Change of exposure with magnification

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Originally Posted by george013

Call it something like the inverse inverse square law. The sensor needs a total amount of light deliverd by the surface of the view. If you half it, then you need twice the time.

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Originally Posted by george013

The so called effective f-stop doesn't explain anything. It's a tool to use. The explanation is on the other side of the lens, the subject side. If the surface of the view is getting smaller, then the amount of reflecting and thus captured light is also getting smaller. Thus to gain the same exposure, your ev must be larger.
There is also another method to make the view smaller, that's zoom in. But don't forget then that when you zoom in, the diameter of the diafragma is growing too. The f-number stays constant.

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No.

A SCENE viewed through the Viewfinder comprises REFLECTED Light from the SUBJECT. This is what the TTL Light Meter reads.

If the SUBJECT's ILLUMINATION remains constant, then the EXPOSURE required also remains constant irrespective of the CAMERA VIEWPOINT. This is so for any shooting scenario EXCEPT for MACRO work - that's what the OP's question is about.

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Maybe this practical example will explain why your supposition is incorrect.

Consider taking a photo of a nude who is under constant illumination and we make a meter reading for correct exposure of the skin tone.

The camera's exposure settings will remain the same no matter if we make an HEAD SHOT or an HALF SHOT or a FULL LENGTH SHOT or a VERY LONG SHOT.

BUT - if we move in very close and begin to make a Close-up Shot and then a MACRO SHOT of a small portion of skin, then we will note that we need to change the Exposure Settings to record the skin tone correctly, even though the Skin's illumination has not changed - this is what the OP's Question is all about.

WW

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Radiance is independent of distance for an extended area source, because the sampled area increases with distance, canceling inverse square losses.

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I think this quote, which Ted had in his last post, is probably the key to George's misunderstanding. George's statement ignores the inverse square law. I am not a physicist, but I think George's assertion would be approximately correct if the light source were a laser (so the light would not spread out appreciably with increased distance) with a beam wider than the largest image in question. Not a circumstance I run into very often when taking pictures.

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Originally Posted by DanK

I think this quote, which Ted had in his last post, is probably the key to George's misunderstanding. George's statement ignores the inverse square law. I am not a physicist, but I think George's assertion would be approximately correct if the light source were a laser (so the light would not spread out appreciably with increased distance) with a beam wider than the largest image in question. Not a circumstance I run into very often when taking pictures.

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Another way to look at it is that the 'extended source' such as your monitor screen set to some monochromatic color, consists of a satisfyingly large number of point sources (screen pixels) - all busily obeying the inverse square law 'cos they are point sources. As the camera is moved back (without messing with the exposure) more and more point sources are brought into the frame. For twice the distance, the light from each point source is reduced 4 times. But the sensor is now seeing 4X the point sources, thereby exactly balancing that reduction.

A perfect 'extended source' would have an infinite number of points (ignoring quanta) but the principle remains the same.

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radiance is independent of distance for an extended area source, because the sampled area increases with distance, canceling inverse square losses.

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Originally Posted by dank

i think this quote, which ted had in his last post, is probably the key to george's misunderstanding.

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+1

ww

First of all, I'm struggling a bit with light like all of us here. Would it be water the there wasn't any problem with me.

Second, nobody, including me, is denying the needed exposure adjustment when doing close macro. The question is only why. The tutorial here doesn't give an explanation.

I'll think about it today and coming back, for sure. I don't think Ted's post does answers the question.


George

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Originally Posted by george013

First of all, I'm struggling a bit with light like all of us here. . . .

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Speaking for myself I am NOT struggling with the concepts articulated in this thread.

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Originally Posted by george013

Second, nobody, including me, is denying the needed exposure adjustment when doing close macro. The question is only why.

The tutorial here doesn't give an explanation. I'll think about it today and coming back, for sure. I don't think Ted's post does answers the question.

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In respect of your posts #12, #15 & #18 and the responses #14 #16, #17, #19, #21, #22 and #24:


Although you were asked to present Data or Practical Examples or Mathematics to support your claims -you do not yet outline the REASONS why you consider your assumptions to be correct – or conversely accept that they are flawed -

THEREFORE one remains perplexed as to specifically what you do – and – what you do not understand.

