In this case also, focusing at H/n causes the depth of field to extend from H/(n+1) to H/(n-1). As you see, close focusing causes the depths of field to become considerably shallower than infinity focusing.In the case of close focusing with large optical magnification, the depth-of-field values differ considerably from the results of the calculation shown in Section 2.4. It is therefore recommended to consider the above depth-of-field values as rough estimates at an early stage of system design.

Depth offield vsdepth of focusmicroscope

Depths of focus are arranged as a sequence to show the ranges over which an object remains in acceptably sharp focus. This sequence is called a hyperfocal sequence (HS) or consecutive depths of focus.

Hi, could you please edit the source code for MCU, so that the last 10 seconds before completing it started “beep”? to create port PD4 a piezo speaker. Thanks.

Sören Sure depends o the tube type and distance. I used single tube at high distance and maybe that is why the time was long.

When a lens is focused on an object at a distance of H, a depth of field extends from infinity to H/2. In this case, H is called a hyperfocal distance.

Just thought I’d add my two-penneth on this, I built a similar design about a month ago, however I used standard clear case 5mm UV LEDs and drilled a PCB allowing for a much higher density with around 300 LEDs mounted on a 160×160 with SMD resistors mounted on the rear of the PCB. Not expecting fantastic exposure times I produced a test strip with exposure times of 1min, 2min, 3min etc and developing wiped it clean. Subsequently tested again with much shorter times, 10sec, 15sec, 20sec etc.

The ideal wavelength for UV photoresists seems to be about 365nm. At that wavelength glass is a significantly better transmitter than plexi. 400nm works pretty well and is a long enough wavelength that it doesn’t matter whether you use glass or plexi.

regarding the transmittance of the glass/plexi for uv, loz is right. however, it is always depending on the thickness of the glass. e.g. if you compare 6mm of standard window glass to 2 or 3mm of plexi it is obvious that the plexi will be better.

These are all true, but this information does not help you determine which lens provides a greater depth of field because cameras with different sensor sizes require lenses with different focal lengths to obtain the same angle of view.Note that the depth of field is related to optical magnification. In the case of machine vision applications requiring close focusing at relatively high magnification, it would be safe to consider as follows:

But what is more important, that the copper is now etched with copper sulfate evenly, meaning no more overetching, because of different etching times of different corners, and possibility to make traces thinner and nearer. With fluorescent bulb, the best result was 12mil, 10mil was achievable but hard way. 8mil was rarely achievable. Should test what can I achieve now.

The hyperfocal sequence is a sequence of successive H/n values, where n is an integer. The following table shows the values of H to H/5.(The unit of measure is mm. The negative sign that represents a direction is omitted.)

Depth of focusin eye

I’ll take pictures when it looks a bit better (right now it runs underneath an IKEA box). I’m gonna experiment a bit with some glass cause I’ve read that it lets long UV through (360nm and beyond). Some thin glass should be rigid enough to keep the film flat against the PCB as well as let most of the necessary UVs through… just keep your fingers crossed, I’ll try it tonight (if I can get some glass cut in time). Of course, this being of awesome importance, results will be posted. 😀

An image of a point source that is not in perfect focus appears as a blur spot called a circle of confusion (CoC). The size of the smallest spot that an image sensor cannot recognize as a blur is called the permissible circle of confusion or simply a circle of confusion.Even today when digital cameras are the mainstream cameras, the CoC diameters (δ) commonly referenced on the Internet (e.g., 0.033 mm, or 1/1300th of the image diagonal) are for images on silver halide films. These values are specified for images that are printed on a photographic paper of a certain size and viewed at a certain distance. In machine vision, which processes each pixel of an image sensor at high brightness levels, the permissible circle of confusion (δ) is calculated based on the pixel pitch (Ppix) or the diameter of the Airy disk (DAiry) that represents a limit to the optical resolution of an image created by a lens. In the case of monochrome cameras, the larger of these values is used as δ. (For color cameras with an on-chip color Bayer array filter, a value equal to two to three times δ is generally used.)

Focusing at H/n (where n is an integer) causes the depth of field to extend from N/(n+1) to H/(n-1). A hyperfocal sequence is a sequence of N/n:

In contrast, this (somewhat blocky) transmittance graph for various Plexiglass materials shows (if I’m reading it right) that the Plexiglass blocks even more of the UV light.

