Is my thinking about equivalence right?

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tammons Veteran Member • Posts: 6,581
Re: Is my thinking about equivalence right?

SrMi wrote:

Olymore wrote:

Based on that argument the image captured by my 20mp phone needs enlarging the same as my m43 20mp image if I am viewing the results at the same size, which is not correct.

The size of the image captured will determine how much enlargement is required to view the image at a given output size.

What does it mean in practice?

Hypothetically, if I shoot a scene with m43 at f/2.8, 1/100, and FF at f/5.6, 1/100, both 20MP, what kind of theoretical difference should I expect, assuming the necessary enlargement is the only factor?

Not much.

Iliah Borg Forum Pro • Posts: 28,755
Re: Is my thinking about equivalence right?
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rogerstpierre wrote:

Never said that IQ would be similar.

In that case you are comparing different images and it's meaningless in the context.

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bobn2
bobn2 Forum Pro • Posts: 69,811
Re: Is my thinking about equivalence right?
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SrMi wrote:

Olymore wrote:

Based on that argument the image captured by my 20mp phone needs enlarging the same as my m43 20mp image if I am viewing the results at the same size, which is not correct.

The size of the image captured will determine how much enlargement is required to view the image at a given output size.

What does it mean in practice?

Hypothetically, if I shoot a scene with m43 at f/2.8, 1/100, and FF at f/5.6, 1/100, both 20MP, what kind of theoretical difference should I expect, assuming the necessary enlargement is the only factor?

If your lenses project the same MTF (in cycles per millimetre) onto the sensor, which I assume must come along with 'enlargement being the only factor', then if you enlarge them to the same viewing size, then the FF image will look sharper. But that's where we started from....

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Anders W
Anders W Forum Pro • Posts: 22,144
Re: Is my thinking about equivalence right?
2

Muster Mark wrote:

Hi all, I realize this is NOT the topic everyone loves to discuss and I totally understand. I hope no-one feels annoyed by this post. That said, after some investigation I have come to the conclusion that "traditional" equivalence math is only useful as a rough heuristic to get a sense of what DOF will be and NOT a reasonable way to compare individual lenses or judge cost/performance of lenses in different systems. I wanted to see what other geeky 4/3 sensor users think and if I am missing something in my calculations. I should add I fully understand the traditional argument that total light is 4x on FF camera (for example) at the same aperture since we need to multiply (integrate, actually) brightness by the sensor size to calculate S/N. This idea only works if you have an IDEAL lens and an ideal sensor. Lenses are never ideal though, so I wanted to dig deeper.

For the sake of simplicity let's limit ourselves to looking at "normal" lenses: e.g. fov roughly 1 radian. This FoV tends to "favor" larger formats as far as I can tell especially if the goal is to maximize bokeh-ball size/$. Furthermore, to limit permutations I only looked at examples of lenses for m43 and Sony E mount, specifically trying to find an "equivalent" lens to the M.Zuiko 25mm f1.2

My analysis boils down to 2 points:

1: No two lenses are actually "equivalent". This seems obvious. There is more to evaluating a lens than pupil diameter and FoV. People seem to forget this when comparing between systems.

2: Talking about "total light" without accounting for vingetting is either lazy or disingenuous. I am guessing this is 100% laziness as calculus is annoying. However FF lenses have much worse vingetting than m43 lenses for obvious reasons. Is this a hugely important aspect to image quality? No not really, IMHO, it's pretty easy to correct in post if you have the lens profile, but you are losing some of the signal to noise ratio advantage in the process. How much? that's what I wanted to figure out. By my calculation (of course it depends on lens a huge deal) FF lenses at 50mm lose about half a stop of light to vingetting MORE than the 25 f1.2. Specifically, the Sony GM 50mm 1.2 at 1.2 gathers about 2.9x the total light gathered by the olympus 25mm 1.2 (at 1.2). Is this a huge deal? No probably not, but it's not quite as impressive as the 4x factor everyone is repeating is it? I just thought it was interesting, mainly.

