BetterPhoto.com - Become a better photographer today!
EMAIL:
PASSWORD:
remember me:     
     


BetterPhoto Q&A
Category: New Questions

To participate in the Forum, become a BetterPhoto member or Sign In.

 
Photography Question 
Bernard 

member since: 3/25/2005
 

Metering Off the Sky: Why?


Why meter off the sky, then re-compose on a ground-based subject? Thank you.

8/18/2007 1:37:07 AM

 
Bob Cammarata
BetterPhoto Member
cammphoto.com

member since: 7/17/2003
  Metering off a deep blue sky (with the sun at your back) will have an exposure value similar to that off a gray card. In-camera meters measure reflected light so even if the foreground subjects are in "perfect light", the exposure values may be different (... like if a black dog and a gray cat were frolicking in a patch of white flowers). Metering the sky in this case will expose the scene correctly ... as long as you keep the sun behind you.
Bob

8/18/2007 2:24:55 AM

 
Alan N. Marcus

member since: 3/4/2006
  Hi Bernard,

For most of the history of photography, exposure determination was trial and error. In the 1930’s, the electric light meter was just being marketed. At about the same time, Kodak research labs in Rochester (Messrs. Jones and Condit) concluded that the majority of scenes integrated to middle gray. Stated another way; if you examined a typical scene by optically scrambling the light, the tones and shades commingle to battleship gray. This was found to be about equal to a surface that reflected 18% of the ambient light. Kodak products were marketed in yellow boxes. One Kodak recommendation, as an aid to exposure determination, was to measure a Kodak yellow box placed in the scene with an electric meter. Setting a camera as indicated by the meter most often resulted in a near perfect exposure. This was OK for black & white work. The yellow box technique evolved to a gray card or placard advocated in 1941 by the famous nature photographer Ansel Adams and his cohort Fred Archer. Soon serious photographers were carrying gray cards and new fangled electronic light meters. Carrying a gray card can be a pain. Photographers are always looking for gray card substitutes to be found in nature. One can use blue sky or tree bark or parking meters and the like. Most agree, an actual gray card is best.
Meter makers have solved the problem. Some meters are of the incident type; they measure the light by pointing the meter back at the camera from the subject’s position. The result is exactly the same as a measurement off a gray card by reflected light. Modern cameras now incorporate chip logic into their metering circuit. These camera systems are so good that most of the time the camera with its logic and mode setting exceeds the accuracy of a hand-held meter, especially true if the user is not properly acquainted with metering techniques.
Alan Marcus (beware often dispenses marginal technical advice)
ammarcus@earthlink.net

8/18/2007 8:25:52 AM

 
Bernard 

member since: 3/25/2005
  Alan and Bob' I understand perfectly what you are saying, your answers have created another question, please bare with me. With the advancements achived in the chip and circuitry of the internal light meter of the D80, is it necessary to purchase an external light meter and gray card? again please bare with me. thank you.

8/18/2007 12:08:44 PM

 
Bob Cammarata
BetterPhoto Member
cammphoto.com

member since: 7/17/2003
  I've never felt the need for an external light meter for outdoor work except when I'm using flash (...with which I'm not real comfortable).
I do carry a small gray card in my pack for tricky lighting situations with slide films since the margin for error is so narrow.

More often than not though, you'll be OK seeking out mid-tones in the same light and metering there.
You can check the results as you go (...a definate plus with digital) and make minor adjustments if necessary.

Green grass, fall foliage and bright red objects are a few more to add to Alan's list that meter accurately.
The aforementioned deep blue sky works well for winter snowscapes, at high altitudes, or wherever the atmosphere is clean and free of haze.
Another trick for accurately metering snow and ice scenics in winter is to wear a bright red hat or scarf.
You can toss the piece of clothing into the scene to get your meter reading and as long as the rest of the scene is in the same light, the exposure will be correct.

Bob

8/18/2007 3:26:25 PM

 
Alan N. Marcus

member since: 3/4/2006
  Hi again Barnard,

First I am a firm believer that every serious photographer should have a quality light meter in the gadget bag. To me, the exposure meter is about the equivalent of a compass on a deep water sailing ship. The ideal meter would read both reflected light and incident light. The ideal meter would allow spot reading. The ideal meter would also serve to read light emanating from electronic flash. This gadget bag should also contain a gray card.