The CiC Tutorial clearly articulates (to me) in simple lay terms, a clear understanding as to why the "effective" focal length changes - and - I already knew the theory, anyway.

The ancillary comments on this thread add various confirmations by way of both practical and mathematical examples.

Reading the tutorial (the whole document) provides one a very comprehensive explanation.

Maybe reading the “Light Measurement Handbook” by Ryder will also assist you to clear up the confusion which you seem to have concerning how TTL LIGHT METER works when reading a SCENE and how a Digital Sensor works in respect of SCENE ILLUMINATION and EXPOSURE SETTINGS

Sorry, but at this point I am at the end of the tether to submit any further explanations.

WW

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Originally Posted by george013

First of all, I'm struggling a bit with light like all of us here.

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Like William I am not struggling with light at all.

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Would it be water then there wasn't any problem with me.

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OK, here's a hydraulic analogy for us to not understand and argue with:

A wall is punctured with many small orifices designed such that water under pressure is sprayed out with the flow lines dividing angularly, possible obeying a cosine rule. A surface of known area is mounted close to the wall at a known distance. The water mass flow m-dot through the surface is measured. The surface is moved to twice the distance away. m-dot is measured again has not changed.

As a self-professed expert in hydraulics, you will be able to state many reasons why the above is not precisely accurate, but you should agree that it is correct when used as an analogy, i.e. it is correct in principle.

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Second, nobody, including me, is denying the needed exposure adjustment when doing close macro. The question is only "why?". The tutorial here doesn't give an explanation.

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The majority here find the tutorial satisfying.

I haven't read it myself because I already knew everything about macro ;) . (That was a joke, George . . .).

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I'll think about it today and coming back, for sure. I don't think Ted's post does answers the question.

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Correct, my post did not answer "the question" if, by that, you meant the question posed in the OP - which included focus adjustments necessitated by a changing distance.

Myself, I was not accounting for focus changes. I made that perfectly clear in my second post starting with "Aha". However, your argument with me is about the content of my posts, not the content of the original post in this thread.

Waiting impatiently for your comeback which, hopefully, will include something of substance . . . .

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Originally Posted by William W

Maybe reading the “Light Measurement Handbook” by Ryder will also assist you to clear up the confusion which you seem to have . . .

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Just in case:

http://kronometric.org/phot/xfer/lig...20handbook.pdf
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Well. if everybody has laughed enough, I know what I wanted to know.

The question was why to change the EV with macro at 1:1. I diidn't read the answer here in this thread. But I found it.

George

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Originally Posted by george013

Well. if everybody has laughed enough, I know what I wanted to know.

The question was why to change the EV with macro at 1:1. I diidn't read the answer here in this thread. But I found it.

George

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Good.

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Originally Posted by xpatUSA

Just in case:

http://kronometric.org/phot/xfer/lig...20handbook.pdf
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:rolleyes: Why not switch the camera to automatic if people can't cope with setting and exposure to suit the shot. That oddly happens in several camera modes automatically because exposure is set to suit the sensor and hopefully avoid clipping - magnification or not. ;) Or of course everyone could go back to using exposure meters where it may matter.

John
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Originally Posted by ajohnw

:rolleyes: Why not switch the camera to automatic if people can't cope with setting and exposure to suit the shot?

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Because the selector on my camera is seized in the Manual position, John. Probably needs WD40 . . . ;)

P.S. I realize the question wasn't aimed at my good self . . . hopefully :)

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Originally Posted by xpatUSA

Because the selector on my camera is seized in the Manual position, John. Probably needs WD40 . . . ;)

P.S. I realize the question wasn't aimed at my good self . . . hopefully :)

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Definitely not Ted. :D Try a smart blow with a hammer.

:( I've just ordered a flash meter. So my mind is part on the problems of external exposure metering. :eek: :rolleyes: It's just got to be better than TTL.

John
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