Calculating total light (for point 2): This is pretty handwavey, but after squinting at the data on lenstip.com, and specifically the level curves for brightness I thought it looked remarkably parabolic. Thus I approximated the brightness function of a lens wide open as a decreasing parabaloid -(x^2+y^2)K, where K is a constant that depends on the specific lens (I compared contour plots of these functions to the level curves provided by lens tip and they are remarkably close). When comparing between lenses of different aperture, there is a shift applied: e.g. approximating 1.4 as half a stop slower than 1.2, we would shift by -1/2. We then exponentiate and integrate over the frame. For these calculations I am assuming Fstop equals Tstop. Which is not correct, though lenses with similar numbers of elements will not have much difference in T stop I don't think (am no expert!). I couldn't find information on T stops for a lot of lenses which is why I left it out of consideration. I also couldn't find information on how it's measured (are they integrating total lite or measuring intensity at the center of the frame?) which further complicated trying to incorporate it.

Comparing lenses holistically: When more is taken into account than just pupil diameter, it seems to me at least, m43 lenses are not overpriced. For example the 25mm1.2 is optically pretty amazing when it comes to corner to corner sharpness, longitudinal CA correction (which is hard to correct in post, if my understanding is correct) and bokeh quality (not size).

For half the price one could use the Sony 50mm 2.5. This amounts to being about half a stop dimmer due to vingetting, the bokeh while the same size is much busier. Longitudinal CA is only a smidge worse though. Oh and yeah if you want a sharp full frame you need to stop down to f4 (the 50mm 1.8 by sony is not really worth looking at if optical performance is something you care about).

The Sony GM 50mm 1.2 lens performs very well when stopped down (and is pretty good wide open), but now we are back to being much heavier and costlier than the olympus. I am NOT saying it's a rip off, but I also don't think the lens needs defending as no one seems to doubt it.

The Zeiss 55mm 1.8 has lovely bokeh, but bad Long. CA and needs to be stopped down to 2.8 to be sharp.

Sigma ART lenses start to really shine honestly. Generally very well corrected optically, much less vingetting on FF than other lenses when wide open, and even slightly cheaper priced to the olympus. They are significantly bigger though (the 50mm 1.4 is 815g, versus 410g for the oly).

So what do we make of all this? My take away is that there is no free lunch. All lenses are compromises and on a system-wide level nobody is scamming anyone. If size of the bokeh balls is not your biggest concern, and you care about system size/weight and optical performance, m43 makes a lot of sense. I guess you already know that

I personally am shopping around at the moment (trying to decide if I want to adapt old 4/3 glass to m4/3 bodies, or buy new lenses and potentially change systems or buy new m4/3 lenses) and thus did this analysis to try to compare systems more accurately. Hopefully it was moderately interesting?

If there is anything I should be accounting for that I am not (and you know how I might, e.g. t stops) I'd be curious to hear.
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To me equivalence simply means that a lens design with focal length x shot at f-stop y on MFT has the same effect as the same design scaled upwards to focal length 2x and shot at f-stop 2y on FF with regard to angle of view (diagonally), light collection, depth of field, and diffraction. It means no more and no less.

This is useful knowledge in the field when you move from one format to another and want to translate your past experiences to the new format. It is also useful when comparing lenses across image formats, e.g. for the purpose of finding out which format is best suited to your preferences.

Can’t recall anyone who made the mistake you mention, i.e., to think that this was all you would need to consider when comparing two specific lenses designed for different formats.

Additionally, I have no idea what you have in mind when you say that FF lenses have more vignetting and that this is so for obvious reasons. As a matter of fact, if you take an MFT lens design and scale it up to FF, the two lenses will perform identically with regard to all optical aberrations when shot at the same f-stop on the respective format. The only differences will be those following from the principle of equivalence as described above.

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Olymore
Olymore Senior Member • Posts: 1,868
Re: Is my thinking about equivalence right?
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And presumably any lens aberrations will also be enlarged to a greater degree for the smaller image capture.

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bobn2
bobn2 Forum Pro • Posts: 69,811
Re: Is my thinking about equivalence right?

Olymore wrote:

And presumably any lens aberrations will also be enlarged to a greater degree for the smaller image capture.