That being said: I would suggest that currently the vast majority of outdoor shots are exposed via the logic of the in-camera meter and its associated chip logic. I feel strongly that this method is likely all that is needed especially true when one can promptly see results on the digital camera’s LCD screen.

The light meter evolved around film technology. It stated as a light thermometer that loosely tied temperature to exposure. Next, actual film or paper was exposed to sunlight. The materials naturally blackened with exposure, no chemicals needed. Timing the blackening action allowed calculating exposure settings. Tables describing lighting conditions vs. camera setting deserve mention. The first meters were solar sells, no batteries needed. Next solid state and chip logic, we know no limits. Much of my career revolved around design of light meters used in color printing and more sophisticated instruments known as densitometers used to measure the blackening of films and papers, employed as quality control tools used to control film making and developing and printing. Now retired age 70.

Alan Marcus (dispensed questionable techno babble)
ammarcus@earthlink.net

8/18/2007 5:34:20 PM

 
Bernard 

member since: 3/25/2005
  Alan, Bob thanks, my next purchase will be a Nikon flash, a light meter, and gray card, my Dad knows nothing about photography, and the only way I got him to agree to dish out the funds for this equipment is for him to read these answers.

8/20/2007 12:30:16 AM

 
Rom A.G.

member since: 2/16/2005
  I am a bit confused by the replies. I guess the point of metering off the blue sky is to make sure it stayes blue in the photo, not grey or washed out.
Thus, you have to set EV+1 as shutter speed will be 1/1000th on a bright day, point camera at sky, then re-compose on subject.
I tried this trick on Bow bridge in central park. With the sun behind me, the bridge is too bright;however, metering off it makes the boats too dark. Problem solves using EV+1.

8/20/2007 6:13:29 PM

 
Bernard 

member since: 3/25/2005
  Rom , I'll add to that logic, be advised my addition is probably questionable tecno babble. seeing that the camara tries to render everything a neutral gray, if metered off the blue sky, woundn't the camara try to render the blue sky as neutral gray.

8/20/2007 8:07:14 PM

 
Alan N. Marcus

member since: 3/4/2006
  Hi Rom,
Exposure in this instance is the allotment of light energy delivered to the film or digital chip. Gross over or under exposure results in a spoiled picture. Somewhere in-between is a span that will yield an acceptable picture. This span dictates how tones will reproduce. As an example human skin can be reproduced too dark or to light. A skilled photographer can measure scene brightness with a light meter and adjust the camera setting to obtain a suitable skin tone. Such adjustments and setting are not limited solely to a skin tone. A skilled photographer can apply exposure control and exercise control over the rendering of memory colors (tones) such as snow, or sand, or water, or sky, or the like.

Logic would tell you that the center tone we call middle gray should be replicated by a surface with a 50% reflection. However, the human eye brain tends see in tone steps that are not equal. As a result an object with an 18% reflectance appears to us as middle gray. See the works of Albert H. Munsell.

As to exposure theory: If a scene in nature is exposed so that an 18% is rendered correctly, all other tones will be rendered correctly by law (law of reciprocity (Hurter & Driffield). This is true if the tones do not exceed the range of the film or chip (dynamic range). Now the
light meter is calibrated to properly render a gray card with a surface reflection of 18%.

Colored objects like oranges or sky or lemons, are identified first by hue (red, yellow, green, purple-blue and red-purple). Then by value or scale of reflectance i.e. dark, medium, light etc.

Now in this instance we are trying to render the sky correct as well as most all other tones. We know that if we had a gray card and a correctly calibrated meter we could measure the gray card, set our camera as indicated and voila a correct exposure results. In lieu of a gray card we hunt for a substitute. We can choose blue sky, not because it is blue but because its value (intensity) is about at the middle of the scale thus it can serve as a suitable substitute.

Alan Marcus (often dispenses questionable techno-babble)
ammarcus@earthlink.net

8/20/2007 11:18:56 PM

 
Bob Cammarata
BetterPhoto Member
cammphoto.com

member since: 7/17/2003
 
 
  Puff...Puff
Puff...Puff
Arches NP, Utah

Nikkor 180 mm, Provia 100...@4:00 pm
© Bob Cammarata
cammphoto.com
Nikon FM2 Manual E...

 
 
"I am a bit confused by the replies. I guess the point of metering off the blue sky is to make sure it stayes blue in the photo, not grey or washed out."

