how about a DOF tutorial??

Re: how about a DOF tutorial??

This forum could be renamed the Circle of Confusion.
The amount of light only affects DoF becasue it affects the aperature settings. Faster shutter speeds in still photography decrease the amount of light entering to allow you to open the iris. A wide aperature allows a wider cone of light to pass through limiting the range of focus. The iris in our eyes work the same way. On a bright day everything in the distance seems to be in focus. Thow on your neutral density sunglasses and your eyes are forced to focus more selectively.
 
Re: how about a DOF tutorial??

... which is also why we frequently need glasses for driving at night, whereas we might get away without them during the day...
 
borderlinefilms said:
On a bright day everything in the distance seems to be in focus. Thow on your neutral density sunglasses and your eyes are forced to focus more selectively.
Click to expand...

yeah, but WHY is this? I'm asking how the light particles bouncing off things will affect what is in focus and what is not?:undecided

borderlinefilms said:
On a bright day everything in the distance seems to be in focus. Thow on your neutral density sunglasses and your eyes are forced to focus more selectively.
Click to expand...

I thought that you could give the illusion of a shallower DOF but putting on your ND filter so that you can get your iris as close to open as possible. ( in conjunction with getting back and zooming in ) But that statement it seems like your saying that the less light you let in the shallower your depth will be.
 
You're saying the same thing. In both cases you're trying to get the iris more open; some people advocate doing it by just having less light in the scene (which dictates a more-open iris), others say that no matter how much light there is, you can always add ND filters to get the iris where you want.

It's the open iris that causes the shallow DOF effect in these cases.
 
ok yeah i get that much. :) I guess what I'm asking is a little more in depth.

Let's me phrase it this way. Say you had your ap. open all the way and were using no ND filters. Then you back up a bit and zoom in. Now, keeping all that the same; no changing ap. settings or distance, you switch to ND 2. Would that cause your DOF to get deeper?

I'm just wondering how the amount of light itself affects what can be focused upon.

Or does the DOF just seems to get shallower the closer you get your ap. to open and then at that point if you add more light you will just overexpose ( if the amount of light exceeded what your NDs could filter out )?
 
No engaging the ND filter does not, by itself, cause a change in DOF. What the ND filter does do is allow you to open the iris more without blowing out your image. It's the opening of the iris which makes the difference :)
 
Correct. The ND filter has no effect whatsoever on DOF, it just allows you to use a different aperture, and it's the aperture that affects DOF.
 
Check this out, guys. I don't remember exactly when I 'got it' with DOF - some time around the end of high school. So I thought...

What was pretty obvious to me in photography didn't click outside of it UNTIL: I saw volume II of Hollywod Camera Works Master Course. Their explanation helped me realize why older people see better with more light. Thought it was obvious? Well, I know all about increased contrast and blah blah blah, but now I think it's mainly because with more light, the pupils in their eyes contract more, providing more depth of field, and therefore sharper image.

Whatdayathink?

Footnote:
I find it funny how my wife is starting to use the 'let me see' thing - putting her 'normal' glasses on the tip of her nose so she can look at things 'closer'. I am not laughing, though, for I know I am not far behind :thumbsup:
 
Barry_Green said:
Correct. The ND filter has no effect whatsoever on DOF, it just allows you to use a different aperture, and it's the aperture that affects DOF.
Click to expand...


i get that the aperture affects DOF but what i was asking is , why doesn't the depth of field get shallower when you let more light in?
 
reflex said:
i get that the aperture affects DOF but what i was asking is , why doesn't the depth of field get shallower when you let more light in?
Click to expand...
Because depth of field has nothing to do with amount of light. The 'letting more light in' is a consequence of opening the iris, and that (assuming focus, distance to subject and the focal lenght remain constant) is the only variable that would make DOF shallower.
 
okay. so then if more light entering doens't effect it , which i can understand that, since you'd usually put on the ND filters to get the amount of light back to the right amount for proper exposure, how does making that opening bigger lessen what you can get into focus (depth wise) ? It just allows you to use more of the lens? thinking that the larger the lens the shallower the dof i guess?

i'm just confused. I mean , how does opening that aperture / making that hole bigger limit your range of focus?

back on page 6 from borderlinefilms
A wide aperature allows a wider cone of light to pass through limiting the range of focus.
Click to expand...

Is that true? I thought light doens't affect things...
 
reflex said:
how does opening that aperture / making that hole bigger limit your range of focus?