Exactly.

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RSTP14 Veteran Member • Posts: 5,090
Re: Is my thinking about equivalence right?

bobn2 wrote:

rogerstpierre wrote:

Never said that IQ would be similar. I am only saying that there is no correlation between the physical size of the sensor and the size of the rendered image. That is all.

Well, before you were saying that there is no such thing as 'magnification' or 'enlargement' in a digital image. Isn't it very clear that if a lens projects an image which is 17.3 by 13mm and you view it at 173 by 130mm, there is a relationship between those two sizes, like one is ten times the other. Now you decide to view it at 346 x 260mm. The relationship is now that one is 20 times the other. So, let's agree that relationship could be called 'enbigification', however the enbigification happens.

Totally agree on the relationship betwen projected size of the image on the sensor and representational size of the data captured hence a different ratio for a M43  image than a FF one of same resolution.  It's a relationship to the physical size of the image circle though not a cause and effect as it is for film where a smaller negative must be enlarged more as per your original statement. As such, the ratio of projected image to representation will be the same if the larger sensor has a lower resolution equal to that of the ratio between sensor sizes.

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RSTP14 Veteran Member • Posts: 5,090
Re: Is my thinking about equivalence right?

I am not disagreeing, but the discussion was about the need to enlarge an image more if captured with a smaller sensor, and I am saying you can't talk enlargment in a digital world without talking resolution.   How is an image taken with a sensor 1/2 the physical size compare to one taken with the other at 1/2 the resolution?

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bobn2
bobn2 Forum Pro • Posts: 69,811
Re: Is my thinking about equivalence right?
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rogerstpierre wrote:

bobn2 wrote:

rogerstpierre wrote:

Never said that IQ would be similar. I am only saying that there is no correlation between the physical size of the sensor and the size of the rendered image. That is all.

Well, before you were saying that there is no such thing as 'magnification' or 'enlargement' in a digital image. Isn't it very clear that if a lens projects an image which is 17.3 by 13mm and you view it at 173 by 130mm, there is a relationship between those two sizes, like one is ten times the other. Now you decide to view it at 346 x 260mm. The relationship is now that one is 20 times the other. So, let's agree that relationship could be called 'enbigification', however the enbigification happens.

Totally agree on the relationship betwen projected size of the image on the sensor and representational size of the data captured hence a different ratio for a M43 image than a FF one of same resolution.

You see, you start out OK...

It's a relationship to the physical size of the image circle though not a cause and effect as it is for film where a smaller negative must be enlarged more as per your original statement.

Then you veer off again. No. It has nothing to do with the size of the image circle. The lens' image circle might be any size so long as it is large enough to cover the image frame of the camera.

It has to do with the ratio between the physical dimensions of the image frame in the camera, whatever the sensitive medium is, and the physical dimensions of the display frame.

As such, the ratio of projected image to representation will be the same if the larger sensor has a lower resolution equal to that of the ratio between sensor sizes.

The image circle is immaterial. If I buy a Sigma lens for my mFT camera, it will have been designed for APS-C so will have a larger image circle than is needed for the image frame I'm using it with. That doesn't make the degree of enlargement any different from a designed for mFT lens at all when I display or print the result at the size I want to view it.

What you're doing is confusing mechanism and result. With film enlargement occurs optically, using an enlarger to project a bigger image onto another piece of sensitive medium. With digital the process is electronic, performed by resampling from the native pixel array of the sensor to whatever is the required pixel array for the output device or medium at the desired display size. That resampling might happen in a number of different places, such as your image editor or the device drivers for the printer or display, but it will happen. That's the mechanism, not the result. The result is that the image gets enlarged.

Of course, if you use a digital projector, then you're back to optical enlargement fro a part of the process....

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prsc Regular Member • Posts: 246
Re: Is my thinking about equivalence right?
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rogerstpierre wrote:

I am not disagreeing, but the discussion was about the need to enlarge an image more if captured with a smaller sensor, and I am saying you can't talk enlargment in a digital world without talking resolution. How is an image taken with a sensor 1/2 the physical size compare to one taken with the other at 1/2 the resolution?

