Well, there's one way to find out.....
Looks like out of focus light, but maintains shape when partially obscured.
- Thread starter Officepolicy
- Start date
I took video while cleaning the front surface of the camera lens, and none of the dots changed. So, it seems they are internal.
They can't all be on the back surface, though, as there are different depths of motion.
Last edited:
The shape certainly does. Wide open, it will be a circle; stopped down, it shows the shape of the aperture.
Here's what that looks like in this case.
jarlrmai
Senior Member.
What do you think is better to explain to someone that Bledsoe is filming out of focus lights, a link to the mathematical optical formulas of circles of confusion etc or a visual demonstration of the same effects?
Ravi
Senior Member.
Indeed, the latter, that what I meant as well.
FatPhil
Senior Member.
It's not a binary choice - my post #14 was an attempt to get across the reasons why the effect seen should be expected.
Z.W. Wolf
Senior Member.
These are "shadow circles." Defocused shadows. They can appear in the shape of the camera aperture. They're not caused by a diffraction effect. Just scattered light.
In the olden days of film cameras, this usually meant that the rear element of the lens was dusty. But if the outside surface (rear surface that you can see) of the rear element was clean, it could mean there was dust on the inside surface of the rear element. Or there were pits or scratches. Or maybe even flaws in the glass. Back then, flaws in the glass could be there even with quality brands. So this was a test some hard core photographer might use.
This was the guide as to where the dust/pit/flaw might be:
-Closer to the film - creates smaller, sharper disks.
-Farther from the film - creates larger, softer disks.
But I don't think this could be caused by any flaw or dust on the objective. Too far away. The shadow would spread out too wide to be noticed. I doubt it would even happen on anything farther away from the film than the inner surface of the rear element.
A camera with a short flange distance would be more likely to show these shadows, if the problem involves the rear lens element.
In these here modern days, it might involve dust on the sensor. These circles are very tight and crisp. So I reckon these here shadow circles are most likely caused by dust on the sensor surface. Does this camera have an anti-aliasing filter and/or sensor cover in front of the sensor? In that case there might be dust on both surfaces. (Or three surfaces? As I understand it, the anti-aliasing filter also functions as the sensor cover?)
I'm waiting for someone to tell me that there's no such thing as shadow circles, as I was recently told about blur circles. Maybe I've outlived my time and the photography jargon of the 1970's has fallen out of common use. But that's what we called them in my day... by cracky.
Last edited:
Ann K
Senior Member.
In the olden days of film cameras you could also get dust on the negative, or during the process of printing it. Back in the year dot my father made a blackout screen for the bathroom window, got a red lightbulb, and built an enlarger, and my mother would print and enlarge all our family photos, fighting fluffies at every step.
Z.W. Wolf
Senior Member.
That's on the processed negative. Film cameras have film gates that are designed to wipe dust off the film as the film advances. Polished metal; sometimes with a brush or felt as well. You might get dust on the film inside the camera, but that would most likely manifest as scratches.
(35mm film cartridges have visible felt pads, but those are meant to be light traps.)
(35mm film cartridges have visible felt pads, but those are meant to be light traps.)
Last edited:
elvenwear
Active Member
Analysis of defocussed point sources (ie star testing), is commonly used by amateur telescope makers to verify the quality of the optics and to verify collimation after the telescope has been set up. Most amateur telescopes have only two optical surfaces so it is possible to fix figuring errors in fabrication and to recognise collimation errors when setting up a telescope for a night.
For photographers, you wouldn't really be able to do much to fix any problems you found, so I suppose a photographer might use this test to verify the quality of the optics before purchasing a lens. An old friend discovered a flaw in one of their prized lenses they had been using for years and they hated looking at their images after that.
Similar threads
