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Exposure metering by the camera


3 years 4 months ago #703517 by AzrielY
Hi,
I take two pictures of the same exact object (say a picture on a wall) with same exact lighting conditions. First shot is with the lens set to 100mm focal length, then i step back to double the distance to the object and set the zoom to 200mm focal length so as to get the same exact composition as before. The camera sets the exposure for both shots at exactly the same value. And the images look identical.
HOW CAN THAT BE?
Doesn't that contradict the fact that light reflected by the object looses its intensity by square of the distance between the object and the camera sensor? I mean that's basic physics... So where did i get it wrong?  
,
3 years 4 months ago #703532 by Nikon Shooter
So, your understanding of the inverse square law is correct
but that doesn't means that something went wrong. There
are three values in the exposure triangle and one, or more,
MUST have change to adapt to the lighting conditions.

Check all three: SS, ƒ, and ISO.
Light is free… capturing it is not!
Photo Comments
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3 years 4 months ago #703706 by AzrielY
Nikon Shooter,

That's exactly my point: All three values remained exactly the same for both shots. SS 125, f4.0 and ISO 2500. The only thing different was the focal length and the distance to object.
I used Canon 5d M4 with Canon 70-200 f4.0 Lens. But i really don't think it is important which equipment you use. Its the principal that puzzles me.

Its a simple enough experiment, you can repeat it if you have a few spare minutes...
,
3 years 4 months ago #703754 by Ozzie_Traveller
G'day Azriel

I am not sure as to exactly what point you are trying to make here
As someone who has been using SLRs since the 1960s, I have been metering "millions" of frames over thousands of subjects ..........

It is quite feasible that a 'close-up' metering of an object will be the same as a 'distant' metering of the same object under many circumstances. Equally, it is quite normal to find that a 'close-up' metering of an object becomes different once one moves back from that object and the lens / camera starts to see other parts of the scene

Introducing the inverse-square-law does not arise here, as the objects being photographed are / seem to be / part of a wider scene illuminated from a light source (the sun?) ie- so far away that light falling on the scene appears uniform

Phil from the great land Downunder
www.flickr.com/photos/ozzie_traveller/sets/
Phil from the great land Downunder
www.flickr.com/photos/ozzie_traveller/sets/
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3 years 4 months ago #703756 by John Landolfi
What metering mode were you using? Matrix? Spot?
Photo Comments
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3 years 4 months ago #704550 by AzrielY
Hi Ozzie Traveler,

Thank you for your response.
I'm really not trying to make a point. Just seeking an answer to a question.
In both shots the viewing angle of the lens was adjusted so that the composition (in this case a picture on the wall) will be exactly the same, filling the entire aperture. So the image projected on the camera sensor is the same. Since i have doubled the distance to subject, and zoomed in, the view angle was reduced by approximately a factor of two. I would argue that in both cases the camera "sees" the same scene. Meaning that the light creating the image on the camera sensor has come from the same exact object. As the distance to the object was doubled, i would expect the light power to drop by a factor of 2 - in other words a hole stop of the exposure. But, it does not: the camera metering shows the same exact values for both shots and amazingly enough the images themselves look identically exposed! 
Not that this upsets my work in any way, i just find this puzzling.
By the way i used spot metering keeping the metered spot at the center of the scene in both shots.  
,
3 years 4 months ago #704551 by AzrielY
I have used spot metering, keeping the metered spot at the same place for both shots
,

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