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I am photographing a very thin plate (0.01" thick) that is partially submerged in a tank of water. My goal is to find the angle that the water surface makes with plate. When I focus on the front edge of the plate (focal plane 1), I can see the plate clearly but the interface appears blurry, see first picture. When I focus further back on the plate (focal plane 2), the plate seems to disappear, see second picture.

In the second image, why does the plate disappear and what am I seeing in the space where the plate should be?

If it helps, I am using a nikon D7000 with a sigma 105mm macro lens. I shot these photos at f6.3, shutter speed 100, ISO 125.

set up focal plane 1 focal plane 2

  • Do you want to know why, or do you want a work-around which is to use a pin rather than a plate as the meniscus will be the same as at a plane perpendicular to the plate. – Stan Sep 27 '18 at 15:46
  • @Stan I want to know why it seems like I'm looking "through" the plate and if the meniscus I'm seeing in the last picture accurately captures the meniscus shape. I want to keep the plate and not use a pin because it is better for other aspects of my experiment. – Ragnar Sep 27 '18 at 16:04
  • Questions about using a camera as a measuring device are not generally well received - but your question about the focus is a common occurrence when shooting through objects, like a chain link fence or plexiglass. So, for that aspect of it, +1 from me and thanks for providing pictures! – Hueco Sep 27 '18 at 16:09
  • Tip: Your set-up is introducing an experimental error (an optical bias called the point spread function) due to the characteristics of depth of field with the use of a plate. You don't (and can't) know the precise location of the meniscus or its depth. The edge of the meniscus you want to measure is indefinite. Use the point of best focus for a pin rather than a plate and your alignment and imaging will not be an issue for the reasons in all the answers here. – Stan Sep 28 '18 at 15:20
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In the second image, why does the plate disappear and what am I seeing in the space where the plate should be?

The plate "disappears" because it's out of focus. At short focus distances, depth of field is always very shallow. When you focus farther back, the edge of the plate goes out of focus, and because it's so narrow it seems to disappear completely. I approximated your situation using a zip tie held in a spring clamp, and you can see that as the focus moves farther back the zip tie also goes far enough out of focus that it starts to disappear:

I know of three options for increasing the depth of field in your image:

  1. smaller aperture: Moving to a smaller aperture, like f/16, will help to increase the depth of field. At such a close range it won't help as much as you'd probably like, though. Using a depth of field calculator you can see that your 105mm lens at 30cm distance and f/6.4 gives only 1.3mm depth of field. Switching to f/16 increases the depth of field to 3.2mm, and at f/22 you get 4.6mm.

  2. focus stacking: You can take a series of photos with the focus changed slightly from one to the next, and then combine them using focus stacking software to create a final image that's "in focus" over a the total range of all the images. I'm not sure this is a great option for your particular case, though — at the very least you'd have to validate the process to make sure that the stacking process doesn't give you incorrect results.

  3. greater distance: By moving the camera away from the subject, you can increase the depth of field in a single shot. Using the calculator again, you can see that increasing the distance to subject to 100cm gets you almost 20mm DoF at f/6.4 and 49mm DoF at f/16. The down side is that if you want to keep the subject the same size in the final image, you'll have to crop the photo. Using a longer lens won't help — the longer focal length will offset the increased distance. Following is a photo I took of the same zip tie from maybe 6x greater distance, in which you can see the increase in depth of field. But I had to crop the image quite a lot, so there's much less resolution:

(Full disclosure: the image above was taken using a slightly smaller aperture, f/4, than the preceding images, which were taken at f/3.5. The change was inadvertent, and I don't think it changes the larger point, but it's still important to point out.)

I can also think of a few other options that don't involve changing the depth of field:

  1. use your second image: There's a region in your second photo where the interface between the water and the card is in focus, and maybe that's all you really need for your measurement. I'm sure it'd be nice to have the edge of the card in focus, but if it's not actually needed then maybe you already have what you need in the second photo.

  2. photograph the card's shadow: You could set up a collimated light source at the far end of the tank and in line with the card. A collimated source produces parallel light rays, and if those rays are also parallel to the card then they should form a sharp 2-dimensional image. If you project that image onto a semi-transparent screen, you could photograph the shadow of the card and interface.

