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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Mon Jan 23, 2012 3:36 pm 
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Using a higher aperture was not for dof G4. The point if increasing it is that wide open apertures cause distortion and lack of sharpness. By stopping down 1-2 stops you greatly increase the sharpness of the image with most any camera lens


Image quality is determined by the image that is able to be formed by the objective lens, in this case a Greenough style stereo objective not intended for terribly high res photography. In spite of this cascaillou has gotten some pretty sharp pictures at least at the center. Greenough type stereo objectives are almost never flat field. Hence the softness at the outer third.

The classsic technique of stopping down "one or two stops" is intended mainly to reduce the effects of insufficiently corrected spherical aberation. But it doesn't have much effect when the image is formed by the scope objective. It is however, in this instance, reducing the entrance pupil enough, that it is causing a vignetting problem, which is what cascaillou is trying to overcome.

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ps: nice forum G4Lab
I like it. Tim has been known to post there and one or two others from here. As usual I am one of the archives of ancient and arcane information. :roll:

I agree with John Ux4. Keep at it. It may be that you will just want to put up with the vignetting and even perhaps just take that sharp center section. Since your scope only has two mag settings zooming the camera lens to change mag would probably be handier than worrying about whether or not you cover you whole chip. I didn't look up how many pixels your camera has but your image doesn't look to me like you are wasting any resolution.


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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Mon Jan 23, 2012 5:53 pm 
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I think I'm going shoot this labradorite again (that will be like the 100th time), and try to improve both with sharpness of details at the center, and improve with focusing on the desired inclusion.
I keep shooting this same labradorite as I'm quite curious to see how good my scope/cam setting can get at it (I'm not expecting miracles though), and this stone is a good test subject for that purpose.


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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Tue Jan 24, 2012 1:11 am 
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Stopping down has more effect than that. It also reduces lens issues other than sharpness. Many of these P&S cameras use very poor lenses in them. I'm not familiar with the camera in question, just speaking in general, Panasonic aside which is one reason I recommend them over others. Stopping down helps reduce lens issues as the lenses are their weakest wide open on everything from sharpness to chromatic aberrations. Vignetting caused by stopping down can be easily cropped out and is not a real issue, but pixel level sharpness, chromatic aberrations in this type of photography, etc. are not near as simple a fix if can be fixed without destroying the results.

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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Tue Jan 24, 2012 8:34 am 
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G4Lab, Swishman, Ux4:

If you have some labradorite (with tiny ilmenite inclusions) on hand, I'd be quite curious to see the pics you can get (using the same scope settings: 30x scope magnification with 10x scope oculars), both with and without camera zooming when taking the pictures.
That would give me like a goal, or at least a frustrating perspective of what can be done.


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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Tue Jan 24, 2012 9:13 am 
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I'd be happy to, but I have no material on hand.

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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Tue Jan 24, 2012 10:38 am 
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This is such a great discussion! My gemology/craft/sewing room is in a transitional phase (again :roll: ) so I'm unable to try any of the suggestions yet. Is there an easy way to copy this entire thread for future reference?

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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Tue Jan 24, 2012 12:44 pm 
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You don't need to copy the thread Barb. I am at your service. :D

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If you have some labradorite

I have some that is sort of beige with blue schiller.(If I can lay my hands on it)
Shoot a macro shot of the type you are working with and curious about.


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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Tue Jan 24, 2012 1:12 pm 
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You are just too sweet :smt058 .

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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Tue Jan 24, 2012 1:19 pm 
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:smt060

You are permanently on the "Can ask Gene for help anytime." List


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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Tue Jan 24, 2012 7:48 pm 
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Stopping down helps reduce lens issues as the lenses are their weakest wide open on everything from sharpness to chromatic aberrations.


Jamie you have this exactly backwards. Lens resolution is at its maximum when the lens is wide open. As you stop it down its numerical aperture and hence its resolving power is reduced. It is true that stopping down increases the depth of field. But the sharpness in the sharpest zone is correspondingly reduced. This is why it is counterintuitive. It may appear sharper but if you start enlarging the image the one shot with the lens stopped down will go soft before the one with the lens wide open. Gathering more light (ie bigger aperture diameter) means gathering more information. Stopping down means gathering less.

This is exactly why the instructions for the Zeiss Luminar , the Wild MakroSkopes, Apo process lenses, and other high quality lenses, always say to use them wide open, never stopped down.
Microscope objectives have no diaphragm. They are only used wide open.
If you convert microscope na numbers to regular f/numbers you will discover that microscope objectives are very fast lenses. Almost always faster than camera lenses.

Chromatic aberrations are unaffected by the aperture setting because they are longitudinal effects. Only spherical aberrations improve with stopping down.

You might want to review this thread:
viewtopic.php?f=58&t=12497
where the subject got pounded pretty well. Or go to the PM site where it is demonstrated over and over again.

The phenomenon of some lenses performing better when "stopped down one or two stops" follow from the practice of consumer lens makers leaving some extra uncorrected glass at the periphery so that the consumer can grab a shot in darker conditions. But if the lenses were to be sold for a critical scientific purpose they would not have settings that could open beyond where the corrections meet the desired specs.

In examining cascaillou's recent posts he might actually have some advantage to stopping down and vignetting enough to optically trim the soft periphery of the image formed by the Gem Oro's not very flat field objective. (its really small diameter to begin with) But he needs to have it big enough to get the sharp center portion as big as possible on the camera's sensor. And his initial question was how to make it STOP vignetting.


