Eye correction diopter # ?

So my glasses are like +275 or something but I think the prescription is not perfect as I feel like I can never quite focus properly. And my eyes get tired wearing my glasses all day.... But that is just the back story to my question(s)

So with modern cameras and built in diopter dials, I turn it almost as far as it will go to get the image sharp in the viewfinder.... Sometimes with glasses on , sometimes without, depending on Nikon or Canon. the range of the correction seems different??

But with my film cameras I wanted a correction diopter for my nikons . Not knowing the number system I decided to go to B&H to try them on for size..... I was expecting a +2 or something.... Turns out a +1 without glasses makes the viewfinder so so so sharp.... I havent been able to see that good in a long time.... So how do the numbers correlate to eye glasses?? And why arn't the modern dial diopters as good???

The diopter unit is the same, camera viewfinder or eyeglass prescription.

The power of a lens to magnify is based on its focal length. This is the distance lens to projected image when the lens is imaging a far distance object. You can use a magnifying lens as a burning glass. Focus the image of the sun on white paper. Move the lens towards and away from the paper to focus an image of the disc of the sun. When focused the spot of light that is the sun’s image will be at its tiniest. Measure the distance lens to paper; this measure will be focal length.

The focal length is most commonly expressed in millimeters however sometimes inches or even feet or meters are used, depending on need.

Optometrist and the eyeglass industry prefer a unit called the diopter. If the focal length of a lens is 1000mm, then this industry labels the lens 1 diopter. The diopter unit is 1/focal length in millimeters multiplied by 1000. Thus a 500mm lens = 1/500 X 1000 = 2 diopters. A 300mm lens = 3.33 diopters a 250mm focal length = 4 diopters.

The diopter is a strange unit but its advantage outweighs the oddity. Suppose you combine two lens, one has a focal length of 50mm and the other 120mm. What is the final focal length? Convert both to diopters and add. Thus 50mm = 20 diopters and 120mm = 8.33 diopters. The total is 28.33 diopters. Now we convert back to millimeters thus 1/28.33 X 1000 = 35.3mm (the focal length of the combination).

The power of the reading glasses in diopters and the adjustment of the camera eyepiece correction are the same. When we gaze into the camera we want our eyes relaxed. For most, this happens when we are looking at a distant object. The camera has an eyepiece that is configured to cause the image in the viewfinder to appear to be a view of a distant object. In other words, people with normal vision are able to see the viewfinder view with relaxed eye muscles. If you are nearsighted or farsighted, without glasses, the viewfinder view will be blurred. The idea of the viewfinder diopter correction is to allow you use the camera without your glasses. If this adjustment does not work for you, the prescription you need is outside the range of the camera’s diopter correction. An experienced optometrist can make a special lens that you can overlay on the viewfinder or it can be incorporated into specialized reading glasses.

The Nikon film SLR finder is +1 as is. My readers are +2.5 so I add a +1 diopter for shooting without glasses. For me a +2 will not work yet.

The F/F2/Nikkormat are the same size (FM/FE too I think) and the FE2/FM2/FA/FG take a smaller size. The N90s, 8008s and F3HP take the same size.

Hard to find even from KEH but so much better than squinting with glasses. My only camera with built in diopter is the F4. Do not mess with digital SLR cameras.

Chris

Optometrist and the eyeglass industry prefer a unit called the diopter. If the focal length of a lens is 1000mm, then this industry labels the lens 1 diopter.

The technical definition of a dioptre is 1/focal length in meters. Your definition is a bit vague as to what is multiplied by 1000.

It's (1/focal length in mm)*1000 - as you correctly used it - but could have been misread as 1/(focal length in mm *1000) which is a million fold smaller.

One other advantage of diopters is they give bigger numbers with more powerful lenses (unlike focal lengths).

I'm sure the OPs prescription is not +275 but is actually nearer +2.75, but will include other factors such as a cylindrical element (quoted in both dioptre & the required angle) and the distance between pupils a reading addition is often also involved. Having the glasses get these wrong will give rise to issues.

