Photographic image quality – the roles of sensor size, pixels and other factors

Written by Gary on August 5th, 2010

There is a lot of confusion around on the internet regarding what is the best camera, etc to buy and most people buying cameras will be hoping for the best quality photos for their money.

Hopefully I wont be adding to this confusion, but rather provide some general rules of thumb.

My Rule Number 1:

For most photographic subjects, the MOST important factors in the quality of the photograph are:

  • the subject itself
  • the lighting of the subject
  • the vision of the photographer in selecting a composition, point of view, perspective, depth of field, point of focus, exposure duration, exposure, contrast management and white balance (if a color image is chosen).
  • timing of the photograph may be critical to its success in capturing a critical moment, expression or emotion.

Note, the camera equipment itself is very much a secondary component, although for some subjects, the capabilities and characteristics of the camera and lens +/- tripod and filters, etc MAY be critical to enabling the photograph.

For many photographic opportunities, what is far more important than the characteristics of the camera, is whether or not you brought the camera with you – the best camera is the one you are willing to take with you – for many, this means small with adequate image quality is KING.

Amateur photographers tend to pontificate far too much on relative technical aspects of camera and lens image quality and pixel peeping, and perhaps should take a leaf out of the professionals books – professionals concentrate more on lighting than the camera or lens, although they generally have a high end camera and lens anyway so that image quality will be good enough as long as they get the other factors right as outlined above.

My rule number 2:

For most people the number of pixels is largely irrelevant.

For the same size sensor with the same sensor technology, the more pixels means less image quality in terms of dynamic range, exposure latitude, more noise at high ISO, but perhaps less obvious noise due to smaller pixels.

5-8 megapixels is sufficient to allow very acceptable printed enlargements up to 11″x16″ if the sensor size is reasonable and attention has been paid to ensuring image sharpness if sharpness is important to the image.

A high quality image taken with a 10mp cropped sensor from a dSLR can be printed with acceptable quality to 20″x30″  – very few people need larger prints.

20+ megapixels can allow more detail or cropping to attain a 20″x30″ print, but means your storage space will need to double compared to a 10 megapixel sensor.

The professionals who wish to have greater image quality will generally consider a medium format digital with 50+ megapixels but at substantial cost and loss of portability and functionality compared to dSLRs.

My rule number 3:

Take care in selecting your sensor size – sensor size DOES matter and has a number of important implications.

The benefits of increasing sensor size given similar sensor technology:

  • the greater the possibility of having higher image quality in terms of image detail, dynamic range, exposure latitude, lower noise at high ISO
  • the greater the possibility of choosing narrow depth of field to make your subject “pop” with added dimensionality, or to selectively focus on a single plane of focus
  • physical limitations of resolution due to diffraction effects comes in at smaller apertures – thus before diffraction starts adversely affecting resolution, a cropped sensor dSLR may be sharpest at f/5.6-8.0, a full frame camera may be sharpest at f/8-11 , conversely a point and shoot camera may require an aperture of f/2.8 to avoid diffraction effects.

The problems with increasing sensor size:

  • increased cost of sensor and lenses – camera and lenses need to be bigger and heavier to cover the larger image circle
  • shallower depth of field for a given aperture, focal length and subject distance which may be quite problematic when you wish to make everything “sharp” such as in landscapes
  • more difficult to use because the shallower depth of field forces you to select the point of focus and any errors will be much more objectionable
  • image quality towards the edges tends to decline according to the laws of physics – the aberrations increase exponentially the further from the centre of the image – this means larger sensors will expose any weakness in optical designs and mandate more expensive optical designs to ensure edge-to-edge of frame image quality.
  • for the above reason, it is often easier to make higher performing wide aperture lenses if there is only a smaller image circle required – however, this is yet to be realised by Olympus other than its range of unique, superb f/2.0 zoom lenses.
  • the increased details usually mandate closer attention to minimising camera shake or mirror vibration effects often requiring use of image stabilisation, tripod, mirror lockup or a higher shutter speed.
  • the larger the equipment size, the less likely you will take it with you and the more intrusive it can be on your subject which can adversely affect your photo opportunities.

