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DSLRs v Compact Cameras - Sensor Sizes, Megapixels and Image Quality

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This article will take a look at how compact digital cameras differ from digital single lens reflex cameras (DSLRs), and also explain where mirrorless interchangeable lens cameras fit in to the story. It will provide you with an understanding of sensor sizes and megapixels in affecting image quality and resolution, as well as explain why too many megapixels can actually hurt your image quality! Lastly, we will take a quick look at a few other significant differences between DSLRs and compact digital cameras.

Compact Digital Cameras (Hybrid Cameras)

Compact Digital Cameras or Compact System Cameras or Hybrid Cameras have come a long way over the years, to the degree that they have now confused the classification of cameras entirely! The prime hero / villain of this story is the mirrorless camera.

In the beginning there were SLRs… or so goes this expurgated version of the story. Single lens reflex cameras – whether film or digital - have large sensors and a unique single lens refractive system that (literally) defines them as SLRs. This involves a 45 degree mirror reflecting up to a prism assembly and viewfinder eyepiece resulting in the photographer viewing the image exactly as the camera does. This is a magnificent system, with the only drawback being that it takes up a lot of space, resulting in bulkier, heavier cameras. For a detailed look at how the DSLR mirror system with optical viewfinder works take a look at this article Digital Single Lens Reflex cameras.

Back then DSLRs (Digital Single Lens Reflex cameras) had interchangeable lenses, whereas compact cameras were all about minimal size and cost and did not. Compact cameras were roughly split into true compacts (ultra compacts) or bridge cameras (merely compacts with large zoom magnification systems, meaning they had bigger lenses and so were not quite as compact).

Interchangeable Lens Mirrorless Cameras

                      

And then along came the mirrorless interchangeable lens camera! Manufacturers such as Olympus and Panasonic began using mirrorless camera systems that nevertheless also came with a relatively large sensor and interchangeable lenses. These Mirrorless Interchangeable Lens Cameras (MILC) or Interchangeable Lens Cameras (ILC) are mirrorless, hybrid cameras that have many of the features of digital single lens reflex cameras but without the mirror. Some even have large DSLR sized sensors, while some have smaller ones. The biggest difference is that in place of the single lens reflex mirror system in DSLRs these mirrorless cameras have either an electronic viewfinder or even just an LCD screen and no viewfinder at all. This means that top end compact system cameras offer high image quality, creative, manual control and also the option to attach different lenses.

 

Disadvantage of Interchangeable Lens Mirrorless Cameras

The biggest disadvantage of Mirrorless Interchangeable Lens Cameras is that they tend to have slower and perhaps less accurate focusing systems. Some MILCs use the contrast detect method of focusing, which is slower than the phase detect method used by DSLRs and can make them less suitable for action, sports and wildlife photography. This has always put this author off them – but only because one of my hobbies is photographing kingfishers stooping into the water – which is all about understanding bird behavior, lightning quick shutter speeds, the photographer’s reflexes, and of course superfast camera tracking abilities! Having said that though I am aware that tracking speed and accuracy in mirrorless systems is certainly improving, to the extent that a top mirrorless camera is now suitable for many action situations.

 

Okay, so now it is time to poke the bear by explaining the truth about sensor size and megapixels and how they affect image quality!

Sensor sizes, megapixels and image quality

 

Spoiler alert – the first section for Resolution and Image Pixel Size, entitled a. Total Number of Megapixels That Sensor Contains is somewhat technical and for the photography geeks (like myself). If you don’t wish to get down and dirty with the technical aspects of image resolution then just skip ahead to b. The Cameras Sensor Size. The conclusions of section a, b and c have also been placed in bold for anyone less interested in the theory and more in the conclusions.

 

  1. Resolution and image pixel size
  1. Total number of megapixels that sensor contains

Resolution could be defined as the capability of the sensor to observe or measure the smallest object clearly with distinct boundaries. The smaller the size of a pixel, the higher the resolution, and therefore the better the image will be. This is because the more pixels for a set sensor size, the more ‘divisions’ in the sensor. A sensor divided into 1000 pixels high by 1000 pixels wide would have 1000 X 1000 = 1 000 000 pixels or approximately 1 Megapixels. However, let’s subdivide each pixel, allowing for a greater total number of pixels in the set space (sensor size). We shall take one pixel – basically a square block – and halve its height, generating 1000 X 2 = 2000 pixels of height. Let’s also halve the width of the pixels, so now we have twice as many pixels in width too i.e. 2000 pixels. Now our total number of pixels would be 2000 X 2000 = 4 megapixels.

