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.

 

[chris]

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

 

[Timofey Gorlov]

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?

 

[Designer]

Good ol' Wikipedia to the rescue.

Again.

Refractive index - Wikipedia, the free encyclopedia

 

[Trever1t]

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

 

[Timofey Gorlov]

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?

 

[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.

 

[Timofey Gorlov]

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.

 

[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

 

[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.

 

[Timofey Gorlov]

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.

 

[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?

 

[astroNikon]

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.

 

[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.

 

[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.