Resolution does not matter here, it is not a measure of size but a sampling rate. Enlargement is the difference of projected image (that is what the sensor captures) and the viewing size. Again, resolution is irrelevant, if you view the same image on a 24" screen the enlargement is the same with the original photo and one downsampled by a factor of 4. Or if you have a FF sensor sized screen, then a FF image has 1x enlargement and MFT image about 2x (when both are shown at maximum size without cropping), no matter whether there is 1Mpix, 100Mpix, or anything in between.

Yxa
Yxa Senior Member • Posts: 2,657
Re: Is my thinking about equivalence right?

rogerstpierre wrote:

Never said that IQ would be similar. I am only saying that there is no correlation between the physical size of the sensor and the size of the rendered image. That is all.

And there you lost some of the EQ thinking

What was left of it I think

silentstorm Senior Member • Posts: 1,482
IMO the QE of the sensor is more important than size
5

Personally I think the sensor size is not important at all. The technology behind the sensor is more important.

Here's a chart from photonstophotos:

My case in point:

From the 1st Oly E1 FT to the current 20MP sensor

From the 1st Canon 1Ds to the current R5

From the Nikon D3 to the current Z7ii

From the Sony Nex7 to the A6600

Everyone can see from the chart that the 20MP mFT sensor is pretty much up there with the best APSc and FF from yester years. With advancing sensor tech and materials, I believe the sensor size in our cameras is no longer relevant.

How many of you here think of equivalency when you bring the camera to your eyes? How many of you here think of equivalency when you bring the handphone camera to a scene?

jonas ar
jonas ar Contributing Member • Posts: 915
Re: IMO the QE of the sensor is more important than size
4

silentstorm wrote:

Personally I think the sensor size is not important at all. The technology behind the sensor is more important.

Here's a chart from photonstophotos:

My case in point:

From the 1st Oly E1 FT to the current 20MP sensor

From the 1st Canon 1Ds to the current R5

From the Nikon D3 to the current Z7ii

From the Sony Nex7 to the A6600

Everyone can see from the chart that the 20MP mFT sensor is pretty much up there with the best APSc and FF from yester years. With advancing sensor tech and materials, I believe the sensor size in our cameras is no longer relevant.

How many of you here think of equivalency when you bring the camera to your eyes? How many of you here think of equivalency when you bring the handphone camera to a scene?

I think most do. That’s why you don’t see a club f/64 on this forum for example. Perhaps you could start the f/4 club 😉

robert1955 Veteran Member • Posts: 6,187
Re: IMO the QE of the sensor is more important than size
3

silentstorm wrote:

Personally I think the sensor size is not important at all. The technology behind the sensor is more important.

Here's a chart from photonstophotos:

My case in point:

From the 1st Oly E1 FT to the current 20MP sensor

From the 1st Canon 1Ds to the current R5

From the Nikon D3 to the current Z7ii

From the Sony Nex7 to the A6600

Everyone can see from the chart that the 20MP mFT sensor is pretty much up there with the best APSc and FF from yester years. With advancing sensor tech and materials, I believe the sensor size in our cameras is no longer relevant.

But is that a fair comparison? after all the MFT sensor will always stay one step behind.

BTW: the Y axis in this graph is normalized for the CoC appropriate for the sensor size, which I think changes the comparison

How many of you here think of equivalency when you bring the camera to your eyes?

A bit of a straw man, as equivalence only claims to be a tool for comparing formats

How many of you here think of equivalency when you bring the handphone camera to a scene?

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sybersitizen Forum Pro • Posts: 21,330
Re: Is my thinking about equivalence right?
1

ahaslett wrote:

For many purposes MFT gear is overkill, but it is the smallest MILC system ...

Just a correction: It isn't unless you're confining your comment to 'living' MILC systems.

bobn2
bobn2 Forum Pro • Posts: 69,811
Re: IMO the QE of the sensor is more important than size
1

silentstorm wrote:

Personally I think the sensor size is not important at all. The technology behind the sensor is more important.

