In article , Eric Stevens wrote:

Sandman:
There's a difference between "exposure" and "equal exposure". An
exposure is a given amount of light in a given unit area. With a
smaller sensor, you create an equal image by making an equal
exposure, meaning more light on that smaller area.

I'm breaking my vows but I sense progress. You may be saying that
the amount of information carried to the camera depends upon the
number of photons which reach it. From this it follows that the
small sensor has to handle the same amount of light as the larger
sensor. Have I got it right?

Pretty much. In order to create an equal exposure, the smaller sensor needs the
same amount of total light (equal exposure), not the same amount of light per
unit area (same exposure).

Sandman:
Those that care about noise, since less light means
more amplification and more noise.

Eric Stevens:
Why should there be more noise?

Sandman:
Amplification leads no noise. Always.

Noise is always generated in a camera.

As a product of the signal amplification, i.e. the ISO setting.

Eric Stevens:
Providing everything has been kept in proportion as the camera
has been scaled up or down, the light intensity on the sensor
remains the same irrespective of the size of the sensor.
Lighting of the sensor remains te same. Sensor remains the same
...

Sandman:
If the amount of total light is the same, the exposure is equal,
but not the *same* exposure (see above).

It's this sort of statement which makes it hard to follow exactly
what you are saying. James defines exposure as (see below) "The
total light per area (photons / mm²) that falls on the sensor while
the shutter is open". In the scaling up/down situation, if the total
light is the same, the total light per area (photons / mm²) cannot
be the same.

Exactly.

"Same exposure" = "Same amount of light pr unit area"
"Equal exposure" = "Same amount of light on whole sensor"

Making an equal exposure means you're not using the same light per unit area.
This is what I have been saying the entire time. You need to give the smaller
sensor the same amount of *total light*, not the same exposure.

This is the same exposu

FF: 1/250, f5.6, ISO 800
MFT: 1/250, f5.6, ISO 800

That will create two images that are equally bright. But they are ONLY equally
bright because the MFT sensor has amplified its signal MORE than the FF sensor
has, since it has received less light.

This is an equal exposu

FF: 1/250, f5.6, ISO 800
MFT: 1/250, f2.8, ISO 200

Now we have given the MFT sensor the same amount of *total* light, not light
per unit area. This means that during the exposure, it has received as many
photons totally as the FF sensor (using an equivalent aperture). To balance
this, we have to adjust the ISO by the crop factor squared, to amplify the
signal LESS. This means that at ISO 200, the MFT sensor is amplifying the
signal equally to the FF sensor at ISO 800.

This tells us a couple of things:

1. "ISO" says nothing about sensor amplification (called "sensitivity" to most
people) when comparing between sensor formats

2. Smaller sensors aren't noisier, unlike what most people think. They just
need the same amount of light to create an equal image

Sandman:
Using the same amount of light, you adjust the ISO down by the
crop factor squared, which means the signal is amplified the same
amount (give or take, sensor tech).

I understand you better now that I know that by 'brightness' you
don't mean the density of the light which falls on the sensor, but
instead the amplified output of the sensor. At least James does. Do
you mean that also?

I have only ever talked about brightness in relation to the generated image.
I.e. I have mentioned several times that the ISO setting is not so much a way
to know how sensitive (amplified) the sensor is, but rather what brightness
level to expect.

So yes, "Brightness" has nothing to do with exposure (other than being a result
of it). It is how bright the image created by the camera is. Brightness is
exposure + amplification.

Eric Stevens:
The point is that the amount of light falling
on each square millimetre (or each square inch) remains
exactly the same. As far as the sensor is concerned there
has been no change.

Sandman:
Not sure what supposed "change" you are talking about
here? Changing the size of the sensor means that comparing
"exposure" falls apart.

Eric Stevens:
The behaviour of a small patch of sensor is not affected by how
much more sensor there is around it.

Sandman:
Of course not, but on a smaller sensor, that "small patch" is a
part of the whole image that is rendered by the camera. On a
larger sensor, the same region of the resulting image is made out
of a larger "patch" of the sensor, which receives more light. So
there is more signal going in to that part of the sensor that
makes out that part of the image.

There comes a point where the message has got through, no matter how
many extra photons you have poured through.

Yes, but the fewer the photons, the "weaker" the message, and without more
amplification, it will not be as bright.

Eric Stevens:
Certainly the sensitivity is not affected.

Sandman:
No, but to create an equally bright image, the smaller sensor
needs to amplify the signal more, since it has received less
light to being with. I'm not sure how many times I've said this
in various forms.

And the statement seemed meaningless, mad or daft before I realised
what you meant by brightness.

Well, good we got that sorted out, then.

