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I own a DSLR. I want to buy some not-too-expensive compact, and there are a lot of compacts in the range 100-250$ with 1/2.3" sensors.

Can such a sensor/camera set give a quality somewhat comparable with a mid level/low end lens?

In particular does a small sensor give good results at ISO 100 or 200, or will I notice the difference so badly I will miss my DSLR?

Are small sensors an advantage when designing lenses? In other word does a bigger sensor require a more expensive lens to keep the same lens quality?

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In short, pretty much yes! In detail, there are several issues involved and noise performance is only one.

The point is, in low iso you might not feel the noise difference comparing to a higher sensor but one fact that some people forget is the depth of field. I made it bold because it has bold effect on your photos. The shallow dof that a big sensor can give you is almost impossible to achieve on a small sensor.

Another important factor is the dynamic range. Again, the dynamic range of a full frame or aps-c sensor is more than a 1/2.3" and if you have accurate eyes, you can detect it easily.

Regarding your question about a small sensor being an advantage for lens design; it is somehow true. When dealing with smaller sensors (at least on dslrs), the rear element of the lens can be closer to the sensor*. From wikipedia:

The proximity of the rear element to the image sensor greatly enhances the possibilities for wide angle and very wide angle lenses, enabling them to be made smaller, lighter (containing less glass), faster (larger aperture) and less expensive.

But the problem is, compact cameras are designed to be cheap and the above statement can not be considered as huge privilege for compact cameras.

Compacts can give you good results, for a compact, but their image quality is not close to a DSLR (specially if you own a full frame) because of many reasons: Noise, dynamic range, depth of field, vignetting, chromatic aberration and more.

So, they are good for an always-in-your-pocket camera and also very good for their cheap prices but you can not compare them to a good DSLR.

* that's why you can not mount a ef-s lens on full-frame body: the mirror will hit the rear element of the lens!

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    \$\begingroup\$ The depth of field advantage works both ways - a smaller sensor helps with landscape shots. This has been highlighter in the book Understanding Exposure by Bryan Peterson \$\endgroup\$
    – ab.aditya
    Sep 11, 2012 at 5:54
  • \$\begingroup\$ @ab.aditya I've not read that book and would be delighted if you quote the phrase from the book or just explain authors viewpoint briefly. I think on a dslr, for a landscape shot, you can achieve the same dof as a compact (e.g. playing with aperture). But you can not do the contrary (achieving dslr like dof with a compact). \$\endgroup\$
    – Pouya
    Sep 11, 2012 at 8:14
  • \$\begingroup\$ His views are from the Aperture section (p54-55 in the 3rd edition): "Your fixed-lens camera is hopelessly plagued with an uncanny ability to render a tremendous amount of depth of field ... if your lens goes to f/11, you're at a whopping f/64! Those of us who use SLRs can only dream of the vast depth of field...", "One added benefit of apertures that render such great depth of field is in the area of exposure times" \$\endgroup\$
    – ab.aditya
    Sep 11, 2012 at 14:35
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    \$\begingroup\$ So, small sensors help in getting a greater depth of field (useful for landscape shots & macro photography) without compromising shutter speed. Not sure about the diffraction effects though. \$\endgroup\$
    – ab.aditya
    Sep 11, 2012 at 14:37
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    \$\begingroup\$ @ab.aditya - granted, it's worth pointing out that that only is true if the image resolution is the same for both sensors. If the FF has a similar image density (ie, the same amount of pixels come from the same area) then the result would be the same with a crop on the FF. That said, I do agree that it is a street that cuts both ways when doing a simple comparison at the same total resolution. \$\endgroup\$
    – AJ Henderson ♦
    May 14, 2013 at 18:10
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It depends on your perception. In absolute terms, the quality is drastically better on a DSLR. Modern small sensors show noise and muddied details even at their base ISO when you look at pixels.

Now take a DSLR image and one from a small-sensor camera and print a 4x6 out of it and you will see fewer differences. You might even consider them close.

Remember than image noise is not the only difference. Dynamic-range is probably the most noticeable when looking at small prints and most small sensors have much less dynamic-range than a DSLR.

The only way for you to judge is to look yourself at sample images from a small sensor camera you may like. Here is a gallery from the Nikon P310 which is one of the nicer small-sensor cameras. If these do not seem good enough, you will have to consider something with a larger sensor, which nowadays does not have to be a DSLR.

Small sensors are difficult on lenses. You need less glass but the density of pixels means that a camera with a small sensor has small pixels and those require the lens to resolve better. Most small cameras are diffraction limited very quickly which is why the rarely close down beyond F/8 and some simulate aperture with an ND filter to avoid stopping down the lens.

