# Auto focusing problem



## Timofey Gorlov

Hello, I have a problem with my camera autofocusing.

I take two pictures with my Canon camera. The first one (16:25) is taken with a camera and the second one (16:38) is taken with the camera through the acrylic glass 1/8" thickness located right at the lens (like filter). The second image is slightly off-focus whatever I tried to do. Can somebody explain me why? The glass is clear and not scratched. I know that camera may focus on the glass if the glass is located on some distance from the camera but this is not the case. I don't ask how to fix it (I just need to focus it manually to infinity) but I ask why it happens in autofocusing mode. I appreciate any ideas.


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## chris

The acrylic is probably not of optical quality and what you see is due to uneven dispersion of light passing through it.


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## Timofey Gorlov

chris said:


> The acrylic is probably not of optical quality and what you see is due to uneven dispersion of light passing through it.


Thanks, I'm trying to find some online material about what you said. Do you have anything? Uneven dispersion is a roughness or something different?


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## Designer

Good ol' Wikipedia to the rescue.

Again.

Refractive index - Wikipedia, the free encyclopedia


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## Trever1t

Why do you want to shoot through the acrylic? Underwater housings should be underwater when used.


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## Timofey Gorlov

Trever1t said:


> Why do you want to shoot through the acrylic? Underwater housings should be underwater when used.


It's a long story. I need stronger protection for my camera to take a special pictures. May be I'll find alternative but I just want to know why it's not working. From the previous reference it's absolutely clear - what is refractive index but the mechanism of how it affects  on focusing is not clear. Do you know?


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## astroNikon

Essentially the acrylic is making the photo lose detail. 
What if you took a picture with the acrylic only HALF way across the lens.  What are the results from left to right ?


If you look up cheap filters you'll get the same situation.


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## Timofey Gorlov

astroNikon said:


> Essentially the acrylic is making the photo lose detail.
> What if you took a picture with the acrylic only HALF way across the lens.  What are the results from left to right ?


That's a good Idea. I'll try it.


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## astroNikon

You'll find quite a variation.
Essentially, you question is "what is optical quality glass versus regular glass" as acrylic is not even close to optical quality glass and it's not glass.  Anything less than optical quality can create image quality reduction from minor to major imperfections.

This from What is Optical Glass? (with picture)


> Once the glass is made, it can be cut and ground into lenses for various applications. Lensmaking is a very precise art which was once accomplished entirely by hand, with some modern facilities using lasers to produce very reliable and precise lenses. Depending on how the lens is cut and ground, it will behave in different ways, allowing it to be used to correct vision, take photographs, or scan the heavens to look at the stars.
> 
> Some people refer to optical glass as precision or technical glass, referencing the fact that a great deal of skill is involved in its manufacture. Glass making companies which make optical glass can command a very high price to their products, as can companies which grind lenses. Various alternatives to this glass, such as specialty plastics have been explored, but for some applications, glass really is the best material to use.



But Not only is the technical specifications of how to "grind" the glass but also the type of material used is very specific. For instance Shott glass creates high level optical glass in 120 forms ==> Optical Glasses - Over 120 Glass Types: SCHOTT Advanced Optics | SCHOTT North America

types of glass ==. Types of Glass - British Glass

some more 
==> http://www.edmundoptics.com/technical-resources-center/optics/understanding-optical-specifications


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## dennybeall

How about getting a neutral lens filter and cut a circle in the acrylic of that size. Glue the filter into the hole and mount that on the camera . Should give you a clear shot.


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## Timofey Gorlov

dennybeall said:


> How about getting a neutral lens filter and cut a circle in the acrylic of that size. Glue the filter into the hole and mount that on the camera . Should give you a clear shot.


I figured out what was wrong and it become more confusing to me.
The first focused picture was done in autofocusing mode and in "infinity" or "landscape" searching focus.
The second picture made through the glass with the same conditions but the "focusing distance" or optical distance between object and camera "reduced" so the autofocusing cannot find focus within "infinity" setting anymore. Everything clarified when I changed focusing range from "infinity" to "portrait".
The question is how the acrylic window can change the distance that much? The flat glass can make a shift smaller than the glass thickness but it's not explanation.
I cannot understand that big effect with a flat glass.


