beginner question regarding aperture

[ramathorne]

hi

i'm just curious as to why you get a smaller depth of field with a wider aperture? if the lens is wider open and letting in more light, why is the depth of field smaller, as opposed to larger as i thought it would be?

please don't hate me for what might seem like a stupid question :)

[CyberPet]

I sort of used this as a way to remember why:
Wide aperture = shorter shutterspeed = less time to record the whole scene.
Small aperture = longer shutterspeed = longer time to record the whole scene.

I know it's a bit silly, but it worked for me. :D

[tim]

hi

i'm just curious as to why you get a smaller depth of field with a wider aperture? if the lens is wider open and letting in more light, why is the depth of field smaller, as opposed to larger as i thought it would be?

please don't hate me for what might seem like a stupid question :)

It's not a stupid question at all, but it's one of the more complex questions i've seen asked here. I honestly don't understand it well myself, but I figure it has to do with angles, refraction, and using the outer portion of the lens. Hopefully someone who knows more about optics can tell us about it, or direct us to a good web site that explains it.

[ramathorne]

yeah it's got me a bit puzzled, but maybe i have things a bit mixed up.

i'm currently reading a book that tells you about aperture changing brightness and depth of field, and i can understand the brightness as it correlates to the wider aperture, but not the depth of field.

unfortunately my book doesn't explain why and i feel if i understand why certain things are happening then i can become a better photographer

[Mercy299]

think of it as a faucet- opened a little, it takes longer for the cup to fill up (slow shutter)
opened a lot, it fills the cup up faster (fast shutter)
Go to http://www.canon.co.jp/Imaging/enjoydslr/p_2_005.html they explain it in very simple easy to understand terms

[tim]

Good newbie site there, it explains what happens but not why. I think the why could be complex.

[jfrancho]

So far no one has answered the question defintively. I think tim is on to something, since the shallow/deep DOF can be achieved on motion cameras, as well. Shutter speed would have nothing to do with the DOF, and this can be proven using a DOF calculator, as this value is completely disregarded in the formulas. So, it must be the way the light gets to the film or sensor. Perhaps the smaller aperture concentrates the light into beams that they oriented perpendicular to the sensor plane, and creates a greater DOF. I'd love to hear the real story behind this, perhaps some formally trained photographers can weigh in?

[Goofup]

I don't know for a fact either, but I'm just trying to think this thing through logically in my head. If the lens is wide open, all the curvature of the lens is used and the light is "bent" more giving you a smaller area of sharp focus (think magnifying glass). On the other hand, a stopped down lens uses only the center part of the lense that has very little curvature to it and the light is needed to be "bent" (distorted) a whole lot less and therefore much more is in focus (extreme example: a filter with no curvature).

In other words, the more the light is having to be bent (using more and more of the curvature of the lens to get the whole thing focused on the sensor/film) the more critical the focus.

(Course, DOF is also determined by distance to the subject and focal length, but we'll ignore those for now.)

But what do I know.... I could be totally off, and probably am.

[robertwgross]

Goofup, that is about the same way they teach it in Physics and Optical Engineering classes. Sounds good enough to me.

---Bob Gross---

[Andy_T]

Haha ... don't know how to explain it, but you can experience it yourself.

If you squint your eye, you are effectively reducing the aperture, and the resulting image is sharper.

Best regards,
Andy

[lostdoggy]

I just look it up in the Canon's book and it gave me a headahe. It talks about the cicle of confusion and the permissible circle of confusion. Its basically what goofup said. The aperature reduces the diameter of light to the focus plane and in turn increases the permissible circle of confusion. Circle of confusion is the largest dot of light that is consider to be in focus.

[lostdoggy]

BTW, I just accept that the phenomena exist and never questioned why. So, its not a stupid question because now I know why.

[rent]

goofup: you definately got the concept right. it can take a short essay and a couple of good optics lessons to explain this in all its glory details. but for those who are interested in a bit more theoretical details, check out this (http://www.oncloserinspection.com/Photomacrography/Photo_Myths/DOF2/DOF2.htm) site.

-alex

[Andy_T]

Alex,

thank you , the website is very helpful!

I also has the very-often-asked question whether some lenses give better perspective than others (they don't. Your distance from the subject does) answered with very understandable examples.

Best regards,
Andy

[tim]

If you squint your eye, you are effectively reducing the aperture, and the resulting image is sharper.

All that does is shows me my rather long eyelashes!

Another way to do it is with pinholes in a piece of paper.

[ramathorne]

thanks for the replies guys!

i honestly never realised how complex this might be when i asked it.

[robertwgross]

i honestly never realised how complex this might be when i asked it.

It is not necessary to have a perfect command of all of the optics laws and their application to photography. However, it is a very good idea to have at least a loose idea of optics running around in the back of your mind. Then, when you practice your photography, some little problem or question will come up, and you can guess the correct solution. If you have zero understanding of optics, then your guesses will fail and it will become frustrating to keep "shooting in the dark."

---Bob Gross---