Some dumb questions I have been wondering about

Started Mar 28, 2013 | Discussions
Wellington100
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Some dumb questions I have been wondering about
Mar 28, 2013

1) Why are camera sensors not square? Surely a square sensor is the most efficient way to get the best out of a lens?

2) Does using a Polarising filter reduce the Dynamic Range of an image taken in sunlight with a digital camera?

3) Why do most cameras have IQ reducing AA filters when the few cameras that don't have them jump in IQ and moire is nowhere to be seen in 99.99% of the images?

4) What is the optimum resolution for small camera sensors? High resolution sensors seem to add significant file size for little discernible improvement in resolution, so what is the cut off for a functional and well rounded small sensor?

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Zone8
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Re: Some dumb questions I have been wondering about
In reply to Wellington100, Mar 28, 2013

Wellington100 wrote:  1) Why are camera sensors not square? Surely a square sensor is the most efficient way to get the best out of a lens?

Only a quickie as somewhat busy - other will comment on rest.  In fact, a round image would be even more efficient as that is what any lens projects.  Even with film square format cameras (Rollei etc.) most people ended producing rectangular images in print format.

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Re: Some dumb questions I have been wondering about
In reply to Wellington100, Mar 28, 2013

Wellington100 wrote:

1) Why are camera sensors not square? Surely a square sensor is the most efficient way to get the best out of a lens?

2) Does using a Polarising filter reduce the Dynamic Range of an image taken in sunlight with a digital camera?

It should do if the polarised bright light is blocked.

3) Why do most cameras have IQ reducing AA filters when the few cameras that don't have them jump in IQ and moire is nowhere to be seen in 99.99% of the images?

Why equate resolution to IQ, surely the only resolution that is needed is the amount that is needed for the size view or print that you will reproduce, and its not just .01% that has moire problems, that is the amount of visibly annoying ones, many others will be degraded by moire but you won't notice it, and this is a real reduction in IQ.  In my view any moire is undesirable.

4) What is the optimum resolution for small camera sensors? High resolution sensors seem to add significant file size for little discernible improvement in resolution, so what is the cut off for a functional and well rounded small sensor?

I would say about 8mp would cover most ordinary persons needs.

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mike703
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Re: Some dumb questions I have been wondering about
In reply to Wellington100, Mar 28, 2013

Interesting questions.  Some entirely subjective responses...

1) Why are camera sensors not square? Surely a square sensor is the most efficient way to get the best out of a lens?

You'd think so (or even better, round to match the image circle).  but most people seem to prefer rectangles... the golden ratio of about 1.6:1 is supposed to be the most aesthetically pleasing proportion and crops up in paintings a lot for that reasons. A square somehow looks too symmetrical.  So it's just an aesthetic thing, not a technical one.

2) Does using a Polarising filter reduce the Dynamic Range of an image taken in sunlight with a digital camera?

Sort of.  First there is the underlying neutral-density-filter effect which just costs a couple of stops of light.  That contribution affects the whole image equally so doesn't alter the DR.

The additional polarising effect selectively reduces in intensity specular refections (off surfaces such as glass, leaves, metal, snow, rock or a shiny forehead).  So on that basis it would reduce the DR.... except that other bright areas (direct sunlight, bright clouds) are not affected in the same way.  In an 'average' scene, whatever that is, some - maybe even most - of the highlights will be brought down in intensity, but not all of them.  So strictly speaking the DR of the scene is the same, even though the proportion of highlights is less.  if you shot a scene where basically all the bright areas were specular reflections (e.g. a rock face with the sun behind you) then all of the bright areas would be reduced in intensity and the DR of the scene would decrease.

3) Why do most cameras have IQ reducing AA filters when the few cameras that don't have them jump in IQ and moire is nowhere to be seen in 99.99% of the images?