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## chris

Your problem has nothing to do with apparent changes in optical distance due to the thickness of glass or acrylic; it is because the acrylic is not of optical quality and it causes some dispersion or random refraction of the light passing through it so degrading the image.

One of the reasons why high performing filters and lenses are expensive is because of the quality of the glass used in them. Optical quality glass is expensive to manufacture; it is not the sort of glass commonly found in windows. Similarly optical quality acrylic (if there is such a thing) would likely be equally expensive; where did you get yours from and what are you hoping to achieve by shooting through it?


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## astroNikon

Timofey Gorlov said:


> I figured out what was wrong and it become more confusing to me.
> The first focused picture was done in autofocusing mode and in "infinity" or "landscape" searching focus.
> The second picture made through the glass with the same conditions but the "focusing distance" or optical distance between object and camera "reduced" so the autofocusing cannot find focus within "infinity" setting anymore. Everything clarified when I changed focusing range from "infinity" to "portrait".
> The question is how the acrylic window can change the distance that much? The flat glass can make a shift smaller than the glass thickness but it's not explanation.
> I cannot understand that big effect with a flat glass.


As mentioned previously about the quality.

It's actually not the camera that isn't focusing, though it can have issues focusing with the problem.

Think of it this way.
IF you take a glass of water and pour say vodka or some other thick liquid in it, try looking through it.  What is wrong with the "detail" and "accuracy" of what you see on the opposite side of the glass.
OR
just with an empty glass?  What detail do you see lost just looking through a glass ?
OR just test with plastic.

That is the same problem the image in your photos are having.

The Acrylic probably is causing the same issues in seeing a detail, crisp image as you would looking through a glass. Even if you manually make all the settings as if the acrylic wasn't there.

put the acrylic in front of your computer screen and you may not like the results.


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## Timofey Gorlov

Thanks for your reply. But as I mentioned, I could achieve perfect sharp image by manually changing the focus. So the problem was not in the optical - non optical quality of the glass.
In other words I can get picture IMG_0380.JPG without glass with focus distance about 1000mm and
I can get the same picture IMG_0380.JPG with acrylic glass with the focus about 150mm.
Optically it cannot be explained at all. May be I need to find some other "hidden" conditions to explain this case.


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## astroNikon

Truthfully the 2 images you provided are massively different in quality and clarity.

the 380  is pretty good
the 100 is majorly missing clarity.  Just look at the house at the far end on the left.  The window with it's separations is seen (though not very sharp).  Whereas in the 100 you cannot see those separations.

The acrylic is not optical quality.
can you provide a picture of the acrylic that you are using ?

Any see through material in front of a camera can affect the image.  The less optically clear it is, the worse it makes things.


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## Timofey Gorlov

I can show the real focused image through the same acrylic glass that has the same clarity as IMG_0380.JPG.
The focusing distance is about 150 mm.
The focusing distance for IMG_0380.JPG is about 1000mm.
The clarities and sharpness of both pictures are the same. May be illumination is different because of times.
The Focuses are different! So the problem not in quality of glass. I think I have a good logic.


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## astroNikon

To me I see a massive difference in quality.  give me a few to show some crops


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## Timofey Gorlov

astroNikon said:


> To me I see a massive difference in quality.  give me a few to show some crops


There is another one with the glass that has a similar illumination


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## astroNikon

the original 038 shows details in the windows, the cross members and such.  The other ones, including the one you just uploaded, with the acryliv have lost ALL detail.

It's even worse if you look at the house to the left.  I thought they put a sheet of plastic over the house in one of the photos - zoom in and look at the windows.


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## wfooshee

You keep asking, "Why does the flat glass affect the pictures so much?" but you won't accept the answer, which is:

It's not glass. Acrylic is plastic. It's not transparent. It's "fuzzy." 