Eric Stevens:
As I almost said "the point is that the amount of light falling
on each square millimetre remains exactly the same. As far as
the sensor is concerned there has been no change." Certainly it
needs neither more nor less amplification than it did before".

Sandman:
Take it to the extreme, then. You have two sensors. One is
100x100mm and one is 10x10mm, both are 10MP sensors. Using the
same exposure, the smaller sensor will receive one hundredth of
the "signal" as the larger sensor.

Both generate a 10MP image out of this signal, but the smaller
sensor can't possibly make it as bright as the larger sensor
without amplification. And that amplification creates noise.

It is likely that even at their native ISO both sensors will require
amplification.

No, that's not "likely" at all. That would only be "likely" if you knew exactly
how many photons that were transmitted, which wasn't a factor in the input.

It is also a big jump to assume that noise is
proportional to amplification,

You have different kind of noise, of course. Usually grouped together by
"photon noise" (different kind of noise from how the light hits the sensor,
travels through the lens etc) and "read noise" (noise created by the sensor and
hardware). But even though photon noise affect the end result, the read noise
is usually a lot higher and affects the resulting image much more.

Sandman:
Sandman:
http://www.josephjamesphotography.com/equivalence/#8

"For a given scene, perspective, and framing, the total light
depends only on the aperture diameter and shutter speed (as
opposed to the f-ratio and shutter speed for exposure).

Eric Stevens:
That's fine, if you are talking about a porthole letting light
into a room. But we are talking about a lense which takes light
from a series of point sources in limited external area and
focusses that light onto a series of point images on the sensor.

Sandman:
Yeah? Do you have a point?

Providing everything has been scaled correctly, the intensity of
that point of light on the sensor remains the same.

But that's the thing, using the same exposure *doesn't* scale everything
correctly.

Using a 25mm/f1.4 lens on MFT is usually called "equivalent" to a 50mm on FF,
but it's actually equivalent to a 50mm/f2.8 if scaled "correctly".

So:

FF: 50mm, f2.8, 1/250, ISO 800
MFT: 25mm, f2.8, 1/250, ISO 800

Will result in the *same* exposure, i.e the same amount of light per unit area.
But:

FF: 50mm, f2.8, 1/250, ISO 800
MFT: 25mm, f1.4, 1/250, ISO 200

Will result in an *equal* exposure, meaning you get the same amount of total
light through the lens on the sensor, you get the same depth of field, you get
the same signal amplification and the same field of view (i.e. collecting the
same scene).

This is what I have been saying for the last two weeks.

The image is constructed by the array of those points of light on
the sensor. Despite the fact that there are fewer photons finding
the way to the smaller sensor the intensity of the light (what I
thought you meant by brightness) remains the same across all sizes
of cameras.

*only* if you adjust the exposure as outlined above.

If that is the case, and I believe it is, all sizes of
sensors will require the same amount of amplification to reach the
same brightness as defined by James.

Absolutely, which is what I've said this entire time.

Eric Stevens:
I assume you have been using the terms in the same way as James,
although you have not previously made that clear.

Sandman:
I have said so many many many many many times.

Yes you have said that about total light many many times but you
have never explained it. At several points in this thread I
variously enquired after or taken you to task over the terms you
have been using and you have responded by accusing me of semantic
trolling and similar. It wasn't semantic trolling: it was me trying
to find out *exactly* what you were trying to tell me.

I "accused" you of semantic trolling when you cut out the context and claimed
that you can't change the sensitivity of the sensor. Which, while technically
true, was beside the point. A lot of people call the ISO setting on the camera
as a way to change the sensor sensitivity. If you hadn't cut out the context,
and just made a remark but signaled that you understood what I meant even so,
then you wouldn't have been trolling.

Eric Stevens:
His important definitions a

"Exposu The total light per area (photons / mm²) that falls
on the sensor while the shutter is open, which is usually
expressed as the product of the illuminance of the sensor and
the time the shutter is open (lux · seconds). The only factors
in the exposure are the scene luminance, t-stop (where the
f-ratio is often a good approximation for the t-stop), and
the shutter speed (note that neither sensor size nor ISO are
factors in exposure)."

Sandman:
Indeed.

Eric Stevens:
Here is a trap:

"Brightness: The brightness of an image (what people usually
think of as "exposure" -- same units as exposure): Brightness
Exposure x Amplification."

Sandman:
Exactly like I've said.

But you have never explained that by 'brightness' you meant the
output of the amplifier.

I was not aware that it was unclear. This is the first time I used it:

Sandman
How to measure ISO
11/11/2015

"ISO was created decades ago measures amount of light
gathered per square inch on film. On a smaller sensor, ISO
breaks down because there are less such "inches". Using the
same ISO setting may result in similar brightness, but much
higher signal to noise ratio.

