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I'm thinking of getting a Sony A6000 with a Sony SEL35F18 Lens.

I'm a little confused about the actual focal length (and/or field of view) of the lens when attached to an APS-C sized sensor.

The Lens specifications claim that:

35MM EQUIVALENT FOCAL LENGTH WHEN USED WITH APS-C SENSOR CAMERA

52,5

Notice the 52.5 underneath. My understanding was that, by attaching a lens to a smaller sensor, the focal length would be forced up (not down). The specifications here seem to suggest that 52.5 is the lenses normal focal length, but when attached to the APS-C sensor (not full-frame), the equivalent will be 35mm.

Is it possible Sony have done some jiggery pokery on these lenses? Or have I misunderstood focal length and sensors?

Also, am I right in assuming that the focal length and the angle of view are inherently related mathematically?

Or is it just that Sony has phrased this poorly? To me, their specification sounds a lot like "The equivalent focal length of this lens, when attached to an APS-C sensor, will be 35mm.".

  • Re: your update. No, Sony hasn't misphrased anything - see Matt Grum's answer. It's just that "full frame" is also called "35mm" because that is/was the size of film stock used. – Philip Kendall Jul 22 '15 at 12:31
  • Yes, I'm aware that 35mm and 'full-frame' are the same. But to me, the statement I've quoted in their specification sounds like my paraphrasing below it. You cannot deny that my paraphrased version of that statement is wrong, hence the original quote is misleading. – shennan Jul 22 '15 at 12:37
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    Actually, I think Sony's statement is precisely correct. They can't necessarily be blamed for the fact that you have misinterpreted it. – Philip Kendall Jul 22 '15 at 12:52
  • @PhilipKendall I'm not blaming them. I'm simply asserting that my understanding of focal systems is not the issue here: it is clearly my understanding of their quote. If it offends you that I find the specification misleading, then I apologise. – shennan Jul 22 '15 at 12:56
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    I think the core question here is really covered by one of our existing answers on crop factor. If not What is crop factor and how does it relate to focal length?, then perhaps Does my crop sensor camera actually turn my lenses into a longer focal length? or even What is “angle of view” in photography?. The only thing remaining is why Sony phrases this the way they do; I'm going to edit the question to focus specifically on that. – mattdm Jul 22 '15 at 13:11
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The confusion here comes in entirely because "35mm" is the common name for the 135 film format, also known as "full frame" — and it happens to be the focal length of the lens you are looking at.

The specification is:

35mm equivalent focal length when used with APS-C sensor camera

52,5

And you are reading this as:

Equivalent focal length = 35mm when used with APS-C sensor camera

52,5 (presumably the "real" focal length or something)

But it's actually saying:

This lens's focal length gives a field of view equivalent to the following focal length on a 35mm-format camera:

52,5

This confusion is easily resolved by looking at the specifications of a lens which does not happen to have this ambiguous coincidence. For example, the 50mm Sony SEL50F18 Lens, which says:

35mm equivalent focal length when used with APS-C sensor camera

75

Note the 35mm hasn't changed to 50mm, because they're talking about the film format with that name, not the lens.

I personally don't find the wording particularly confusing, but maybe they could have used an extra hyphen or two for extra clarity.

So, once that's resolved, just for the record, you're right on the math. See What is crop factor and how does it relate to focal length? or What is "angle of view" in photography? for more.

  • I actually find this answer to be the most helpful as it is fuller and provides evidence. Indeed, seeing the way the other specifications are written provides much greater clarity on where I misunderstood this particular specification. – shennan Jul 22 '15 at 13:52
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What's causing confusion here is that "35mm" in the spec refers to the format (more commonly known as "full-frame"). So the part of the lens spec that states:

35mm equivalent focal length when used with APS-C sensor camera

52,5

Should be read as:

Full-frame equivalent focal length when used with APS-C sensor camera: 52.5mm

  • But the name of a lens (as I understand it) usually contains the focal length, not the format. Is it fair to assume that Sony have completely fudged the name of this lens? – shennan Jul 22 '15 at 10:34
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    @shennan Sony haven't fudged anything, the lens is called a 35mm lens because it's focal length is 35mm. It's 35mm when mounted on a full frame camera, it's 35mm when mounted on an APS-C camera, it's 35mm when it's in the box and not mounted on any camera at all... the focal length helps you work out the FOV, but it doesn't define the FOV by itself. Sony state the focal length that gives the same FOV on a full frame camera, to help people who understand the relationship between focal lengh and FOV on full frame cameras estimate the FOV of the lens on APS-C cameras. – Matt Grum Jul 22 '15 at 10:49
  • So why restrict this to an APS-C sensor? Why not make it for full-frame, as the FOV/EFL will be the same (52.2mm) surely? – shennan Jul 22 '15 at 11:03
  • @shennan it seems you're not getting what happens when you put a lens on a smaller sensor - you get a smaller image, which has the same field of view as a longer focal length on a larger sensor. The lens is not "restricted" to an APS-C sensor, it's just if you put it on full frame you'll get darker corners. They could fix that but the lens would be heavier and more expensive. But even if they did that, it wouldn't change how it behaves on a smaller sensor, it would still give the same field of view as a 52.2mm lens would on full frame. – Matt Grum Jul 22 '15 at 11:07
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    "SEL-35F18: £399". "SEL35F28Z: £699". I think that should be fairly easy to understand. – Philip Kendall Jul 22 '15 at 12:10
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The more correct term for this case is "equivalent field of view". The focal length of the lens doesn't change, the crop factor changes. Using a crop-sensor camera with this lens would be the same as using a full-frame camera with the same lens and extracting a crop from the center of the frame with the same size as the cropped sensor.

