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I have a Sigma 18-35 1.8 sigma lens. The latest number on the distance scale of this lens is 2m. At subjects further than 4.22m the lens focusses at the infinity mark. The exif data confirms this, as it seems that any object further than 4.22m has focussing distance 4.22m in the exif data.

I was now wondering about the following. Clearly, the hyper focal distance at 35mm,1.8F is a lot further than 4.22m. Shouldn't the maximum distance focus distance, before the lens reaches infinity, not be beyond the hyper focal distance?

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Focusing distance scales and depth of field scales are holdovers from a time before many cameras had a way of viewing the scene through the lens used to take the image.

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They've never been extremely precise, but with a fixed focal length lens they can be more accurately marked than with a zoom lens. This is because with most zoom lenses the scale needs to shift and expand or contract as the lens is zoomed out or zoomed in.

In the autofocus era this has been even more the case as the amount of rotational movement needed to change focus from near to far has been reduced. Manual focus lenses often moved from somewhere around 180° to almost 360° between the minimum focus distance (MFD) and infinity. Modern AF lenses rarely use more than 90-135° rotation between MFD and infinity. The smaller rotational movement reduces the size of the focus distance scale. The scale is also a logarithmic one. There's more rotational difference between your lens' MFD (28mm) and the mark at two meters (200mm) than there is between two meters and infinity. This makes marking longer distances accurately problematic even for fixed focal length lenses, much less for zoom lenses.

At least part of the issue with the focus distance reported by the lens that is recorded in the EXIF info might be due to the Sigma being a "third party" lens. Sigma does not license the communication protocols between cameras and lenses from the camera manufacturers, they reverse engineer them. Camera makers often include things in their protocol system that are not specifically used in any cameras/lenses and then a new model is released that does use that part of the protocol. Because reverse engineering earlier cameras/lenses does not reveal the parts of the protocol not actually used by those models, the third party lens makers aren't aware of that part of the protocol until it actually shows up in in use with a new camera model. They then have to update the firmware of the lenses they have already made in order to make them fully compatible with the new camera models.

Beyond all of that, depth of field is an arbitrary and subjective value. There is no magical line inside of which everything is perfectly in focus and outside of which everything is totally blurred. Only one distance is in sharpest focus. Everything else is blurred to one degree or another. What we call depth of field is the range of distance on either side of the distance in sharpest focus that looks acceptably sharp to our eyes when we view a photograph.

A word about what depth of field is and isn't:

In a way, depth-of-field is an illusion. There is only one plane of focus. Everything in front of or behind the point of focus is out of focus to one degree or another. What we call DoF is the area where things look, to our eyes, like they are in focus. This is based on the ability of the human eye to resolve certain minute differences at a particular distance. If the slightly out-of-focus blur is smaller than our eye's capability to resolve the detail then it appears to be in focus. When you magnify a portion of an image by making it larger or moving closer to it you allow your eye to see details that before were too close together to be seen by your eyes as separate pieces of the image.

Since things are gradually blurrier the further they are from the point of focus, as you gradually magnify the image the perceived depth of field gets narrower as the near and far points where your eyes can resolve fine details moves closer to the plane of focus.

Many variables affect what will look focused and what will look blurred to our eyes:

  • The focus distance. DoF increase with longer focus distances and decreases with shorter focus distances.
  • The lens focal length. DoF increases with shorter focal lengths and decreases with longer focal lengths.
  • The f-number of the aperture. DoF increases with narrower apertures (higher f-numbers) and decreases with wider apertures (lower f-numbers).
  • The enlargement ratio. The more the image projected on the sensor or film by the lens must be enlarged to be displayed at a certain size, the more the blur in the photo is also enlarged and the DoF decreases. Blur that looks sharp to us at a small enlargement ratio will eventually be enlarged enough that we can see it as blur. Sensor size and display size both affect the perceived depth of field because they both affect the enlargement ratio, which is the ratio between the sensor/film size and the display size. An image from a larger sensor does not need as much enlargement as an image from a smaller sensor needs to be displayed at the same size.
  • The viewing distance and visual acuity of the viewer. The greater the viewing distance, the fewer fine details we can see and the more depth of field we will perceive. The shorter the viewing distance, the less depth of field we will perceive. A person with greater visual acuity can see finer details at the same distance than a person with lesser visual acuity. Most lens makers assumed 20/20 vision when they published lens data back in the day. Zeiss based their computations on 20/15 vision.