I’m sorry, I don’t have the UV box where I live since I don’t use it that much anymore. I will do a video once I get the chance.

Two optical systems provide the same depth of field when their lenses have the same optical magnification and f-number and their image sensors have the same pixel pitch.

What you can see in the picture above, from 2 to 7 minutes exposure if perfect. At 7 minutes some over exposure starts to show up, only in large copper areas, due to the fact that the film was printed with a laser printer. This result is very different from what I have obtained when using a fluorescent tube: below 12 minutes it was clearly underexposed and above 14 it was clearly overexposed, leaving only a small interval where it was right.

To form the cylindrical coordinates of a point P, simply project it down to a point Q in the xy-plane (see the below figure). Then, take the polar coordinates ( ...

Nice! I used the same LED’s for exposure! Something about wavelength and dager to the eyes: (did some research before trying) The led’s are called UV, but they’re actually just violet, not ultra! Their wavelength is 405..408nm. (check this with your datasheet if you wish) UVA starts at 400nm (and below). So no UV. So no UV danger to your eyes. (UVB is the more dangerouse wavelength, but that’s far away from this wavelength, 300nm as i recall). The optimum wavelengt of the PCB resist is 410nm, so this is an almost perfect match to the ‘uv’ LED. That’s why it works so great! Nice timer you built!

I suspect that with more use (exposure), the plexiglass may become yellowed or fogged, transmitting less blue/UVA over time. Still, a neat project, and well done!

I have just finished testing the solder mask with LED box. 13-15 minutes are OK. 12- minutes is a clear underexposure and mask go off when developing. Starting from 16 minutes there are bits of overexposure start to appear within thin traces. Starting from 21 minutes is a clear overexposure.

The following shows an equation for calculating the depth of field using the distance to a subject (x) from the focal point.Newton’s lens formula uses the front focal point as an origin to measure the distance to a subject (x). For typical shooting, a point on the opposite side of an image takes a negative value.The front depth of field (DoFN) is positive whereas the rear depth of field (DoFF) is negative. Since the total depth of field represents a distance, it is expressed as an absolute value.As effective f-numbers, the values calculated from the optical magnification at the front and rear depths of focus (FeN and FeF) are used. For general applications, FeN and FeF can be replaced with the Fe value that is calculated from optical magnification at a distance to the subject (x).

The final setup will contain a LM317 for stabilizing as I don’t plan on using my variable regulated supply all the time. There is another reason for this,  due to the non linear nature of the LEDs, varying 12V with +/-5% creates much larger current variations which influences the required time. Putting a regulator on the box simply allows more flexibility in choosing the power supply.

I tested with a small board from 2 minutes up to 8 minutes. Starting with the minimum of 2 minutes proved not to be the best idea, but my long exposure time old setup made me choose this value.

Thank you for your blog post with details. I have checked the cheap chinese LEDs from eBay, flat tops, 400nm, 3.2Vf, 20mA, and now my exposure time is only 4 minutes, instead of 15-18 with fluorescent bulb.

When a lens is focused at a hyperfocal distance (H) of about 362.3 m, the depth of field extends from infinity to H/2. So, the farthest point is infinity, and the nearest point is roughly 181.2 m. When the lens is focused at H/3 (≈120.8 m), the depth of field extends from about 181.2 m to about 90.6 m.

1. Exposure time is now SHORTER. Yes, glass, worked better. 4 minutes for perfect exposure it seems. You may stack more glass to make the film flatter. 2. Making a box is pricey. Unless you’re really enthusiastic about it, recycle something:) 3. Nothing matters more than film-quality it seems. Make it high dpi and high-quality film so it doesn’t curl. One small curl and it’s all over. 4. Watch the developer solution concentration. If you’re not sure, make it weaker, not stronger, and let it cool down before you use it:)

Great project. It’s great people like you sharing your experience & talent that make everyone else who wish to learn better individuals. I was searching for something I could build to kill germs in well water coming into my home. (don’t have any problem, just being careful). They are commercially available but very expensive. Found some info on UV exposure you might need to be aware of. http://www.ehs.washington.edu/rsononion/uvlight.shtm Thanks

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There’s also an expiration date on the material, which is very important – the older the material is, the longer it needs to be exposed to uv light, while “very old” (2 years after expiration date or so) did render the pcb board unusable.