  3. use a laser: If you hit the interface with a narrow laser beam from a known angle, you could measure the angle of the interface by seeing where the beam is reflected. Move the laser to measure the angle at different points determine the shape of the curve. As a bonus, studies show that experiments that involve laser beams are much cooler than those that don't.

  • Re: point #3 - Cropping and enlarging back to the original display size has the same effect as using a longer focal length lens, because you have increased the overall magnification, which means you're also magnifying blur to a greater degree. – Michael C Sep 28 '18 at 11:33
  • @MichaelClark That's a good point, but I don't think that cropping to increase magnification exactly offsets the increase in DoF that you get from increasing distance in the same way that changing focal length does. A 100mm lens at distance d has exactly the same DoF as a 200mm lens at distance 2*d*. But using the same 100mm lens and just increasing the distance to 2*d*, and then cropping to get the angle of view of the 200mm lens, seems to give more DoF. I might be wrong on that, but my sample image seems to bear it out (though, do see the note I added about aperture). – Caleb Sep 28 '18 at 14:11
  • If one plugs the numbers into a DoF calculator that allows for variables such as cropping (via changing the sensor size from FF to µ4/3 for a 2X crop), it's pretty significant. A 100mm lens on a FF sensor @ f/4 gives DoF of 2.3cm @ 100 cm. A 100mm lens on a FF sensor @ f/4 gives DoF of 9.7 cm @ 200 cm. A 100mm lens on µ4/3 (2X crop) @ f/4 gives DoF of 4.9 cm @ 200 cm. Doubling the enlargement ratio halves the DoF given before cropping. 4.9 cm is closer to 2.3 cm than 9.7 cm by almost a factor of 2. – Michael C Sep 28 '18 at 14:34
  • So I think you're saying that increasing distance does buy you more DoF, but if you then crop the image you lose some but not all of that benefit because you're also magnifying the blur. If so, then we're in agreement. – Caleb Sep 28 '18 at 14:43
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    @caleb Thanks for all the suggestions, I will give them a try and see what works best. Lasers sounds like a good option too – Ragnar Oct 1 '18 at 13:55
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enter image description here

When your camera is focused on the far end of the plate, light rays from the far end of the plate (red lines) that strike any part of the front of the lens are being refracted to a point on the camera's sensor or film (let's just imagine it is the back of the camera).

At the same focus distance, light rays from the front of the plate (yellow lines) that strike different points on the front of the lens are being focused to a point well behind the film/sensor plane. The light from the front edge of the plate is being spread out over a blur circle as it strikes the film/sensor.

If the well focused light from other areas of the scene the same distance away from the lens as the rear of the plate is bright enough, or even if blurred light from a uniform surface beyond the back of the plate is bright enough, it will more or less completely cover up the dissipated light from the front of the plate that is spread over such a large area as to be very weak at any one point on the film/sensor.

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The forward edge of the plate is simply out of focus, and the angle of view turns the sides of the plate into mirrors, so all you see is a reflected view of the meniscus and background. Some things you could try would be a double exposure of the forward edge of the glass plate and the region of interest, 'focus stacking' to keep the entire plate and meniscus in focus (more or less), and increasing the aperture to f22. Another thing to try might be to coat the part of the plate not actually in the fluid with non reflective material (black paint?).

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    "and the angle of view turns the sides of the plate into mirrors": they aren't even visible, they are hidden behind the front edge (see top pic), which is itself blurred to oblivion. – xenoid Sep 27 '18 at 20:39
  • Good point... I thought about adding that but got distracted. Thanks for pointing it out... – BobT Sep 27 '18 at 21:11
  • They aren't fully hidden if the front element of the lens is wider than the plate any more than everything in the center of the field of view when using a mirror lens with an obstruction in the center of the lens is hidden. – Michael C Sep 28 '18 at 1:15
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The reason you cannot resolve the object is due to a phenomenon called the point-spread function of an image.

An image does not have an exact point of focus you can choose precisely. The point of best focus is a narrowing within a zone due to diffraction. The "point" is actually a narrowing of a converging beam of light to a "waist" before it diverges again.

The actual point is indiscrete.

Your lab set-up (or interpretation of the results) must compensate for these "biases" in the final image. It is not a trivial thing.

Photographic authentication of an event is not conclusive as it is subject to the vagaries of physical and psychological phenomena for interpretation. It is not bias free as you are discovering.

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