This is the second installment of the Understanding Your Lens series of tutorials. If you have not read the first part I suggest that you go through it otherwise this lesson will be a bit tricky to comprehend.
Part one discussed the effects of varying focal lengths in terms of lens compression; how a wide angle lens seem to make the background more distant while a telephoto lens brings the background closer. This is a very important concept in understanding this next topic which is crop factor.
Technically, crop factor is not a feature of the lens but of the camera sensor. You are probably familiar with the different digital sensor formats. We have the full frame sensor which has the same size as a 35mm film frame, the APS-C sensor which is approximately 2/3 the size of a full frame and the 4/3rds format which is 1/2 the full frame size.
Full frame cameras include the Nikon D700/D800 and the Canon 5D series and most high end versions from both manufacturers. Sony also has full frame cameras, the A800 and A900. APS-C sensor cameras are the most common. Examples are the Nikon D300/D7000/D5000, Canon 7D/60D/600D, Sony A77/A65/A55 and Pentax K5/K30/K7. Then we have the 4/3rds format like the Olympus E-5/E-P3/OM-D and the Panasonic G/GF series.
Different sensor sizes generally require different types of lenses. In the case of Nikon we have FX for full frame and DX for APS-C. With Canon, EF lenses are for full frame and EF-S are for APS-C. A full frame lens will work with an APS-C sensor but not the other way around (although there are still exceptions).
A common source of confusion is in what photographers call the lens zoom factor. You have probably heard of a normal 50mm full frame lens becoming a mid telephoto (75mm) when attached to an APS-C sensor camera or why you should get a 35mm lens instead if you want a “normal” lens because your camera uses a crop sensor or how a 200mm lens magically becomes a 300mm.
What exactly is this zoom factor?
Zoom is used incorrectly in this context. The lens actually remains the same. A 50mm is still a 50mm no matter which type of sensor it is attached to. What this means is that the effects of lens compression does NOT change. The perceived distance between the foreground and the background remains the same for the same focal length. There is no zoom at all.
Let’s use the same (crappy) shot that I took in part one. This is a full frame shot of Thomas the tank engine:
If the same scene is captured by the same focal length at the same distance to subject by a crop sensor camera, this is how it will look like:
At first sight, it seems that you have magically zoomed in. Well not really. Have a closer look at the perceived distance between Thomas and the house. It is exactly the same. The same amount of lens compression is taking place. The “zoom” is purely an illusion. A fake zoom produced by the smaller sensor’s tighter field of view. The “zoom” is a side effect of the cropped sensor.
So again, zoom factor is a misnomer.
A common follow up question is, so which one is better, a full frame or a crop sensor camera?
There are advantages to using a crop sensor camera:
1. Lenses are sharpest at the center and gradually become softer around the edges. Crop sensors utliize only the center of the lens thus producing generally sharper images compared to full frame cameras.
2. Lenses designed for crop sensors are smaller because they only have to cover a smaller image circle.
3. For the same field of view and aperture, crop sensor lenses/cameras have greater depth of field. This is a huge advantage for landscape, low light and macro photography where focus is very important. For example, while a full frame camera might require f5.6 at ISO 6400 to shoot a concert while keeping all the band members in focus, a micro 4/3rds camera can shoot the same scene at f2.8 ISO 1600 thus producing much cleaner images. In macro photography, full frame cameras will stop down to f16 or f22 but a micro 4/3rds can shoot at f8 or f11 respectively for the same depth of field therefore gaining two stops of light advantage.
Of course there are disadvantages for using crop sensor lenses as well. If shallow depth of field is your thing, full frame is the way to go. Full frame cameras also offer bigger, brighter view finders which to me is very very important. Bigger sensors generally produce cleaner images at the same resolution because of the larger sensel dot pitch.
So let me summarize everything:
1. There is no such thing as zoom factor. It does not exist. It is purely an illusion.
2. A 50mm lens is a 50mm lens no matter which camera it is attached to. The same lens compression is produced. The perceived distance between foreground and background remains the same.
3. Full frame and crop sensor cameras have their own advantages and disadvantages. What matters is whether you understand the implications and whether you can fully utilize those in your photography.