Here's a chart from photonstophotos:

My case in point:

From the 1st Oly E1 FT to the current 20MP sensor

From the 1st Canon 1Ds to the current R5

From the Nikon D3 to the current Z7ii

From the Sony Nex7 to the A6600

Everyone can see from the chart that the 20MP mFT sensor is pretty much up there with the best APSc and FF from yester years. With advancing sensor tech and materials, I believe the sensor size in our cameras is no longer relevant.

How many of you here think of equivalency when you bring the camera to your eyes? How many of you here think of equivalency when you bring the handphone camera to a scene?

The question is, what do the curves actually mean? 'PDR' doesn't measure anything of much interest.

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Jonas Palm Senior Member • Posts: 1,109
Not much further to go then.
1

silentstorm wrote:

Personally I think the sensor size is not important at all. The technology behind the sensor is more important.

Here's a chart from photonstophotos:

My case in point:

From the 1st Oly E1 FT to the current 20MP sensor

From the 1st Canon 1Ds to the current R5

From the Nikon D3 to the current Z7ii

From the Sony Nex7 to the A6600

Everyone can see from the chart that the 20MP mFT sensor is pretty much up there with the best APSc and FF from yester years. With advancing sensor tech and materials, I believe the sensor size in our cameras is no longer relevant.

How many of you here think of equivalency when you bring the camera to your eyes? How many of you here think of equivalency when you bring the handphone camera to a scene?

While you are quite deliberate in avoiding all recent larger sensor cameras, you do well to bring up QE.

The quantum efficiency of todays full frame sensors, (and presumably the quite decent 20MP sensor used the last few years in m43), is above 50% currently just under 60%. 
Which means that even theoretically you cannot improve QE a full f-stop worth.

At best, you might see a third to half an f-stop improvement, and that in itself would be a great achievement. We’re pretty much at the end of the road, it is probably more rewarding to focus ones attention in other areas of sensor/system design where there is greater room for improvement. (Read-out speeds and resolution being particularly low hanging fruit, but there are others.)

tehdehzeh Forum Member • Posts: 57
Re: Not much further to go then.
1

Jonas Palm wrote:

The quantum efficiency of todays full frame sensors, (and presumably the quite decent 20MP sensor used the last few years in m43), is above 50% currently just under 60%.
Which means that even theoretically you cannot improve QE a full f-stop worth.

Personally I would include the losses in the CFA into the quantum efficiency of the sensor.

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ahaslett
ahaslett Veteran Member • Posts: 9,077
Re: Is my thinking about equivalence right?

sybersitizen wrote:

ahaslett wrote:

For many purposes MFT gear is overkill, but it is the smallest MILC system ...

Just a correction: It isn't unless you're confining your comment to 'living' MILC systems.

I was, but good build.

Andrew

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alfn
alfn Regular Member • Posts: 167
Re: Is my thinking about equivalence right?
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Mark Ransom wrote:

RobBobW wrote:

- the argument about total light is pointless as what is important is light density. Yes FF will bring in 4 times the light, but FF also has 4 times the surface area of sensor to illuminate, so it is a wash. Faster lenses bring in more light per unit area, period.

This is simply false. The reason apertures are measured by F-stops is because this equalizes the light density per unit area between lenses of different characteristics. A lens at F/1.2 will produce the same light density, no matter the focal length of the lens or the size of the sensor behind it. This means that a sensor with 4x the area really will collect 4x the light, when measured over the whole image, as long as the F-stops/T-stops of the lenses are the same.

The reason this matters is the nature of noise. The majority of noise in today's cameras is from photon shot noise, which is a property of the light itself and not of the lens or sensor or any other camera electronics. The only way to reduce shot noise is to collect more light. Whether you do this with a larger sensor, a larger aperture, or a slower shutter speed is immaterial.

That depends on how you are measuring the "noise". The signal to noise ratio (SNR) for each pixel remains the same for example, all else being equal. As does the SNR per unit area of the image.

It is true that the SNR of the entire image increases statistically but how does that translate to visible "noise"?

As for the majority of "noise" in today's cameras being from shot noise, that depends entirely on the intensity of the light being recorded. Shot noise is virtually irrelevant in very bright areas of an image.

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