For safety reasons and extra functionality I added a switch to turn off the UV light when the box opens, turn on the red light and stop the timer. This makes the box more high tech and  gives it a professional behavior.

I used nuts glued on the interior walls of the box to hold the bottom 4 mm thick plexiglas panel. The choice was simply of what was easily available: nuts.  Don’t use normal glass as it blocks ultraviolet light. There is no top panel mounted, because it is part of the aligning procedure: I put the first film on the bottom, then the board and the second film is taped to a smaller piece of plexiglas. Aligning means aligning the smaller piece of the plexiglas so that the two layers overlap perfectly. Having the film attached to it makes it easier. Extra markings outside the board help this (ghost vias). Part two should contain a more detailed tutorial on making double sided boards.

Depth of focusformula

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Depth of focuscalculator

I bought 25 LEDs and began to experiment. This way I was able to find a compromise between LED density (spacing), distance between LEDs and board and cost. I wanted something compact, I don’t make large PCBs and those 30cm tubes were not right for this job.

In order to determine the depth of focus (DoFo) accurately, it is necessary to consider the optical aberrations of a lens such as an image plane curvature. However, since such accuracy is generally unnecessary, the depth of focus is calculated based only on parameters along the optical axis.The DoFo value obtained by the above equation represents the most stringent case in terms of accuracy. For actual applications, a more relaxed value can be used as necessary. (For details, see the white paper Consideration for depth of field in machine vision from Toshiba Teli.)

Nowadays, surveillance cameras in distant-view shooting mode employ image shooting and processing techniques for machine vision cameras, for example, to synchronize trigger control with lighting.Suppose, for example, a wavelength of 550 nm, a camera with a pixel pitch of 3.45 μm, and an F2 lens with a focal length (f) of 50 mm. Let us calculate consecutive depths of field under these conditions.The specifications for the camera and the lens are:

When a lens is focused at H/n, H/(n-1) and H/(n+1) are the rear and front depths of field respectively.For example, a lens focused at H holds a depth of field from H/2 to infinity.As shown in the previous subsection, when a lens is focused at infinity, the rear depth of field is infinite, going slightly “over infinity,” while the front depth of field is H.Dividing a hyperfocal distance (H) by an integer (n) means that the focal point shifts along the optical axis by half the depth of focus (δ*Fe) times n in relation to the lens extension (x’) at infinity.

The final assembly allows an area of about 10x15cm of board size, with the interior dimensions of the box being 18cm long, 11cm wide and 5.5cm deep for each of the two halves. Exterior dimensions will depend on the thickness of the material used. Each board contains 7×12 UV LEDs, with 3 and respectively 4 holes between them( the actual spacing between the center of the LEDs is the same). The distance between the top of the LEDs and the board is 3.5cm, including the 4mm plexiglas panel. Check out the detail below to see the LED arrangement on a normal 0.1″ or 1.27 spaced board:

adrian, you are correct, it is dependent on thickness. Also, if that particular thickness of plexi does not extend the time for too long, there is no problem. You have to think of this from an amateur point of view. How much time will you actually save for a single/few board(s) if you exposure time is shorter than my 2 minutes? probably not that much.

Many machine vision applications shoot subjects at close distances (for example, at a distance of 300 mm). For close-up shooting, the depth of field can be calculated using optical magnification. The previous subsection explained the depth of field in relation to longitudinal magnification (α). The following shows an equation for calculating the depth of field using linear magnification, β (also called lateral or transverse magnification) since optical magnification generally means linear magnification. α and β have the following relationship: α=β2.

As for the boards, I can purchase boards from two manufacturers, like I mentioned one is Bungard and I dont know the other’s name, but results are very similar, i never changed the exposure time. Never had a board stay for as long as two years before being used.

I have bought this solder mask and tested with fluorescent bulb. I had to cure it for 25 minutes from the height of 2cm.

the best would probably be a borofloat glass as thin as possible. 2mm is a standard thickness there. http://www.pgo-online.com/de/katalog/kurven/boro_kurve.html

I’m sure it really isn’t required, but have you thought about sanding the tops of the leds to get a more diffused spread? You could sand with something fine like 400 grit and not lose the bevel, so you’d still get the 140 degree spread, albeit a little more diffused and even.