Point and shoot digital cameras have tiny sensors which substantially limits image quality so that enlargements printed are generally limited to 8″x12″  while image noise or loss of image detail is problematic above ISO 200-400, and there is NO capacity to make your subject “pop” by selective focus unless it is a macro shot.

This is one of the reasons for the success of the Micro Four Thirds system – it offers many a perfect balance between size and image quality while its versatility in lens selection brings back the fun to photography.

Those wanting even shallower DOF, higher ISO capability or dynamic range should consider a full frame dSLR instead or to supplement a Micro Four Thirds system.

My rule number 3:

Telephoto reach for a given lens is proportional to the pixel density of the sensor as long as the lens can match the sensor resolution and as long as camera shake or subject movement can be minimised.

Thus, for cameras with reasonable dSLR quality image quality, the Micro Four Thirds and Four Thirds cameras have the highest pixel density and thus can give you the most image detail for the same size lens – this is one reason why I love the Olympus Four Thirds dSLR combined with an Olympus ZD 50-200mm f/2.8-3.5 – it gives the most compact, high quality 100-400mm f/2.8-3.5 telephoto reach available.

Not far off is the Canon 7D or Canon 1D Mark IV with their 17mp but larger sensors.

Many sports venues now limit people to using 200mm focal length lenses – if you want the most reach at 200mm, get an Olympus Four Thirds dSLR.

My rule number 4:

Those wanting to shoot fast action will generally need a camera with fast AF for action photography, preferably with a fast burst rate, and probably with further telephoto reach.

A mirror-less camera system such as Micro Four Thirds, Sony NEX or Samsung NX relies on contrast detect AF which, currently is much slower than high end dSLR phase contrast AF systems when trying to autofocus on fast moving subjects, and thus cannot be recommended for this usage at present.

Thus, if this is critical, then one should select a camera such as:

  • Canon 1D Mark IV
  • Canon 7D
  • Nikon D300s
  • Olympus E-3 or E-30

Note that a “budget” full frame dSLR such as the Canon 5D Mark II will not be as suitable as it’s AF system has been limited to cut costs.

Be aware that professional sports cameras such as the Canon 1D mark IV are complicated beasts to use as you need to tailor the many options for AF functionality to your subject – this is not for the faint hearted!

My rule number 5:

Usually the characteristics of the lens is MORE important than the characteristics of the camera.

When choosing a camera system, look at what lens you will need, what you can afford and what you would be prepared to bring with you given size and weight considerations.

Just a little explanatory note – remember when comparing lenses used on different cropped sensors, it is handy to convert them into equivalences for a lens used on a 35mm full frame camera:

  • to get lens on a 35mm with an equivalent field of view, multiple the lens focal length by the crop factor
  • to get lens on a 35mm with an equivalent depth of field wide open at the above focal length, multiply the aperture by the crop factor
  • hence a 50mm f/2.0 lens on a 2x crop sensor (eg. Olympus dSLR) will give the same field of view and depth of field as a 100mm f/4.0 lens on a 35mm camera BUT it will allow 2 stops MORE light in which means you can get away with two stops lower ISO than on a full frame camera which negates any difference in image noise.
  • of course, on the full frame camera, you could use a 100mm f/2.0 lens which would give even shallower depth of field and the same amount of light in, but probably less edge-to-edge image quality and perhaps too shallow DOF. To match this on a Micro Four Thirds, you could use a Leica-M Nokton 50mm f/0.95 or similar.

A few short examples:

Head and shoulder portraits often are most flattering when using a lens such as one with 35mm camera equivalence of a 100mm f/2.8-4.0 or similar. Too short a focal length means you have to get too close which creates unflattering distortions of facial features.

