card at both stages. Technically we measure the gray card using a photographic instrumentation called a densitometer. The measurement will be 0.75 density units. This corresponds to an asci value of 128. If these measurement is taken from a color image and a color print on paper, the readings will 0.75 Red – 0.75 Green – 0.75 Blue or asci 128 R – 128 G --- 128 Blue (the three light primary light colors).
The popularity of the gray card stems from the period when electric light meters became available in the 1930’s. Many photographers supported using the gray card as a target to obtain the correct camera exposure using a reflective light meter as opposed to taking measurements directly off the subject to be images. Both usages have merit as well as a measurement of the incident light falling on the scene. The bottom line, the gray card is just one more tool in our exposure finding inventory.
When it comes to post processing, the gray card value is, it has a known reflectivity thus how it images can be predicted with great accuracy. This the gray card can be help greatly when making evaluations and adjustments to an image. The size of the image of the gray card within the image is generally unimportant provided the measuring probe is able to take unadulterated readings, and the gray card image is large enough to allow visual evaluation. As to location and orientation of the card, both are important because the incident light playing on the card must have the same characteristics as the incident light playing on the subject.
How big does the gray card have to be in the frame?
It will vary from one camera to the next, but in general it needs to be anywhere from a significant part of the frame to all of the frame.
Do I maintain my camera position and try to zoom in as much as possible to the card being held in front of the store (which means it is very small in the frame)?
Or do I have the person holding the gray card come close to me, as long as the lighting on the card is the same as on the store?
Neither. You get close enough to the card being held immediately in front of your subject, take your reference shot, then back up to your desired shooting position to frame the shot.
I will use it exclusively as a reference for post processing. So if I am shooting a wide shot of a storefront, should the assistant stand right in front of the storefront with the card and I keep the camera position and try to zoom in? Or, if that means the gray card is very small in the frame, may I pick the camera up and get closer to the gray card?
It means the assistant stands right in front of the storefront and you get close enough to the assistant for the card to fill enough of your frame. Then you move back to your shooting position to frame the shot.
