Bokeh Episode IV (A New Hope)

OK, this next bit is lengthy so we’ll split it into two posts. We’re going to talk about crop factor. For those of you who already know what this is, feel free to skip this post and check back the next one when I go into why this matters for bokeh. For the rest of you, read on!

OK, so. Way back in the bad old days of photography people used all sorts of fun and exotic stuff to capture images on. Most of these were big, and I mean really big, and had to be prepared by hand and used chemicals you’re probably not allowed on an airplane with these days. Eventually, these things were replaced by film, which found it’s most popular form by far in the 35mm negative. Then, near the end of film’s rule as king, and right before the dawn of digital, there was a format called APS-C (the history of which I’m not going to get into) which was somewhat smaller than the frame of a 35mm negative. More or less, and for various reasons, this smaller format became popular with DSLRsand we’re only now starting to see a resurgence of 35mm-sized sensors. But, alas, APS-C isn’t the only smaller sensor size, and for a whole slew of reasons you’ll find that pretty much every camera maunfacturer out there uses a different sensor size, most of the time smaller than 35mm.

Now, since 35mm was used for so long, its way of thinking and seeing is basically the common language of DSLRs. In 35mm, a 50mm lens gave us a Field of View (FOV) of 45 degrees, which as I’ve mentioned is about the same as what we as humans actually see in real life. But, these smaller sensors, they effectively “crop” from the center of the image the lens is projecting, which changes the FOV we see. For example, let’s look at the picture on the right. The big picture there represents a “full-frame” picture with a given lens. The yellow area represents the difference in size for a standard APS-C sensor, in this case the same difference in size as you’d find in the avergae Canon Digital Rebel camera. You can see how less of the image falls onto that sensor. But, since only what hits the sensor is shown on the final pictutre, it looks like it was shot using a more telephoto lens. See the crops below for an example of what the final output from the two sensors would show for the same lens.

So, let’s say the lens was a 100mm. In the “full-frame” 35mm shot it gives a field of view of around 23 degrees. Put that same 100mm lens on an APS-C sensor (which has a 1,6x crop, that is, the diagonal size of the APS-C sensor will go into the diagonal size of the 35mm 1.6 times), and that field of view is now 14 or so degrees. The length of the lens didn’t change, but since we’re only seeing a chunk out of the middle it looks like it did. Now, since most people don’t know what 23 degrees FOV looks like, but a lot of people know what a 100mm on a film SLR did, it’s the frequent practice to express field of view in 35mm equivalents. To do this, you take the actual length and multiply by the crop factor. So, the APS-C picture looks like it was taken with a 160mm lens in 35mm equivalent.

Now, there are a lot of sensor sizes, as I mentioned, and they all have different crop values. I made up a little chart you can click on there to the left and see the different sizes relative to each other, as well as their crop values. Looking at it you can see, for example, that Olympus digital cameras like my E-3 have a 2x crop. That 100mm lens on my camera will give me the same field of view as a 200mm would have on a film SLR. (Please note only the FOV changes as a result, most of the rest of the lens is still just a 100mm, we’re just seeing it different now). A DX Nikon digital camera system as you can see has a 1.5x crop, so that 100mm will have the field of view of a 150mm from the film days.

It’s important in this age of modern cameras to understand this, because the real focal length of these lenses never changes. A 100mm lens always has a focal length of 100mm no matter which crop factor you use it with. Crop factor merely changes the field of view.

Crop factor also has a few other effects on the final image, and I’ll talk about those next time now that we have the basic groundwork in place. If you’re still confused please do a Google search for “crop factor explained,” or read these posts here or for more in depth and complete information, here.

Next time, crop factor and depth of field. Your questions answered, your mysts demystified.