The Airy Disk and the Real World

The Airy Disk is used as a basis for many explanations, such as those re: DOF or resolution for example. However such explanations, although they are helpful in context, often miss - or fail to emphasize - an important connection to the Real World where real photographers ply their trade!

It all stems from the use of the phrase "The Airy Disk", along with it's various formulae, as if the said disk is a uniquely visible item. You see, any formula for the disk, radius, diameter or even the Rayleigh criterion contains the value of the light wavelength, usually called lambda. And there we have it - by using a unique value for lambda (usually 555nm, a.k.a. green) the explanation is calculating an Airy Disk property for monochromatic light which, by and large, does not exist in the Real World - with the possible exceptions of laser-lit nightclubs or futuristic battlefields.

So, even with a 1um pixel pitch and a lens set to f/45, you are most unlikely to capture one of these:



"What's he on about?" I hear y'all say . . .

Well, a real image might, for example, contain a yellow flower. A yellow flower does have some reflection of our perfect monochromatic green at 555nm, but it also has reflection up in the reds and infra-reds, as shown by this Real World image (alpine meadow, Tibetan Plateau) from this paper:



So, in my view, the diffraction effect causes round blobs at the cameras image plane - not fancy-looking disks with rings. You could think of the blob as being made up of many, many Airy Disks, one for each wavelength, all averaged together in your image, i.e. integrated. It would definitely be a blob for the flower above, which spans the wavelengths from c. 500nm to 1000nm (thank heaven for IR blocking filters). Blob range: at f/8 the Airy Disk diameter for 500nm is about 10um; for 1000nm, about 20um. Blobby, blurry, blecchh . . how do we even manage to take good pictures?



Disclaimer: Maybe I've just discovered axial chromatic abberation! And, for sure, I'll never see an Airy Disk on my Sigma SD10 with its nice big fat pixels :-).

But, if you're waiting for a 36MP 4/3" camera, . . . beware.

Best regards, xpatUSA

Comparing JPEG Compression Quality - Nikon vs PhotoShop

The Gory Details

Was curious as to how Nikon's D50 in-camera JPEG compression compared to that of PhotoShop Elements 6 (PSE6). According to the D50 manual, Nikon's "Fine" jpeg compression is 1:4 and their "Basic" compression (i.e. smaller file size) is 1:16. I was thinking of using Nikon's "Basic" compression quality to save a few steps in PSE6 when making images for auctions or quick forum posts. So, I took two identical shots of the trusty Citizen Wingman, with the only difference being the selected image size: everything else set manually, and no change in lighting. Here they are both, "Fine" image on top, click either one to see it's full-size image.

The above are the images as delivered by the camera without any editing other than cropping and re-sizing to act as clickable thumbnails. As you can see, they look much of a muchness. However, at the pixel level, a different story unfolds . .

Above is the logo from the Fine image, blown up 16X time fullsize in PSE6 (but not resized, I used Windows screen capture to get the image). Not bad, really. The Basic image detail below, however, came as a bit of a shock! (scroll up and down to compare the two).

The color rendition of the logo is pretty poor and there may even be an artifact or two to be seen. Gone is any notion of cutting out jpeg compression from my auction image workflow in PhotoShop!

Next, out of interest, I used PSE6 jpeg compression (level 7, whatever that means) on the Fine image and, for a somewhat smaller resultant file size than that given by Nikon's "Basic", got this:

Pretty good, I thought. So, at least in the compression department, PhotoShop beats Nikon handsomely, IMHO.

Best regards, xpatUSA

Nice Lady's Antique Wristwatch

1900's Stadler Mabel Enameled Solid Silver Lady's Watch

White porcelain dial with no hairline cracks. Breguet-style numerals with red 12. Classic poire shaped hands. Sunken seconds sub-dial. Purple enameled bezel. Clear mineral glass crystal with no dings or major scratches.

Fine Swiss hallmarked 0.935 silver case by the Stadler Watch Co. in Solothurn, Switzerland sometime between 1916 and 1930. The silver used is a little finer than Sterling which is itself only 0.925.

Solid silver snap-on back with purple basse-taille enamel. Basse taille is the rarer technique of first engraving the metal and then firing a smooth coat of glass enamel on top. This makes the engraving stand out with much nicer reflections and eliminates cleaning wear on the engravure.