 

 

Now consider this diagram for a second. As we can see there are two factors in the diagram affecting the clarity (quality) of the image:

  1. The first is whether the projected detail happens to fall in the center of a pixel or on the edge – with the edge increasing the detail. In general an object has to fall on at least three pixels for it to even vaguely begin to resemble the object. As you can see when it falls on just one the circle here is represented as a square. Once it falls on several pixels (even if the same size) it begins to be represented as something closer to a circle!
  2. The second has to do with the size of the pixels (square blocks) relative to the size of the object, and this is highly significant. Let us assume for a second that the reason the object is larger in the lower rows of images is not due to magnification of the object but rather a reduction in pixel size (in both instances the relative size of the pixels versus the object is decreasing). Immediately we can see how much the detail improves as the pixel size decreases relative to the object!

Let us consider just the change from row two (labelled ‘1’) and row three (labelled ‘2’) - for either of columns 1 & 2 (image in center of pixel) or columns 3 & 4 (image at edge of pixel). From the second row to the third row (for our hypothetical example) both the width and height of each pixel has been halved, resulting in the same sized camera sensor being able contain twice as many pixels in height and twice as many pixels in width. So if the sensor in the second row had 1 megapixels (1000 X 1000), the same sized sensor in the third row would contain 4 megapixels (2000 X 2000). The large difference in image quality between the two rows is also clearly demonstrated! If we consider the second and third rows in the diagram (for either of columns 1 & 2 or columns 3 & 4).

With images that are not going to be enlarged much this difference in pixel number may well not be important, but it can really make a difference if the images are going to be professionally exhibited or blown up to A1 size or similar.

So if all other factors are equal AND THE SENSOR SIZE IS LARGE ENOUGH, the larger the number of megapixels the better the image quality if a particular image is enlarged. 

 

  1. The camera’s sensor size

 

The size of the camera’s sensor is the most important determinant of image quality, and is way more significant than the total number of megapixels. A strong, brighter signal requires less amplification and this means there's less opportunity for image noise to be introduced or magnified.

 

Let us take a moment and digest this. The size of the sensor determines how much light the camera can use to create an image. Simply put, image sensors could be visualized as consisting of millions of light-sensitive spots called photosites which are used to record information about what is seen through the lens. Obviously a larger sensor can obtain more information and thus produce better images. A smaller sensor with smaller photosites has darker pixels that have to be greatly magnified to produce the same sized image as a larger sensor camera of the same number of megapixels. As the image is magnified so are all the flaws in it.

 

A way of better understanding this is may be to consider what happens (in a similar type of process) when you begin to digitally zoom in on a compact camera image. It doesn’t take long for you to start seeing these imperfections raise their ugly heads on your LCD monitor as they become magnified. And if you shot the image in low light conditions on a relatively high ISO the image quality deteriorates even faster!

 

 

Image quality as a product of sensor size and photosite size (pixel pitch)

Format

Sensor size

Pixel Pitch (10MP)

Camera description

Max. ISO*

 

 

 

1/2.6”

5.76×4.29mm

<1 micron

Smartphone camera

100-200

 

 

 

1/1.8”

7.18×3.2mm

1.4 microns

Ultra compact camera, internal zoom

200-400

 

 

 

1/1.7”

7.6×5.7mm

1.7 microns

Bridge camera / compact camera, external zoom

400-800

 

 

 

Four Thirds

17.3x13mm

4.8 microns

Interchangeable lens camera (DSLR or MILC)

400-800

 

 

 

APS-C

22.2×14.8-23.6×15.7mm

5.7 microns

Consumer/Hobbyist DSLR

800-1600

 

 

 

APS-H

28.7x19mm

6.3 microns

Prosumer DSLR

800-3200

 

 

 

Full Frame

36x24mm

8.4 microns

Pro DSLR or Rangefinder

1600-6400

 

 

 

 

*The Maximum ISO figures in the diagram are an estimate of the highest ISO setting that can really be recommended, based on typical signal-noise ratio and dynamic range sensitivity. Obviously this is a generalization and can vary somewhat depending on other factors such as processing ability.

 

If you look at the second diagram on image quality as a product of sensor size and photosite size (pixel pitch) you will notice that the sensor size between different cameras can vary vastly! A compact camera or mobile phone with a typically small image sensor, and yet the same number of megapixels as a DSLR, would have photosites (‘pixels’) that are a tiny fraction of the size of the photosites on the DSLR. The DSLR is thus capable of creating images with a better dynamic range, less noise and improved low light performance than the ultra compact mirrorless camera, mobile phone or action camera.