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    \$\begingroup\$ small sensors have adequate dynamic range (usually over 10 stops) at low ISO speeds, it's only in low light that dynamic range really suffers \$\endgroup\$
    – Matt Grum
    Sep 10, 2012 at 16:27
  • \$\begingroup\$ I didn't say inadequate, just noticeable. With a print side-by-side from even a high-end compact and a DSLR, it is my opinion that dynamic-range is the most noticeable difference. \$\endgroup\$
    – Itai
    Sep 11, 2012 at 0:37
  • \$\begingroup\$ Thanks. P310 seems a really cool camera for its price :) \$\endgroup\$
    – Paolo
    Sep 11, 2012 at 8:17
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    \$\begingroup\$ @Itai Good photo paper only offers dynamic range of about 8 stops so you'll only see a difference if you tonemap your images prior to printing. Some compacts even exceed the dynamic range of DSLRs through lower read noise. I think you're much more likely to see differences in micro contrast and depth of field than dynamic range in prints. \$\endgroup\$
    – Matt Grum
    Sep 11, 2012 at 12:10
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Small sensors are often are a bad thing, because it means that each pixel is smaller and can capture less light, resulting in poor high-ISO/low-light performance. It also means that they are diffraction limited at lower f-stops, limiting the potential for aperture control and placing higher demands for lenses (better designs, tighter manufacturing tolerances).

However, there are advantages. Most notably, small sensors make it easier to design and manufacture superzoom lenses. This would be infeasible with a larger sensor, because a lens of reasonable quality with 20-25x or more zoom for a sensor of at least Four Thirds size would have to be too large, too heavy, and too expensive to be practical. They also enable highly compact camera designs and substantially lower cost.

Furthermore, with advances in sensor technology, a small sensor does not mean that you will always get poor image quality in low light. For example, the Pentax Q system shows that it is possible to deliver a remarkably compact MSC using a 1/2.3-inch sensor, with better-than-average image quality enabled through state-of-the-art BSI CMOS sensor technology, quality optics, and advanced image processing. For more information, see What are the advantages or disadvantages of a back illuminated CMOS sensor?

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A large number (tens of millions) of smartphone users are getting good photos with small sensors. Many professionals with podcasts (Scott Bourne, Alex Lindsay, etc.) predict that the low-end DSLR market will radically shrink because folks are happy with the rapidly improving quality of smartphone cameras. Nearly all of the pundits expect that the really small point-n-shoot camera market will be eaten by smartphones.

Whether you will miss your DSLR is a personal opinion question. Its not even a great question for this forum.

From a technical standpoint, photographers need better pixels, not more pixels. Larger pixels let each photosite gather more photons of light in a given exposure. This makes for better quality photos. The marketing folks have not replaced the laws of physics, no matter what they say.

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    \$\begingroup\$ Is this a joke? Quote: "A large number (tens of millions) of smartphone users are getting good photos with small sensors." If you mean "looks good on a smartphone screen" is "good", maybe. Print quality? No. Desktop background... not really... Tiny webpic: Maybe (same resolution as smartphone). In most cases you can spot a phone has been used - without going to the pixel level. And what some "professional" or "self-proclaimed guru" or what not predicts is also not helpful. Look at what the people buy. Also, there is a reasona micro 4/3 is successful - pretty good quality in a small package. \$\endgroup\$
    – DetlevCM
    Sep 10, 2012 at 17:32
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    \$\begingroup\$ No, its not a joke. Your "look at what people buy" is correct, and they are not buying point-n-shoots, they are using smartphones. Smartphone photos are the biggest source of shots on photo sharing sites like Flickr and Facebook. Get used to it. \$\endgroup\$
    – Pat Farrell
    Sep 10, 2012 at 18:01
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    \$\begingroup\$ I think a distinction needs to be made between people that are looking to buy a camera and choose a smartphone and people that use the cameraphone because it's handy. Just because Flickr and Facebook are plagued with cameraphone snapshots doesn't mean that's the future of photography. \$\endgroup\$
    – tenmiles
    Sep 10, 2012 at 21:11
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    \$\begingroup\$ Most people eat fast food hamburgers rather than foie gras. That says nothing about the qulity of the two. Comparing smartphones and entry level DSLR is somewhat rash, don't you think? The fact that most image on the internet are from Smartphone is because it is so easy to upload a picture from a phone, while it is more complicated to download you photo on a pc, then possibly resize them and upload to some photo site. \$\endgroup\$
    – Paolo
    Sep 11, 2012 at 8:13
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    \$\begingroup\$ I think your point is valid for people who wants just a camera to take some self-portraits and have photo-galleries for their last trip. For those who do photography as work of art or a serious hobby, a smart phone has nothing to offer but low weight. \$\endgroup\$
    – Pouya
    Sep 11, 2012 at 8:20
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If you value cameras that are generally smaller, lighter, and cheaper and provide plentiful depth of field with lenses that are generally smaller, lighter, and cheaper, then maybe a small sensor is a good thing.

If you value ultimate image quality, shallow depth-of-field, or low noise in low light, then a smaller sensor is probably going to be disappointing.

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Small sensors are not always bad, as they can give you a greater depth of field at larger apertures openings (smaller f numbers) which does have its uses for landscape & macro photography. Quoting from Bryan Peterson's Understanding Exposure (p54-55 in the 3rd edition):

Your fixed-lens camera is hopelessly plagued with an uncanny ability to render a tremendous amount of depth of field ... if your lens goes to f/11, you're at a whopping f/64! Those of us who use SLRs can only dream of the vast depth of field ...

One added benefit of apertures that render such great depth of field is in the area of exposure times

So, small sensors can help in getting a greater depth of field without compromising shutter speed. Effects of diffraction are also be likely to be less pronounced.

Apart from this, the benefit of smaller sensors is that the cameras are likely to be smaller and more portable than ones with larger sensors. There are of course exceptions to this.

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