Crop frame cameras – why your field of view may not equal your focal length

Confused by the title? I thought you might be. Today I’m going to introduce you to the idea of digital sensor sizes and “crop” vs. “full frame” digital cameras. It may seem a bit complicated at first but there are really only a few things you need to know and take away from today’s lesson and I’ll be sure to point them out as we go along and give you a recap at the end as well.

Digital SLR cameras for the most part replace consumer 35mm film SLR cameras. They work in a very similar manner, but instead of a strip of film capturing the light, a little digital sensor does the job. You can even use many of the very same lenses you had for your old film SLR on your digital SLR. Where it gets tricky is that the sensors used are not all necessarily the same size as a 35mm film plane. So what you say? Well, what that means to you as the photographer is that your sensor is not capturing the entire image that your lens is seeing. In essence your smaller sensor is “cropping” your photo for you before it’s even taken. And to make it even more complicated, the “crop factor” depends on the particular camera you’re shooting, with most consumer Canon cameras having a factor of 1.6 and most Nikon cameras using 1.5. Nikon also refers to their full frame cameras as “fx” sensors and crops as “dx”… just to throw a little extra terminology into the mix.

Still confused? Let’s try another angle (get the pun?) You just bought your very first Canon Rebel on the recommendation of all of your fellow photography enthusiasts. Those very same friends of yours encourage you to ditch the kit lens and go for the “thrify 50” – the 50mm 1.8 lens. It’s a great choice. Way to go. You slap that puppy onto your camera, look through the viewfinder, and WOAH… everything is super zoomed in, right? You can hardly shoot a picture of the person sitting across the dinner table from you without stepping a few feet backwards. You scratch your head, confused. “But I thought you said in your post on focal length that a 50mm lens replicated the angle of view of the human eye, this doesn’t seem right”. Well good for you for remembering that little nugget of information. And you’re right. So why do things seem so close through the viewfinder of your camera? Blame it on the field of view crop factor. Your camera’s sensor only captures around 62% of the image that the very same lens would capture on a full frame camera. This idea is usually shown by a set of concentric rectangles, the outermost one representing the “full frame” shot and the inner ones representing the crop. Let’s take a look, shall we?

The full image is what would have been captured with either a 35mm film camera or a full frame digital camera, in this case a 5d. The image was shot with a 50mm lens. The grayed out portion of the image is 62.5% of the image – or in other words approximately what would be captured with a crop frame camera. Big difference right? Now let’s see if this really pans out. Let’s take a look at the same shot (or as close to it as I could get to show you). I didn’t move myself or my lovely assistant (Mr. Potato) and only swapped the lens to another body, this time a crop frame sensor. Here’s the result.

Looks about the same as what’s in the box on the other image right? The framing is a little off, but this is just a demonstration and hey, I’m not perfect. You get the idea. So where does the 62% come from and what’s with the 1.5 or 1.6 crop? What do those numbers mean? Well someone out there came up with the idea of calling the sensor by the “field of view crop factor”, that is the value by which your “field of view” is multiplied when you use one of these sensors (I know, I know we’re getting complicated again).

Let me try to break it down. For the purpose of this explanation we’re going to pretend that “field of view” is the same thing as focal length… it’s not really but let’s just say it is. So you have your 50mm lens – that number, 50 is your focal length or field of view. And on a 1.6 crop sensor you need to multiply that number (50) by 1.6 to get the “effective” field of view (or focal length) of your lens. That is, what the focal length will appear to be. So your 50mm lens, times 1.6 crop, will have an effective focal length (or field of view more accurately) of 80mm. So basically, in essence, that 50mm lens you thought you bought is acting on your camera more like an 80mm lens, which is decidedly a telephoto length.

To explain it more in pictures here are a few more direct comparisons between the full and crop frame cameras. Here is a 28mm lens on a full frame

and on a crop frame. Note here that a 28mm on a 1.6 crop camera will give an effective field of view of 44.8, which is much closer to that 50 you were looking for in the first place. You can see that it is indeed a similar shot to the one taken with the 50mm on the full frame.

and just in case you need to see it one more time, here it is with an 85mm lens. Full frame first.

and the crop frame

So what does this all mean to you, the crop frame camera owner? Well it means you need to think carefully before you purchase your lenses. Realize that lenses designated as “wide angle” for a full frame camera might actually be more middle of the road, and lenses like the 50 will be telephoto. This is good news if you are a lover of telephoto photography and background compression. Not so great news if you are a lover of wide angle, because as I’m sure you’ve realized, you’d have to go REALLY wide to get a lens that is wide on your camera. The good news is that there are a few lenses out there (one of my favorites is the Tamron 17-50mm 2.8, which is available for a variety of camera bodies) that are made specifically for crop frame cameras and that will allow you get the wide angle you crave. The downfall of these lenses is that if you ever decide to upgrade to full frame, the lens will not be able to be used on your new body. A small price to pay for the ability to shoot wide angle on your Rebel, wouldn’t you say?

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Crop frame cameras – why your field of view may not equal your focal length

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