So far I have made 4 boards with 2 minute exposure and they came out perfect. When free time will be available I will investigate what the results are for less than 2 minute exposure. As any time between 2 and 7 minutes seems to work, there is no immediate need for a timer. Still, part two should contain a basic circuit for this.

Feb 16, 2023 — Focal length is the main optical property of a camera lens. It's displayed in millimeters and by a number that looks like this: 35mm, 50mm, ...

For a detailed description of the depth of field, see Basics of camera lenses: Guidelines for lens selection and Consideration for depth of field in machine vision on our website. This section provides an overview of the depth of field and some added information about it.

Remember that the previous subsection mentioned that the depth of focus is the conjugate of the depth of field. A change in the object-side depth of focus caused by a displacement of the image plane by half the depth of focus is equal to half the depth of field. A slight displacement of an image plane causes the object-side focal point to shift by as much as a change in the image-side depth of focus divided by the longitudinal magnification of the lens. Because the depth of focus is symmetrical around the image plane, the depth of field is also symmetrical around the image plane.

12 to 14 Minutes with tubes? That sounds odd – my (not-selfmade) exposure box needs 3 to 4 minutes to give good results. Though I found that it well depends on the material used and how old this material is. For example cheap noname products gave moderate to bad results while some little more expensive material gave me quite good results.

On eBay you can find the same mask cheaper when buying larger can, instead of syringe. There is also white solder mask available, which I want to try to use instead of silkscreen.

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Hello Alessandro. I cannot help you, as I did not build that PCB. It is my design, but i don’t know who made it, just hope that it worked well for the builder. I found a video with someone trying the same thing: http://www.youtube.com/watch?v=WTuF28TqwT0

Product Details. Great vintage Magnivision magnifying glasses in great condition.

From Newton’s lens formula, the lens extension (x’) is expressed as follows using the camera-to-subject distance (x): The lens extensions (xN’ and xF’) at the nearest point of the front rear depth of field (DoFN) and the farthest point of the rear depth of field (DoFF) are calculated as follows using the effective f-number at x, Fex (denoted as Fe in the above paragraph): The linear magnification factors (βN and βF) and the effective f-numbers (FeN and FeF) at xN’ and xF’ are calculated as follows:

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I’ve got my LEDs from TME too, the price was about the same. You are right about the soldering, it’s very boring, not to mention the red LEDs I’ve used. I actually recommend using a red light bulb in your desk lamp for aligning, it’s much simpler. On the safe part, mine has no problem, it won’t led the UV LEDs turn on with the box open and the box is not transparent. For the supply part I used local regulation because I had more than one power supply that could be used, so i decided that it was worth to have the box work the same way for various input voltages.

On the bottom board I have added red LEDs. These LEDs help me align the top and bottom films. I have used low brightness, cheap, matte LEDs as I have to look into them when aligning the films. Connection is similar, 3 LEDs in series with a 220Ω resistor, except for the last row which contains 2 LEDs and a 330Ω resistor. These are not really necessary, aligning the films can be done in a dim ambient light.

While I agree that Plexiglass is reasonably transparent at the wavelengths you are using, your comment earlier that glass is _less_ transparent to UV than plexiglass is incorrect.

-“safe”-ish wavelengths for the UV light (which is actually kind of violet at 400-405 nm but is smack in the middle of the photoresist’s maximum sensitivity range. But just play it safe and don’t look at the LEDS while they’re on, or let the light fall on your skin; -the mentioned LEDs are cheap (got mine from TME – http://www.tme.eu, a Polish supplier) for about 30 Euros for 200 pcs; -simple construction (just LEDs, resistors and perf board, after that just add power and box and plexiglass); -If you need 2 amps of 12 volt power, an old computer power supply has all the necessary wires. Just don’t leave it attached to the computer when you use the box; -tubes are fragile, LEDS aren’t.

Depth of focusvsdepth offield

With fluorescent bulb I had to print the templates 3 times on the same sheet of film. Luckily I have a photo printer which matches the printout position 4 times out of 5. Now I can print once and the density is enough. There was no overexposure from 3.5 to 6.5 minutes with 30 second steps tested.