All of these things must be taken into account when calculating depth of field.

  • camera format (sensor or film size)
  • lens focal length
  • lens aperture
  • focus/subject distance
  • display size
  • viewing distance
  • viewer's vision

Any time any one of these variable change, the DoF also changes (assuming another of the variables has not already limited the DoF by more than the change in other variables). To see how all of these variable can affect DoF and hyperfocal distance, please try this DoF calculator and click on 'show advanced' to be able to enter all of the variables.

Hyperfocal distance is a special case of depth of field. All of the variables mentioned above, other than the focus distance which is what we are computing when calculating hyperfocal distance, must be taken into account to accurately compute "the" hyperfocal distance. If any of those variables change, "the" hyperfocal distance also changes and must be recomputed based on the increase or decrease in the DoF. If the Dof decreases, the hyperfocal distance will increase (it will move closer to infinity). If the DoF increases, the hyperfocal distance will decrease (it will move closer to the camera).

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Hyperfocal distance depends on focal length and aperture, and also sensor size. 36 meters is about right for a crop factor 1.6 sensor (at 35 mm f/1.8). The number also depends on the arbitrary Circle of Confusion spec used. So... DOF and CoC and Hyperfocal are all just ballpark numbers. And you will Not measure the 36 meters accurately either, if at all, so it all works out good enough.

Lenses focus continuously from close to far. For example, they will focus at 3.8 meters and also at 5.2 meters or at 20.6 meters... at any point really. Your 2 meters is just where they were able to mark it, and 4.2 is just where the Exif is able to report it. Exif does not know distance, it can only report lens focus rotation angle and assume distance, but due to different zooms changing things inside (between 18 and 35 mm focal length), that Exif distance is not necessarily accurate.

There is very little focus rotation between say 10 meters and infinity. There is simply no room to mark the numbers there (and different zoom focal length values change things too). But lenses focus continuously, and hyperfocal in this one case might be 36 meters (if crop 1.6).

The significance of hyperfocal is:

If the lens is focused at infinity, DOF barely reaches back to hyperfocal.

If the lens is focused at hyperfocal, DOF barely reaches from infinity back to half of hyperfocal.

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Depth-of-field calculation is subjective, and the hyperfocal distance is a subset of this calculation. Now the image projected by a lens consists of super tiny indistinct circles of light called circles of confusion. If an image is to be declared acceptably sharp, these circles must be too tiny to see. This works out to 1/2mm when viewed from normal reading distance. Now our modern cameras make a tiny image, too tiny to be of value, unless we enlarge to get an image we can comfortably look at. Most depth-of-field tables are based on an 8X10 inch displayed image. Your camera is an APS-C format, the imaging chip measuring approximately 16 X 24 millimeters. To make an 8X10 inch display, we must enlarge the camera image 12X.

The point is, the circles of confusion that play on the chip must be so tiny that they can withstand this 12X enlargement and remain under 1/2mm when displayed. By the way, this is for an image viewed in good light from customary reading distance. There are tons of variables here so the industry typically sets the circle size in the camera at 1/1000 of the focal length. For the 35mm lens, that’s 0.035mm. Does a 0.035mm circle at the image plane meet the criteria? Check this out 0.035 X 12 = 0.42 -- thus after 12X magnification the circle size is just under 1/2mm.

We can compute a hyperfocal distance using simple math. First we find the working diameter of the aperture at the f-number setting used. For 35mm set to f/1.8, that’s 35 ÷ 1.8 = 19.4mm. Now we multiply by 1000 -- that’s 19.4 X 1000 = 19,444mm. That’s the hyperfocal distance – converted to meters = 19.4m or 63 ½ feet.

Set the camera’s distance scale to the hyperfocal distance, and the span of acceptable focus is ½ the hyperfocal distance to infinity = 9.7 meters to infinity ( 31 ¾ feet to infinity).

The distance scale on the lens does not have enough room to display the distance. As you approach infinity the scale is too compressed to be of value.

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