Full Frame vs. Crop: An Explanation


50mm f/1.8 Shot in Full Frame

The whole crop-sensor versus full frame sensor thing never quite made sense to me, ’til I saw the difference between a 50mm lens on a crop camera versus the exact same lens on a full frame sensor. If this whole thing already made perfect sense to you, feel free to skip this post. For those of you to whom the whole thing makes about as much sense as Finnegan’s Wake — in Swahili — read on.

Sensors come in several sizes, from the thumbnail-sized sensors in your average point-and-shoot to the 120mm sensors in medium-format cameras. If we’re taking a 35mm film frame as our point of reference (also the size of the sensor in a “full frame” camera), any sensor smaller than 35mm is going to have a crop/multiplier factor when used with 35mm lenses. You’ll recall that some time back we talked about the difference between digital and optical zoom, where digital zoom essentially crops the image captured by a sensor at its native resolution; the crop factor introduced by a smaller sensor does the same thing, minus the software trickery.

Here’s why it didn’t exactly make sense to me. Lenses have different fields of view at different focal lengths. A fisheye lens (say, 8mm) can give you a 180 degree angle of view. A 50mm lens, sometimes called a “normal” lens, closely approximates your natural field of vision. A much longer lens, like a 300mm lens, gives a much tighter field of view (around 8 degrees).* You’d think (or I thought, at least) that regardless of the size of the sensor, the photo would be the same because the lens’s field of view at a given focal length would be the same in any case, so a lens racked out to 300mm would have the same FOV whether you used it on a full-frame sensor, a crop sensor, or a point-and-shoot.

50mm f/1.8 Shot in Crop Format

Only it isn’t quite. So how’s this crop thing work? By way of analogy, think of it like this. Let’s say you’ve got a slide projector that’s ten feet from a three foot wide screen. The image fills the screen with no problems. The projector is your lens; the image coming out of the projector is the “image circle”; the screen is your sensor. When you’re using a crop sensor, you’re not moving the projector relative to the screen; it’s simply changing the area covered by the projected image. So if you put a two foot wide screen in front of the projector, you’re going to notice that a much smaller part of the image is visible on the screen (with some of the image spilling over to the area beyond it). A DX lens has a smaller image circle (in essence, focusing the “projector’s” beam more tightly), so it’s going to fill a smaller “screen” (sensor) easily enough, but it’s going to come up short on anything larger.

This is also, incidentally, why a lens that exhibits light falloff or softness in the edges and/or corners on a FF camera generally looks better on a crop camera. Most lenses — at least once you stop them down a bit — are going to be reasonably sharp in the center. The part that’s sharp is the part that’s being projected onto the smaller sensor, whereas a larger sensor’s going to also incorporate the dodgy bits from the perimeter of the frame.

Therein lies a lesson. Some people — and I was one of them — purchase full frame lenses when they have a crop sensor camera just in case we decide at some point to jump to a full frame camera. When you’re reading reviews of lenses, therefore, one of the things to pay attention to is who’s using the lens as they review it. It’s not unheard of for a lens to get rave reviews from DX/crop users only for the FF people to point out flaws in the lens’s image quality. If you have no plans to switch formats, you may not have much to worry about (though other issues, like lens flare, coma and color fringing will typically manifest no matter what body you’re using). But if you’re going to be switching at some point, pay attention to those flaws. You may be willing to put up with them, but at the very least, go in with eyes open.

Oh, and about the images accompanying this post: the camera wasn’t moved relative to the bookcase (it was on a tripod). I’m also using the same lens (a 50mm 1.8) in both shots. The only change is that the first shot was taken in the camera’s full-frame mode, while the other uses its crop mode.  So on full frame, the 50mm looks… well, like a 50mm looks. In crop mode, it acts more like a 75mm.

