3

Why not just show the effective depth of field, by default?

What's the point of having it display a picture that's different to how the image will actually turn out?

My first thought was, it must be to allow more light in while composing a shot. But it's weird, because when I attach/enable a flash; the display goes from pitch black to perfect vision (without actually firing the flash, or using the DOF button). So, it seems obvious there are other ways to harness enough light to adequately compose an image.

  • 1
    With what kind of camera are you shooting? What kind of viewfinder does it have? – Michael C Aug 21 '17 at 6:00
  • @MichaelClark currently, a Canon EOS 650D. I dont know the details of the viewfinder, but it's usually covered. I shoot a lot of macro imagery, at weird angles, so I tend to use the articulated LCD monitor more than anything. – voices Aug 21 '17 at 6:04
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    You should make it clear in the question that you are talking about the LCD display. – Carsten S Aug 21 '17 at 12:52
  • @CarstenS the question isn't specifically about one or the other. – voices Aug 22 '17 at 13:19
18

Hitting the DOF preview with a stopped down aperture should make it obvious: what you see can be very dim, dim enough it can be very difficult to compose and focus.

That's why cameras have the lens wide open until actually taking a picture, so you can get the brightest image by which to setup your shot.

  • Not on mirrorless – downrep_nation Aug 21 '17 at 10:10
  • For Sony Alpha mirrorless cameras it depends on firmware, lens, camera model, seleted aperture (and whatever more) whether the camera uses open aperture or stopped down before exposure. – Gerhardh Aug 21 '17 at 11:26
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    @downrep_nation Actually, mirrorless cameras are starting to use phase detect, on the sensor (e.g. NX300 was one of the earlier models with it) and it is advantageous for them to have more light to focus with (even for edge detection it is usually helpful anyway). So, the real DOF may not be shown by default so that the camera has the best focusing conditions rather than for the user per se. Other than that distinction (it is for the camera hardware to 'see' rather than for the user to) the answer is sound. Ah... actually you may have meant that the display wouldn't necessarily dim... true. – ttbek Aug 22 '17 at 12:11
  • @ttbek i definitely meant the display, good comment tho – downrep_nation Aug 22 '17 at 12:12
13

Why do we need a DOF preview button?

So that you can see how much depth of field you'll get when the shot is actually taken.

Why not just show the effective depth of field, by default?

Because your eyeballs don't have variable shutter speed or auto ISO. And because autofocus systems need plenty of light to work well. Try setting your camera to f/22, hitting DoF Preview, and looking through the viewfinder. Next, release the DoF Preview button and look again. Which view would you rather work with?

What's the point of having it display a picture that's different to how the image will actually turn out?

That's a funny question, because the whole point of the single lens reflex design (whether film or digital) is to let you see, as much as possible, the same view that the sensor does. And giving you the best preview of the image is also the whole point of including a DoF Preview feature. But again, your eye works a bit differently than the sensor does in that it cannot accumulate light during a long exposure, which means that even when stopped down, what you see through the viewfinder isn't quite the same as what the sensor will record. Also, being able to see what is or isn't in focus isn't essential: many photographers will know from experience, and even those that don't probably don't need to see the true DoF from one moment to the next.

  • My eyes definitely have what amounts to auto iso. It just does not necceserily react that fast. – joojaa Aug 22 '17 at 13:04
  • NO DoF button shows exactly anything because 1) the ground glass in the viewscreen (or its modern plastic counterpart) 'cuts off' the aperture above the fastest lenses, 2) the angular size of the image in the viewfinder is not the same as typical display size. – Michael C Aug 24 '17 at 8:06
  • @MichaelClark Fair enough -- edited. – Caleb Aug 24 '17 at 12:46
5

Your confusion comes from a mixup of viewfinder and live-view use:

Through the viewfinder, you physically, through purely optical means, see through the lens. The only way to get a bright image is to open the aperture as wide as possible. Pressing the DoF button closes it down to the set value to enable you to check the DoF the photo will have, albeit at the cost of a darker view.

In live-view mode, you see an electronic representation on the display, which can and is modified and enhanced by the camera's electronic. If you activate the flash, the display will simulate a brighter picture, since it will often be quite dark and you need the enhancement to see anything at all. This enhancement is purely electronically and probably rather grainy. In any case, nothing that can be applied to the purely optical view-finder view.

