Lenses – An overview on field of view, focal length and crop factors

Aug 21, 2018

John Aldred

John Aldred is a photographer with over 20 years of experience in the portrait and commercial worlds. He is based in Scotland and has been an early adopter – and occasional beta tester – of almost every digital imaging technology in that time. As well as his creative visual work, John uses 3D printing, electronics and programming to create his own photography and filmmaking tools and consults for a number of brands across the industry.

Lenses – An overview on field of view, focal length and crop factors

Aug 21, 2018

John Aldred

John Aldred is a photographer with over 20 years of experience in the portrait and commercial worlds. He is based in Scotland and has been an early adopter – and occasional beta tester – of almost every digital imaging technology in that time. As well as his creative visual work, John uses 3D printing, electronics and programming to create his own photography and filmmaking tools and consults for a number of brands across the industry.

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Lenses are an integral part of photography or filmmaking. Well, unless you’re using a pinhole camera. But field of view, focal length, and crop factors can be confusing for newer photographers. This video from The Basic Filmmaker goes over the basics of what they all mean and how to convert “focal length equivalency” for non-full frame sensors.

Kevin goes through a lot of information in the video. He uses several lenses to help illustrate his points. The Canon 24mm f/1.8, 50mm f/1.4, 85mm f/1.8 and 24-105mm f/4L. If you’re very new to photography, you might have to watch some parts several times to understand it all. But I want to explore one aspect of it a little here, and that’s crop factors.

Kevin mentions that a lens of a certain focal length acts like a lens of a different focal length when you place it on a camera with a sensor other than full frame. It doesn’t actually change the focal length of the lens. That stays the same. It doesn’t magically make the lens get longer. A smaller sensor simply captures a smaller area of what the lens sees and alters the field of view to match that of another lens when used on a full frame body.

That sounds a lot more complicated than it really is. So, let’s look at a standard 50mm lens. On a full frame camera, that lens has the field of view that one would expect from a 50mm lens.

But if you put that 50mm lens on a Nikon crop body, you effectively get a 1.5x multiplier. This means that 50mm lens now offers the same field of view as a 75mm lens on a full frame body. If you put it on a Canon body, it’s 1.6x multiplier, so you get the same field of view as 80mm on full frame. If you’re shooting Micro Four Thirds with a 2x crop factor, then that 50mm lens has a similar field of view to a 100mm lens on a full frame camera.

Again, it doesn’t actually change the focal length of the lens, just the field of view that the sensor sees.

This change in the field of view will also affect the relative depth of field – how much of your scene is in focus. You get more depth of field as you go smaller in sensor size for a given field of view and aperture combination. This is why a Nikon 105mm f/2.8 lens on a full frame body offers a shallower depth of field than a lens at 70mm f/2.8 on a Nikon crop body, even though their field of view would be identical.

Look at the photos you take on your own DSLR or mirrorless camera compared to those you take with your phone. Even though you may match the field of view, the entire world will be in focus on your phone whereas you might only get a few inches of depth of field on your “real camera” at the same aperture value. This is why a lot of people prefer larger sensors for things like portraits, so they can more easily throw the background out of focus.

There’s a whole lot more to lenses than can be put across in a 5-minute video. So, be sure to do your own research for further reading. But this is a great introduction to the terms, what they all mean, and how it can affect your photography.

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John Aldred

John Aldred

John Aldred is a photographer with over 20 years of experience in the portrait and commercial worlds. He is based in Scotland and has been an early adopter – and occasional beta tester – of almost every digital imaging technology in that time. As well as his creative visual work, John uses 3D printing, electronics and programming to create his own photography and filmmaking tools and consults for a number of brands across the industry.

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9 responses to “Lenses – An overview on field of view, focal length and crop factors”

  1. Stewart Norton Avatar
    Stewart Norton

    35mm on full frame…love it !

  2. stewart norton Avatar
    stewart norton

    I love the look of 35mm on full frame.

  3. JustChristoph Avatar
    JustChristoph

    Here’s what he said but without the rocket science/confusion –

    A lens is the same no matter what camera you put it on. Forget all the stuff to the contrary. And especially forget about f-stop equivalences, whatever they are. Here is EVERYTHING anybody needs to know about putting a lens on a different camera with a different sensor. A lens DOES NOT CHANGE no matter what camera/sensor you put it on. The maximum aperture stays the same and the focal length stays the same. And here’s the surprise… the resulting image will be exactly the same. What! You heard different? Let me say that again – the image will be exactly the same. Same aspect ratios, same depth of field.

    Oh, yes, OK, there is one small difference. If the sensor in the camera is smaller than the one the lens was designed for, the image the lens projects won’t all fit on to it. Pretty obvious when you think about it. So the photos that result will be missing the outer part of the image – in other words, it will be cropped. Obviously the smaller the sensor the less of the image can fit on it, so the more that is cropped off. It’s called the crop factor.

    The end.

    1. JustChristoph Avatar
      JustChristoph

      There’s been so much rubbish said about lens equivalence, I just had to get that off my chest.

    2. Kaouthia Avatar
      Kaouthia

      “And especially forget about f-stop equivalence, whatever that is.”

      Well, nobody mentioned the term “f-stop equivalence”, but I would’ve thought it was quite obvious.

      Let’s say you’ve got two camera setups, say a Nikon D5 and 300mm f/2.8 prime, and a D500 with a 70-200mm zoom.

      If you shoot the crop body’s 70-200mm at 200mm and the full frame is shooting at 300mm, then both cameras get the same field of view, right? But do they have the same apparent depth of field relative to that field of view? No, they don’t.

      You need to stop that 300mm lens down to get the same field of view. If you’re at 200mm f/2.8 on the crop body, you need to be at 300mm f/4 on the full frame body to get the same field of view AND depth of field.

      That’s what it is. Everything is relative to something else. If you want the same shot on a different setup, then when you change one thing, you have to change other things to balance it out and achieve the same (or as close to it as possible) result.

      “Here is EVERYTHING anybody needs to know”

      Statements like that are dangerous. :)

      1. JustChristoph Avatar
        JustChristoph

        You have also missed the point: The cropped image has exactly the same depth of field as it would have done if you had taken a photo with a full-frame camera/sensor, printed it, then cut off the outer part of the print with a pair of scissors.

        This only seems a dangerous statement because you, too, seem to have not understood its simplicity and exactness. The depth of field effect you describe arises as a direct result of cropping because more of the image will appear to be in the focal zone. But if the cause of cropping is made simple, it becomes much easier to understand the resulting implications. Bamboozling people with numbers just serves to muddy the issues.

        1. Kaouthia Avatar
          Kaouthia

          “he cropped image has exactly the same depth of field as it would have done if you had taken a photo with a full-frame camera/sensor, printed it, then cut off the outer part of the print with a pair of scissors.”

          Yes, but NOT the same field of view. I’m talking about taking the SAME identical shot on two cameras with different sensor sizes and focal lengths.

          It’s you who seems to be missing that point. :)

          1. JustChristoph Avatar
            JustChristoph

            Kaouthia, do you have a twin, because John Aldred looks a whole lot like you.

            “I’m talking about taking the SAME identical shot on two cameras with different sensor sizes and focal lengths.”

            If you have different focal lengths, you are not comparing identical lens setups. And that was what the video is all about.

          2. Kaouthia Avatar
            Kaouthia

            If you only want people to understand half the story, then knock yourself out. :)