Light bends every time it passes through a surface. The acrylic surface is not anywhere close to as smooth as a real glass surface, so the light entering and exiting the acrylic bends differently at every different point of the surface. Then there is the lack of transparency, the cloudiness of the acrylic. Every microscopic bubble or bit of uneven material density provides another light-bending surface.

A flat sheet of optical glass minimizes that by being pure. The surface is smooth with no irregularities, down to a microscopic flatness, and the material inside the surface is uniform and clear. A flat sheet of glass in front of the lens, and parallel to the focal plane, with no light behind it causing reflections, would have a minimal impact on the image clarity.

A sheet of plastic, though, cannot help but scatter the light. There's a difference between "I can see through it" and optically transparent.

Please stop asking, "Yeah, but why?" You've been told why. If you have a need for the acrylic sheet in order to protect yourself or the camera from something on the other side, then you have to accept the optical limitation, or go find something more transparent but still with impact protection, which will be _expensive_.


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## astroNikon

At first I thought the one picture with the acrylic that you just uploaded, the owners put plastic over the windows to keep drafts out.  But when I checked the houses to the right it was the same problem.

But on the #62 image if you just look at the windows it looses all detail.  Even the shutters are nearly gone in the image of the small window.  The windows look like it has plastic over them.
The bricks has lost all detail.  The roof.  If you see the full image the trees are all fuzzy.

You may not be used to looking at photographs and comparing but once you do, you'll know the differences.  this is all due to the "acrylic" not being "optical quality".


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## Timofey Gorlov

astroNikon said:


> At first I thought the one picture with the acrylic that you just uploaded, the owners put plastic over the windows to keep drafts out.  But when I checked the houses to the right it was the same problem.
> 
> But on the #62 image if you just look at the windows it looses all detail.  Even the shutters are nearly gone in the image of the small window.  The windows look like it has plastic over them.
> The bricks has lost all detail.  The roof.  If you see the full image the trees are all fuzzy.
> 
> You may not be used to looking at photographs and comparing but once you do, you'll know the differences.  this is all due to the "acrylic" not being "optical quality".
> View attachment 94196 View attachment 94197


The post became little bit confusing. The last two pictures that you compare made in a different times of the day (that is my fault). Originally I supposed to show focusing issue. Just compare IMG_0380.JPG and IMG_0176.JPG that I attached that is still made in a different time-shades distribution.
Ok, forget about it.


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## astroNikon

but we're not talking about the light / time of day.
we're talking about the fuzziness of the image, or "washed" out image due to the acrylic.
The window detailed are totally different.  bricks are washed out, and roof, trees .. not due to lighting variances.


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## tecboy

Take the acrylic glass off from your lens, and you will not have anymore problem.


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## Timofey Gorlov

Thanks for discussions. I'll continue this thread with more strict pictures/questions if I get one.


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## Timofey Gorlov

After all I figured out that infinite auto focusing gives a bad result and average distance focusing setting gives sharp image through the same acrylic glass. In this way I could resolve my problem without understanding why it happens. There is no real physical reason for it. I would assume that the glass somehow affects focusing mechanism or algorithm when focusing is setup for infinity. This is my big project and I cannot avoid using acrylic screen because it's a waterproof case for my camera.
I think my case can be useful for other people who use outdoor case. Just don't use "infinity" setting for it.


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## astroNikon

I use manual focus to shoot through any material.  The Autofocus system wants to use any contrast to use for identifying where the object is.  If the glass is not perfectly clear (to the camera and not to you), then it will have an issue with focusing and clarity.

and see post # 2


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## wfooshee

With a modern lens, you probably shouldn't ever use infinity anyway. To accommodate manufacturing tolerances, camera-to-camera differences, and so on, a lens set to infinity may actually be set _past_ infinity.

You may well ask, "How can it be set farther???" but it's a simple answer. As you turn the focus ring towards infinity, the elements in the lens move physically. There is a point at which they are focused on something as far away as it's physically possible to focus on. Most lenses, however, can keep turning, and thus the elements move even farther, and move _past_ the point of achieving focus at infinity, giving you your unexpectedly blurry image.

Just cranking the ring over to the stop is absolutely no guaranty of good focus at infinity!


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