D.O.F. Quest

[CMoore]

OK.....
1. I am new to photography, so pardon the "not this again" question.
2. I DID do a search, but.....Depth Of Field...brings up so much info I could not sift through it all to find the answer.

Lets assume I hold my hands together to mimic the (Hole/Opening) aperture blades of a camera lens, and I look through the "opening" that my hands create. So my hands are a camera and lens, and I am taking a picture of a lamp on a table.
At f2 I can see a pretty wide angle of view. I see the lamp (the subject of the picture) in the center of my hands, but I can also see quite a bit to the left and the right of the lamp.
When I stop-down my hands to f16, I loose a lot of the wide angle.....I can, mostly, just see the lamp on the table.....the wide angle of view is gone.
But that Is Not what happens with a camera lens.
At f2 the amount of "stuff" in the picture is the same when the camera is at f16. You do not loose that wide angle when you stop-down a camera lens. Why is that.? Why does the angle of view stay pretty much the same at f2 and f16.?
I hope you guys can understand what I am asking.
Thank You

 

[mooseontheloose]

Your hands are the wrong analogy - look at the pupils in your eyes instead. When you are in bright light, they are very small (ex. f/16 - I know it's more probably more than that), yet in dim light they can be really big, almost covering your irises (f/2 or smaller). You still have the same angle of view, it's just your eyes are letting in different amounts of light to allow you to see - a camera lens works the same way. (of course, there are other differences between eyes and lenses, but I won't (can't) get into that here).

 

[Gerald C Koch]

The field of view, that portion of the surroundings that a lens images, is determined by the focal length of the lens and not[ by its aperture. So your analogy with your fingers is faulty.

https://en.wikipedia.org/wiki/Field_of_view

 

[Bill Burk]

Take off your glasses and pinch thumb and forefingers together. Hold them right up to your eye and look through the opening. Notice how the tighter you pinch the sharper everything gets? That's what's happening. (Works best if you're nearsighted)...

 

[Peltigera]

The reason that looking through a hole made by your hands alters the field of view is because you are holding them well in front of your eyes. The reason the aperture in a camera does not alter the field of view is because the camera aperture is inside the lens, not in front of it. Technically, the aperture needs to be at the optical centre of the lens to be fully effective.

 

[Nodda Duma]

The official term for what you're making with your hands is called a field stop. So yes introducing a field stop in front of the camera lens will replicate what your hands are doing: vignette the field of view of the lens. But that's not the same as changing the f/# as explained above.


Depth of field refers to the range of distances in object space that are in focus.

In a camera lens, the f/# indicates how much light passes through the optics. The diameter of the pupil, or aperture stop, controls the actual amount. Since its part of the lens itself, it affects light equally from all field angles, so changing the f/# of a lens doesn't vignette the field of view any. The poster's mention of the eye pupil above is a great example.

Here's what I really wanted to make you aware of: For a reference on optical terms and definitions, do a search for and download "MIL-STD-1241A". Keep it handy as a reference as you get further into the hobby. Good luck!

Hope this helps,
Jason

 

[RobC]

what you are describing is not DoF (Depth of Field). see following for expalnation of DoF...

http://www.dofmaster.com/dofjs.html

What you are getting at is the FoV (Field of View) which is the angle the lens is able to see in front of the Lens. This is often called angle of view but in fact angle of view is the cone of light the lens projects onto the film which is not the same.

The FoV is fixed(more or less) for any focal length. The angle of view onto the film is determined by the lens design (not focal length) and must at least cover the film format. For large format cameras the angle of view can be much larger than film format so that you can "shift" the film around in it (rise, fall, left shift, right shift). For 35mm and medium format cameras the lenses are usually designed with angle of view which just covers the film format (there are exceptions) and shift is not available(normally).
So essentially FoV (field of view) remains constant for any lens. However, it is cropped by the film format and depending on the lens design and how big the AoV is, a varying amount of the FoV (in front of the lens) is used.

As others have explained, what you are doing is not adjusting DoF, it is adjusting(cropping) FoV which is not something that lenses normally do.

For an explanation of how aperture adjusts DoF see:

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

 

[CMoore]

Yeah...sorry. I do know what DOF is..... but I did not know a better term for the title of my question.
A post above has lead me to what I am experiencing and why it happens. https://en.wikipedia.org/wiki/Angle_of_view
Thanks for all the Reply's/Info.
I Appreciate It