Aperture and DOF

ShutteredEye · Jun 7, 2005

Probably showing my noobile tendancies here, but can someone answer this for me?

I understand that apeture affects DOF, that a wide open aperture results in a small DOF, and a small aperture gives a large DOF.

My question is, in an SLR, while composing the picture and adjusting the aperture, should I see the effects of this changing DOF in the viewfinder?

I would break out the SLR and try and figure it out myself, but I'm stuck in class. Biochem. Ugh.

Artemis · Jun 7, 2005

No, unless (Like 300D and 300V Canon cameras) you may have a Depth Of Field Preview, but this simply shows how dark the picture is, and you work it out from there.

But...to be answer your question...no...

Unimaxium · Jun 7, 2005

Like Artemis said, you generally should not see the DOF when looking through the viewfinder. The aperture is supposed to stay wide open until the shutter is released (in order to make the viewfinder brighter and to make focusing easier due to the smaller DOF). Some cameras, however, do have a DOF preview button on them which will temporarily close the aperture to whatever it's set at until you let go of the button.

ferny · Jun 7, 2005

Artemis said:
No, unless (Like 300D and 300V Canon cameras) you may have a Depth Of Field Preview, but this simply shows how dark the picture is, and you work it out from there.

But...to be answer your question...no...

DOF Preview buttons give you a preview of the DOF. They don't show you how dark the picture will be because with correct exposure the picture will be no darker at different apertures. It may be hard to see, but they do show you what the DOF will be when you press the shutter.

Meysha · Jun 7, 2005

Ahh that's something I've always wondered too. So when you focus on something does the DOF center on that.... or is the DOF always in the same spot of the photo.

Argh! that doesn't make sense. What I mean is does the DOF move around the photo or is it always in the same spot.
Hmmm writing this - I'm starting to think that the dof is just the width of the focus.... so you can focus anywhere (within your lens' capability) and the DOF is the width of that focus.

Wow anyone else confused?

ferny · Jun 7, 2005

It's quicker to paste than type. :lazy:

Check out the related links as well.

http://www.answers.com/depth of field

ShutteredEye · Jun 7, 2005

ferny said:
It's quicker to paste than type. :lazy:

Check out the related links as well.

uhhh....what link?

Big Mike · Jun 7, 2005

You've got it right Meysha, DOF is just a width (distance from the camera) that is in focus. It doesn't matter where in the frame the objects are, if they are within the DOF, they will be in focus. As for how that all works, there is something called the circle of confusion, you will have to ask Hertz for an explanation on that one.

Artemis · Jun 7, 2005

Think of it as levels in a picture...for example.

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII Is a photo..
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
Low aparature
IIIIII
High aparature...as you can see the lines represent what is in focus certain distance from camera....

probe1957 · Jun 7, 2005

Meysha said:
snipped
Wow anyone else confused?

It seems you might be a tad confused, yes.

Depth of field is simply the distance from the nearest object in focus to the farthest object in focus.

For example, and these are just numbers I am pulling out of the air...if you set your lens at f4 and focus at 25 feet, objects from 12 feet to 35 feet might be in focus. So your depth of field in this case would be 12 to 35 feet.

Depth of field changes with aperture and focal length. For example, using the same prime lens as in the above example, focusing on a subject 25 feet away but changing your f stop to f16, objects from 4 feet to infinity might be in focus, so you depth of field is now 4 feet to infinity.

ferny · Jun 7, 2005

mountainlander said:
uhhh....what link?

The one I lost when I edited the post last time.

I put it back.

etaf · Jun 7, 2005

lots of links and info here
http://www.dpforums.com/forum/showthread.php?p=10266#post10266

the screen goes dark on DoF preview because the apeture shuts down and lets less light in

ksmattfish · Jun 7, 2005

probe1957 said:
Depth of field is simply the distance from the nearest object in focus to the farthest object in focus.

I would expand slightly on this: DOF is the area of the photo that is in acceptable focus. Of course, "acceptable" can vary from one photog to the next.

A general rule, that is not particularly accurate, but you see and hear it all the time anyway, is that the DOF extends approx 1/3 in front of the point of focus, and 2/3 behind the point of focus.

The most accurate method of measuring DOF is using DOF preview, if your camera has it. Although, as has been mentioned, when you stop down the lens it can be hard to see. If you are using a camera with a ground glass you can use a loupe to get a better look.

Prime lenses used to come with DOF scales on them, so you could see at least what the manufacturer considered acceptable focus. For some reason they don't always have them anymore. In my cynical opinion it's because they really don't want you to understand photography; they want you to rely on their fancy smancy auto-programs.

You can also check out DOF scales and calculators on the web. There are a million of them. Some are better than others. Just search for "DOF calculator".

Hertz van Rental · Jun 7, 2005

F = focal length of lens
u = distance of object from lens (the object being the point focused on)
f = f-number
c = diameter of the circle of confusion
The diameter of the circle of confusion varies slightly from person to person, it is also affected by viewing distance. It is normally taken as 0.25mm viewed from a distance of 25cm. Compensation needs to be built in to allow for the fact that prints are made by enlargement - usually from smaller negs.
The rule of thumb is: 0.25mm divided by the degree of enlargement anticipated.
Eg: 8x10 enlargement from a 35mm neg is a magnification of 8x. Therefore CofC = 0.25/8.

Depth of field calculations.
Distance from lens to nearest point in focus:
= Fu(F+cf)/(F squared + ucf)

Distance from lens to farthest point in focus:
= Fu(F - cf)/(F squared - ucf)

ksmattfish · Jun 7, 2005

Here is an extensive DOF calculator that lists a lot of info on digital point-n-shoot camera lenses, as well as the typical film and DSLR sensor formats.

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

If you search for "DOF software" or "depth of field software" you can download programs for your computer, laptop, palm pilot, etc... that has the formulas that Hertz posted ready to go.