People often get far too excited about super narrow depth of field and spend a fortune on lenses such as the superb Canon 85mm f/1.2 which is renown for its shallow DOF – but at f/1.2 it is much too shallow a DOF for a head and shoulders portrait where you wish to get everything in focus from ears to tip of nose, and it’s very slow AF may mean you miss a lot of opportunities.

Many Canon users would be better off with the far cheaper 85mm f/1.8 lens although it is does have more aberrations wide open, or perhaps a 135mm f/2.0 lens.

Nikon users have the luxury compromise of an 85mm f/1.4 lens.

There is no such AF wide aperture short telephoto lenses available in the Four Thirds system so Olympus dSLR users would have to consider the superb ZD lenses such as the 50mm f/2.0 macro, the 35-100mm f/2.0 zoom or resort to using the  Nikon or other lenses in manual focus mode only.

On the other hand, there is no Canon or Nikon lens that can match the size, features and price of the Olympus ZD 50-200mm f/2.8-3.5.

Environmental portraits often are better with a wide lens to allow some context to be included and thus a 35mm equivalent of a 50mm f/2.8 tends to give good results (for a cropped sensor Canon or Nikon, a 35mm f/1.4 lens, in the Four Thirds world, the Leica-D 25mm f/1.4, or in Micro Four Thirds, the Panasonic 20mm f/1.7 pancake lens are well suited to this).

Specialist lenses such as the 85mm f/1.2, tilt-shift lenses, ultra wide angle lenses and super telephotos are MUCH more difficult to use and often have limited uses – think twice before you buy them!

Lens bokeh can be a very important quality which can detract from your photos, each lens has its own characteristics of out of focus areas and many photographers end up with a bucket load of lenses trying to find the one that suits them best.

Professional sports photographers will want to use their super telephotos on a monopod – eg. 300mm f/2.8, 500mm f/4, or if shooting indoors, perhaps the 70-200mm f/2.8 IS. For wide angle shots, the superb Nikon 14-24mm f/2.8G lens can’t be beaten on image quality for that focal length range.

Wedding and fashion photographers will usually want their mandatory 70-200mm f/2.8 IS lens.

Astrophotographers will want a lens with minimal aberrations wide open – this means NO optical image stabilisation elements and no zoom – hence a favorite is the Canon 200mm f/2.8 lens.

Tilt-shift lenses used to be the domain of Canon and Nikon, but Micro Four Thirds may change that dramatically by allowing one to convert essentially any legacy lens into a tilt-shift lens while allowing the easiest assisted manual focus system available courtesy of the absence of mirror.

Of course, almost any lens ever made can be mounted on a Micro Four Thirds camera albeit in manual focus and with a 2x crop factor, although it seems Olympus is working on a special 0.5x AF adapter which may provide AF and normal field of view for these lenses which would be extremely exciting indeed!

My rule number 6:

Consider the judicious use of filters.

A polarising filter is almost mandatory when shooting nature scenes with significant amounts of foliage such as streams in a rain forest.

A square or rectangular gradient filter is almost mandatory when shooting landscapes with substantial areas of sky which is not lit from the sun behind you or which contains light coloured clouds.

My rule number 7:

To attain maximum image detail to enable high quality enlargements greater than 20″x30″, attention to obsessive compulsive detail is mandatory.

This means a large enough sensor, accurate focus, heavy duty tripod, and mirror lockup unless you are shooting with electronic flash which will minimise camera shake.

This will also mean maximum optical resolution, which for larger sensors usually means stopping down to f/5.6-11.


3 Comments so far ↓

  1. markus says:

    Good article.
    But it’s sad that even the F2 Zoom lenses by Olympus with smaller image circle don’t have any advantage over full frame lenses. The Nikon 24-70 F2,8 for example has shallower depth of field and gives us more light because the sensor has less noise.

  2. Murray Lord says:

    I was intrigued by your comment “Many sports venues now limit people to using 200mm focal length lenses”. Can you give any examples?

    • admin says:

      Most of the sports venues in Australia have this limitation of 200mm – Australian Tennis Open, Australian Football games at Melbourne Cricket Ground and Etihad Stadium.