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The magnification depends on the focal length and the subject distance, and sometimes it can be difficult to estimate. When the magnification is small, the formula simplifies to
However, they’re cheap because their lenses are plastic. My problem with them was that I either wound up scratching them (always in a spot directly in front of my eyes!) or I splashed a drop or two of some solvent-based paint, glue, filler, etc. on them which etched permanent defects.
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I never found myself using the smaller lens on the swivel, and it in fact eventually broke off from being rammed into my work light when leaning forward.
I have one similar to SableLiger’s; I think mine is an Optivisor or a knock-off. It came with a set of different magnification inserts depending on your needs.
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Use your experience with the magnification and working distance you get with the cheap ones to select and buy the appropriate ground-glass magnification power lens plate (or plates). You don’t need to buy the entire Optivisor boxed set with all of the available lens plates at once. You can buy them individually, as needed.
In astronomy, the depth of focus Δ f {\displaystyle \Delta f} is the amount of defocus that introduces a ± λ / 4 {\displaystyle \pm \lambda /4} wavefront error. It can be calculated as[4][5]
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While depth of field is generally measured in macroscopic units such as meters and feet, depth of focus is typically measured in microscopic units such as fractions of a millimeter or thousandths of an inch. In optometry depth of focus is usually measured in dioptres.
I finally pulled the trigger on a pair of genuine Optivisors with a couple of different magnification power lenses. As @flatfour observes, the lenses are glass and so are nearly impervious to scratches and anything I’ve every splashed on them has wiped and cleaned away with no problems.
If you wear glasses, then ones like SableLiger recommended are great, as they fit down over your prescription glasses. If you don’t wear prescription glasses then look for “cheaters”, “readers”, whatever you want to call them. The higher the number, the more magnification. I use 1.5 for reading and computer work. I use 3.5 for model working. I think you can get them as high as 6.0, check Amazon.
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I’ve been doing ok so far whilst building 1:35 armor. However the Takom Hetzer full interior kit has me thinking about more help to see the tiny parts which challenge these 91 year old eyes… I currently use one of those magnifiers thats on a huge boom and can swivel… etc. It doesn’t give me room to work under it when I really really need it. Perhaps a set of magnifying eyeglasses. that focus at 12-14". That’ll let me see and still swing tools around.
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Depth of focus can have two slightly different meanings. The first is the distance over which the image plane can be displaced while a single object plane remains in acceptably sharp focus;[1][2][clarify] the second is the image-side conjugate of depth of field.[2][clarify] With the first meaning, the depth of focus is symmetrical about the image plane; with the second, the depth of focus is slightly greater on the far side of the image plane.
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Following historical convention, the circle of confusion is sometimes taken as the lens focal length divided by 1000 (with the result in same units as the focal length);[2][3] this formula makes most sense in the case of normal lens (as opposed to wide-angle or telephoto), where the focal length is a representation of the format size. This practice is now deprecated; it is more common to base the circle of confusion on the format size (for example, the diagonal divided by 1000 or 1500).[3]
In small-format cameras, the smaller circle of confusion limit yields a proportionately smaller depth of focus. In motion-picture cameras, different lens mount and camera gate combinations have exact flange focal distance measurements to which lenses are calibrated.
Depth of focus is a lens optics concept that measures the tolerance of placement of the image plane (the film plane in a camera) in relation to the lens. In a camera, depth of focus indicates the tolerance of the film's displacement within the camera and is therefore sometimes referred to as "lens-to-film tolerance".
The simple formula is often used as a guideline, as it is much easier to calculate, and in many cases, the difference from the exact formula is insignificant. Moreover, the simple formula will always err on the conservative side (i.e., depth of focus will always be greater than calculated).
Screenshot_20240223-0830011080×2460 137 KB Have the this pair. $15.95. + shipping. They have bright led lights that are rechargeable. Work great with my reading glasses . interchangeable. Lens.
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@corsutton Hits the mark here. I have reader magnifier eyeglasses from +2 to +6 that I use for all my modeling. I keep them lined up at the back of my bench and just switch among them when I need more or less magnification. They are much lighter and less claustrophobic than a visor and you can easily look over them to locate tools / read instructions. They let in more light. They’re cheaper than an Optivisor, too. You can order a whole set online from any number of sources or find them at the dollar store.
Once you get used to wearing them, you’ll never work without them again. First thing I do when I walk into my workroom is to put them on and taking them off is the last thing I do before turning out the lights.
But I manage to get by with my 4x Opti Visor, two 1.7x ring-light magnifiers, a 5x hand loupe, a 10x jeweler’s loupe and a 17x geologist’s magnifier, and lots of light.
The same factors that determine depth of field also determine depth of focus, but these factors can have different effects than they have in depth of field. Both depth of field and depth of focus increase with smaller apertures. For distant subjects (beyond macro range), depth of focus is relatively insensitive to focal length and subject distance, for a fixed f-number. In the macro region, depth of focus increases with longer focal length or closer subject distance, while depth of field decreases.
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The choice to place gels or other filters behind the lens becomes a much more critical decision when dealing with smaller formats. Placement of items behind the lens will alter the optics pathway, shifting the focal plane. Therefore, often this insertion must be done in concert with stopping down the lens in order to compensate enough to make any shift negligible given a greater depth of focus. It is often advised in 35 mm motion-picture filmmaking not to use filters behind the lens if the lens is wider than 25 mm.
The phrase depth of focus is sometimes erroneously used to refer to depth of field (DOF), which is the distance from the lens in acceptable focus, whereas the true meaning of depth of focus refers to the zone behind the lens wherein the film plane or sensor is placed to produce an in-focus image. Depth of field depends on the focus distance, while depth of focus does not.
where t is the total depth of focus, N is the lens f-number, c is the circle of confusion, v is the image distance, and f is the lens focal length. In most cases, the image distance (not to be confused with subject distance) is not easily determined; the depth of focus can also be given in terms of magnification m:
Having said all of the above, I do have a number of modeling friends who really like the inexpensive drug-store magnified “reading glasses.” These might be worth checking out (especially since you can just walk into the drug store and try them on).