Typical Swiss hand-decorated 7-jewel movement, also by Stadler. Sets correctly, runs and keeps good time for an antique watch. The silver wire lugs were straightened after this picture was taken.

Enameled silver back and bezel snap firmly on to the case. Comes with a 10mm lightly-used Swiss oiled leather band with matching silver-tone buckle. Onion-style crown makes the watch easy to wind and set. Watch is 27½mm diameter, excluding the crown. 9mm thick, from front to back, and weighs 22½ grams (¾ oz),including the band.

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Best regards, xpatUSA

Watch Valuation the Hard Way

You want to sell a watch but you're not sure what to ask for it. Too high - it may not sell; too low - you lose out. If you have the time, here's a scientific way to figure a market value.

First go to your country's eBay and do a search for the same model as your watch. Then, staying in that search, click on the checkbox that says "completed listing only" or words to that effect. Now start writing down all the prices where the watch actually sold (at auction, not buy-it-now). You might like to weed out prices for watches that are not your actual model or are not in a similar condition as yours. But a little diversity can be beneficial, for example when there are only a few watches of your exact model, dial color, etc. Something like 20 prices should do it, more if you have the patience. Less than 6 prices would only give you a rough idea.

Now you select some price ranges, somewhere between 10 and 20% of the total range of the prices that you found. For example, if the "spread" (max minus min) is 600-100 = $500 and you found a good few sold items then your ranges could be 500/10 = $50, i.e. 100 to 149, 150 to 199, and so on up to 600-649 (just in case a watch was actually $600, if not - the last range would be 550-599).

Next you count up the prices that fall within each range and list them with an "X" for each watch found:

100-149	X
150-199
200-249	X
250-299	XX
300-349	XX
350-399	XXX
400-449	XXXXX
450-499	XXXX
500-549	XX
550-599	XX

You can see that the 'X's form a horizontal bar chart which shows quite clearly that the range with the most sales is the $400-449 range. The other ranges indicate either worse or better condition, if the market***  is to be believed. So, if you consider your watch to be in average condition, a selling price in the $400-449 range would give you a reasonable chance of a sale. The higher ranges would be appropriate for a better condition and vice-versa. The shape of this particular  bar chart tells you more about the model of watch. The majority of the 'X's are at the higher end of the total price, indicating a sought-after model for whatever reason - quality, name, rarity, etc.

***Remember, "the market" in this example is eBay in the country or countries where you intend to sell. If instead you had researched prices obtained by Retailers in the open market, prices would be higher and less realizable in a different market.

Here's some results from a search for Luminox "Navy Seals". About 20 results were found, of which 6 were rejected for various reasons - like, one item was a Luminox box but no watch.

Column A shows the prices found on eBay, sold recently, and searched for as Luminox "Navy Seals". The average of the 14 prices is shown in row 16. At $119, this value falls into price range 3 (100-129) but the chart shows that the average is quite a bit higher than price range 2 (70-99) which has the highest count. This is because the chart values are "skewed" toward the lower end of the total price range, indicating a much less well-regarded watch than our other example above, The chart is telling you that, for a watch in average condition, a selling price selected from price range 2 would have a better chance of selling. It is also implying that simply taking the average of the prices as the sole criterion for your sales price would have put you too high. Worse, had you been selling an expensive, high quality watch, your price could have been 10-20% too low.

Row 17 shows a price of $110 as the "median" a word much-loved by statisticians and much avoided by the rest of us. However, this median value is a lot closer to price range 2 than the average is, so there must be something to it. Having said that, the median is not easy to figure out unless you use a spreadsheet function like in the above. Simplistically, it's a value which is in the "middle" of an ascending range of prices. With the 14 prices as shown above, it lies somewhere between the 7th and 8th price, i.e. the median value is between $95 and $125, QED.

In conclusion, it's worth at least picking some ranges and filling in the 'X's. Picking the longest bar gets you closer to establish a good criterion by which you can judge and price your own watch for the market. Simply figuring the average from the prices can throw you off quite a bit, either way. Doing a spread-sheet is fun for geeks but is quite un-necessary.

Best regards, xpatUSA

Kronometric.org Web Site Updated

In the beginning, there was simple HTML which stands for HyperText Markup Language. Couldn't do a lot with it but it was nice to be able to include "hyper" links to related documents or to be able to link to other places within a long document. The ability to link to pictures stored elsewhere was especially welcome as means of creating an illustrated document without having to learn how to "embed" pictures or drawings in it.

