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Old Feb 16, 2007, 2:23 PM   #1
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It seems all DSLR's have some DOF preview function/button.
Its mentioned in all reviews, but I haven't ever seen it explained.

I guess its got something to do with a DSLR viewfinder only using the center part of the image to get a brighter viewfinder, and this gives wrong DOF view, or something.

Anyway, can anyone in detail explain about this DOF preview, what it does, how it works, why its needed.

Many thanks in advance to anyone who try
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Old Feb 16, 2007, 4:13 PM   #2
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When you are looking through the viewfinder of a dSLR, in order to give you the brightest possible view, the camera opensthe aperature of the lens as wide as possible. But when it does that, it also gives you the smallest DoF (large aperature = small DoF). When you pressthe DoF Preview button the camera closes down the aperature to the setting it would use when you release the shutter (whether it's determined by the automatic exposure system or it's an aperature you've specified manually.) In this way, you can see through the viewfinder what the DoF will be when you take the shot. Butanotherconsequence of closing down the aperature is thatthe viewfinder becomes dimmer. This isbecause, in order to show you what the DoF will be, it needs to close down the aperature, whichlimits the amount of light coming into the viewfinder.

Does that help?
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Old Feb 16, 2007, 4:20 PM   #3
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TCav wrote:
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When you are looking through the viewfinder of a dSLR, in order to give you the brightest possible view, the camera opensthe aperature of the lens as wide as possible. But when it does that, it also gives you the smallest DoF (large aperature = small DoF). When you pressthe DoF Preview button the camera closes down the aperature to the setting it would use when you release the shutter (whether it's determined by the automatic exposure system or it's an aperature you've specified manually.) In this way, you can see through the viewfinder what the DoF will be when you take the shot. Butanotherconsequence of closing down the aperature is thatthe viewfinder becomes dimmer. This isbecause, in order to show you what the DoF will be, it needs to close down the aperature, whichlimits the amount of light coming into the viewfinder.

Does that help?
Ah, ok, thanks a lot

But does that mean if you step down to F/11 or so, and its not a bright sunny day (you choose very high ISO and long shuttertime instead) you might hardly even see anything in the viewfinder?
I mean, if its so dim that it automatically assumes you want widest apperture to see what you shoot...

Anyway, with DOF preview do you see the correct DOF in a DSLR, or are there something like only the center part of the lens is used for reflection up to the viewfinder and cause of that you never ever get the real DOF? I know thats another issue and off topic, but do you know anything about that too? Maybe its a minor difference, but I think I've read something about that as a downside of DSLR's (compared to compacts with EVF's).

Thanks again
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Old Feb 16, 2007, 4:50 PM   #4
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ASBR wrote
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But does that mean if you step down to F/11 or so, and its not a bright sunny day (you choose very high ISO and long shuttertime instead) you might hardly even see anything in the viewfinder?
I mean, if its so dim that it automatically assumes you want widest apperture to see what you shoot...
If you mean that using DOF preview at that setting means the viewfinder will be really dim, then yes. The normal view will always be at the widest aperture regardless of anything else. Remember that on a DSLR viewfinder, you're just looking at what the lens sees reflected by a mirror. The camera will keep the aperture at it's widest until right when the photo is taken, or you push the preview button.

Some cameras, like the Pentax K100d, come with another preview function that allows you to snap a photo without saving it so you can quickly see how it will turn out on the viewfinder.

DOF preview on a DSLR will give you an extremely accurate assesment of DOF since you are seeing what the camera sees.

As far as I know, most p&s cameras don't give you any kind of DOF preview, they just use the widest aperture (if it has an aperture) at a shutter speed that will approximate the exposure levels you would get. So, if you set a digicam to shoot f/5 at 1 second in a dark setting, the camera would probably use something more like f/2.8 at 1/15 with an ISO boost to give you your live preview. DOF on a digicam is so large though that a preview is rarely useful.
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Old Feb 16, 2007, 5:08 PM   #5
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Ok, thanks you too

But you agreed that when using DOF preview, under some conditions it might get too dark for it to be of any use?


I'm somewhat sure I've read somewhere that DOF seen through a DSLR ain't the real DOF, since the EVF only uses the middle part of the light through the lens, to get more light, and therefor the DOF (in some advanced way having to do with physics I guess) shows up somewhat incorrect. As opposed to a compact where you really see what the camera sees.

