It’s one of the weird relics from the transition to digital. We’ve still got the old lenses, but the cameras we’re attaching them to are no longer the same. When you use an old lens with a digital camera, you have to multiply the lens’ focal length by a number known as the crop factor. I’d like to take a quick moment to tell you what a crop factor is, how it affects your photography, and why it’s important.
If there is any big takeaway, it’s this. Digital camera sensors are smaller than 35 millimeter film. When you attach a film lens to a digital camera, only a portion of the image gets projected onto the sensor. The rest is cut off around the edges.
Imagine using a flashlight to shine the famous Bat Signal onto a square piece of paper. If you were to cut some of the edges off of the paper and discard them, less of the Bat Signal would appear on the paper, and more would appear on the surface surrounding the paper. That’s what’s going on in your digital camera when you attach a film lens. In effect, what you get is an image that looks a little more zoomed in. The degree to which it is zoomed in is known as the crop factor.
So, what you do to get the Bat Signal to fit into the now smaller piece of paper? That’s right, you’d have to take a few steps back until it fits. We also know this as zooming out. When you know the crop factor, you can figure out how much to zoom in or zoom out to get the image to fit in the way you want. That’s why it’s important. The following math will help you use it. (Update: Changed 'zoom in' to 'zoom out'. Thanks to Armond in the comments for noticing).
How to figure out how much your lens crops the image
The crop factor is a useful number because it tells us what our image will look like on the sensor. We can multiply our crop factor by the focal length of the lens to determine how much more zoomed in we’ll be.
It goes like this:
Lens Focal Length * Crop Factor = Cropped Picture Focal Length (what it is zoomed to)
So, if you were to have a 50mm Nikon lens with a crop factor of 1.5, you’d end up with a picture that appears as though it were taken with a 75mm lens. It would be zoomed in to that focal length.
The above picture would look like this:
Some film lenses crop a little more than others
That’s where the 1.5 crop factor for Nikons and the 1.6 crop factor for Canon lenses comes from. The sensors inside the cameras are slightly different from one another, so they produce slightly different results. As long as you know the math above, you’ll be fine.
What about DX and digital lenses?
You don’t have to worry about anything with these kinds of lenses. They’re designed for digital, so there is no crop factor. Just keep shooting away as you normally would, and what you see is what you will get.
In fact, I highly recommend purchasing and using digital camera specific lenses. It removes a ton of hassle.
If you have any more questions or anything else you’re curious about, don’t hesitate to ask. Feel free to leave a comment below or send me an email.
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