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News & Insight

Depth Of Field And Lens Equivalents | Shattering Myths & Heating Debates

By Kishore Sawh on January 21st 2016

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I feel comfortable enough somewhat to safely say, I am not a ‘thickie’. I am not a forever-a-student, fully certified scholar with a handful of letters after my name plucked from a Scrabble bag. But, I know to eat not only when hungry, that breaking a sweat daily is some of the best medicine, that helping someone hinders no one since a rising tide floats all boats. I study aeronautical physics for enjoyment and know enough to know that, really, I know nothing.

I, like many of you, am still learning about camera technology and theory, and I also know that some pieces of information are better delivered by some than others. The source can mean so much in terms of acceptance of credibility, and that despite some of my formal scholastic education in photography, some things are better explained by others. In that vein, I humbly bring you the video below where John Hess from Filmmaker IQ goes into some detail, for a full 17 minutes, on depth of field, and lens equivalents.

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Depth of field is such a primary and fundamental part of photography, something we see and consider in every image, but something so few seem to understand. And that’s ok, because it is rather technical; you don’t always need to know how something works in order to work it. However, in today’s multi-format digital camera world, these topics become a bit more complex, causing mass confusion, and heated debates. It helps to have a grasp on it – not for argument’s sake, but for your own. That is to say watching this video and learning the material is autotelic.

Some of this has been touched on in former posts of mine and others on this site. Regardless of the science behind it, meets non-believers and causes conflict, so this is sure to do some of the same, though Hess leaves little wiggle room for argument; physics, on this level anyway, doesn’t lie. You’ll learn about the difference between depth of field and depth of focus, how sensor size and lens equivalency plays a major role in DoF, and many myths will be dispelled. For example, I’ve mentioned it before, but given the same lenses and apertures, essentially all other parts being equal, a smaller sensor will have a shallower depth of field. That’s right, but it’s when crop factors are taken into consideration and more, things begin to change. But then higher resolution means shallow DoF.

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[REWIND: THE PROPERTIES OF CAMERA LENSES]

I’ll leave that right there for fear that how I phrase things may interfere with your uptake from the video. But watch it, learn it, and if you challenge what’s in it, please tell us what and why. Let’s see if as a community we can come to an explanation where everyone understands.

About

A photographer and writer based in Miami, he can often be found at dog parks, and airports in London and Toronto. He is also a tremendous fan of flossing and the happiest guy around when the company’s good.

Q&A Discussions

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  1. Dave Haynie

    Ok, first, as a dedicated science geek, I have to object to part of this. When you have a perfect point source of light projected through a lens in perfect focus, you don’t get a perfect point source on the sensor. You get a disc… the Airy Disc, after George Airy, the first guy to “do the math” on this. The size of the Airy Disc is entirely based on aperture, because it’s due to light diffraction.

    Ok, so then there’s depth-of-field. He’s kind of mixing two things here. The depth of field for any given lens focal length/aperture setting will always be the same, no matter what camera you’re on. In short, the lens doesn’t know if it’s on a FF or a m43 or something else. That should make sense.

    The acceptable range of DOF is a different thing, and for any given lens and aperture, that’s based ENTIRELY on pixel size. You could compute DOF chats based entirely on aperture, focal length, and pixel size, and they’d work on any camera with any size sensor. The effect, of course, is that that 50mm lens on my FF 20Mpixel camera has more acceptable DOF than that same lens on a 20Mpixel APS or 20Mpixel m43, that’s absolutely true. But the reason is the pixel size… 6.55um on the FF, 3.34um on the m43.

    What I found interesting was the demonstration of matching the magnification factor. Not sure if that’s necessary in regular use, but still very interesting. And one instance in which you’ll want those magnification factor numbers in your head.

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  2. Holger Foysi

    A lot of the confusion could be avoided in my opinion, if people just took five minutes to look at the DOF equations directly (http://toothwalker.org/optics/dofderivation.html for example, eq 12,13 in terms of magnification, for simplicity). This is simple highschool math and gives you all the knowledge needed.