So the DoF button really is useless in live-view mode, you'd only use it when looking through the viewfinder.

2

The Depth of Field (DoF) button is one of those legacy items from the film era that may or may not be very useful to someone using a digital camera. No camera absolutely needs one. There were many cameras in the film era that did not have one, even cameras that used wide open metering. Many others, including most advanced and higher end 35mm cameras made after wide open metering became popular, had one.

The purpose of the DoF button is, of course, to stop the lens down to the selected aperture value without triggering the shutter release. This is needed because most modern SLRs (film and digital) hold the lens at it's maximum aperture until the instant before the image is actually taken. Focusing, metering, composing - all of these are done with the lens wide open. There are numerous benefits to doing it this way.

  • The lens is at it's brightest when wide open, so it is easier to see things in the viewfinder.
  • The depth of field is at its narrowest, so it is easier for the AF system or the manually focusing photographer to see the point of sharpest focus easier.
  • The light meter gets more signal with which to work which makes it more accurate. Microprocessors compute the difference between the aperture when metering takes place wide open and the selected aperture. The camera compensates for the difference in the reading displayed by the light meter and in the calculation of exposure if the camera is used in an automatic/semi-automatic exposure mode.

With the advent of digital sensors the need for a DoF button is less obvious. The camera's rear LCD screen can be set in most cameras to "simulate" exposure by adjusting the brightness of the screen. Many cameras will go ahead and stop down the lens when in Live View (or at least provide the option to do so via a menu selection). This has the same function as using the DoF button when using the viewfinder to compose a scene. Of course the screen must be adjusted properly in terms of brightness that matches ambient conditions for this to be effective. The instant feedback available with a digital camera also makes it much easier to take a shot, instantly review it, and examine the effect of a selected aperture/focal length/subject distance/exposure value on the DoF and exposure of the image.

  • Low to midrange SLRs in the film era sometimes did NOT come with one, always showing a wide aperture, bright image in the viewfinder - these were a nightmare... – rackandboneman Aug 21 '17 at 21:54
  • @rackandboneman Which is exactly what the third sentence of the answer says: There were many cameras in the film era that did not have one, even cameras that used wide open metering. Though they could be inconvenient at times, I wouldn't exactly call using such a camera a 'nightmare', though. – Michael C Aug 24 '17 at 8:02
0

The other answers are correct, but slightly miss the point.

The viewfinder help you to prepare the shot. It is (it was) not made to have exactly the same results as in the film/sensor. Now it is seldom, but on high end cameras you can change the viewfinder type: now as default we have the most neutral one, but in the past (and especially on film cameras) the viewfinders made much more evident the focus of image, making the rest of image more out-of-focus. The priority of viewfinder was and it is still focus.

So the priority of viewfinder were: focus, then composition. DOF had lower priority, and not really compatible with focus.

The Actual setting: full open lens helps to view and select the zone which are in focus (it has minimal DOF). Also as other answers: it is also more luminous, so it helps also composition.

With the DOF button press, you have less light, and an estimate of DOF: it is in any case an estimate because viewfinder is small, so your eyes and brain will see the image much more on focus as it is really [make a photos to a far/small text. You can read it when you zoom in, but at a certain point, when you zoom in it is more difficult to read]

It was also an estimate, because with old viewfinders, the out-of-focus parts were accentuated.

So with DOF-button you have some indication of DOF but not a definitive answer.

I also never seen any professional photographers to use DOF-button. It is good for student, to help getting DOF and so aperture by heart, as finally it should be.

  • 1
    Why the negative vote? This explain why the button is there, historically. In any case viewfinder cannot be trusted 100%. On film machines: different "screen" which exaggerate the out-of-focus. With electronic viewfinder: not enough pixels.In many DSRL: too small, and other gears (Leica M) per design, different optical path. – Giacomo Catenazzi Aug 22 '17 at 6:52
0

My recollection is that SLR focusing systems rely on having a wide aperture (look up phase detection and/or split focusing screen). It's not simply about the amount of light; even in broad daylight the autofocus might simply not work beyond 5.6. That's why the default state is to have the aperture open, but there's a DOF preview button to see what the focus/bokeh will actually look like. If you have an SLR where you can manually focus through the prism with and without the DOF preview pressed, you'll find it to be easier with the aperture open.

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