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 Replies: 17 / Views: 3,893 |
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Pillar of the Community
2087 Posts |
After reading this thread: https://goccf.com/t/147634I thought I would give stacking a try. I tried two coins, a 1741 Maria Theresa thaler. The coin would grade somewhere above VF. I used a manual focus 50mm lens...and moved the camera using the fine adjustment I built into the copy stand. The camera: a Sony A7r. I took 11 photos in all and then tried out the Helicon soft ware. I was so impressed with the result that I am too embarrassed to post it here. Thinking it might have been the camera and stacking that was highlighting all the coin faults. I switched to my sigma 105mm ex lens and tried a different coin. I used the same technique and again I was not too happy with the result. Here is the second go. The coin is thaler sized( aprox 39-41mm in diametre) WB was custom set and the lighting consisted of three separate 5600K LED sources. To get the narrowest possible DOF I used F2.8 So what am I doing wrong?  |
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Pillar of the Community
 2087 Posts |
Just to give a comparison. Here is a single photograph. F8, ISO 400, 1/250 sec. Same lighting. It may not be too obvious but this shot is heaps better than the stacked  |
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Pillar of the Community
 United States
4038 Posts |
I'm not sure what is wrong with either shot! It looks like the exposures are different, and of course the colors are as well. Different apertures will emphasize color differently. Smaller usually produces more color as it narrows the angle of the light cones that are allowed to get to the sensor.
By the way, that coin is probably too flat and too large to justify stacking. Also, your camera can't resolve the necessary detail at f2.8, so you may actually cause some problems by being at such a large aperture. It's usually not a good idea to be larger than the DLA of your sensor...
Contact me for photographic equipment or visit my home page at: http://macrocoins.com |
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Pillar of the Community
2087 Posts |
OK that makes sense. I am perhaps being overzealous and wanting more than my camera lenses and skills allow. I take the point about the coin being too large. Although my 105mm macro goes 1:1, the 1:1 is just too much for thaler( silver dollar) sized coins. 1:2 sees such coins filling the sensor and with 1:1 I can't fit the coin on the sensor. I have found that for thalers a 50mm macro with 1:2 magnification is perfect. From my experience so far with stacking. the technique seems, for my purposes, overkill. The only time I can see it being useful is when I want to photograph a small part of a coin.
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Pillar of the Community
2087 Posts |
I took some time to do some experimenting. I used the default stacking method simply because I wasn't familiar with changing it to another option. I tried an ancient coin and a modern both very small diameter. The picture of the ancient is stunning. Clearly this technique works well with high relief.  With the modern the result was shocking. I have the impression that with out clear contrast the programme actually ends up blurring parts of the coin so the flatter the image the worse the performance.  From this brief play I believe if you collect ancients stacking is going to produce some phenomenal Photos |
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Valued Member
Canada
478 Posts |
Ok this might be a silly question but what is "Stacking"  I tried to figure it out by reading various posts but its all greek to me Edited by rmc
12/30/2013 09:48 am
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Pillar of the Community
2087 Posts |
Have a look at the link in the first post. Some cameras ( my Sony A65 and A7r) will take 3 pictures to construct one image. The soft ware Helicon takes a multitude of photos and combines them to create one picture. To do this effectively you take a series of photographs changing the focus slightly between each shot. The software combines the photos so that, in theory at least, the final result has a much greater resolution and sharpness than a single shot on its own. The soft ware can also produce a 3d view of the image..but I don't believe I can post the file in the forum.
Edited by austrokiwi
12/30/2013 10:14 am
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Pillar of the Community
United States
4038 Posts |
He's using the "Focus Stacking" technique where a sequence of images is taken at slightly different reference planes. Software is used to create a composite image using only the in-focus parts from each source image.
The problem with flat coins, large apertures, and low magnification is the depth of field. Stacking is used when the DOF is too small to capture a single image with good sharpness. The rule I use is if the DOF is insufficient at the DLA of the camera, then I stack. This almost always means stacking at 1:1 or above for the Lincoln Cents I spend so much time with.
Ray
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Pillar of the Community
United States
4038 Posts |
Here's a "crosseyed" 3D made using Helicon.  And here is a zoomed-in detail image. This was taken straight-on, but the software was used to 3D render the perspective image:  Contact me for photographic equipment or visit my home page at: http://macrocoins.com |
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Valued Member
Canada
478 Posts |
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Pillar of the Community
2087 Posts |
Quote:
Also, your camera can't resolve the necessary detail at f2.8, so you may actually cause some problems by being at such a large aperture. It's usually not a good idea to be larger than the DLA of your sensor... ? Ok theres something I am not getting about DLA. I thought DLA (Diffraction limited aperture) occured at smaller apertures( ie: high F numbers) at 2.8 (Wide open)there should be no DLA problems. |
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Moderator
United States
23522 Posts |
Quote:
? Ok theres something I am not getting about DLA. I thought DLA (Diffraction limited aperture) occured at smaller apertures( ie: high F numbers) at 2.8 (Wide open)there should be no DLA problems.
