Tip of the Week: Understanding Sensor Crop Factors, Part 1Tips & Tricks
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You’ve likely heard the term “Crop Sensor” before, and if you’re new to the world of digital photography, then you may only have a rudimentary understanding of what that means.
In Part 1 of this series, we take a look at three different types of sensors and the practical effect they have on lens selection. In Part 2, we’ll take a look at what this means for depth of field, and that “bokeh” thing everyone’s talking about.
What’s a “Crop Sensor” camera anyway?
To understand what a “Crop Sensor” camera is, you first have to understand what a “Full Frame” sensor camera is, and that takes us back to the days of film photography.
A piece of 35mm film measures approximately 36 x 24mm in size, and that’s the size of the sensor in “Full Frame” cameras like the Nikon D4 and the Canon 5D Mark III. Cameras with these sensors typically occupy the higher end of Canon, Nikon and Sony’s offerings, and are also among the most expensive DSLRs you can buy from them.
All three manufacturers also make cameras with smaller chips. Nikon and Sony have cameras like the Nikon D7000 and the Sony A77 that have “APS-C”-sized sensors measuring 23.6 x 15.7mm. Canon’s APS-C sensor is a bit smaller, measuring 22.2 x 14.8mm. Canon also has an APS-H sensor that sits between the two, and measures 28.7 x 19mm in size.
Great! Now what does that mean for me?
When you rent or buy a lens, the type of lens it is is described in two ways: focal length, and max aperture. For example, you can get 50mm f/1.4 lenses from all three camera manufacturers I mentioned earlier, as well as third-party lens makers.
The max aperture – in this case it would be f/1.4 – stays the same regardless of camera. The focal length, however, is subjective.
On a “Full Frame” camera like the 5D Mark III, a 50mm lens is a 50mm lens. That’s because the focal length of the lens, is measured based on the standard 35mm film size.
However, on a “Crop-Sensor” camera, like the Nikon D7000, your 50mm lens becomes a 75mm lens. Since the sensor is smaller, it’s only seeing a slice of image the lens is projecting onto it, and that makes it mimic a lens with a “longer” focal length.
In effect, the smaller sensor is “cropping” the image being transmitted to it by the lens – hence the term “Crop Sensor.”
On the Canon side of things, since the APS-C sensor is smaller than the Nikon’s, that 50mm lens becomes an 80mm lens.
Now hold on! How do I figure out what lens is what on what camera?
Here’s where the phrase “Crop Factor” comes in. Since the size of the sensor is known, we can easily calculate the “effective” focal length of a lens.
For Canons, multiply whatever you see in the lens description by 1.6 when you’re using a camera like the 7D, or the T2i.
For Nikons and Sonys, multiply the same thing by 1.5.
But wait! There’s more!
Canon decided to add another sensor size into the mix with the APS-H sensor size, which I mentioned above. There are only two cameras in our inventory that have this sensor, the Canon 1D Mark III and the Canon 1D Mark IV. To figure out the effective focal length of a lens on these cameras, multiply what’s in the lens description by 1.3, and you have your answer.
Oookay… And this means what, in practical terms?
Visual aid time. There are three images in each of the graphics below. All were shot with a Canon 70-200mm lens. The first graphic shows that lens zoomed all the way out at 70mm, but on three different cameras. Clockwise from the top-left, they are the Canon 5D Mark II (Full Frame sensor), the Canon 1D Mark IV (APS-H sensor), and the Canon 7D (APS-C sensor).
As you can see, the smaller the sensor, the closer you get to your subject with the same exact lens.
Here’s the same thing, this time with the lens zoomed in at 200mm.
Once again, we see the same effect. Here, however, it’s a bit more pronounced, and that’s important.
Going from a 5D Mark II to a 7D means that your long lenses, like the 400m f/5.6, for example, will be longer (it’ll be a 640mm f/5.6). That’s great news for people who want to do, say, bird photography, but don’t want to lug around a huge lens.
But for landscape photographers, it means that the 14mm f/2.8L lens you rented is actually a 22.4mm lens. Still pretty wide, but maybe not as wide as you’d like.
Fortunately, to counter this issue, lenses made specifically for “Crop Sensor” cameras are available from a number of different sources. Tokina, for example, makes an 11-16mm f/2.8 lens that is one of my favorite lenses for “Crop Sensor” cameras.
Because the distance between the back of the lens and the mirror is different for “Crop Sensor” cameras than it is for “Full Frame” cameras, these lenses can’t be used with “Full Frame” cameras. Happily, “Crop Sensor” camera users have access to lenses made for “Full Frame” sensors, so sometimes, depending on what you’re doing, it might make more sense to rent a D7000 vs. a D3.
One of my favorite things to do is shoot birds, so I often take a 1D Mark IV and a 600mm f/4 lens out for a day of shooting. That’s a big, honking lens, however, and I don’t want to carry that on a hike, so I’ll sometimes take a smaller body and lens, like a Canon 7D and a 400mm f/5.6, or a Sigma 50-500mm lens. The results are often surprisingly decent, as you can see from the image here.
I hope this gives you a better understanding of what a “Crop Sensor” camera will mean for your lens selection, versus a “Full Frame” camera. In part II of this article, I’ll talk about the effect this has on depth of field. If you have questions or feedback, please feel free to leave them in the comments section below.
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