There's a can of worms.  Maybe moire used to be a major problem in some far-off age and we've over-compensated ever since.  Apparently it's now quite easy to get rid of - it used not to be and was a major hassle if you got it.  Maybe that's the answer and we will now see more and more cameras with no AA filters as the benefits become apparent.

4) What is the optimum resolution for small camera sensors? High resolution sensors seem to add significant file size for little discernible improvement in resolution, so what is the cut off for a functional and well rounded small sensor?

You will never lose detail by having more and more pixels, and (as long as you print at the same size) noise effects cancel out, more or less.  but as you imply there is a point beyond which your lens quality and technique need to be immaculate to capture any more detail.  Unless you are shooting with top-drawer lenses under very controlled conditions using a tripod and mirror lock up my impression is that we already have enough resolution and pixel count is not the limiting factor in the technical quality of most pics.  But there are a few people with demanding requirements who do shoot like that and will make the most of higher resolution.

Best wishes

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ZorSy
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Re: Some dumb questions I have been wondering about
In reply to Wellington100, Mar 28, 2013

For the question 1, I would suggest reading about "golden ratio" (Phi) and amazing influence it have had though human history in almost every aspect of human (and not only so) activities. Somewhere along those lines you may find the answer why square is not considered "perfect".

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Mark B.
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Re: Some dumb questions I have been wondering about
In reply to Wellington100, Mar 28, 2013

Wellington100 wrote:

1) Why are camera sensors not square? Surely a square sensor is the most efficient way to get the best out of a lens?

Because standard print formats are not square, nor are any display devices.  That's fine for some pros who don't mind the format or cropping, but for the vast majority of point 'n shoot users, as well as DSLR users, square just wouldn't make sense.  For sure I wouldn't want to crop every single image.  You can argue about most efficient use of the area all day long, it doesn't change the fact that a square format doesn't work for my shooting.

Mark

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Barrie Davis
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Re: Some dumb questions I have been wondering about
In reply to mike703, Mar 28, 2013

mike703 wrote:

snip

2) Does using a Polarising filter reduce the Dynamic Range of an image taken in sunlight with a digital camera?

Sort of.  First there is the underlying neutral-density-filter effect which just costs a couple of stops of light.  That contribution affects the whole image equally so doesn't alter the DR.

The additional polarising effect selectively reduces in intensity specular refections (off surfaces such as glass, leaves, metal,

Polarising filters do NOT work on reflections from metal surfaces.

Polarising works on non-metal surfaces only...

... unless the light reflected is pre-polarised by a pola filter positioned over the light source itself.

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Tom Axford
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Re: Some dumb questions I have been wondering about
In reply to Wellington100, Mar 28, 2013

Wellington100 wrote:

3) Why do most cameras have IQ reducing AA filters when the few cameras that don't have them jump in IQ and moire is nowhere to be seen in 99.99% of the images?

This is a big subject and there is no simple answer, but you can make a start by looking at the Wikipedia article on "aliasing".

A rather oversimplified answer is that if you omit the anti-aliasing (AA) filter from a camera, some of the extra resolution you obtain is illusory. Apart from the obvious problems of moire patterns (which are mathematically impossible to completely remove in post-processing), in any image taken without an AA filter, some of the finest detail will be false detail.

A simple example of this is than almost any straight line will show jaggies which were not there in the image produced by the lens - they are an artefact of the pixel array used both in the sensor and the resultant digital image.

For these reasons, most cameras still use AA filters. The non-AA cameras are really for those who put resolution as their very top priority and don't mind if the images are sometimes a slightly less accurate representation of reality.

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ultimitsu
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Re: Some dumb questions I have been wondering about
In reply to Wellington100, Mar 28, 2013

Wellington100 wrote:

1) Why are camera sensors not square? Surely a square sensor is the most efficient way to get the best out of a lens?

Square would not use the wider side of the lens that current 3:2 sensor use. So overall area is more, but not significantly more. Ideally we should have a large squasheens or that cover both maximum 16:9 ration and 1:1 square, with some part outside image circle and is wasted, like panasonic gh series and lx series. But those are too expensive to make for larger lenses such as ff.

