Is crop-factor a bad thing?

Question

It seems to me that there is a preference for full-frame sensors rather than cropped sensors, and I'm curious as to why. It seems to me, that the cropped sensor means that I get more bang for my buck with zoom lenses. True, I suppose it means I would need to a shorter lens to get the same wide-angle effect on the short end, but it seems like wide-angle lenses are (generally) cheaper than telephoto lenses. Am I missing something?

Answer 1

No, it is not a bad thing. It is not really "good" or "bad" in any sense. Its simply a different format than full-frame, which is different than medium format, etc. There are pros and cons to each. The smaller APS-C style "cropped" sensors do have some effects on lens focal length due to their field of view, and that can be beneficial or detrimental, depending on how you choose to see it. Here are some facts about sensors:

1. Cropped Sensor Formats (APS-C)
   • These are smaller sensors
     • They have higher manufacturing "yield" than larger sensors
     • As such, they are generally much cheaper
   • Photosites are generally smaller and more densely packed
     • This generally results in lower signal-to-noise ratio, more noisy pictures
     • This also means the maximum dynamic range (contrast ratio) of cropped senors is lower (less light gathering power per photosite)
   • They have a narrower field of view compared to larger sensors
   • Their narrower FOV has the effect of multiplying the focal length of any lens
     • This may be beneficial if you need super telephoto lengths (i.e. 400mm on FF ~= 640mm on APS-C, effectively)
     • This may be detrimental if you need ultra wide angle lengths (i.e. 16mm on FF ~= 26mm on APS-C, effectively)
   • The additional "effective magnification" offered by a cropped sensor is only illusory, and is not actual magnification
     • Given a large enough sensor with enough megapixels, and the same exact "crop" provided by a cropped senor can be achieved with a full-frame or medium format (however, the larger sensor would need some SERIOUS megapixels to achieve this.)
       • The 1.6x crop sensor of a Canon 450D would require a full-Frame sensor with 31mp to achieve the same crop
       • The 1.6x crop sensor of a Canon 550D would require a full-frame sensor with 46mp to achieve the same crop
2. Full-Frame Sensor Formats
   • These sensors provide the same "usable" pixel area as 35mm film
   • These sensors are larger, and have lower manufacturing yield
     • This generally means they are more expensive
   • The photosites are larger and often less densely packed
     • This results in better signal-to-noise ratio, less noisy pictures
     • Dynamic range is generally higher with larger photosites.
       • (The new Canon 1Ds IV with a 30mp+ sensor is touted as having full 16bit RAW capability, which offers much greater dynamic range than the general 12bit RAW of cropped sensors)
   • Their field of view is "normal" from the perspective of the bulk of the photography community and equipment
   • A lenses focal length is as stated when used on a full frame
3. Medium Format Sensors
   • These sensors are often much larger than full-frame (up to 57mm or larger)
     • They have extremely low yield, and thus their cost is extremely high
   • They have high density, but large photosites
     • This results in some of the best dynamic range possible in a digital sensor
     • Leica and Hasselblad's latest medium-format sensors tout 24bit RAW
   • They may have a much wider field of view than normal 35mm for a given focal length
     • A lens of a normal 35mm focal length would be shorter on medium format, providing even greater field of view
     • As with cropped sensors, the effect is illusory, and only useful when describing things at a technical level

(Note that the effect of sensor size on focal length or the apparent magnification assumes a common lens system. Medium format cameras tend to be rather specialized, so a direct comparison here is likely impossible. For the sake of discussion, the effect given similar lens system and focal lengths would thread throughout the range of sensor sizes.)

Answer 2

Full-frame sensors have more megapixels, better noise, or both; in other words, they permit better image quality which is why many people prefer them. However, crop sensors yield extremely good IQ as well. You're correct that the make lenses longer, which is an advantage in many cases; one issue is that there's much less legacy glass which is wide on crop sensors. They also yield smaller and lighter cameras.

Wide-angle lenses aren't cheap, but there's a limit as to how wide you can get. On the other hand, you can make a lens as long as you can afford, which is why there are truly huge and expensive telephoto lenses but no (well, very few) wide-angle exotics.

Answer 3

If you want more "zooming", I think you're right, it's an advantage. One advantage to a larger sensor is reduced noise and/or better high ISO performance.

Here are a couple of other references to consult:

http://digital-photography-school.com/full-frame-sensor-vs-crop-sensor-which-is-right-for-you

http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm

Answer 4

Cropped sensors have longer depth of field at similar f-number and field of view, which might be beneficial in some cases (macro, photo-journalism, low-light, manual focus based on distance scale) or detrimental in others (portraiture, cluttered background, selective focus).

Answer 5

There is another aspect of Crop Sensors: in a crop sensor the incidence angle is closer to 90 degrees at the corners of the sensor, and is less perpendicular for large sensors. This can lead to some "vigneting" caused by the light transmitting less "power" to the sensor at such angles, in a way similar as a solar cell being less effective if not perpendicular, the crop sensor is less prone to this. I have seen this effect in rather cheap cameras, the image is slightly darker when far from the center, however, I have never handled a camera with a big sensor. (I use Four Thirds)

Answer 6

The sensor size will make a difference in the visibility of lens aberrations. If the larger sensor has the same number of megapixels as the smaller one, it will show more center sharpness as the image plane is less magnified. On the other hand it might show more corner softness and vignetting as it is using parts of the lens that just aren't visible on the crop sensor.