By 'full frame' we mean that the EOS 5D's sensor is the same size as a 35 mm negative, this means that lenses used on the camera will produce the exact same field of view as they would on a 35 mm film camera (as they were designed). This is especially advantageous when shooting wide angle as we will get a much wider field of view than we would on a digital SLR which 'crops' (such a the EOS 20D). The diagram below demonstrates the difference in field of view between the EOS 5D and the EOS 20D using the same lens with a focal length of 17 mm. The EOS 20D would only be able to capture a portion of the center of the field of view produced by the lens, the EOS 5D captures the entire view.
Example of an picture taken with a focal length of 17 mm
On the downside a full frame sensor puts a higher requirement on the quality of the lens (as you are now also capture the 'less good' edge and corner of the view), with certain lenses this may lead to softness, chromatic aberrations and light fall-off near the edges of the image. Lastly many current digital photographers who don't shoot wide angle may appreciate the effective 'multiplication factor' produced with telephoto lenses (although obviously you can always crop an EOS 5D image in post processing).
Full frame vs. Cropped sensor
Below is a simplified summary of the pros and cons of Full Frame and Cropped (APS sized) sensors. As you can see it's a bit of a minefield, both formats have their advantages. The biggest thing to take from this is that Full Frame is not the 'answer to everything' that many think, and that thanks to years of development of so called cropped sensors and lenses the advantages of Full Frame aren't as attractive as they may have been.
|Full frame|| FOV matches indicated focal length
Only way to achieve super-wide angle
Pixel pitch larger (lower noise)
Lower lp/mm requirement for lens
Viewfinder view large and bright
Matched prime lenses
Shallower depth of field *1
| Lens-sensitive, requires good lenses
Edge / corner softness / CA
Fall-off (vignetting) with some lenses
Can not use smaller 'digital only' lenses
Expensive to manufacture
|Cropped|| Uses best part of the lens (center)
Not as lens-sensitive
Advantage for telephoto (FOV crop)
Compact, light 'digital only' lenses *3
Increased depth of field *2
| Wide angle requires even wider lens
Viewfinder view smaller, darker
|*1||Only shallower because for the same field of view you would need to get closer to the subject than with a 'cropped sensor' camera.|
Just as a shallower depth of field may be an advantage to one photographer so a slightly larger depth of field may be an advantage to the next. Again this is because of subject distance.
|*3||Although as we will demonstrate later in this review these are seldom any better from a performance point of view than a normal 35 mm lens on a FF camera.|
Potential optical disadvantages
As you can see from the 'Cons' list above the primary disadvantages of full frame appear to be related to lenses. The simple truth is that most digital photographers who have used cropped sensor cameras (virtually all digital SLR's) have lived with the luxury of always using the best part of their lenses. This means that you are avoiding the corners and edges of the elements of your lens which are more prone to aberrations and softness. A full frame sensor is far less forgiving, in fact it's not forgiving at all, it 'reveals' a lens fully, which means good lenses and good practice (such as stopping down; using a smaller aperture) are important on a full frame camera.
Depth of field differences
Depth of field is the amount of depth in the image that will be 'in focus' (acceptably sharp) as a distance around the focus position, typically more behind the subject than in front. A shallow depth of field can deliver the soft 'bokeh' background that is the signature of SLR photography, by contrast a larger depth of field can be useful for getting more of the image in focus. Depth of field is a function of the absolute focal length, aperture, subject distance and film / sensor format.
Using the same 105 mm focal length on a cropped sensor camera (say 1.6x, the EOS 20D) and a full frame sensor camera we would have to move 1.6x further away (subject distance) with the cropped sensor camera to get the same field of view. Imagine our subject is 1 m away from our EOS 5D and is perfectly framed, we would have to step back 0.6 m (1.6 m subject distance) to get the same framing with an EOS 20D. The depth of field at F4 in the EOS 5D shot would be approximately 2 cm, for the EOS 20D shot it would be 3 cm.
Below is a table demonstrating the difference in depth of field using the same lens / focal length at different apertures. Note that for simplicity we have chosen a subject distance of 1 m to frame the shot perfectly with the full frame camera, hence a 1.6 m subject distance would be required for the same framing (with the same lens / focal length) with the cropped camera. As you can see a cropped sensor does effectively increase depth of field (although we are simplifying what actual happens).
|EOS 5D (1 m subj dist.)
(36 x 24 mm sensor)
|EOS 20D (1.6 m subj dist.)
(23 x 15 mm sensor)
|24 mm @ F4||42 cm||70 cm|
|24 mm @ F5.6||63 cm||103 cm|
|24 mm @ F8||97 cm||161 cm|
|105 mm @ F4||2 cm||3 cm|
|105 mm @ F8||4 cm||7 cm|
Sensors compared, a microscopic view
We used this diagrammatically view for the first time in our review of the Nikon D2X. It's a useful reference as to the relative pixel pitch of each camera's sensor. These diagrams represent an area of the surface of the sensor measuring just 0.1 x 0.05 mm (1/254 x 1/508 in). Another theoretical advantage of a much large sensor is of course that its pixel pitch can be much larger and hence each photosite capable of capturing more photons, delivering a larger 'signal' which (should) mean lower noise and higher dynamic range.
|Canon EOS 5D
CMOS 13.3 MP, 8.2 x 8.2 µm
|Canon EOS 20D
CMOS 8.2 MP, 6.4 x 6.4 µm
|Canon EOS 1Ds Mark II
CMOS 17.2 MP, 7.2 x 7.2 µm
|Canon EOS 1D Mark II
CMOS 8.5 MP, 8.2 x 8.2 µm
CMOS 12.8 MP, 5.5 x 5.5 µm
CCD 5.5 MP, 5.9 x 11.9 µm