DoF Preview - I Don't Get It

[ShooterDan]

I've owned my D7000 since January, but only just discovered the DoF Preview button. I dutifully grabbed my manual to see what this button did. Of course, I had never heard of DoF preview before, and now I see I have one. "How nice", I thought, "I wonder what it does"?

I've spent a couple of hours searching for and reading explanations of how this works; what it does. I have found plenty of articles about this, but I simply cannot see anything that seems to help me understand what this feature is doing.

I've tried fairly large apertures, focusing on objects near and far - pressing the DoF Preview and trying to understand what I'm looking at. I've repeated this focusing exercise with smaller apertures. I've read comments concerning using this feature to help spot some issues more to do with highlights or shadows, but I cannot see, no matter how hard I try, how this tells me anything about focus.

We are talking about focus aren't we? I mean, through the viewfinder alone, once the camera is focused I can see where objects are outside the DoF (if any). All the DoF Preview appears to do for me is darken the image I see.

What am I missing here?

 

[amolitor]

If you look closely, when the finder image darkens there should also be more stuff that's "sharp" than there was before. Things that were blurry and out of focus -- but only a bit -- should now be sharper.

It's a "depth of field preview" which should show you what the depth of field will look like in the final picture. It's only marginally useful, since as the effect gets more pronounced (deeper depth of field is revealed) the image you're looking at in the finder is getting darker (more DoF is given by very small apertures, which let in very little light, producing a dark finder image). Once you get the hang of it, it will be easier to see what's going on, though.

Put your fastest lens on the camera (the one with the smallest f-number). Focus on something pretty close. Notice how the background is all fuzzy. In fact pretty much everything except what you focused on should be pretty soft. Make a note of something that is, back to front, pretty close to what you focused on. That thing should be "a little bit fuzzy" since it's not quite at the same distance as whatever you focused on.

Set the aperture to something quite small (larger f-number, say about f/8.0).

Without changing the focus, press the DoF preview button.

Find that object that was a little bit fuzzy. It should be pretty sharp now. Or at least, sharper. Whatever you focused on should also still be sharp. The background in general may be a bit less fuzzy at this point as well.

 

[astroNikon]

Truthfully when I found that button too it really didn't make much difference or sense with the kit lens.

AFTER I bought a 50mm f/1.8 lens and started playing/testing with f/1.8 thru 3.5 THEN I definitely saw the differences.
And now I can see the difference using the kit lens, though it's not as apparent as with the 50mm lens at large apertures

 

[molested_cow]

Your view finder will always be at the largest aperture.

To see what the DOF preview does, set your camera in aperture priority mode.
Set the aperture to something small, like F14 or something.
Look into the view finder (it should show how it is like at the largest aperture).
While still looking, press the DOF preview button.
You should now see a significant difference in the DOF, as well as a dip in exposure.

 

[ShooterDan]

astroNikon: Yeah, this is all with the 18-105 kit lens. I plan on buying the AF-S NIKKOR 28-300mm f/3.5-5.6G ED VR very soon, so I'll look at this again then.

amolitor: I understand what you are saying. The thing is, when I hold the camera up and just focus, I can clearly see the viewfinder image and I can see what is blurred; this method seems to give predictable results on the actual pictures taken. If I change aperture I need only refocus and I can see what has changed.

If I understand, the difference with DoF Preview is that I press THAT instead of refocusing. Except too I have a darker image that is harder to see. Both methods appear to show me what is and is not in focus, except DoF Preview makes the image darker.

Now, everyoe seems to say it is a good thing, but I still don't see the advantage. I figure that means I'm still missing something important.

 

[astroNikon]

Yes, it becomes more self-explaining after you have a lens that you can bounce between f.1,8 and f/22 and then f/2.8 f/11 bouncing back and forth.

But, it just allows you to preview the DOF before taking a photo, to make adjustments. So you don't have to shoot as many photos as much.

 

[amolitor]

With the ability to take a picture and chimp, especially with the ability to quickly zoom in on the picture, the DoF preview button is about 95% irrelevant. As far as I can tell, it's useful only in cases when:

- you haven't got the time to chimp and really get an accurate notion of what's going on
- your experience isn't giving you good enough information, or you'd at least like a check, to make an aperture decision "blind"

are both true. This is pretty rare, I suspect. If you're in a hurry, your experience should jolly well be enough to get you into the right territory or you have problems. If you're not in a hurry, just chimp. It's going to give you a much better picture of what's going on anyways.

The button doesn't even exist on all low-end stuff. It's there to keep the pros and, uh, "high-end" amateurs happy, I suspect, not because they need it but because they're used to it. It's also a lot more useful on top-end very bright finders with fast lenses.

 

[molested_cow]

Whether a feature is useful or not is entirely subjective.

Personally, "chimp" means I am wasting time and missing critical moments because I have to look at the LCD review screen too many times. I have seen many photographers spending more time looking at the LCD screen as opposed to looking into the view finder and capturing moments. I personally find that very silly.

Using the DOF preview button, I don't even have to take my eye off the subject, let alone waste too much time. It's a split second action.