Is that true? I thought light doens't affect things...
Click to expand...
But the SHAPE of the light cone does. There are many explanations on the net (try to search google). A simple diagram explains how a bigger apperture also means a larger circle of confusion:

DOF-concept.jpg


The point where the horizontal ray intersects the lens and bends toward the focal point represents the aperture. Try to mentally play with this picture and make the point closer to the optical axis (closing the iris) or farther away from it (opening the iris). The cone of light that has its vertex in the focal point gets narrower (or wider). That affects the size of the circle of confusion, which gets smaller (or larger). The smaller the circle of confusion, the sharper the resulting image. So, as you close the aperture, more of the area in front of and behind the plane of focus becomes 'sharp enough', so we see it as sharp. Or vice versa - as you open the aperture, more of the area becomes blurry, so we see it as out of focus. That's all to it.
 
In addition to all that Barry said, I'd like to add that shutter speed IS also a factor. The same as in still photography. Increase shutter speed, you HAVETO open the iris, the more you open the iris the shallower the DOF. Use a long lens and move a good distance from the subject and then zoom in all the way. You create what's called a compression shot. For example: you see someone running down railroad tracks and towardthe camera -- it looks like the train is about to run him over ... not really. The person is quite a safe distance from the train and the camera is even farther away. Because of the optical illusion created, they look like they're only mere feet away.
 
Undoubtedly, that was the most comprehensive tutorial on the DOF. I have a comment.

> So: to maximize the area that's in focus [...snip...] back away from the subject.

I know I need to have excessive confidence to make corrections to what Barry said here in
this DOF tutorial. But here is what my understanding is: for a given Focal Length and Aperture, the "area that is in focus" is always constant. By going far away from the subject, we can't alter the area ("distance" seems to be a better term) that is in focus.

Did Barry want to say was this (?): Once we focus "accurately" on a subject, the distance
behind the subject (ie., away from the camera) that is in "acceptable focus" is twice that of
the distance infront of the subject (ie., towards the camera) that is in acceptable focus.

Putting it in other way: after setting the focus on a specific subject for a given Focal
Length and Aperture, moving away (or towards) only makes the subject look "less in focus".

Requesting the members to correct me if it is wrong.
 
Yes, for a given focal length and given aperture, the area that is in focus is always constant. But the statement I was making was to describe how someone could get the deepest focus possible. Zooming out, stopping down, and backing away are all three methods that will lead to deeper focus. If you're at 45mm and 10 feet away from your subject, and focused at 10 feet, there will be a certain depth of field. If you're at the same 45mm but 20 feet away from the subject, and focused at 20 feet, the depth of field will be deeper (but the subject will be smaller in the frame).

You are correct that if you set focus distance, and then move the camera without changing that focus distance, then the subject will be less in focus. That's not what I meant though, I was talking about changing the focus point so that the object is in the best focus it can be at whatever distance you're at. The further that distance is, the deeper the DOF will be, for any given zoom setting. If you change the zoom setting to keep the subject size the same, however, the DOF will appear to be shallower.
 
Reflex - perhaps you need to look at it from the other side? If you have a very small aperture then everything is in focus. Did you ever make a pin-hole camera? Because the aperture is very very small on a pin-hole camera the image on the film is in focus even though you're not actually using a lens.
As you increase the aperture then lenses are required to correct the focus of the object onto the image plane, the larger the aperture the harder it is for the lens to provide focus all the way to infinity - hence Depth of Field.
 
grab 1, camera about 10 feet from subject, subject is about 8 feet from wall. not what I wanted because wall is in perfect focus, not a good DOF shot. I worked hard to get DOF on this and it just didn't work.

grab 2, camera is about 8 feet from subject, subject is about 7 feet from wall. perfect blurring of background. total accident, and it was just me playing around in my kitchen.

why does one work and not the other? they are virtually the same shot distance wise.

so frustrating!

grab1.jpg


grab2.jpg
 
Aperture. Outdoors was probably very bright, you were probably shooting at f/8 or f/11, right? Whereas indoors, it was much darker and you probably opened up to around f/2?
 
Barry_Green said:
4.5mm is equivalent to about 32.5mm on a 35mm still camera, or about 18.9mm on a 35mm movie camera.

Using the Century 0.6 adapter gets you to about 2.7mm on the DVX, which equates to about 20mm on a 35mm still camera, and 11.5mm on a 35mm movie camera.
Click to expand...

Why is this? I don't understand why the measurements are different across different formats. Why is 4.5mm on the DVX not 4.5mm on a 35mm movie cam? A millimeter is a millimeter is a millimeter right? What am I missing?

Thanks alot.

BTW this thread has been very enlightening. Reinforced a lot of things I was unclear on. Thanks to all.
 
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