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  3. Vít Třebický

    Hello,
    to sum it up: we can get equivalent field of view (or angle of view) and depth-of-field if we would multiply object-to-camera distance and F-stop by crop factor using the same lens on FF- and Aps-c-body. However, the images will be equivalent only in terms of DoF, FoV and object magnification, not in perspective because of perspective distortion – which could be seen in the the video (15:37), see the shape of the sofa or tree in the background. In the end, if I am getting it correct, we cannot get 100% equivalent images from FF and APS-C cameras. Right, or am I missing something?

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    • Martijn van Eeten

      Perspective of a photo is determined by distance only. A zoom lens _at the same distance_ has the same perspective as a wide-angle lens. It just shows only a tiny fraction of the field-of-view of the wide angle lens.

      As long as you use a _completely_ equivalent lens and equivalent settings, you can obtain the same picture. For example, taking a crop factor c = 1.5, you would get the same picture from the same distance using a 50mm, f/4 at ISO800 on a FF or a 35mm, f/2.8 at ISO400 on an APS-C. Even the noise is comparable as a the noise performance of FF is typically one stop better.

      However, the problem is that there usually exists no APS-C equivalent of a FF f/1.8 lens or even a FF f/1.4 lens (which would be an f/1.0 lens for an APS-C camers)…

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    • Vít Třebický

      Thanks. Somebody equipped with say Nikon FF and APS-C bodies and having lenses like 60mm and 90mm (because 60mm×1.5 crop factor=90mm) could simply test this out. Take pictures of static object from fixed distance, one with 60mm mounted onto APS-C camera a second taken with 90mm mounted onto FF camera.

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    • Martijn van Eeten

      Actually, Tony Northrop did that – though with a different set of lenses. See http://youtu.be/DtDotqLx6nA

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    • Kishore Sawh

      Yes, I spoke to Tony about it and we covered that here on SLRL, however, the foundational points weren’t really touched upon near the same way was this was. However, Tony’s was good, maybe better as seeing relative difference.

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    • Lauchlan Toal

      Pretty much. As far as I know, the only way to get 100% match-up between two formats is to either crop the larger format or stitch frames from the smaller one. I could be wrong on this though – there’s a lot of conflicting information out there.

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  4. Stephen Glass

    Best explanation I’ve seen yet.

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  5. Ralph Hightower

    The Filmmaker IQ series is always informative. But John Hess said that lens-equivalents only came into being with the digital age. Lens equivalency has been around with film. I found a Mamiya lens catalog on the web for their 6×4.5 camera and it gave the equivalent focal length in 35mm for their lenses. A normal lens for 35mm is 50mm; a normal lens for 6×4.5 format is 80mm.

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    • Ben Perrin

      Yeah, I thought I heard him mis-quote a few different things but it can’t be easy getting all that info correct. He probably just mis-spoke a couple of times.

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  6. Jim Johnson

    I have heard all the arguments about cropped v. full frame depth of field, aperture settings, and exposure so many times it makes my head spin.

    This is the first time I full understood it, though. Thanks.

    I think all the confusion comes from the statement highlighted in the article: “All things being equal…” No one would ever shoot like that— if you changed to a camera with different sensor size, you would change something else, like the lens or the distance to the subject — so the idea makes no sense in a practical situation. In practical shooting, getting the same framing on two different sensors (i.e. changing the lens/angle of view), the full frame has a shallower depth of field.

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    • Kishore Sawh

      I know what you mean John, it wasn’t something I was particularly concerned about at all before I began to seriously shoot digital. Shooting film for me was almost always 35mm versus the larger formats and thus hardly did that theory dictate I should absolutely internalize it. The ‘all things being equal’ is, while possibly confusing, the only way to explain it effectively I think.

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    • Jim Johnson

      You are absolutely correct; it is the only way to explain it. I was just always trying to apply it to the practical way I shoot. I’m glad you posted the article; now I understand it, and more importantly, I understand when it will and won’t affect the way I shoot. Thanks.

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