dirtpusher Loc: tulsa oklahoma

i do think the link i posted Explained it did it not?
no they were not bogoling to me or you. but will it be for others. the comment trig and alg. was a meaningless comment, no foul intended. ggssshhhh :roll: :roll: :lol: :lol:

thom w Loc: San Jose, CA

I don't think I care about the math. I'm looking for the principle or theory. Dirts article mostly showed me what I already knew. I haven't had time to check out the second link. I know it won't help me take pictures I just really like to know how and why things work.

Wallbanger Loc: Madison, WI

This one explains why:

http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/950/depth-of-field.html

Apaflo Loc: Anchorage, Alaska

Unfortunately, it does not explain why. It shows a lot of pictures of apertures, and explains what happens with bigger or smaller ones.

I posted a pretty good article, and now Wallbanger has posted another that also does a good job of discussing why the DOF changes as the aperture changes.

Apaflo Loc: Anchorage, Alaska

It actually can help you take better pictures. Consider the relationship between the light paths, as shown in the two good articles that do tell why, and think about how the angles involved affect images in other ways. Lower angles are can be more precisely controlled with lenses. Perspective distortion and diffraction are related. It all adds to the body of knowledge you use when your finger says "NOW!" and pushes the button to take an exposure.

As an exercise, look at either of the two good articles and see if it makes sense why dust spots on the sensor do show up at f/22 and don't show up at f/4. Useful concept to be aware of...

oldtigger Loc: Roanoke Virginia-USA

That one doesn't explain squat about 'why' DOF is altered.
"less light = smaller circle of confusion"
guess a neutral density filter would do the same?

http://www.exposureguide.com/focusing-basics.htm
didn't even pretend to explain anything, just said "change aperture"

http://www.uscoles.com/fstop.htm
doesn't address the issue of DOF at all

69rdstr

I'm new here and to photography. I've done a bit of research and this article helped me considerably.

http://www.uscoles.com/fstop.htm

EdM Loc: FN30JS

it has to do with PATHLENGTH. THE photons have to travel longer (further) from the top of the lens to the bottom of the image and they essentially arrive out of phase with those that travel from the same source via the bottom of the lens.... whew! actually out of phase with itself!! well , someone asked!

jerryc41 Loc: Catskill Mts of NY

Without reading all the replies on these two pages, I'll add my two cents worth. Closing the aperture eliminates extraneous light, making the image potentially sharper, so you range of sharp focus is larger.

That's why people squint. If you squint or curl you fingers into a tube and look at something, it will be sharper.

http://astronomy.stackexchange.com/questions/520/why-does-squinting-make-hard-to-see-objects-clearer

rehess Loc: South Bend, Indiana, USA

Many years ago I graduated from Purdue University, an engineering school, with a degree in Mathematics and minor in Physics, including a course entitled "Intermediate Optics"; as far as I remember, even Purdue didn't offer an optics course beyond that. I don't remember seeing much in the ways of equations, mostly ray diagrams. With modern computers, I'm sure that you can do some actual calculations to predict lens behavior, but the "old guys" designed some mighty fine lenses without that kind of thing, and you don't need those calculations to have a basic understanding of what is happening.

The basic answer is simple: in order to build a "perfect lens" the cross-section of the lens would need to be a parabola ... but we don't have a reliable, inexpensive way of building parabolic lenses, so optics normally uses what we can work with, namely lenses each having a circular cross-section. If you expose just a small portion of the lens, then it basically looks like a parabolic surface to the light, so it all focuses to the "focus point"; the more of the surface you use, the less it looks like a parabolic surface, so it does an increasingly poor job of focusing everything to a single point. In a nutshell, that is what causes DOF.

Apaflo Loc: Anchorage, Alaska

Referring to the cite that you quoted, it certainly does explain it. And it does not say "less light = smaller circle of confusion". What it does is provide a very informative graphic, which along with the text explains "As the subject-lens distance increases, the size of this circle increases." And by looking at the graph you can see what is happening, and exactly why.


Yes, that cite was useless.

Apaflo Loc: Anchorage, Alaska

That's not DOF, but comes pretty close to describing Spherical Aberration, where the outer edges may not be able to focus light (particularly of different colors) all at the same point as does the center of the lens.

Apaflo Loc: Anchorage, Alaska

Lots of very nice information... but it does not answer the OP's question!

EdM Loc: FN30JS

same with your eyeball, pathlength, in low light your iris is open and things are fuzzyer.

Chase Loc: Paintsville, KY

Nailed it CaptainC. That's like having to learn to write software before you can use your computer.