  • That doesn't explain why the focal length goes down when attached to an APS-C sensor, though. Surely Sony should have described the equivalent focal length as 52.5 on an APS-C sensor, and 35mm on a full-frame? – shennan Jul 22 '15 at 10:18
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    See this article. – K. Minkov Jul 22 '15 at 10:20
  • Yes, so the crop factor of an APS-C sensor is 1.6. Meaning that, in order for the EFL of this lens to be 35mm when used with an APS-C sensor, the full-frame focal length would have to be 21.875 (21.875 * 1.6 = 35). – shennan Jul 22 '15 at 10:31
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    @shennan No, interchangeable lenses are always sold by their actual focal length. This lens is a 35mm lens. The EFL when used with an APS-C sensor is 52.5mm – Matt Grum Jul 22 '15 at 10:50
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I would read that as the lens being eqivalent to a 52.5mm lens on a 35mm camera, when attached to an APS-C sensor camera.

  • Ah, ok. So it's possible that this is just a poorly worded specification? The specifications seem to suggest that this lens is specifically for an APS-C sensor. If the equivalent focal length of the lens is different on an APS-S sensor, why do they bother claiming it is for an APS-C while naming it a 35mm lens? – shennan Jul 22 '15 at 10:23
  • They name it as a 35mm lens because it is a 35mm lens, i.e. it brings parallel lines into focus 35mm behind the second principal plane. This is the definition of focal length, a property of the lens only, one that doesn't change when mounting the lens on different cameras. Focal length does not by itself define the field of view. – Matt Grum Jul 22 '15 at 10:32
  • So if this is indeed a 35mm lens (on full-frame) then why do they market it as an APS-C lens? What, if anything, makes it for a smaller sensor, other than the fact that they've included some fairly basic EFL calculations for an APS-C in their spec? – shennan Jul 22 '15 at 10:37
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    @shennan It's an APS-C lens because the image circle it illuminates is only big enough for an APS-C sensor. When mounted on a full frame camera the corners will be black. – Matt Grum Jul 22 '15 at 10:47
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    @shennan: concerning your previous comment: yes, that's exactly correct. But note that a full frame capable lens will nearly always be bigger, heavier and more expensive than one for APS-C, all else being equal. – Michael Borgwardt Jul 22 '15 at 13:18
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The lens has a focal length of 35mm. Sony APS-C sensors have a crop-factor of 1.5 (same a Nikon - surprise - but different from the 1.6 of Canon's APS-C sensors). So when mounted on a camera with an APS-C sized sensor, the 'equivalent' focal length is 35 * 1.5 = 52.5mm.

So this lens on an APS-C camera will have the same field-of-view as a 35mm lens on a full-frame (i.e, 35mm - don't get confused with the format being the same as the focal length - it's just a coincidence) camera.

The easiest way to visualise this is to picture the image circle projected by the lens. Then imagine how much would fall on a big rectangle (the 35mm/full-frame sensor) compared to a small rectangle (the APS-C sensor). The bit of the image that falls on the rectangle is what you capture.

This should also help you understand why no matter how you mount the lens, the size of that image circle doesn't change (unless you move the lens closer to the surface that the image is projected on, which you don't normally).

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The Focal Length of a Lens cannot physically be altered. It is the distance between the centre of a Lens and its focusing point, the camera sensor. Therefore, the Sony 35mm Lens has distance of 35mm from the sensor to the lens. (Please note, a lens may have several corrective elements within it, the measure is taken from the final glass nearest to the camera sensor).

Therefore, if you imagine a pinhole camera from school science experiments where light converges from the top and bottom of your subject (the angle of view) and meets at the pin hole. it then goes out the other end and displayed at the back of the camera, inverted. the distance between this display and the pinhole is the focal length. this cannot be altered.

The same physics apply to modern cameras, but with sophisticated lenses, both fixed or zoom capable focal lengths.

A full frame sensor, (also referred to 35mm cameras, based on the old 35mm film formats) has a measure of 36mm across with 24mm down. Therefore, a lens when used with a camera that has a FF of 36mmx24mm, the light coming in to the camera, will cover the entire size of the sensor exactly.

However, when the sensor size is smaller, IE the Sony A6000, the light coming through will be greater than the sensor and therefore the image displayed will be greater and therefore be cropped by a factor of 1.5.

To capture an image with the same crop factor as the A6000 using a full frame camera such as Sony A7, you will need a 52.2mm Lens.

This is the reason why Sony say, “ 35mm Equivalent focal length when used with APS-C Camera 52,5”, because that is in effect what you are getting with the A6000, a 52.5mm equivalent image.

  • The measure is actually taken from the rear nodal point of a theoretical thin lens that would yield the same Field of View as the (usually) more complex lens. – Michael C Jul 24 '15 at 2:18

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