 

[Big Mike]

amolitor: I understand what you are saying. The thing is, when I hold the camera up and just focus, I can clearly see the viewfinder image and I can see what is blurred; this method seems to give predictable results on the actual pictures taken. If I change aperture I need only refocus and I can see what has changed.

You are confusing distance to point of focus (what you are focusing on) and Depth of Field.

When you focus on things that are closer to you, your DOF will naturally be more shallow, which may mean that you see the background getting more blurry. If you focus on something else, you will see that change of focus in the view finder but you may also see a change in the DOF, based on how far away you are focusing.
But either way, when you are looking through the viewfinder, you are always seeing through the lens at the maximum aperture, which will show you the shallowest DOF.

DOF preview stops down the lens so that you can look though the lens at the selected aperture, which will then show you the actual DOF in the view finder.

To really see the difference, you pretty much have to keep the same point of focus (don't re-focus). Push the button, let your eyes adjust to the darker viewfinder and see how the DOF will have changed. Obviously, the smaller the aperture you choose, the bigger a difference you will see in the DOF.

Of course, this really isn't all that useful in the digital era. It's much easier to just take the photo and examine the resulting DOF in the image on the LCD. DOF preview is really a holdover from the days of film cameras, where you couldn't see your photo until it had be developed.

 

[Designer]

IIRC that feature wasn't uncommon on film cameras. I don't remember which one(s) have that, but I remember using it.

 

[Derrel]

The DOF preview button is useful mostly in good lighting levels, for focusing by hand, with the right middle finger pressing the button inward, thus stopping the lens down, and allowing the photographer to manually move the point of focus for a desired creative effect. As stated, in good light levels, it's possible to evaluate the focus placement, and how that placement affects front- and rear-defocus areas. This is an advanced technique, and one I have used for almost 30 years. It's called "stopped down focusing". It's not for every situation; it's much more applicable to telephoto work, where the image is fairly highly magnified, and easy to see the degree of focus being either "in" or "out" or as "suggesting detail", or "not suggesting enough detail".

It's the little things that separate good photos from average ones, and being able to look through the lens, press a button, and then simply twist the focusing ring while focusing stopped down can allow an experienced shooter to avoid the need for chimping; it's simply easier, and faster. When you move away from the pentamirror cameras with the small, low-magnification, low-quality viewfinder images, and into the higher-end camera with bigger, all-glass pentaprism finders and better viewfinder image quality, then the DOF preview button actually becomes a useful tool, in certain situations. Again, the DOF preview button is not a tool for every situation, and its use is well beyond the abilities of inexperienced photographers.

It's a lot like double-clutching on downshifts, or double-haul flycasting...both are things that the "average" practitioner/user will tell you have little practical reason for existing. The difference between using the DOF button and chimping is that the DOF button allows you to place the focus as you desire, and to SHOOT, in about 1/2 second. Chimping is a slower process. Chimping requires a shot to be triggered. If there is a flash, or a self-timer delay, or anything else, then the chimping method takes maybe 8m,10,12,15 seconds to achieve the same things. again...on the cheaper cameras with crappy viewfinders, the DOF preview is a LOT less-useful than on the highp0end cameras, where one can actually SEE useful detail as the focus is moved forward and backward. The DOF preview control goes along with the old idea of pre-visualizing and getting things right in the camera, as opposed to shoot/chimp/re-futz/shoot/chimp/re-futz/chimp/ahhhh finally got it right!

 

[Gavjenks]

Note that modern digital camera viewfinders can't really show a very accurate depth of field at very bright apertures like 1.4 If you have an f/1.2 lens or something and use DOF button to preview f/2, things will look a lot darker but not actually sharper, and the file will have less DOF than either.

I don't quite understand why this happens, but it might explain some amount of confusion about why a DOF might not "seem to be working" depending on the lenses and apertures tested with it.

 

[amolitor]

I think modern digital cameras tend to have appalling finders.

 

[Derrel]

Gavjenks is correct: modern d-slr's with their stock viewfinder screens do NOT SHOW the shallow depth of field of fast lenses. A few years back, Canon's USA PR dude, Chuck Westfall, stated that one of the then-new Canon d-slr models showed an apparent f/4.2 or thereabouts in terms of "apparent depth of field". This has to do with the way the viewfinder screens are ground. The degree of scatter of the light is the difference between newer "autofocus viewfinder screens", and older-style Fresnel groundglass viewfinder screens, which are designed to be focused by hand-and-eye. Modern AF finder screens are very bright and clear; a higher-contrast but dimmer, and "coarser" viewfinder screen is easier to focus manually with wide-aperture lenses.

The AF optimized viewfinder screens in modern d-slrs make focusing wide-aperture lenses like 50mm f/1.2 and 85mm f/1.2 and 85mm f/1.4 lenses, and the like VERY challenging. For people who do a lot of manual focusing, aftermarket viewfinder screens like those sold by KatzEye,and so on, are popular with the manual focus and adapted lens set.

The viewfinders in modern full-0frame cameras are quite good, but the majority of lower-level APS-C bodies have appalling viewfinders, yes.

 

[rambler]

If you are using a small aperture on the D7000 and the view goes dark when you engage the DoF Preview button, turn on, ie. pop up, the built-in flash, the flash will blink rapidly to help lighten up your view.