4) What is the optimum resolution for small camera sensors? High resolution sensors seem to add significant file size for little discernible improvement in resolution, so what is the cut off for a functional and well rounded small sensor?

People could come up with random answers in the past, butnowadays we have Nokia preview which has 40m. On a tiny sensor and itkick dominating mobile imaging. So itpay have shown us that there is nooptimum resolution, the more is always merrier.

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nikkorwatcher
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Re: Some dumb questions I have been wondering about
In reply to Wellington100, Mar 28, 2013

Re 1), it would be very boring not to have an in-camera choice of framing the subject, don't you think? With the gigantic film area, medium format photographers could happily crop, but I think the 3:2 format for 35mm was quite realistic given the plebs would take a lot of scenics with it. I think they tried panoramic film in some slimline cameras similar to 110 cartridge film but they were not a big part of the market.

I find 3:2 useful to shoot sports in the portrait orientation. If they made a 2:3 sensor I wouldn't need a battery grip. Getting the camera level for landscapes might be trickier though.

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FractalFlame
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Re: Some dumb questions I have been wondering about
In reply to Wellington100, Mar 28, 2013

Wellington100 wrote:

1) Why are camera sensors not square? Surely a square sensor is the most efficient way to get the best out of a lens?

Humans tend to look left right more than up down - our field of vision is rectangular.

Not necessarily by eye limitation, more of a brain limitation. When we look up we lift our chin and use our head to look. when we look sideways we turn our eyeballs (peripheral vision is generally accepted as side to side).

Note that a few groups of people have bypasses this - usually martial arts (not meaning kung-fu type, but military arts AND kung-fu type). People who are extraordinary aware of surrounding, some sports where you have to be aware of incoming balls, military (enemy up top), etc.

The general, average man-on-th-street is a horizontal viewer more than a vertical viewer - our dangers are level with our eyes but sideways (cars, pedestrians, doors, etc).

That's why, I believe, the rectangular film frame came about - dig sensors are just an extrapolation of them.

Even 6x6 format is often cropped to rectangular.
Some people even feel unusually 'unsettled' by a square painting or photo.

2) Does using a Polarising filter reduce the Dynamic Range of an image taken in sunlight with a digital camera?

Yes, unlike a ND filter (which just 'shifts' the light downwards, blocking all evenly), a pol filter actually blocks some areas more than others.
If a bright reflection is polarised then you will get a DR shift.

Bear in mind how it works - a pol filter stops certain wave directions from passing and lets other wave directions through.
A differential blocking of light = DR change. A ND filter does a non-differential light block.

3) Why do most cameras have IQ reducing AA filters when the few cameras that don't have them jump in IQ and moire is nowhere to be seen in 99.99% of the images?

I can only offer a surmise here -  consumer cameras are aimed at ordinary people.
If they take photos and end up with a lot of moire in them they will not know how to remove it and might turn away from the camera (of even digital photography in general).

So they make them with the AA filter, accepting that the loss of resolution is offset by the loss of aliasing... ??

4) What is the optimum resolution for small camera sensors? High resolution sensors seem to add significant file size for little discernible improvement in resolution, so what is the cut off for a functional and well rounded small sensor?

A sensor is more than just the resolution - it's about the actual size (and quality, et al) of the sensor sites.

You are physically limited by the sensor size:

A 1" square sensor at 10 Mpxl might have the sensor sites at 30 microns = good light gathering ability but lower actual image resolution.

A 1" square sensor at 20 Mpxl might have the sensor sites at 15 microns = lower light gathering ability but higher actual image resolution.

A 4" square sensor at 20 Mpxl might have a sensor size of 60 microns = good light gathering AN high resolution.

(No, I didn't do the math - that's why i chose 1" and 4" - these numbers are examples only!)

This is why an apc-c camera of 16 mpxl can sometime outperform an acp-c camera of 20 mpxl.

