Crop Factor Nonsense.

I hate crop factors!

Crop factors create more confusion than almost any other subject. Why? Because they get applied to a lenses focal length which is completely incorrect. The focal length of a lens does not change, no matter how big or small the sensor is.

What changes is the field of view. A bigger sensor gives a wider field of view, a smaller sensor gives a narrower field of view. The focal length stays the same and it’s important to know the true focal length of the lens as this will govern your Depth of Field and any DoF calculations you make must be based on the lenses actual focal length, not some converted equivalent focal length. Use an equivalent focal length and all your calculations will be wrong as may be any estimations of the expected DoF.

All you need to know is how the sensor size in the camera your using affects the Field of View. Compared to a full frame 35mm camera, for any given focal length, a super35mm film or video camera will give a 34% narrower field of view for any given focal length, so 1/3rd narrower.

Cinematographers and DoP’s with film based backgrounds don’t use crop factors. For them a 20mm lens is a 20mm lens, a 50mm lens is a 50mm lens. They know the field of view that lens will give them on their camera. If you start talking about “50mm equivalent” etc then it just gets confusing….. equivalent to what? The only thing a 50mm lens is the equivalent to is a 50mm lens. What changes is the camera or more specifically the cameras sensor size and as a result different cameras result in different field of views, but the lens itself does not somehow change focal length.

Sensor size does not change focal length, only field of view.

16 thoughts on “Crop Factor Nonsense.”

  1. Blimey Al, has someone upset you? I thought you should be relaxed after your hols.

    You are of course absolutely correct. However as a result of the DSLR wave having the massive impact it had on our industry the ‘crop factor’ phrase and ‘equivalent focal length is just an easier way for the vast majority of shooters who haven;t come from Film to equate the magnified field of view.

    I personally find it easier to work in focal length numbers so for example I know a 50mm lens is 50mm regardless of what canera it’s on. But if I’m shooting on a Super 35mm sensor then I know a 35mm will give me a similar field of view to that of a 50mm on a full frame 24x36mm DSLR. I guess it cimes down to how your brain processes the info. I find it harder to picture a 26 degree angle of view over say what a 50mm lens will look like…

    Anyway pal, hope you are well and had a great holiday.

    Den

    1. I’ve just spent hours trying to trouble shoot some shots where the person was complaining that his 28mm lens wasn’t giving him shallow DoF at f1.8. Turns out it wasn’t a 28mm lens but a 5.6mm lens on a 1/2″ sensor block camera and he was using the “equivalent” focal length as that’s what the lens manufacturer had used in the spec sheets. What a waste of time.

      If I need to visualise the different FoV’s then I just imagine the shot with either 1/3rd added to the frame sides or 1/3rd removed from the frame edges. The problem is that the way crop factors are applied at the moment makes people visualise what the lens is doing when they should be considering what the camera is doing.

      You and I Den understand the difference, but many newcomers struggle with the concept of a lenses focal length being a constant no matter what the sensor size due to the way crop factors are often incorrectly described or applied and this then leads to confusion. More care needs to be taken to ensure that the reason why a crop factor needs to be applied is understood correctly. I blame the camera manufacturers as it’s is admittedly very easy to just quote equivalent focal lengths or provide a focal length multiplier rather than actually explain the more difficult concept of FoV equivalents. It’s a case of dumbing down something to make it initially easy to grasp without considering the longer term ramifications of incorrect education.

  2. I think the clue with crop factor is in the term it’s just a crop! The relationship between DOF, sensor size, FOV and distance to subject can be tricky to understand. I’ve attempted to explain it in a blog post: http://www.andytaplin.co.uk/2011/04/30/large-sensor-cameras-depth-of-field-field-of-view-and-bokeh-–-it’s-a-circle-of-confusion/

    Alistair, as a much better educator than me I’d be interested to know if you think my explanation is useful or just more confusion!

  3. Imagine a head and shoulders shot, with the camera about 1 metre away from the subject.

    Using a full frame camera like the 5D or NEX-VG900, we might use a 50mm lens to achieve the right field of view to get the top of the head down to below the shoulders.

    But swapping the camera for a C100 or an F3, to frame the same head and shoulders shot, you would need to use a 35mm lens. (35 x 1.5 crop factor = 50). And the result would be that the subject’s nose would look bigger and the portrait would be less flattering with the cinema sensor size than with the full frame sensor size.

    Or alternatively you would need move the camera back further to use the same 50mm lens and to keep a more natural looking focal length for the subject’s face.

    Taking the thought experiment one step further, to a 16mm film camera, then the focal lengths would have to get even smaller to frame the same shot and I guess you would need more like a 24mm lens for the same head and shoulders shot.

    Thinking of the number of high budget commercials and music videos that have been produced over the years on 16mm, I suppose they would have needed even more room to get back from the subject to get a flattering portrait focal length like an 85mm.

    1. Your thought process completely confirms my annoyance with the whole “crop factor” misunderstanding. Certain focal lengths, for example 85mm do not have magic properties. It is not the focal length that has the magic property but the image field of view relative to the subject to camera distance.

      A simple experiment. Stand 5ft from a mirror and look at your face, does you nose look big? Now stand a couple of inches from the mirror. Notice how much bigger you nose looks. Has your focal length or FOV changed? No, but the distance has. It is not the focal length of 85mm that is flattering, it is the distance from the camera to the subject required to achieve the field of view for a head and shoulders shot that is flattering. Move the camera closer to the subject and the nose will look proportionally bigger no matter what the focal length.

      What is important to know is what field of view you get with different focal lengths, not what the “equivalent” focal length is, so you can choose the lens that gives the appropriate FoV for your desired shot.