A deep blue sky WILL stay blue and whites will stay white...not blown out.
In the attached example, the scene was metered off the sky without the clouds in the frame. When I re-composed, I noticed that the red rock monolith metered exactly the same with the sun behind me.
(Keep in mind though that metering a LIGHT blue sky will result in under-exposure of the entire scene.)

"...seeing that the camera tries to render everything a neutral gray, if metered off the blue sky, woundn't the camera try to render the blue sky as neutral gray?"

Yes,...and it will with black and white film.

Bob

8/20/2007 11:47:02 PM

 
Rich Collins
BetterPhoto Member

member since: 6/24/2005
  "I am a bit confused by the replies. I guess the point of metering off the blue sky is to make sure it stayes blue in the photo, not grey or washed out."

A deep blue sky WILL stay blue and whites will stay white...not blown out.

And...."...seeing that the camera tries to render everything a neutral gray, if metered off the blue sky, woundn't the camera try to render the blue sky as neutral gray?"

Yes,...and it will with black and white film.

This is a good thing, the intent, yes?

8/21/2007 6:49:33 AM

 
Nancy Donnell
BetterPhoto Member

member since: 3/23/2004
  Hi, Do you guys have time for another part to this. You say to meter off the sky, then recompose, but what if the sky is pale bluish grey, not blue? I know to put on a polarizer, somtimes that helps, but is there a rule of tumb in this instance?

I am guessing to just find something mid grey,(18%) or the red scarf, or green grass or my hand, then recompose? But is it ok NOT to do this in manual, just get a reading then recompose if I am in AP, or another mode?
Thanks

8/21/2007 7:43:32 AM

 
Marianne Fortin
BetterPhoto Member

member since: 1/23/2006
  Bernard, you might wish to read "Understanding Exposure" by Bryan Peterson. This is one of the most popular books on the subject and answers your question very well.

8/21/2007 8:18:43 AM

 
Alan N. Marcus

member since: 3/4/2006
  Hi Nancy,
Most advise surrounding the use of the light meter attempts to expand on the instructions provided. In general meters see the world with an angle of view equal to about 60°. This angle is selected because it matches the angle of view of a camera mounted with a normal lens (not wide angle not telephoto). When such a meter is pointed from the camera position, it reads and averages what it sees. This is OK for average scenes however the possibility is high that it will error if the scene is unusual. The idea of restricting the meter to read only a gray 18% reflective surface is sound. The premise is; the gray card receives the same light intensity as other objects and if the camera is sets to properly render the card, all other objects will fall correctly by law. The idea that a sky or other objects can be substituted for a gray card is also sound provided the photographer can choose fitting targets. Pale blue sky can have the same intensity and may work just as well as a rich blue sky. So too red scruffs or green grass can serve as gray card substitutes. The human hand generally reads one f/stop more reflective than a gray card. We are not choosing objects based on color, we are choosing based on intensity.
Now consider: As time marched on, the cameras received brains via chip logic and sophisticated light sensors. Additionally the camera is viewing the scene through a lens and the sensors are through-the-lens looking devices. Additionally, mode settings add logic by altering what portion of the scene will be metered and the weight to which the data will be applied. These are now considered to be spot measurements that play on a carefully selected grid pattern. You are well advised to take advantage of these marvelous innovations. Also keep in mind that camera makers stake their reputations on the superior abilities of their camera models. To this end they pack the camera with “pixie dust”. This is the new magic that will win every time over the not so suffocated user and his/her hand held meter.
Alan Marcus (spot on techno babble this time)
ammarcus@earthlink.net

8/21/2007 10:45:07 AM

 
Nancy Donnell
BetterPhoto Member

member since: 3/23/2004
  Hi Alan,
Thanks so much for the in depth answer. I really appreciate it! Ahh..pixie dust.. I thought it was pixel dust. :-)
Nancy

8/21/2007 5:24:10 PM

 
Bernard 

member since: 3/25/2005
  marianne' speaking of books, one of the two photography books I received this year is Understanding Exposure" by Bryan Peterson, I only made it up to page 85 so far, I'm going to skip ahead to page 114. and to think I put Alan through all that work. the other book is "the art of photography by Bambi cantrell (portraits), I can't remember if she stated why she shoots mostly in aperture priority. ALAN WHEN IS YOUR BOOK DUE OUT? I'll purchase it for sure.

8/21/2007 9:50:36 PM

 

To participate in the Forum, become a BetterPhoto member or Sign In.
 

Copyright © 1996-2014 BetterPhoto.com, Inc.® All Rights Reserved.