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olliess
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Re: Some dumb questions I have been wondering about
In reply to FractalFlame, Mar 28, 2013

FractalFlame wrote:

Wellington100 wrote:

2) Does using a Polarising filter reduce the Dynamic Range of an image taken in sunlight with a digital camera?

Yes, unlike a ND filter (which just 'shifts' the light downwards, blocking all evenly), a pol filter actually blocks some areas more than others.
If a bright reflection is polarised then you will get a DR shift.

Often you find a wide DR between a bright blue sky and a (darker) foreground. By darkening the blue sky, the polarizer can allow you to expose a bit higher to better capture shadow details in the foreground while still keeping cloud detail. A gradient ND filter can also help with bright skies if the scene really does trend from light to dark, and in particular it works on cloudy skies (where the polarizer is not helpful).

The polarizer also works to tone down sky reflections off water or foliage. Since the reflections are usually the brightest parts of these areas, this also tends to reduces the DR, and also helps the foliage colors look more saturated.

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joejack951
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Re: Some dumb questions I have been wondering about
In reply to nikkorwatcher, Mar 28, 2013

nikkorwatcher wrote:

Re 1), it would be very boring not to have an in-camera choice of framing the subject, don't you think? With the gigantic film area, medium format photographers could happily crop, but I think the 3:2 format for 35mm was quite realistic given the plebs would take a lot of scenics with it. I think they tried panoramic film in some slimline cameras similar to 110 cartridge film but they were not a big part of the market.

I find 3:2 useful to shoot sports in the portrait orientation. If they made a 2:3 sensor I wouldn't need a battery grip. Getting the camera level for landscapes might be trickier though.

As I believe Joseph Wisniewski has pointed out, the 3:2 ratio has design implications as well.

Flash sync speed is determine by how quickly the shutter can move across the sensor. Shutters are vertical plane (travel across the short side) for that reason. If you lengthen the short side, you now need a faster shutter or you deal with a slower flash sync speed.

In order to see 100% of the frame, you'd also need a bigger mirror for a square sensor in an SLR. Mirrors pivot on the long side in order to keep the sensor to lens distance to a minimum. With a square sensor, that distance would need to increase thus making a whole lot of existing SLR lenses obsolete and further complicating wide angle lenses. Of course, mirrorless camera would not have this issue.

All digital cameras would be affected by the change in LCD size required for a square display. Unless you wanted a square image every time, you'd see less of the image on the LCD for a given display size.

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mike703
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Re: Some dumb questions I have been wondering about
In reply to Barrie Davis, Mar 28, 2013

Barrie Davis wrote:

Sort of.  First there is the underlying neutral-density-filter effect which just costs a couple of stops of light.  That contribution affects the whole image equally so doesn't alter the DR.

The additional polarising effect selectively reduces in intensity specular refections (off surfaces such as glass, leaves, metal,

Polarising filters do NOT work on reflections from metal surfaces.

Polarising works on non-metal surfaces only...

... unless the light reflected is pre-polarised by a pola filter positioned over the light source itself.

How interesting (and slightly surprising).  Any idea why a metal surface behaves differently from the others?

Best wishes

Mike
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Joseph S Wisniewski
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In reply to Wellington100, Mar 28, 2013

Wellington100 wrote:

1) Why are camera sensors not square? Surely a square sensor is the most efficient way to get the best out of a lens?

It's not. That's an old myth.

Any time you shoot a square and crop to a rectangle, your corners are no longer on the image circle, so you've lost some of your lens. Since 99.99% of images look better as rectangles, "the most efficient way to get the best out of a lens" is with a rectangular sensor of the same aspect ratio as your final image.

It's also not "the most efficient way to get the best out of" a camera, at least in the decades dominated by SLRs, because the longer swinging mirror of a square format has more vibration (roughly proportional to the cube of the aspect, so it's huge) and it requires a longer clearance distance (the "back focus") between the rear lens element and the sensor, complicating lens designs and decreasing lens quality.