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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Tue Jan 24, 2012 11:04 pm 
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Sorry I don't have it backwards G4. I've been shooting for 25yrs+ and in real life every single lens I own from 40yr+ old ones to brand new 2011 manufacture date perform better stopped down vs wide open as do all P&S cameras I own with fixed lenses. It's a well known fact.

An f/2.8 lens wide open @ f/2.8 is not actually very sharp because you are using the entire lens to create the image, lenses are strongest at their center. They suffer from softness and chromatic aberrations(CA) in the form of fringing around contrast edges. By stopping the lens down to a higher f-stop you can increase sharpness and reduce CA. Almost every lens,not fixed lens cameras, has a peak performance area somewhere in between f/8-f/11. On digital any lens above f/16 starts suffering from lens diffraction which negates the benefits.

This is the exact reason that full frame camera lenses perform better on crop body cameras, because the crop bodies only utilize the peak area of the lens, its center. This is a main reason a lot of folks seek out the older film lenses for use on crop body digitals, because they perform insanely well!

Anyone can test this in their cameras, take a shot wide open @ f/2.8 and the same at f/8 or even f/5.6 or f/4, use a flat subject so that DOF is not at issue. Or simply check resolution chart shots and one can quickly see the increase in quality as a lens is stopped down.

Lens data even supports this on every lens.
Older lenses were prime around f/8-f/11. Newer ones are prime between f/5.6-f/8 typically, especially macro lenses.

Here's lots of tech data on lenses, just picked a common one to get you started. Technical data and sample images.
www.photozone.de/canon-eos/488-canon_100_28_50d?start=1

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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Tue Jan 24, 2012 11:07 pm 
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Actually, stopping down lessens vignetting on cameras. Fixes lens cameras are typically designed so little to none as the lens is quite well matched to the sensor. Where on interchangeable lenses are made for various cameras and stopping down lowers or eliminates vignetting.

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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Wed Jan 25, 2012 4:29 pm 
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Gene's post that begins with

G4Lab wrote:
Jamie you have this exactly backwards.

is precisely correct.

The laws of optics, in this case, produce what seems to be a counter-intuitive result.

So let's look at another experiment that illustrates this result. If you happen to have a magnifying glass, then this is an experiment you can do for yourself at home. To do the experiment, you need:
(1) a magnifying glass
(2) a 3"x5" index card, or a piece of cardboard cut to the same size or bigger
(3) a slightly darkened room (interior lights off)
(4) a window looking out on a sunny day
(5) and a blank light-colored wall space opposite the window

The first thing we are going to do in the experiment is locate the so-called real image of the window, produced by the magnifying lens, on the wall opposite the window. Begin by holding the magnifying lens very close to the wall (almost touching), and slowly move the lens away from the wall. As the distance between the wall and the lens increases, at some point you'll see the real image of the window projected on the wall. This image will not be very big, and it will be upside-down and also reversed left-to-right.

Just locating the real image is a really fun trick if you've never seen it done before. This real image is what a camera's lens is creating on the camera's sensor, which then records the image. So all camera sensor images actually begin upside-down and reversed left-to-right. Fortunately there is that thing called software that can invert the sensor signals for proper rendering on the camera's LCD screen.

But on to the experiment. Once you have located the lens position that creates the real image, hold up the cardboard right behind the lens, so that it is covering half the lens. Take a look at the real image produced on the wall. Then hold the cardboard right in front of the lens, so that it is again covering half the lens. Again, take a look at the image produced on the wall.

After doing the experiment, you should be able to answer the following questions...
Does the real image on the wall change if the cardboard behind the lens covers half the lens, and if so, how?
How does the real image on the wall change if the cardboard behind the lens covers 9/10ths of the lens?
If the cardboard is held in front of the lens when covering half the lens, is the real image any different than when the cardboard is held behind the lens and covers half the lens?

If anyone actually performs this experiment for themselves, please send me a pm detailing your observations. I'd love to read your reactions.


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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Wed Jan 25, 2012 6:38 pm 
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If this is true, please explain how real world usage is just the opposite then!? Seriously.

Just check the sample images on the link I posted. If you check test charts you can even clearly see the visual decrease in fringing by stopping down a lens from wife open.

Do the test I explained above in my previous post yourself. The results are there. Many lenses are even extremely soft wide open, yet stopped down a single stop yields razor sharp results.

If backwards, then why do the lenses even result in higher, better, MTF numbers when stopped down!?

Please explain then. Because what is happening in usage is the exact opposite of what you state.

It doesn't make sense if what you say is fact, yet the fact in actual use is just the opposite?!? I'm not sure how one can argue with what happens in use by staying written text.

I'm all for "books", but often times what gets written isn't always what happens in real world usage. Remember the World was once written as Flat too, lol. But turns out in real life it was anything but.

I guess we'll have to agree to disagree. Because what you say isn't true, is exactly what actually happens when using a camera + lens to take a picture. :/

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 Post subject: Re: dino-lite digital microscopes?
PostPosted: Wed Jan 25, 2012 6:46 pm 
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Just some more proof.
www.slrgear.com/reviews/zproducts/canon ... eindex.htm

Check any number of lenses. One can clearly see the better quality stopped down to f/8 versus the wide open f/2.8 shots.

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