For many years I had my viewfinder adjusted to use with my glasses, but since getting my eyes upgraded (I now only need glasses for reading, or at work where safety glasses are compulsory) I have my viewfinders adjusted for use without. I've never bothered using the prescription to set these up merely adjusting the dioptre till the numbers in the viewfinder are comfortably sharp works well.

Diopter "filters" are negative lenses, consequently have no intrinsic focal length except in context with positive lenses. A +1 diopter lens in conjunction with a positive lens reduces the focal length of that lens, so that the objective (camera) lens focused at infinity now focuses at 1 meter. A +0.5 diopter causes the objective to focus at 2 meters, according to the reciprocal of the strength of the diopter "filter." In a Galilean telescope, the eyepiece is a negative lens, which is focused on the virtual image of the positive objective lens, which is then refocused by the lens in your eye to form a sharp, erect image on the retina as though you are looking at something far away (infinity).

This principle is used in the design of both wide angle and telephoto lenses. Wide angle lenses use a strong negative element in the objective in order to increase the back focus distance. The rear node of the lens can easily be closer to the focal plane than the flange distance. This is the logic behind "inverse-telephoto" lenses for SLRs, even MIL cameras for focal lengths less than 18 mm or so, or for a smaller angle of incidence on the sensor (telecentricity). That is why a highly corrected 35 mm MIL lens is often larger than a simpler 90 mm lens.

A negative lens at the other end increases the effective focal length, so that the lens can be shorter and more portable, called a "telephoto" lens. Thus, "telephoto" refers to the lens design, not the focal length per se, despite the common use to describe any long lens. The rear node (effective focal center) can actually be located beyond the front element of the lens (and the front node).

Theoretically, the perfect human eye, when relaxed, is focused at infinity. It the eyepiece of the viewfinder has a net +1 diopter power, the view will appear to be at a distance of one meter. For rangefinder cameras, that refers to the projected frame lines, if any. For SLRs and EVFs, that is the apparent distance to the ground glass or screen.

Diopter "filters" are negative lenses, consequently have no intrinsic focal length except in context with positive lenses. A +1 diopter lens in conjunction with a positive lens reduces the focal length of that lens, so that the objective (camera) lens focused at infinity now focuses at 1 meter. A +0.5 diopter causes the objective to focus at 2 meters, according to the reciprocal of the strength of the diopter "filter." In a Galilean telescope, the eyepiece is a negative lens, which is focused on the virtual image of the positive objective lens, which is then refocused by the lens in your eye to form a sharp, erect image on the retina as though you are looking at something far away (infinity).

 

This principle is used in the design of both wide angle and telephoto lenses. Wide angle lenses use a strong negative element in the objective in order to increase the back focus distance. The rear node of the lens can easily be closer to the focal plane than the flange distance. This is the logic behind "inverse-telephoto" lenses for SLRs, even MIL cameras for focal lengths less than 18 mm or so, or for a smaller angle of incidence on the sensor (telecentricity). That is why a highly corrected 35 mm MIL lens is often larger than a simpler 90 mm lens.

 

A negative lens at the other end increases the effective focal length, so that the lens can be shorter and more portable, called a "telephoto" lens. Thus, "telephoto" refers to the lens design, not the focal length per se, despite the common use to describe any long lens. The rear node (effective focal center) can actually be located beyond the front element of the lens (and the front node).

 

Theoretically, the perfect human eye, when relaxed, is focused at infinity. It the eyepiece of the viewfinder has a net +1 diopter power, the view will appear to be at a distance of one meter. For rangefinder cameras, that refers to the projected frame lines, if any. For SLRs and EVFs, that is the apparent distance to the ground glass or screen.

@Ingold - The diopter unit of focal length applies both to positive (convex) and negative (concave) lenses. A positive mounted before the camera lens allows closer focusing. A negative lens mounted before the camera lens results in a wider angle of view. In terms of the eyepiece correction most

Specific to camera eyepieces: The ground glass focusing screen is just millimeters away from the viewfinder eyepiece as is the data displayed on the reticle. To facilitate comfortable viewing, the camera’s eyepiece is fitted with a magnifying lens. The focal length of this eyepiece lens is adjusted so that the light rays from the ground glass and reticle exit as parallel. In this way, the eye/brain is fooled to discern this view as if it comes from a far distance (infinity ∞). On fine cameras, the position of the eyepiece as it relates to the ground glass and reticle is a variable. The so-called diopter adjustment simply alters the air-spacing of eyepiece to the ground glass / reticle. This adjustment permits fine tuning for individuals that are near or far sighted (within limits). The diopter scale associated simply provides equivalent diopter values that are loosely related to the power of corrective reading glasses.