Made the box (and added one extra row of LEDs just because I’m a mean prick:P) and with 4.5 mins of exposure I can get 0.1 mm continuous traces, so I guess I can say this works just right, so allow me to review this:

If you will use the same supply with the same load all every time then it is likely that you will be ok. My worries are that if you switch between two 12V supplies, and one gets you 12V-5% and the other 12V+5% you will need to retest for exposure time. But then again, my results show that there is a rather large interval which gives you a good exposure.

I have been using the toner transfer method for about 9 years with great results. Occasionally I would need a board with finer traces and I would use UV method. My exposure setup allowed only a single sided board to be made and it was composed of a 30cm UV tube, some holders, a table, board and film. Exposure time was rather long as the tube was far away from the board.

This is very good news. I think i might get some solder mask myself. How much time does it take you to make a pcb from start to finish? (assuming you have the design done, of course). Can you add some links to some pictures with your results?

Most machine vision applications provide close focusing. The following table shows the consecutive depths of field for relatively close focusing under the same conditions as above.

The box may be made of your choice of material. I’ve built it out of two identical halves, interior dimensions 18 x 11 x 5.5cm each. The material is 8mm thick OSB, for which you will require the following parts:

You are always able to find crystal at shops that work with mirrors and special glasses. It is a little bit more expensive than glass, but for a small piece as necessary for this box, probably you will be able to have it for free.

Depth of focusmicroscope

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Hi, I was interested to see your UV light box build. You say that you use plexiglass instead of glass, but I thought that glass, although it doesn’t transmit far UV, it does transmit quite a bit of the near stuff.

Depth of focuscamera

So when “investigating” use equivalent boards (manufacturer and expiration date), otherwise you won’t get comparable results 😉

I personally use glass because I can use a thinner panel and get the same rigidity that would require a much thicker plexi panel to achieve.

The permissible circle of confusion (δ) is the larger of the pixel pitch (Ppix) and the diameter of the Airy disk (DAiry). So, let us calculate DAiry to determine δ:

Thanks for the tip on crystal. I knew it was UV transparent but never thought about using it. I will look for suitable glass if plexy will show any signs of yellowing or if boards will appear underexposed.

The solder mask itself seems to require more curing after development, since it can go off sometimes when soldering large ground pads even with thermals.

With an exposure time of only 15 seconds I am getting excellent results, any longer and its overexposed which was quite shocking. Thought it was worth mentioning that with a higher density of LEDs the exposure time is dramatically reduced to near that of a professional-level UV exposure unit, however I do think some better guarding will be necessary as that is probably a highly dangerous amount of UV light being produced.

However, if the optical magnification of a lens differs greatly at both ends of the depth of focus, the front and rear depths of field are not equal as described in 3 and 4 above. Equations for the depth of field are presented in the following subsections. In the case of machine vision and other applications requiring relatively close focusing, we recommend using an equation with an optical magnification term. Otherwise, we recommend using an equation with a subject distance term.

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Regarding glass, I have tested with ordinary glass and the time is longer(5 minutes was not enough compared to 2 that I used). The way I read that graph, you have good transmittance for plexiglass around 400nm. Still, I need under two minutes to expose the board, which is reasonably short for amateur work.

It’s a great thing somebody else has built one with same leds. actually, I’ve got two colleagues who replicated the design minus the timer. The leds are emitting UV light too, they do have a peak at 405nm. The boards I use come from bungard.de, and have a max sensitivity at 350-400. They work perfectly. I wanted to add protection after the first experiments i made with these leds, just 1 second of looking at them when on seemed very uncomfortable.

The depth of focus refers to the tolerance of placement of an image sensor in relation to the lens. The depth of focus is the conjugate of the depth of field. Since both the depth of field and the depth of focus are abbreviated as DoF, they are hereinafter referred to as DoFi and DoFo respectively for the sake of clarity.The depth of focus (DoFo) is the distance over which a sensor can be displaced along the optical axis while an object remains in acceptably sharp focus. DoFo can be calculated from the permissible circle of confusion (δ) and the effective f-number (Fe).

