How do I read the depth of field preview?

Joined: 16 Mar 2007 Posts: 16 Location: Essex, England

Hi

When ever I press the dof button all it really seems to do is go dark.
I under stand about the appeture settings but I dont see how this can help me compose pictures better.
Please help.

Posted: Wed Mar 28, 2007 3:18 am

im pretty sure the darkening just shows you how much more of the scene will be in focus, progressingly darker with each higher fstop. there could be something i left out of that but thats what ive always thought.

Joined: 27 Oct 2004 Posts: 452 Location: Gloucester, UK

It's not always easy to see, but the increased depth of field of say f11 over f4, should be apparent in the viewfinder when you press the DOF button.

Premium Member Joined: 02 Apr 2005 Posts: 2020

It also depends a lot on the camera. The viewfinder on the D70 is so dim, that it makes the DOF Preview function almost useless.

Posted: Wed Mar 28, 2007 8:49 am

I've never found the DOF preview function of much use. You are better off just learning through experience what combination of aperture/lens focal length/distance to subject will give you for DOF.

Along these same lines I was wondering if anyone else experiences this and how to explain it: When shooting wide open, lets say f/2.8, and the subject is in focus and the background is not, why does my actual image captured almost always seem to have the background even more blurred than what it looked like in the viewfinder? I mean shooting wide open is equivalent to always seeing a DOF preview whether the preview button is pushed or not because it is the natural state of the aperture. Is it simply our human eye compensating somewhat? Or what is going on here? Or am I the only one that experiences this?

Joined: 04 Dec 2006 Posts: 79

Dermit wrote:

I've never found the DOF preview function of much use. You are better off just learning through experience what combination of aperture/lens focal length/distance to subject will give you for DOF.

Along these same lines I was wondering if anyone else experiences this and how to explain it: When shooting wide open, lets say f/2.8, and the subject is in focus and the background is not, why does my actual image captured almost always seem to have the background even more blurred than what it looked like in the viewfinder? I mean shooting wide open is equivalent to always seeing a DOF preview whether the preview button is pushed or not because it is the natural state of the aperture. Is it simply our human eye compensating somewhat? Or what is going on here? Or am I the only one that experiences this?

Not so much eye compensation as it is the fact that you are looking at a very small representation of what you are really seeing. The eyes cannot that much detail when shrunk that much. Much like seeing a thumbnail on a web page that looks sharp and then clicking on it only to find the full size image is a soft or even blurry.

Premium Member Joined: 02 Apr 2005 Posts: 2020

gryphonslair99 wrote:

Not so much eye compensation as it is the fact that you are looking at a very small representation of what you are really seeing. The eyes cannot that much detail when shrunk that much. Much like seeing a thumbnail on a web page that looks sharp and then clicking on it only to find the full size image is a soft or even blurry.

Yeah... I'd buy that. I've wondered the exact same thing, but never was smart enough to ask.

Posted: Wed Mar 28, 2007 3:05 pm

As good as an explaination as I've ever heard, not sure if it's what it is, but good enough until I hear otherwise. I mean the difference that _I_ notice is pretty significant. For example when looking through the viewfinder at f/2.8 focused on something close a branch of a tree in the background is blurry but I can clearly tell it is a branch. But when looking at the image captured at f/2.8 this same branch is so blurred that you can't even tell it's a branch. It might just look like a brown smudge. I know that optics and the human eye/brain can do some tricky stuff so it wouldn't surprise me if it had something to do with that.

There's an interesting experiment you can do to give you an idea of how the eye sees. Let's see if I can explain this well enough. Get a 35mm film canister (black one or block light of clear ones with tape, etc.) and a standard straight pin (the kind with a flat pinhead on it, not the round ball type pinhead). Now take the lid off the film canister, you will not need it. With the pin, poke a hole dead center in the bottom of the film canister. This next part is tricky. Half way between the top and the bottom (or even a little closer to the top) of the canister poke another hole in the side of the canister. Now you may have to cut some length off the pin for this next part. Poke the pin from the INSIDE of the canister through the hole you made in the last step in the side of the canister. Push it through the hole to the point that the head of the pin is in the center of the open space of the canister. Now hold the canister horizontally with the top opening facing you and the bottom facing away. Hold it so the pin is hanging straight down and the head of the pin is aligned with the hole in the botton of the canister. Now prefereably in bright light, like outdoors in daylight or looking toward a lamp, etc. hold the open end of the canister up to your eye and look through the hole in the bottom so you can see the scene thorugh the hole, but as you do so make it so you can also see the silouette of the pinhead. Then report back here and let everyone know what you see.