2) Does using a Polarising filter reduce the Dynamic Range of an image taken in sunlight with a digital camera?

Often it does, if the brightest thing in the picture happens to be something that the polarizer can darken, like the sky, or a specular reflection such as glare on water. s

3) Why do most cameras have IQ reducing AA filters when the few cameras that don't have them jump in IQ and moire is nowhere to be seen in 99.99% of the images?

That's a nonsequitar, because it requires us to agree with your assessment that "moire is nowhere to be seen in 99.99% of the images". With good technique, moire is visible in about 5% of images, in my experience with cameras lacking AA filters. Whether the moire is bad enough to be a "picture killer" is another thing, but "nowhere to be seen in 99.99%" is wild hyperbole.

The people lacking in good technique (adequate attention to shutter speed, handholding technique, tripod use, aperture choice to avoid diffraction, careful focusing, etc) will have less moire, because their technique provides enough motion blur to act as an effective AA filter. But those people won't notice an IQ change from the lack of a filter.

We're only now entering a time when cameras often have such high resolution that even excellent attention to detail still results in enough blur to take the place of an AA filter. Give things another 5 years, and AA filters will become extinct.

4) What is the optimum resolution for small camera sensors?

About 500mp for APS, 1200mp for FF. Although there are reasons to go even higher.

High resolution sensors seem to add significant file size for little discernible improvement in resolution,

You can't have it both ways. If the 20% increase in resolution from not having an AA filter is "a jump", then the 20% increase from having 40% more pixels is "a jump" too, not "little discernible improvement".

so what is the cut off for a functional and well rounded small sensor

1/2 wavelength in the green, for resonant filters. About 5 gigapixels for APS. That also works well with photon counting techniques.

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Barrie Davis
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Re: Some dumb questions I have been wondering about
In reply to mike703, Mar 28, 2013

mike703 wrote:

Barrie Davis wrote:

Sort of.  First there is the underlying neutral-density-filter effect which just costs a couple of stops of light.  That contribution affects the whole image equally so doesn't alter the DR.

The additional polarising effect selectively reduces in intensity specular refections (off surfaces such as glass, leaves, metal,

Polarising filters do NOT work on reflections from metal surfaces.

Polarising works on non-metal surfaces only...

... unless the light reflected is pre-polarised by a pola filter positioned over the light source itself.

How interesting (and slightly surprising).  Any idea why a metal surface behaves differently from the others?

I think it is something to do with the crystaline nature of metals when considered at the atomic level...

.... but this is only half remembered, I'm afraid, and could be wrong.

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Leonard Migliore
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In reply to mike703, Mar 28, 2013

mike703 wrote:

Barrie Davis wrote:

Sort of.  First there is the underlying neutral-density-filter effect which just costs a couple of stops of light.  That contribution affects the whole image equally so doesn't alter the DR.

The additional polarising effect selectively reduces in intensity specular refections (off surfaces such as glass, leaves, metal,

Polarising filters do NOT work on reflections from metal surfaces.

Polarising works on non-metal surfaces only...

... unless the light reflected is pre-polarised by a pola filter positioned over the light source itself.

How interesting (and slightly surprising).  Any idea why a metal surface behaves differently from the others?

It's because the electrons in metals are not attached to the atoms; the electrons migrate freely through the metal (which is why they conduct electricity).

In dielectric materials, light interacts with electrons in atoms and excites them to higher energy states. The atoms quickly decay to the ground state, emitting a photon. This is the basis of reflection. One may visualize the excited atoms as vibrating, with the vibration in the same plane as the incident wave. The emitted photon shares this polarization direction. Looking at the geometry of reflection and realizing that light is a transverse wave, you can see that no light can be reflected when the plane of vibration is parallel to the reflected ray (this really needs diagrams; I'm looking at Fundamentals of Optics by Jenkins and White, pg. 524). This causes reflections from dielectrics to be polarized since some incoming polarizations are reflected more than others.