      I can see that for some people the notion of equivalent focal lengths is easier to use than FoV equivalents, but the thing is that very often they are not actually understanding what is really going on as the assumption is that some how a 50mm lens on a FF35mm camera is the same as a 35mm lens on a S35mm sensor. It’s not, the DoF will be different.

  4. Ah ha. Right. Ok. I see. Thanks for that Alister, I see what you mean and that changes the way I’m thinking about it. Although I think it will take a while to sink in an realise the implications on lens / camera selection.

    So the “angles” in the picture are defined by the distance of the camera from the subject. Ok. I see. Great.

    The end result of what we are trying to achieve is the relative roundness or flatness of objects in the picture. I did a course in stereography recently and the guy was talking about roundness.

    Or in a scene people sometimes refer to the “compressed” look of telephoto, but what I’m understanding now is that telephoto is not an absolute property of the lens, it’s a combination of lens and sensor size that gives the “image field of view relative to the subject to camera distance” as you described it.

    Brilliant. Thanks.

  5. Think of this:
    In broadcast TV, the standard sensor size for most news TV cameras is 2/3″. An average head and shoulders shot of a person from 4ft away would typically use focal length of about 15mm. Most TV news interviews do not look distorted.

    Use the same 15mm lens on a 35mm DSLR and it would be an extremely wide angle shot. To get a head and shoulders shot you would need to get very close to the subject and there would be a lot of distortion, because you are very close to the subject relative to the field of view.

    In 3D the problem is that as the distance between the lens centers (interaxial) alters the parallax in the image and thus the stereoscopic depth illusion. If the lenses are not the appropriate distance apart for the field of view of the shot, then the depth of the image becomes distorted compared to our normal view of the world and objects appear flattened or distended. The problem is compounded in 3d by the fact that different screen sizes and viewing distances also alter the parallax. Typically for the best roundness you want to match the camera/viewing field of view and interaxial with that of our own FoV and interaxial (interoccular).

  6. That sounds great. Next time I am shooting 3d I will look up this post for reference.

    Moving back to the main topic, I’m wondering if you have maybe left something out of your original post. Because the choice of lens also affects the field of view, not just the sensor size. The confusion comes because lenses are named by their focal length which, you are saying, is unchanged in relation to sensor size. Whereas the lenses’ field of view does change as a “factor”, or multiplication, in relation to the sensor size it is used with? But lenses are not referred to by the field of view factor that they give, only sensors are, so will there always be the tendency for confusion here as long as the lens naming has the bias towards focal length?

    Would it be helpful to have a new word or concept for “image field of view relative to the subject to camera distance” which I sometimes think of as the “angles” in the picture? Or would it be better to have new names for lenses that refer to the field of view that they give?

    Am I disappearing down a rabbit hole here? If I have totally misunderstood, please don’t try and explain, I would prefer to move on to be honest and go and shoot something with my 5D and my new 16-35. Or if there is some truth in this reasoning, it would be good to hear your thoughts…

  7. The problem is that many people apply the crop factor to the lens, for example “a 35mm lens becomes the equivalent of a 50mm lens on “x” camcorder”. This is incorrect and that is borne out by the simple fact that the DoF would be the same no matter what sensor the lens is on.

    Instead people should understand that “a 35mm lens will give the same FoV as a 50mm lens on “x” camcorder, but the lens is still a 35mm lens”.

    One lens does not become the equivalent of another, it is the FoV that becomes the equivalent of the other and “crop factor” deals with how the FoV changes when you use a different sized sensor. When talking about lens equivalents the correct term is Focal Length Multiplier, but this too is misleading as it too implies that somehow the focal length of the lens is changing. The wikipedia article on this subject is very good: http://en.wikipedia.org/wiki/Crop_factor

  8. Of course this is just words and theory, as you suggest, just go shoot something and learn what gives a good shot with the gear you have.

  9. I find it all a but confusing when many cameras call themselves as full frame yet when you put the same lens on different cameras the frame is different…crop factor. field of view …but different
    So what crop factor…is the sony f55 please??? I have seen a number of different things on various forums suggesting no crop to 1.3 but then you see the Abel cine field of view chart and the f55 is more cropped than a c300 which is 1.6 or there a outs….some how I was think we were moving to a world where 50mm meant 50 mm..

    This becomes especially relevant when using wide lens. When Using a the tokina 11-16 (I think accepted the widest for those with a non feature budget) on one of these cameras the crop factor or restricted field of view means you no longer have anywhere the near frame…..no nearly as wide or more more acurately no nearly as broad…

    So what is the truth about the f55.. Please thanks you…

    1. But that’s the thing, 50mm is 50mm on ANY camera. It is still a 50mm lens and it will give exactly the same DoF on any given camera, no matter what the sensor size. The ONLY thing that changes is the Field of View, which is dependant on the sensor size. The very notion of a crop factor changing the lenses focal length is what causes all the confusion.

      The F55 sensor is almost exactly the same size as the C300’s. So, compared to a full frame DSLR the FoV is reduced by approx 1.5 in full 17:9 4K and 1.6 in 16:9 QuadHD and HD. This is almost exactly the same FoV as you would get on a 35mm movie camera. Talk to a film cameraman and they will know what FoV a 20mm lens will give them on a 35mm movie camera, they will not think in terms of crop factors because a 20mm lens on a 35mm film camera is a 20mm lens. We have to get people think in terms of the change of FoV between different sensors, not focal lengths, because the focal length does NOT change and to say that lens “X” on camera “Y” is the equivalent of lens “Z” is simply incorrect because the DoF will be different between lens X and lens Z, so they are not equivalents at all.

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