That's true, but....DLA is a hard number, a mathematical function of pixel density and airy disk size. It's a specific aperture number for a given pixel density. It's not subject to lens/quality or camera settings, excepting the fact that some lenses aren't of sufficient quality to deliver sharp images up to DLA. And very few lenses are at their sharpest wide-open; most want to be stopped down a bit. So, wide-open, you may be introducing sufficient blurriness to every image to defeat the purpose of stacking. Stacking is only for situations where you can't achieve sufficient depth of field at an aperture below (wider than) DLA, and at "normal" mags (magnification also mathematically affects actual aperture) for all but the smallest or very highest-relief coins, you'll very likely be below DLA for full-face images. |
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Pillar of the Community
2087 Posts |
Thanks SsuperDdave That makes sense! Just to check...the principle is the same as using cheap Full frame lenses on APS-C sensor often those lenses will perform far better on the smaller sensor because the light getting to the sensor is passing through the best parts of the optics
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Valued Member
United States
146 Posts |
Reading some of these posts makes me think I should have taken "camera" as a second language in school.  Is a focus rail a must for taking shots for stacking? austrokiwi: Quote:
The picture of the ancient is stunning. Clearly this technique works well with high relief. That is a great shot,would like to see that in the 3D. Between you,rmpsrpms and SsuperDdave the bar is pretty high. regards coffeecup57 |
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Moderator
United States
23522 Posts |
Quote:
the principle is the same as using cheap Full frame lenses on APS-C sensor often those lenses will perform far better on the smaller sensor because the light getting to the sensor is passing through the best parts of the optics That's an absolute truth - most lenses show their shortcomings at the edges first - but a different concept I think. As I mentioned, DLA is entirely separate from any lens characteristic, and purely a mathematical function. Whether or not the lens is presenting the sensor a sharp image capable of resolving to the actual airy disk size doesn't affect DLA. "Airy disk" is defined for my purposes here as the smallest a dot can be while still remaining "sharp" to the eye, from a theoretical "perfect" lens. In this case, DLA is the limiting factor of airy disk size. This is kind of a generalization of the "airy disk" concept, though. Quote:
Is a focus rail a must for taking shots for stacking?
Some method of very fine focus adjustment is necessary. You could do it with a focusing ring on a lens, or even height adjustment on a copystand, but I wouldn't want the  factor inherent in the latter. |
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Pillar of the Community
United States
4038 Posts |
The A7R has no Anti-aliasing filter, so can create false color quite easily. I see that happen with my HRT2i as well. The primary purpose of the AA filter is to spread the light hitting any given pixel across adjacent pixels so that sharp edges can't create moire effects. This is its so-called "blur filter" effect. A secondary (but important) purpose is to eliminate false color occurring on individual pixels due to highlights hitting sub-pixels in the Bayer array.
Because of the blur effect, the DLA of a sensor with an AA filter is a bit smaller (higher f-number) than the pixel pitch would suggest. I've heard of folks measuring as much as a 30% degradation (about one full stop) smaller(!!). This is why many reviewers pan the DLA, as it is skewed so much by the AA filter. The result is that you won't see much actual sharpness degradation until about one stop smaller than the calculated DLA. However, for cameras without AA filters, the calculated DLA is correct within the inaccuracies of Bayer filter geometry. Furthermore, if a camera without AA filter is used with a lens that has aperture larger than the DLA of the sensor, and the subject has details that can be resolved on a sub-pixel level (everything does...), then highlights which happen to fall on individual R, G or B sub-elements of the Bayer array will cause that pixel to have false color. In practice, all pixels will give some level of false color when viewed at 100%, though on average if you downsize the image for web publishing the final image will be close to color-correct.
The false color phenomenon has made me much more cognizant of my aperture settings on the HRT2i. I set my (effective) aperture no larger than the sensor DLA to avoid false color issues. And while it seems this is only a AA-filterless problem, it has made me realize the futility of trying for more sharpness than the AA filter will allow on my other cameras as well. Large apertures are great for certain macro applications where words like "bokeh" are spoken, but for coins they are not at all useful. For coins there is an optimum (effective) aperture that is either the DLA for AA-filterless or somewhere around DLA+1stop for AA-filtered cameras.
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Replies: 17 / Views: 3,893 |
Posted 12/30/2013 01:32 am