 

In conclusion, when it comes to sensors Size Does Matter, and Bigger is Always Better!

 

 

  1. Increasing megapixels can reduce image quality!

 

What you will have noticed is that there is a trade-off between ‘resolution’ as determined by total number of pixels versus image quality as measured by low noise.

 

Cameras with larger sensors (e.g. DSLRs and large sensor mirrorless ILCs) have a huge advantage in that they can increase the megapixel resolution (i.e. have more detailed images) without sacrificing much noticeable in terms of increased noise! As these cameras are professional quality though, you may have noticed that their total megapixel count has (with a few exceptions) actually increased relatively slowly as they are currently optimizing this trade-off.

 

Rather ironically, while top DSLR camera pixel counts have remained pretty static compact system cameras and mobile phone cameras have been pushing the megapixel count often to ridiculous levels.

 

This author suspects that this is partly due to smartphone manufacturers wagging the dog – it is a brilliant strategy to keep the customers from noticing that the sensor size in top smartphones has increased at a snail’s pace. For example the iPhone 7 has a sensor size of 1/3”, which is practically the same size as the iPhone1! Yet it has blown the number of megapixels up 6 fold from 2MP to 12MP. So I suspect that especially camera phone manufacturers are exploiting the general public’s ignorance as to how increasing megapixel size actually affects image quality, where with miniscule sensor sizes a higher megapixels count is often not better.

 

Now don’t get me wrong, today’s smartphones – including the iPhone 7 - take some pretty good images, especially in good lighting. But this is due to a plethora of advances in camera technology over the last decade rather than the desire to squash in megapixels like they were sardines in a can.

 

 

In the two photographs directly above this, the top image has a higher number of pixels, but a lower spatial resolution due to the actual size of the photosites on the sensor (what we refer to as pixel pitch).

 

The images below provide another example of this, by comparing two different Canon cameras. You will notice that all variables in terms of the shot are exactly the same – including the same light, F-stop, exposure and ISO (400). The only difference is the camera – and with a stationary object like in the photo this basically translates into pixel pitch. This should give you a great indication of how important sensor (and thus photosite) size is, with the comparison here being between a 2.3 micron pixel pitch and an 8.2 micron pixel pitch.

 

If the huge difference here isn’t clear enough, then cast your eyes over the second set of images. These are merely an attempt to make the images usable, by adjusting them to show more detail in the shadows and highlights. You can see that this markedly improves the quality of image for the (second) shot that was taken with the camera with the larger pixel pitch. If anything though it actually decreases the quality of the image taken with the smaller pixel pitch camera. This huge amount of additional noise is because less light was collected by the camera with the smaller pixel pitch, and so there is even more noise when this low light level is magnified.



This is the main reason why Samsung dialed back the megapixels from 16 to 12 with the addition of the Samsung Galaxy S7. The lower number of megapixels has significantly increased the photosite / pixel pitch size and thus increased the cameras low light performance!

 

By the way this is also why most experts give the Samsung Galaxy S7 a clear edge over the iPhone 7 when it comes to low light performance. Both cameras have the same number of megapixels but there is a large difference in pixel pitch. And in fact the new kid on the block, the Google Pixel has the best dynamic range of all – and as they continue to improve it this may well translate into the best low light performance (perhaps not quite there yet). Pixel pitch of these three smartphone cameras: iPhone - 1.12µm, Galaxy S7 - 1.40µm, Pixel - 1.55µm.

 

By the way I am not trying to open up some sort of performance war between these three phone cameras – and I actually decided to remove the images demonstrating low light performance. Let us just say that all have great image quality for a smart phone, and shocking quality for a camera. Equally, there are light situations where each of these has an edge over the other two.

 

And never forget, the smartphone easily destroys the humble compact and even the lofty DSLR when it comes to sms!

 

In conclusion though increasing megapixels can reduce image quality in small sensor cameras if the resultant photosites (pixel pitch) is then too small.

 

 

Other advantages of larger sensors

 

  1. Narrow depth of field

 

Larger cameras with larger sensors also have the advantage that they have a narrower depth of field for the same F-stop value. Bigger sensors are better for isolating a subject in focus while having the rest of the image out of focus (blurred). This blurred-background-effect or bokeh is an important part of good photographic images for certain types of shots. Cameras with smaller sensors have to be much closer to the image and so struggle to create the same blurred background effect. Images would be required to be moved much further away from a subject to achieve the same effect – and this would result in a far smaller image in the photo! So although your Samsung S7 accurately tells you it has an F-stop of F1.7 this does not mean that it has even close to the same depth of field or bokeh potential of a full frame DSLR with an F1.7 lens on it!