Many of the readers of this white paper might have had questions before about the depth of field (DoF) not only when using machine vision systems but also when taking photographs with ordinary cameras.The depth of field is the distance between the nearest and the farthest objects in an image that appear acceptably sharp. You can find information about the depth of field on the Internet, in off-the-shelf books, and in trade journals. White papers about the depth of field are also available on our website under the headings Basics of camera lenses: Guidelines for lens selection and Consideration for depth of field in machine vision. These white papers discuss the depth of field from a different perspective than the commonly adopted one in the photography industry.This white paper describes, in an easy-to-understand way, the depth of field in relation to the hyperfocal distance and the hyperfocal sequence. Although these lens properties are closely related to each other, it is difficult to find such information on the Internet. Therefore, you will find this white paper useful.

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In order to build your own you first must decide what the required size is for you. With my setup, boards up to 10 x 15 cm may be exposed. For this, you will need the following materials:

Hi, I found a photo of one of your pcb with solder mask on the tinydeal website: http://www.tinydeal.com/pcb-fbc-uv-solder-mask-repairing-paint-green-oil-prevent-corrosive-arcing-ctl-26735-p-22509.html. I would kindly ask you how many time did you expose the pcb with green ink to the uv light.

he depth of field is the distance between the nearest and the farthest objects in an image that appear acceptably sharp.When you shoot images, you focus a camera on objects at arbitrary distances. The distance to the nearest point in acceptably sharp focus in front of the best-focused point is called the front depth of field whereas the distance to the farthest point in acceptably sharp focus behind the best-focused point is called the rear depth of field. The distance between these points is the total depth of field.The depth of field can be calculated as described in the following subsection. However, objects appear differently, depending on their size and surface characteristics as well as the optical aberrations of the lens used. Therefore, cameras do not always come into sharp focus precisely at the calculated threshold; rather, they gradually come into and out of focus around a threshold.

Since I have now made the LED box, and photoresist curing time is now 3 times faster, I guess, the solder mask curing time should be also faster and results more stable. I will test it in the nearest future and write back the results.

I’m not sure about that, i’ve used some plexi for outdoor applications and it stayed clear after about 2 years in the sun.

A few months ago I was looking for some LEDs at one of my suppliers and found that they also had UV LEDs. My mind went directly to upgrading my not so great setup, so I began looking on the internet for info. It turns out some other people have done it, and this instructable confirmed it was really possible. A LED UV box has a clear advantage over a tube one: it can be made in any size, depending on the needs.

If was possible, use a laminate of crystal, it conducts UV much better. This is the reason why UV lamps are made of crystal, not glass.

Whens a lens is focused at infinity (x=∞, x’=0), the hyperfocal distance is equal to the front depth of field as given by the following equation. (Here, we use Newton’s lens formula since our attention is on the lens extension, x’.)Optical magnification has not been determined yet when you calculate a hyperfocal distance. Therefore, we use the f-number (F) at infinity (that is not modified by optical magnification) instead of the effective f-number (Fe).

The LEDs are OSSV53E1A from OptoSupply and they have an 140° angle. This means that the first plane with uniform density can be obtained at a shorter distance from the LED, for a given spacing between LEDs which means smaller height for the board. Their peak wavelength is 405nm, which is higher than recommended by the boards’ manufacturer. Results showed they emit sufficient UV light in the responsive spectrum of the photo resist to allow for a perfect exposure.

I’ve used a thin piece of paper on the plexiglas to test how diffused the light is, and it seems perfect. Also, after >10 pcbs made with this box there is no sign of any required extra diffusion.

I don’t have anything stronger than 12V at 600mA currently, except for the PC supply, and a 15+V supply at 2A to run through dual LM317s would have cost me quite a lot…

this is probably not as easy to buy as a plexi is, but it will give the best performance when it comes to transmittance of uv light.

So it look slike even though normal glass is not such a great transmitter of UV as silica or quartz glass, it may still do better Plexiglass. If that’s the case, you might get a shorter exposure time by switching to glass.

I use acetone, usually with some colophony dissolved in it. This has the effect of protecting the copper from oxidation and making it very easy to solder.

Gauss’ lens formula (1/(-a)+1/b=1/f) expresses the depth of field using the distance to a subject from the principal point.The subject distance used in Gauss’ lens formula is greater than the one used in Newton’s lens formula by the focal length of a lens (f). Therefore, the depth of field can be easily obtained simply by replacing x in Newton’s lens formula with a+f.The origin of the coordinate system for the subject distance (a) is the front principal point. For typical shooting, a point on the opposite side of an image takes a negative value.