On another subject: Supplemental lenses can be affixed before, after, or between the lens elements of camera and projector lenses. These are used to alter the focal length of fixed focus lenses. Mounted on the camera, positive power supplemental lens allow for close focusing. Others, both positive and negative in powered, transfigure the focal length coveting a prime lens to a moderate telephoto or wide-angle.

Projector lenses can be fitted with supplemental lenses to make fine adjustments to the size of projected image. Photofinishing printers often were augmented with a drawer of primes with supplemental lenses mounted. The operator changed lenses to accommodate the desired print size i.e. 4x6 inch borderless, ¼inch borders etc.

In most cases, these supplemental lenses are labeled in diopter units or multipliers such as 1.5X etc.

You're in good hands

For additional confirmation of much of the above, search "Nikon Viewfinder Eyepiece Application Reference".

In it, it said

I have never found the relationship to be that straightforward. For reading these days I use about a +3 diopter, though I can get by with less, and a good bit less for computer screens. But I still find a +1 on the old Nikon F or F3 quite enough, and once when ordering on line KEH accidentally added a +3 to the order. It's utterly unusable, way too strong.

On digital cameras with built in diopters, the adjustment varies. The D3200, for example, goes only to +.5. That's marginal for me, usable but not comfortable, but I found that an eyepiece magnifier also adds diopter, and I ended up dialing it back with it on. The d7100 has enough native adjustment, going to +1.

For my own purposes, it seems that the viewfinder diopter should be about what you would use to read a desktop computer screen from a normal distance.

The Nikon film SLR finder is +1 as is. My readers are +2.5 so I add a +1 diopter for shooting without glasses. For me a +2 will not work yet.

The F/F2/Nikkormat are the same size (FM/FE too I think) and the FE2/FM2/FA/FG take a smaller size. The N90s, 8008s and F3HP take the same size.

Hard to find even from KEH but so much better than squinting with glasses. My only camera with built in diopter is the F4. Do not mess with digital SLR cameras.

Chris

Actually Nikon consider their viewfinder in neutral as -1 and not +1. Yes the diopter you use should be less than your reader.

I have never found the relationship to be that straightforward. For reading these days I use about a +3 diopter, though I can get by with less, and a good bit less for computer screens. But I still find a +1 on the old Nikon F or F3 quite enough, and once when ordering on line KEH accidentally added a +3 to the order. It's utterly unusable, way too strong.

 

On digital cameras with built in diopters, the adjustment varies. The D3200, for example, goes only to +.5. That's marginal for me, usable but not comfortable, but I found that an eyepiece magnifier also adds diopter, and I ended up dialing it back with it on. The d7100 has enough native adjustment, going to +1.

 

For my own purposes, it seems that the viewfinder diopter should be about what you would use to read a desktop computer screen from a normal distance.

If you set the diopter on the viewfinder at the default position or with camera without the adjustment you need to add a diopter lens that is the same as your eyeglasses to see 1 meter from your eyes. And this is less than your reader glasses.

In addition to the above mentioned uses, diopters are also used for traditional close-up lenses.

Before SLRs, when you had to estimate the focus distance, it was not so hard to figure out the

appropriate distance to focus (or not, for a fixed focus camera), with +1, +2, or +3 diopter

lens on the front.

For a lens focused to infinity, a +1, +2, or +3 lens will shift the focus distance

to 1.0m, 0.5m, or 0.33m, respectively. This worked fairly well with simpler

cameras from the early years.

For two thin lenses, fairly close together, diopters conveniently add.

As the spacing increases, that doesn't quite work, but closer than nothing.

With SLRs, we mostly don't need to worry about this.

My glasses are about -6. I have thought about, but never tried corrective eyepieces.

When using binoculars, though, I most often focus without my glasses.

(This is close to the limit on many of them.) One lens allows correction for any

difference between the two eyes.