Comparing Olympus 4/3lenses to FX "Full Frame" offerings

Started 6 months ago | Discussions thread
Ian Stuart Forsyth
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Re: Wrong
In reply to Tiger1, 6 months ago

Tiger1 wrote:

Ian Stuart Forsyth wrote:

Tiger1 wrote:

bobn2 wrote:

Tiger1 wrote:

bobn2 wrote:

That is a circular argument, since ISO is defined by exposure. Same ISO, same exposure but four times as much light on the FF sensor, four times as many photons, twice the SNR. It's that simple.

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Bob

The surface area is four times bigger Bob so what each unit area "sees" is the same amount of photons regardless the format. Hence the SAME SNR.

You've got yourself confused.

I am not confused.

Lets go through it one step at a time.

1. For the same f-number the amount of light per unit area projected on the sensor is the same.

We agree on something.

2. A FF sensor has four times the area as a FT sensor, so four times the 'unit'.

We agree here too.

3. Thus a FF sensor collects four times the light at the same f-number.

But that light is spread over 4 times the area....

Here’s your problem the intensity of the light remainders the same, it’s not spread over 4 times larger surface area but rather that 4 times larger area is being hit with the same light intensity capturing 4 times more light

I know. That is what I am saying! The intensity of light per unit area is the same...... I thought that I have made that obvious.

Then why would you say just above “But that light is spread over 4 times the area “ when responding to uncle Bobs “ thus a ff collect’s 4 times the light at the same F number”

Don’t you see the fallacy of say” but that light is spread over 4 time the area” when the intensity remains the same ?

4. When you view an image, you will do so at a size independent of the sensor size, the size you want to look at it. Imagine that we view in HD, approximately 2 megapixels. In the viewed image, each output pixel, one 2-millionth of the image represents the photons collected by one 2-millionth of the sensor.

When you view an image the quality you see is a measure of the number of pixels and the quality per pixel.

5. Thus each pixel of the output image from FF sensor represents four times the number of photons as that made from a FT sensor, assuming the same exposure (f-number, shutter speed and scene luminance).

You are only correct if the number of pixels equate the number of photosites AND that the photosites capture the 4 times the light than the smaller photosites. Photosites capture light, pixels DO NOT.

6. Hence the FF image will have double the SNR of the FT image.

You are being simplistic. SNR is dependent on the size of the photosite, its structure, its circuitry and the circuitry that takes its signal away to be processed. All of these things coupled with the way the signal is amplified will affect SNR. It's not a linear relationship to just size of photosite which you are assuming (incorrectly) is four times the size of that found in a 4/3 sensor.

The fact that smaller sensors tend to have smaller photosites so they can have the same number as larger sensors is what determines noise characteristics.

For what you say to be true, given that most noise in the image is photon shot noise, the smaller pixels would have to have a much lower quantum efficiency (collect a smaller proportion of the photons falling on them) than big pixels. This is not the case, they don't - in fact they typically have higher efficiency.

They may be more efficient per unit area but they are less efficient overall. Think about it. A large photosite collects more photons than a small one. Why else are big photosite sensors less noisy than smaller ones?

That is why their SNR is inferior.

Not so, I'm afraid.

You are wrong here.

Nothing to do with format. I'll even give you an example to prove this. Imagine you were able to take a Nikon Df chip and cut it into a 4/3 sensor and then put it into a 4/3 camera. The resulting image would only be 4MP but its noise and low light performance would be identical to the Df at 100% viewing of their pixels.

No-one in real life is interested in 100% viewing of their pixels. We are interested in what is the effect of a real photograph, and if you thing that a photograph taken from a FT size crop of a Df sensor and the resultant frame viewed same size as one taken from the full frame, would have the same SNR, you are mistaken, in fact it will have half the SNR.

You are changing arguments. I am talking about the SNR per pixel. Of course if you have the same quality pixel but have more of them you will achieve better over all image quality.

Simple.

But wrong.

Sorry, but you are wrong Bob.

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Bob

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The Camera is only a tool, photography is deciding how to use it.
The hardest part about capturing wildlife is not the photographing portion; it’s getting them to sign a model release

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The Camera is only a tool, photography is deciding how to use it.
The hardest part about capturing wildlife is not the photographing portion; it’s getting them to sign a model release

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