For the newcomer to photography sensor sizes and camera lens focal lengths can be very baffling, so I am going to try to shed some light on the matter.

In the days of film only cameras, the early cameras used film plates and these were gigantic 8″ x 10″ and 4″ x 5″ – many photographers still use these “large format” film sheet cameras today for the highest levels of image quality, but they are cumbersome and difficult to use.

Then came along the Kodak brownie camera and eventually film cameras for most consumers and professionals settled upon 6cm x 6cm or 6cm x 4.5cm using 120 film and this is known as “medium format”.

Medium format was great but it had several problems – the film was still quite large, and thus the cameras were generally big and heavy, and if they had a reflex mirror, this was big and heavy and caused a lot of camera shake through vibrations. Furthermore, depending on the width of the image, a roll of film only allowed less than 20 photos before one had to change rolls, and this was not only inconvenient, but for the consumer, expensive.

Then someone had the bright idea of using movie film for still cameras and we ended up with 35mm “full frame” film and much smaller and spontaneously shootable cameras which took the world by storm in the 1960’s onwards. Initially these cameras were met with much resistance from the professionals who felt the much smaller film size equated with poorer image quality. This was partly addressed by better lenses and better film, but in the end, the best camera was the one you had with you.

35mm film cameras were so popular, most people of the pre-digital age still think of lens focal lengths and the apparent field of view, and depth of field in terms of 35mm full frame cameras, such that a “normal” lens was of 50mm focal length, a short telephoto portrait lens would be around 100mm focal length, etc.

Enter the digital age, and digital sensors were very expensive to make and thus 35mm full frame sensors, not to mention almost medium format sized sensors were costing in the $10,000’s (and many still are).

Canon and Nikon took an affordability approach and introduced dSLRs as cropped sensor cameras – cameras with a sensor somewhat smaller than 35mm full frame to save cost – in Canon’s case it was called APS-C and had a crop factor of 1.6, while Nikon called their sensor size “DX” and it has a 1.5x crop factor.

The crop factor is applied to a lens focal length to determine the equivalent lens giving the same field of view and depth of field on a 35mm full frame camera.

Thus if one used a normal 50mm f/1.4 lens on a Nikon DX camera, it would now be more telephoto in field of view, similar to a 75mm (50mm x 1.5) lens on a 35mm full frame camera, furthermore, the depth of field at f/1.4 would be similar to the depth of field of a 75mm at f/2.1 (1.4 x 1.5).

To confuse things, Canon also brought out a professional cropped sensor size called APS-H which has a 1.3x crop factor and thus is bigger than APS-C or DX.

Olympus decided that 35mm form factor was now totally irrelevant to photography and that there was no need to be tied to legacy systems so it discontinued its 35mm film camera system – the wonderful, compact, brilliantly engineered OM system – and as part of the Four Thirds consortium they decided that best edge-to-edge image quality would be attained using a 2x crop factor size sensor, and this would also enable smaller cameras and lenses, and this compact size has finally been realised with the Micro Four Thirds mirrorless system.

Even though the Olympus sensors are 1/4 the area of a 35mm full frame sensor, they are still 6 x larger than most point and shoot cameras and this gives them the great advantage of compact size but with far better image quality than point and shoot cameras which have tiny sensors.

So what do you get with increasing sensor size?

• larger and heavier lenses for a given focal length as they need to cover a larger image circle, thus often more expensive, and much more strain on your back, not to mention issues with air flight carry on cabin baggage which is often limited to 7.5kg.
• better high ISO performance for a given sensor technology and sensor photosite density as the photosites can be larger
• more dynamic range (ability to capture the very bright areas with detail as well as the very dark areas with detail)
• allow more megapixels at same pixel density, and thus potentially more image detail but only if high quality lenses are used and they are focussed well with no camera shake.
• shallower depth of field for a given lens focal length and aperture, and thus better ability to blur the background, particularly with wider focal length lenses
• the shallower the depth of field, the more difficult photography can become as focus choice must be more precise, and the focus must be more accurate – pro cameras have a multitude of customisable functions designed to optimise AF for certain subjects – the casual photographer is likely to become confused and have blurred images for moving subjects by not learning how to use these tools correctly.
• shallow depth of field is especially problematic for landscape photographers who generally want everything sharp
• shallow depth of field may become a problem in low light circumstances when one is forced to use a wide aperture, although this is partly offset by being able to use a higher ISO and thus a smaller aperture
• larger photosite size also means ability to use smaller apertures with less degradation of image resolution due to physical issues resulting from light diffraction – apertures smaller than f/8 should be avoided with 2x cropped sensors, while f/22 and smaller should be avoided with 35mm full frame sensors.
• poorer image quality away from the centre – the physics of optics design mandates aberrations increase exponentially the greater the distance from the centre, plus, there is the issues of the outer sensors not receiving as much of the light causing darkening towards the periphery (“vignetting”)
• less telephoto for a given lens (this has more to do with pixel density than sensor size but at pressent 2x crop give far better telephoto for lens size than do 1.6 crop or full frame – 800mm handheld is very doable with Four Thirds)
• higher cost
• ability to print to larger size with same print quality – point and shoot cameras may allow up to 11″ x 16″ prints if carefully used, while 2x to 1.3x cropped sensors print to 20″x30″, and medium format will print to commercial large print sizes well.
• the medium format sizes are currently VERY expensive (eg. $25,000 upwards in general), are slow in use (usually only 1 frame per second), have limited high ISO due to the sensor design, and are big and heavy with expensive lenses to match (eg. the Leica S2 lenses start at $US5000 each).

Everything in photography is about compromise.

For most people, a 2-1.5x cropped sensor size is the best compromise in cost, ease of use, portability and image quality – these are good enough to enlarge a print to 20″x30″, and provide adequate ability to blur the backgrounds for most situations. The difference in image quality between 2x and 1.5x cameras is becoming less and less relevant as technology progresses – the far majority of most people’s photographs are taken at ISO 100-400 because no matter which camera you use, the lower the ISO, the better the sensor image quality and the greater the dynamic range.

If you are using these cameras indoors hand held without a flash, it is highly recommended you use a high quality lens with an aperture less than f/2.0 (eg. 25mm f/1.4), and this should allow ISO to be kept as low as ISO 800 for most situations. Don’t expect to take great photos in low light situations hand held without a flash using an f/3.5-5.6 range “kit” lens – give your sensor a chance with at nice wide aperture.

This is why in the Micro Four Thirds, you can now get 12mm f/2.0, 20mm f/1.7, 25mm f/1.4, 45mm f/1.8 lenses – because wide aperture lenses optimise the relative deficiencies of a 2x crop sensor.

Those wanting better high ISO, dynamic range, larger prints, and shallower DOF with wide angle lenses, will need to consider 35mm full frame or larger (eg. Leica S3 is part-way in size to a medium format dSLR).

You just can’t get the same imagery as a 25mm f/1.4 lens on a 35mm full frame camera on a cropped sensor camera – on a 2x crop camera, you would need a 12.5mm f/0.7 lens, and that is just not going to happen!

Likewise, you can’t currently get the same imagery as a Canon 85mm f/1.2 lens with a 2x crop camera, but with a 50mm f/0.95 lens you could get a little close.

But there is no point buying into full frame just for these reasons if you can never afford the $2000 plus price tag for those lenses.

You can get almost identical imagery of a Canon 135mm f/2.0L lens on a Canon 1D camera with a 2x crop camera – just get the really cheap but excellent Rokinon 85mm f/1.4 lens, or even better, use the super-expensive Canon 85mm f/1.2 lens for even shallow depth of field.

 

see also dpreview’s article.