Joined: 27 Oct 2004 Posts: 452 Location: Gloucester, UK

SingleMalt wrote:

It also depends a lot on the camera. The viewfinder on the D70 is so dim, that it makes the DOF Preview function almost useless.

That's fair comment. I have always used full frame cameras, with bright viewfinders.

Posted: Fri Mar 30, 2007 9:52 am

simonfb11 wrote:

Hi

When ever I press the dof button all it really seems to do is go dark.
I under stand about the appeture settings but I dont see how this can help me compose pictures better.
Please help.

I don't think anyone in this thread has said this explicitely, but what the depth of field preview does is just close down the aperture to whatever you've set it to. This makes it darker.

If you look carefully you might be able to make out important details to see if they are in focus, but I find it not very useful. It's better to make a depth of field chart and focus that way if you're really worried about getting adequate DOF.

Joined: 02 Jan 2007 Posts: 190

The DOF preview does work... I use it on my XT, but having said that, I only find it useful for well lit close-ups and macro shots.

Get a feel for it by placing a long(ish) item under a lamp and dialing in different apertures....check them with your preview button....you'll see that it works well for applications like this.

Joined: 04 Dec 2006 Posts: 79

DOF Preview does work, however it is somthing that takes time an practice to learn. Optically the camera does not really change the vield of view. When you press the DOF preview button with a very shallow depth of field you get very little or no darken of the viewfinder. The greater the DOF the darker the preview becomes until it is very dard at the smallest aperture. It is a matter of learning the shades as they relate to the DOF for the apeture the lens is set at.

Joined: 02 Jan 2007 Posts: 190

Quote:

Optically the camera does not really change the vield of view. When you press the DOF preview button with a very shallow depth of field you get very little or no darken of the viewfinder. The greater the DOF the darker the preview becomes until it is very dard at the smallest aperture. It is a matter of learning the shades as they relate to the DOF for the apeture the lens is set at.

Your reasoning doesn't make sense. First off, "field of view" is something altogether different than "depth of field." DOF preview DOES change the optical DOF in the view finder.... that's the point. It gets darker because it stops down the aperture to the selected f# to show you what the DOF will be. The dark result is a by-product of this feature. The lens is stopping down to a smaller aperture to show you the DOF and thus letting less light in, making your viewfinder image darker.

Learning the "shades" would be pointless because DOF is also determined by camera-to-subject distance and focal length. Available ambient light would ultimately dictate how dark the viewfinder becomes, therfore getting used to a certain darkness would still give you several differing DOF's based on these variables mentioned.

It's just a matter of getting used to seeing what's in focus and what's not under low light. Plain and simple.

Joined: 04 Dec 2006 Posts: 79

ryguy76 wrote:

Your reasoning doesn't make sense. First off, "field of view" is something altogether different than "depth of field." DOF preview DOES change the optical DOF in the view finder.... that's the point. It gets darker because it stops down the aperture to the selected f# to show you what the DOF will be. The dark result is a by-product of this feature. The lens is stopping down to a smaller aperture to show you the DOF and thus letting less light in, making your viewfinder image darker.

Learning the "shades" would be pointless because DOF is also determined by camera-to-subject distance and focal length. Available ambient light would ultimately dictate how dark the viewfinder becomes, therfore getting used to a certain darkness would still give you several differing DOF's based on these variables mentioned.

It's just a matter of getting used to seeing what's in focus and what's not under low light. Plain and simple.

After reading the way I worded my explination you are correct. My brain was writing way faster than my fingers could type, and it came out all wrong. My appoligies for the confusing post above

You are correct, you are not reading DOF from the darkness or lightness of the viewfinder itself. You are reading the DOF through the change in brighness. When you press the DOF button the set apeture is activated in the lens, causing the lens to darken due to light loss depending on the apeture.

What is in focus does change, but optically, it is hard for the human brain to recognize that change in such a size of view such as a view finder. It takes time (at least it did for me) to develop the ability to see what often can be a subtle change of focus and see what is in focus and what is not.

Posted: Wed Aug 15, 2007 7:35 pm

Again this might seem obvious but give your eyes a few seconds to adjust to the dim light. Don't just hit it and release.