If I haven’t been as clear on this as I’ve tried to be, feel free to sound off with your questions (or better examples) in the comments section below. We’ll be revisiting this topic (albeit from a different angle) soon.**

*If reading this is making you trade your confusion over sensors for confusion over angles of view, there’s a very good explanation at the always-reliable Mansurovs Photography: http://photographylife.com/equivalent-focal-length-and-field-of-view

**The “different angle” is a post taking up whether a crop sensor or full frame is “better.” (click the link for my take)

10 Replies to “Full Frame vs. Crop: An Explanation”

  1. Great article! Thank you so much.

    Now, i have a question. You said “So on full frame, the 50mm looks… well, like a 50mm looks. In crop mode, it acts more like a 75mm” about the pictures you included. But it makes me think again how does a 50mm really looks, and actually now i understand it is depending on which body we are using, right? Because when i bought my 50mm 1.8 for portraits and i saw the example pictures (generally portraits) i said “yes, that’s the FOV i’m looking for” and that was what i got, but should it say then at amazon that it is really a 75mm for FF and 50mm for crop sensor? The confusion comes because right now in my mind i have the image reference for a “50mm” lens from my crop sensor camera, and im trying to understand how should i interpretate the “mm” value when i see a lens on amazon. How can i know if it is talking about FF or crop sensor lens? You said you used a 50mm for your example pictures, but now i wonder if it is a 50mm for full frame or 50mm for crop sensor (this is the $100 lens everybody gets, that amazon says it IS 50mm). You also said it acts like a 75mm in crop mode, but i wonder if you mean “it looks like if i take the picture with a Crop Sensor camera using a (FF) 75mm lens”.

    Well, if i really made it complicated than it should be just tell me this 2 things: How can i difference in the store or web page when a lens is XXmm for FF or crop sensor (as i see it, they take crop sensor as reference always), and what is the mm equivalent of my crop sensor 50mm 1.8 lens to get the same FOV in a FF camera? Making it simplier: What lenses are needed for each body to get the “same” pictures?

    Thank you!

    1. You’re welcome, Ronald! Thanks for stopping by, and for commenting. Those are good questions. Let me try to clarify.

      To your first question — how can you tell which lens is intended for which type of sensor — it’ll generally tell you on the lens. Nikon’s lenses made for crop sensors use the designation “DX,” while full-frame lenses, although they’re technically called “FX,” won’t necessarily say so. So if you’re trying to figure out which kind you’re dealing with, you have to find out the manufacturer’s designation for each, or check their website to see which lenses are compatible with which cameras.

      As far as knowing the field of view that an individual lens will give you, that depends on the brand of camera, but either a 1.5x or 1.6x crop factor is the rule of thumb. In plain English, if you want to know what your lens’s equivalent crop is, just multiply that number by the focal distance you’ve got dialed in. So 50×1.5=75mm (Nikon), or 50×1.6=80mm (Canon), for example. This holds true regardless of the type of lens you’re using (in other words, you’re getting the same multiplier even if you use a lens that’s designed for a crop sensor, because of the crop sensor). And I’ve no idea why camera manufacturers don’t just label DX lenses with their equivalent focal lengths, like calling an 18-70mm a 28-105mm instead. Yes, I understand that the 18mm is the actual focal length of the lens, but I still think it’d make more sense the other way.

      Which lenses? If you’re looking for the standard “trinity” of primes (in full frame, those would be the 35mm, 50mm and 85mm) the equivalent crop lenses (again, using Nikon as my frame of reference) would be 24mm, 35mm, and 60mm; to further confuse matters, those will give you equivalent focal lengths of 36mm, 52mm, and 80mm respectively. So if you’re used to the FOV from your nifty fifty on a crop sensor, you’d likely want to get an 85mm lens if you picked up a full-frame camera.

      Hopefully this helped and didn’t just confuse things further. Let me know if there’s anything else I can clarify.