Or maybe it just got something to do with this DOF preview, that it uses widest aperture to get more light, and with preview you get it totally correct....I might have misinterpreted it, dont remember.



Oh, and a compact shows real DOF, at least when you halfpress and it locks focus/exposure. Just checked on my S3 and it really seems that is the case. I can easily see the background change as I change aperture, only when halfpressing though.
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Old Feb 16, 2007, 5:21 PM   #6
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ASBR wrote:
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But does that mean if you step down to F/11 or so, and its not a bright sunny day (you choose very high ISO and long shuttertime instead) you might hardly even see anything in the viewfinder?
Yes.

ASBR wrote:
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I mean, if its so dim that it automatically assumes you want widest apperture to see what you shoot...
Actually, the camera will always use the widest aperature just to keep the viewfinder bright. That has nothing to do with what aperature the camera will use when taking the picture. That will be determined by the automatic exposure system.

ASBR wrote:
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Anyway, with DOF preview do you see the correct DOF in a DSLR, or are there something like only the center part of the lens is used for reflection up to the viewfinder and cause of that you never ever get the real DOF?
When you press the DoF Preview button, you will see the correct depth of field for that aperature. When the aperature closes down, the camera is only using the center part of the lens, but it's using the center part of the lens to get the entire image, not just the center part of the image, so you actually get the real DoF.

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I think I've read something about that as a downside of DSLR's (compared to compacts with EVF's).
No. The downside of EVFs is that there is no DoF preview (primarily because the lenses on most P&S cameras don't offer a lot of DoF anyway.) The downside of dSLRs is that there is no EVF (except for a few newer models.)
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Old Feb 16, 2007, 7:49 PM   #7
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ASBR wrote:
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... But does that mean if you step down to F/11 or so, and its not a bright sunny day (you choose very high ISO and long shuttertime instead) you might hardly even see anything in the viewfinder? ...
You have it right - I have found the depth of field preview to be usless. When it is stopped down to the shooting f/stop, it is to dim to make out what is in focus. Interesting to play with, but of little or no practical value for small aperatures.
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Old Feb 17, 2007, 3:48 AM   #8
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Ok, thanks you guys

But dont say the compacts dont have any DOF preview just cause they have so long DOF its never even interresting.

When shooting macros its very nice to always (when halfpressing anyway) see the correct DOF, and with a ultrazoom bridge-cam you also get very short DOF when hitting like over 400mm and F/3.5 for closer distances, I assure you.

But as I said, thanks for the info. Always nice to learn more. Maybe one day it'll be time to step up into the DSLR-world.
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Old Feb 17, 2007, 8:14 AM   #9
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ASBR wrote:
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But dont say the compacts dont have any DOF preview just cause they have so long DOF its never even interresting.
That's certainly not what I intended to communicate. When I said "The downside of EVFs is that there is no DoF preview (primarily because the lenses on most P&S cameras don't offer a lot of DoF anyway.)" I meant that there are a number of factors that will make a DoF Preview useless on a P&S. A substantial one, to be sure, is that typical P&S cameras don't have a lot of range in the available DoF to start with. But another substantial one is that, compared to the resolution of the recorded image (6 to 8 megapixels), the resolution of the electronic viewfinder is pretty poor (0.2 megapixels), to the point where any possible variations in DoF wouldn't be visible anyway.
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Old Feb 18, 2007, 3:26 AM   #10
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TCav wrote:
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ASBR wrote:
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But dont say the compacts dont have any DOF preview just cause they have so long DOF its never even interresting.
That's certainly not what I intended to communicate. When I said "The downside of EVFs is that there is no DoF preview (primarily because the lenses on most P&S cameras don't offer a lot of DoF anyway.)" I meant that there are a number of factors that will make a DoF Preview useless on a P&S. A substantial one, to be sure, is that typical P&S cameras don't have a lot of range in the available DoF to start with. But another substantial one is that, compared to the resolution of the recorded image (6 to 8 megapixels), the resolution of the electronic viewfinder is pretty poor (0.2 megapixels), to the point where any possible variations in DoF wouldn't be visible anyway.
Well I think its nice seeing the correct DOF when I halfpress on my "P&S", its very visible at close macros or long teles at closer distances. You do notice as you close the aperture that the DOF increases.

On the other hand its hard to see anything precisely, like exactly how much must I step down to the all of that flyes eye into focus and so on. Maybe thats your point also I guess.
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