Lenses and pinholes: what does in focus mean

[stevendaryl]

stevendaryl submitted a new PF Insights post

Lenses and Pinholes: What Does "In Focus" Mean?

Continue reading the Original PF Insights Post.

 

[Greg Bernhardt]

Great addition to our FAQs!

 

[kith]

For an introductory article, I think it would be good to include a small paragraph why we need the pinhole in the first place.

 

[Drakkith]

I like what you've done with figure 2. It shows that if you take 2 different rays from the same point, then they essentially help form two separate images of the same object at slightly different locations which may overlap each other. Taken to the extreme, this means that without a lens, there are an "infinite" number of overlapping images that wind up being a blurry mess. The smaller the aperture, the tighter the grouping of the images and the less blurred the overall image is.

 

[sophiecentaur]

"It seems like a superior way to do cameras, because there is no need for focusing."

Reference https://www.physicsforums.com/insights/lenses-pinholes-focus-mean/

As a matter of fact, extreme depth of focus is not always an advantage. Limiting the depth of the sharp field is an important aspect of many photos for artistic reasons and also for emphasising and isolating detail. Binoculars can often allow you to see a bird behind a bush because the bird can be in sharp focus and the bush can be a blur. Also, of course, diffraction rears its ugly head with small apertures. If you want to image two closely adjacent stars, you will be needing a telescope objective of many (even hundreds of) cm diameter. It's horses for courses, as ever.

 

[Drakkith]

It's horses for courses, as ever.

I'm... not sure I know what that means, lol.

 

[sophiecentaur]

I'm... not sure I know what that means, lol.

http://dictionary.cambridge.org/dictionary/english/horses-for-courses extended by me (and others) to include things / systems etc.

 

[Pushoam]

After reading this , I understood what is exactly meant by " in focus".

Thank you, thanks a lot for posting this insight post.

 

[Frenemy90210]

Why do you need a lens over the hole? The simple answer is: to make sure that the image is in focus.

Even pinhole cameras can give good focus. The reason we use lense is lense can gather much more light thus giving bright sharper picture unlike a tiny hole.

 

[sophiecentaur]

Even pinhole cameras can give good focus.

Good(ish) . The image will have no more detail than the diameter of the pinhole. That is probably not as good as a 20MPx camera sensor can record. But, projected on a big screen, the perceived sharpness can be good. (Darkened room needed, of course)

 

[Frenemy90210]

Good(ish) . The image will have no more detail than the diameter of the pinhole. That is probably not as good as a 20MPx camera sensor can record. But, projected on a big screen, the perceived sharpness can be good. (Darkened room needed, of course)

Different colors within the image refract at different angles at the lens boundary. Thus different colors will have a different focal lengths for a glass lense. A pinhole does not have this problem. Pinholes never give rainbows unlike a powerful lense.

 

[sophiecentaur]

A pinhole does not have this problem.

True. It masks the problem with problems of its own; the image from a pinhole is blurred for a start. It is a really trivial matter to make a lens with an aperture of, say f22 which will produce a sharper image than the equivalent pinhole and in which the Chromatic Aberration is hardly detectable. So in what respect would a pinhole be 'better'? In this game of Top Trumps, the only parameter for which the pinhole scores higher is on Production Cost.
The problem with a pinhole is basically that it doesn't let in enough light. Taking the Sun as the brightest subject for for observation, a pinhole camera is inadequate, even for that, if you want any serious resolution for sunspots etc.

a powerful lense.

How "powerful" are you suggesting and how would a pinhole achieve the same result.