In metals, incoming light is absorbed by the free elections. But the electrons are not coupled to the metal ions so almost all the energy is re-emitted as photons with a 180 degree phase shift. This makes metals opaque because the phase-shifted photons destructively interfere with the incoming ones. It also makes the reflectivity relatively insensitive to the polarization of the incoming light because the plane of vibration of the electrons is nearly parallel to the surface of the metal.

There is a lot of literature on this topic if a more complete explanation is required.

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mike703
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In reply to Leonard Migliore, Mar 28, 2013

Thanks Leonard - that makes perfect sense.  I have a chemistry background and work in spectroscopy so as soon as you pointed out that localised electrons in dielectrics behave differently from delocalised electrons in metals the penny dropped.

Best wishes

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jonikon
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Re: Some dumb questions I have been wondering about
In reply to Wellington100, Mar 28, 2013

Wellington100 wrote:

1) Why are camera sensors not square? Surely a square sensor is the most efficient way to get the best out of a lens?

It's the aesthetics. How many paintings have you seen that are square?

2) Does using a Polarising filter reduce the Dynamic Range of an image taken in sunlight with a digital camera?

Not directly, but indirectly they can. The reason is that a typical polarizing filter reduces the light hitting the sensor by about two stops. If the ISO is raised to compensate for the reduction in light, then the dynamic range will be reduced. (With present day sensors, as the ISO goes up, dynamic range goes down.) An alternative would be to increase the exposure, typically with a longer shutter time which will sometimes require use of  a tripod.

3) Why do most cameras have IQ reducing AA filters when the few cameras that don't have them jump in IQ and moire is nowhere to be seen in 99.99% of the images?

An AA filter is not necessary once the spacing of the photosites gets very small and therefore a 6 MP APS-C  sensor will be prone to moire, while a 24MP sensor will not. Because of the density of their photosites, the small sensor P&S cameras shed their AA filters some time ago, and now DSLRs are doing the same.

4) What is the optimum resolution for small camera sensors? High resolution sensors seem to add significant file size for little discernible improvement in resolution, so what is the cut off for a functional and well rounded small sensor?

Years ago I read a well reasoned article on web that made a case for a maximum of 6 MP for small sensor camera. Based on my experience with P&S cameras, I believe this may be a bit conservative, but certainly not very far off. The P&S camera that gives me the best image quality and most accurate colors is the 6MP  Fujifilm F31FD. To this day there is no better tiny sensor for high ISOs.

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lukaw
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In reply to Wellington100, Mar 28, 2013

Wellington100 wrote:

1) Why are camera sensors not square? Surely a square sensor is the most efficient way to get the best out of a lens?

2) Does using a Polarising filter reduce the Dynamic Range of an image taken in sunlight with a digital camera?

3) Why do most cameras have IQ reducing AA filters when the few cameras that don't have them jump in IQ and moire is nowhere to be seen in 99.99% of the images?

4) What is the optimum resolution for small camera sensors? High resolution sensors seem to add significant file size for little discernible improvement in resolution, so what is the cut off for a functional and well rounded small sensor?

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A square sensor would be the most versatile format and more expensive than the rectangular.  The reason that the rectangular format rules is its clientale, amateur photographers. High end pros had been using a square format for the last 70 years (at least).

Photographers that worked with square format, appreciate its advantages and the level of creativity and versatility it offers.

You can read more about it at the following sites:

http://photo.tutsplus.com/articles/theory/a-guide-to-producing-beautiful-square-format-images/

http://digital-photography-school.com/five-reasons-to-love-the-square-format

http://gilly-walker.com/2011/05/square-shooter/

http://www.ephotozine.com/article/understanding-square-format-18005

http://bandh.tumblr.com/post/10244505807/three-reasons-why-square-format-images-have-so-much

I just named a few sources, if you go on line, there is a wealth of info describing advantages of shooting square format.

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