 

Alternatively, small sensor cameras could use a much wider angle lens and therefore also much faster lens (lower F-stop). And this often becomes impractical to impossible. As an example of this if a mobile phone (approximately a 1/3-inch sensor) was to attempt to replicate the depth of field and therefore bokeh from a full frame DSLR such as the Canon 5D IV with a 28 mm f/2.8 lens, it would require a 4 mm lens with an F-stop of F0.4! There are not many of those floating around...

 

  1. Dynamic Range

 

Larger sensors with larger photosites / pixels can usually capture a greater range of light to dark without having this become solid white or solid black. This range of light is called dynamic range, with large sensor cameras having higher dynamic range. This has several advantages you may not have considered. Have you ever taken a photo you really like, only to discover that some of the light areas are ‘blown’? You end up with those solid white overexposed areas that cannot be darkened even in raw file editors such as Photoshop Lightroom. It is one of very few aspects of a photo (together with out of focus images) that cannot be changed with photo processing software. The other advantage has to do (not surprisingly) with the other end of the light range, where high dynamic range allows a camera to preserve a lot more detail in the deep shadows.

 

Angle of view / crop factor

 

Angle of view / crop factor can be an advantage or disadvantage, depending on what types of images you are trying to shoot. In the above images you can clearly see an example of the concept of angle of view / crop factor. The image on the left was taken with a full frame camera and the one on the right was taken with a cropped sensor 4/3 camera.

 

For the same reason that large sensor cameras require larger lenses, they also have a different angle of view if they are using the same millimeter lens. So a full frame Digital Single Lens Reflex Camera (35mm sensor) has a wider angle of view than a cropped sensor Single Lens Reflex Camera, which has a wider angle of view than a compact camera, which has a wider angle of view than a mobile phone. 35mm sensors are generally seen as the standard for measurement of angle of view / crop factor. So a 50mm lens on a 35mm full frame DSLR would have a focal length of 50mm and thus no crop factor (crop factor of 1X). A cropped sensor camera (they tend to be around 24mm depending on the camera) will have a crop factor of around 1.5 – 1.6.

 

A way of understanding crop factor is to consider how much the magnification of a lens would have to be reduced by, and therefore the width of the image would have to be ‘zoomed out’ or cropped by to show the same image as on a full frame camera.

 

Hopefully you can see that the type of crop factor and angle of view that you get will vary widely depending on your cameras sensor size. So once again you need to begin by considering what types of images you are mainly going to be capturing, and then buy a camera with the type of crop factor / angle of view that is in line with those images.

 

Full frame DSLRs are immediately more useful for wide angle shots such as landscapes. If you use a fisheye lens frequently you will also notice that they only fill the frame on full frame DSLRs. On the other hand cropped sensor DSLRs are great for things like wildlife and most sports as you get an extra 50% to 60% of magnification (1.5X to 1.6X).

 

This the main reason why this author’s go-to camera for wildlife photography is the Canon 7D II. As a cropped sensor camera it has a built in extra magnification (crop factor) of 1.6X (60% extra) when compared to a full frame camera such as the magnificent Canon 1Dx II. This means a 7D camera with a 400mm lens gets an effective focal length of 640mm (400mm X 1.6), which is larger than the 1Dx with a 600mm lens on it! As a quick point of comparison let us consider the size and weight of 400mm and 600mm lenses with the same F4 aperture. The Canon EF 400mm f/4 DO IS II USM Lens costs less than $7000 (still not exactly cheap) and weights around 2.1 kg, whilst the Canon EF 600mm f/4L IS II USM Lens costs a bank breaking $11500 and weighs in at a hefty 3.92 kg.

 

Unfortunately compact system cameras, due to their tiny sensor sizes, tend not to give you a lens focal distance in millimeters. They tend to just report the magnification of the lens (e.g. 12 X). However, if you can work out ratio of the size of the sensor compared to the size of a full frame DSLR sensor you will then know the crop factor as well as the angle of view.

 

Feel free to contact us here at Cameraverse if you would like some advice on which Compact Digital Camera, Digital Single lens Reflex Camera or Mirrorless Interchangeable Lens Camera will best suit your needs.

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