  2. So when using a Canon, the EFS series of lenses are locked into a crop sensor if I’m not mistaken. What series or specs of lenses should I be looking for if I eventually want to upgrade to a Canon full frame? (In the meantime, I’d like the lenses to work on my T3i crop frame)

    1. You’re not mistaken. The EF-S are Canon’s crop lenses. The EF (note there’s no “-S”) are the full frame lenses. Generally speaking, any of Canon’s full-frame lenses will work on your T3i (just don’t try using your EF-S lenses on a full-frame camera). As I allude to in the post, however, just make sure you research ANY lens carefully before buying, ’cause crop sensors are more forgiving on full-frame glass than full-frame sensors are. So pay attention to the results reviewers are getting on FF cameras.

      I can’t speak to one Canon lens over another, as I’m not a Canon shooter. The only exception I can make is for the “plastic fantastic” (Google it), which has gotten raves from nearly everyone I know who owns one… fifties seem to be the one lens that are nigh-impossible to screw up.

  3. Hi Paul

    Thanks for the post, very good to know. With regard to your example images above, you have made them equal size so it gives the impression that the crop sensor captures a smaller portion of the frame and then enlarges it, this is not the case, right?
    If you shoot with a 50mm lens on a crop body, you will capture less in the frame but what is captured will appear the same size/dimensions as on a FF right. The pictures above make it seem as though you really are getting closer with a 50mm on a crop. If this is the case then how is it still considered 50mm?

    1. Thanks for the question, Rich.

      The answer’s a bit complicated, since there are a number of factors in play. First is the sensor’s native resolution, which can also be the same among several different cameras; you could, after all, have a compact, a DX SLR and an FX SLR, each with twelve megapixel sensors, but get different image resolution out of each. This is due first of all to the sensor pitch (the actual size of the photosites/pixels), and also to how the image processor renders the input from those pixels into the final image. There’s not a one-to-one correspondence between what each pixel of the sensor captures, and what each pixel of the finished product is going to be. Especially if you’re shooting in, or once you’ve converted to, JPG, you’re losing information from the initial photograph to compression.

      Then there’s the lens. What’s being captured by the lens, or what it “sees,” isn’t going to vary. What does vary is the image circle (the image projected from the lens onto the senor). The sensor acts like a projector screen (think of it like a slide or filmstrip projector). The 50mm lens used here is a full-frame lens, engineered so that its image circle completely covers the equivalent of a 35mm film frame, so a full-frame sensor is going to be covered by, and take advantage of, everything the lens “sees”. If I shoot in crop mode, or on a camera like the D5200 that has a crop sensor, the lens’s image circle remains the same size, but only the center of it is being projected onto the smaller “screen.” It’s still considered 50mm because the lens’s focal length and image circle haven’t changed; you just have to figure for the crop factor when the image circle is being projected onto a smaller sensor. A DX/crop lens is engineered differently. Same focal length, but a smaller image circle that’s optimized for the smaller sensor.

      I know this sounds (and come to think of it, is) confusing. If I can clarify further, please let me know.

  4. Thanks a lot for your sound explanation. But still, it makes me think, is it anything else than a digital zoom, which you can also get in post processing? Apart from the fact that you van use DX lenses on a FF body?


    1. If I’m understanding your question, the short answer is “no.” The crop sensor gets its “zoom” from the fact that the image circle is projected on a smaller area. A DX sensor is roughly 24mm across, while an FX sensor is 35mm. The image circles for lenses meant for those types of sensors will have image circles that roughly coincide with the size of the sensor for which they’re designed (so a 50mm FX lens will have an image circle meant to cover a 35mm sensor even if the camera it’s attached to only has the 24mm sensor). But the “crop,” in that case, is a function of optics.

      With digital zoom, it’s a different story. Some cameras (your typical mobile phone camera, for instance) have a prime lens and rely on digital zoom exclusively, while others will offer optical zoom and digital zoom will take over once the lens has hit its maximum focal length. In both cases, digital zoom works the same as cropping a photo in Photoshop. In other words, the camera captures the photo at its native resolution (and whatever focal length is available), and then crops to the equivalent of the zoom setting you’ve chosen.

      Hope that helps. I also have a post here that might be helpful on the optical-vs-digital zoom issue: http://www.thefirst10000.com/optical-vs-digital-zoom-what-it-is-why-it-matters/

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