Following on from those good old days, HTML has developed beyond all recognition to the point of being almost unintelligible. Not to mention the emergence of XHMTL with it's Draconian rules and it's incestuous relationship to XML, XSL. There also appeared scripting languages such as JavaScript, which allow a page to be messed with once your Browser of choice has downloaded it and displayed it on your screen.

One good development was something called CSS which allowed you to write up your subject and then to apply rules separately on how it should look. It was also a pleasure to discover that HTML5 has appeared and sounded the death knell for XHTML in all it's horrible varieties.

Meanwhile, like the good techie that I am, I had developed my website into a paragon of scripting such that you couldn't even view my pages properly without having JavaScript enabled in your browser. That meant that every bit of content showed by JavaScript in my pages had to be repeated as plain old HTML with "NOSCRIPT" tags around it - double the pain for no gain.

"Enough already" as they say. I've just changed to HTML5 for my collection pages. Completely separate CSS files now say how they look. Any scripting is used only for minor functions, like "back to the top" links which have little effect if they don't work.

I also took the opportunity to show the collection in thumb-nail format with each thumb-nail being a clickable link to a page showing bigger pics and more information. Previous visitors will notice a simpler horizontal navigation bar up top instead of at the side. Makes room for more watches!!

So, click here and see how it looks now.

Best regards, xpatUSA

An Experiment in Oil Migration on Metal Surfaces

I've often read that oil can spread around inside a watch and that proper watch oils are designed to "stay in place". I was a little skeptical that oil could move around all by itself until I tried an experiment involving a comparison between Moebius 9010 (a Swiss oil designed for general use in wristwatches) and "Mobil 1" 5W-30 (an automotive engine oil). Here are the results:

The areas of interest are indicated by purple arrows. At left, a test to see if an excessively-oiled metal-to-metal bearing would leak oil out onto the surface surrounding the bearing hole. At right, a test to see if drops of oil deposited onto a metal surface would migrate (move or spread). The barrel at right was mounted vertically to show any effect due to gravity.

This is a shot of the comparative test set-up after 36 hrs. The Moebius oil drop at right has stayed in place with a very slight spread delineated by the purple lines. The Mobil oil at left shows some unexpected behavior. The main part of the oil drop has sagged downward due to gravity, a lower surface tension, and a somewhat larger initial drop size than that of the Moebius oil. However, a thin film has formed and continued to grow, apparently following the circular machining marks on the metal surface. It appears that gravity has aided the migration of the lower part of the film. There remains a line of demarcation between the film and the edge of the original drop.

This is an external shot, after 17 hours, of the barrel arbor bearing hole, the inside of which was oiled excessively with the Mobil 1 automotive oil. The purpose of this test was to see if any oil is able to exit by overcoming the capillary force induced by the bearing clearance gap. It is seen that no spreading has occurred so far, although the oil "ring" itself is clearly visible. The small blob of Mobil 1 at left was added later to test spreading of a drop on a horizontal, smoother surface.

I have drawn no firm conclusions yet and am still in research mode. Therefore, conclusions will be published later as additions to this post.

Best regards,

xpatUSA

Domed versus Flat Crystals

Flat crystals have their place in the scheme of things, I suppose, but the one annoying thing is reflections - especially from nearby lamps. By comparison, a domed crystal makes a reflection smaller and, if the underside is equally curved, it's reflection is also smaller. For example, this Stocker & Yale model 490's flat glass crystal shows no mercy when it's under a desk lamp.

The standard flat crystal on this watch is 30mm diameter and 1.5mm thick. A 30mm, 2mm thick, "double domed" mineral glass crystal was purchased for $7, installed, and the result was excellent:

The two pictures were taken at the same distance and in the same position. The lamp distance was about a foot above the watch. The domed crystal has an unpolished chamfer on the top and the crystal projects about a 1/2 mm from the anodised aluminum flat-topped bezel:

The side view is much improved. The angularity of the bezel is relieved by the domes' curve, which itself complements the curved chamfers formed on the side of the plastic body.

 Was thinking of selling this watch, but now it's keeper!!

Best regards, xpatUSA