To get the same picture with two different size sensors, you
need to adjust the total amount of light, which ISO can not
do, it can only handle the amount of light given to it.

Smaller sensors needs a lower ISO to gather the same amount
of light as a larger sensor."

Note the date, that was 19 days ago.

This is the very first time I wrote it in a reply to you:

"A smaller sensor needs a higher sensitivity to produce the
same level of brightness as a larger sensor, meaning that
ISO 400 on MFT is more sensitive (more amplified) than ISO
400 on FF."
/ Sandman- 11/14/2015

Note the "to *produce* the same level of brightness".

To my knowledge, I have only ever used it in relation to the result, i.e. the
generated photo.

Eric Stevens:
"Total Light: The total number of photons that falls on the
sensor (lumen·seconds, or, equivalently, photons): Total
Light Exposure x Effective Sensor Area."

Sandman:
I.e. exactly what I've said.

I think you will find you have talked about 'total light' only once,
im message

Of course not, I've talked about this many many times:

Sandman
11/11/2015

"With less total amount of light, the signal to noise ratio
differs between sensor sizes, meaning that ISO 200 on MFT
has the same s/n ratio as ISO 800 on FF."

Sandman
11/14/2015

"Now, adjust the ISO by the crop factor squared, AND give
each sensor the exact same *total* amount of light:"

Sandman
11/14/2015

"So suddenly, higher pixel density isn't noisier at all.
because if you give the sensors the same amount of *total*
light, the noise level is more or less the same. So with
the same total amount of light, the end result is roughly
the same."

Sandman
11/15/2015

"Each square cm is hit by 10 photons in a given exposure. The
smaller sensor is hit by fewer *total* photos, regardless
of how many photo sites it has. It could be a 10MP MFT or a
24MP MFT, it matters not. The total amount of light that
hits the sensor is what is different."

And so on, and so on. A quick search found more then thirty occupancies of me
talking about the total amount of light. I didn't want to list them all here.

Eric Stevens:
"Many of the misunderstandings come from people using different
definitions for the same words. In particular, "f-ratio" is
often confused with "aperture", and "exposure" is confused
with "brightness" and "total light". The importance of these
distinctions is often overlooked or simply not understood,
... "

Sandman:
Yes, these are some of the terms you and nospam have repeatedly
misunderstood, I agree.

And you consistently failed to explain *exactly* what you meant to
many more people than me and nospam.

Maybe, and that's on me, for sure. But there's a fair level of stubbornness in
play here as well, especially when it comes to nospam. I.e. there is nothing
really "new" brought to the table in these last posts than haven't already been
said by me many many times. You have ignored the support, snipped the support
or just flat out denied the support without offering up a reason
(substantiation) for why the support isn't valid.

Eric Stevens:
B. It has been assumed that noise is inversely proportional to
photons received: the more photons the less noise. To describe
this as a hairy approximation is a gross understatement, but no
matter.

Sandman:
Noise has nothing to do with amount of photons. It has everything
to do with amplification.

Which depends on the number of photons received.

Only if the number of photons require amplification.

Eric Stevens:
C. All this pseudo-mathematics is centred about an attempt to
set out the rules relating the properties of cameras of
different sizes which produce not only the the equivalent
optical geometry but also equal image noise.

Sandman:
I.e. creating equivalent images using different sensor sizes.

And lens sizes also.

Indeed. I talked about that above as well.

Eric Stevens:
In the real world their buckets will not be four times as large
so will be overflowed, with resulting highlight burnout.

Sandman:
This is a false assumption. We are talking about a MFT sensor that
has amplified its signal four times as much as a FF sensor to
create a "normal" photo. Lowering the ISO and opening up the
aperture will result in the exact same photo, only with the exact
same noise as the FF camera.

Here is something you can't prove. It is probably an assumption.

I have proven it, remember:

http://jonaseklundh.se/files/iso_adjusted.png

Eric Stevens:
E. Nobody operates a camera so as to obtain constant noise so
the conclusions, while interesting, are irrelevant to the real
world.

Sandman:
It's very relevant to a number of things:

1. The myth that smaller sensors are noisier, they aren't.
2. The myth the "ISO" is a certain "sensitivity" (amplification),
it is not.

The concept that ISO is a certain amplification is a myth (I hope).

Yes, that's what I just said

Sandman:
3. The problem with using "35mm equivalent" terms for the focal
length but not for aperture to mislead users.

Look up the definition of f/ number (and think about it).

No need, the f-number is a physical function of the lens, but the f-ratio is
what is important when making equivalent comparison between cameras. See above.

--
Sandman