Orquid "Phoenix"

Orquid "Phoenix"

by ceinmart

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I'm fairly new to DSLRs and I've been exploring the functions on my new Canon 60D.

There's a button to the side of the lens described as the "depth-of-field preview button". Based on the limited description in the manual, I thought this would give me a preview of how the photo would look in terms of what's in and out of focus when I'm using Live View.

When I press it though, despite jerking as if it's processing something, I see no difference in the LCD screen. I've played with the aperture with no discernible effect.

Am I missing something? What should I be seeing?

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3 Answers

up vote 29 down vote accepted

DoF preview is difficult to use well. The idea is simple, but application much less so. Without DoF preview, what you see through the viewfinder is shown with the lens "wide open" -- at its largest possible aperture. This provides no guidance about how much depth of field your picture will have, because (unless you happen to be shooting at maximum aperture) the picture you're seeing in the viewfinder won't be at the same aperture as the picture you take.

In theory, DoF preview is a simple answer to that: it just closes the aperture on the lens to whatever aperture you have set for the picture. If, for example, you were going to take a picture at f/11 using an f/2.8 lens, the DoF preview will stop the lens down from f/2.8 to f/11.

Difficulty in application stems from two facts. First of all, stopping the lens down significantly makes the view through the viewfinder darker. Second, and more importantly, the focus screen on most current cameras doesn't portray depth of field very accurately. A focus screen that transmits more light directly through gives a brighter view. Since most people depend on autofocus for focusing and don't worry much about DoF, this is the sort of screen used in most current cameras. A screen can be built to diffuse the light more. This doesn't give as bright of a view, but also makes the apparent sharpness in the viewfinder drop off much more quickly when things are out of focus. This makes the DoF preview in most cameras pretty inaccurate. What you see in the viewfinder will show a lot more DoF than the real picture regardless of the exact aperture the lens is set to.

As such, to use DoF preview at all well, you just about have to do some testing to compare what you see in the viewfinder to what you get in the picture. The two won't be the same (usually, anyway), and it's up to you to train your eye to compensate for the different between what you see and what you'll get to be able to compensate.

One technique I've used with reasonable success is to take some pictures at a particular aperture (e.g., f/8) and set up a laptop screen next to the camera so you can quickly look from the viewfinder to a picture you just took. You can then adjust the aperture (with DoF preview) to get close to the same DoF through the viewfinder as shows in the picture. What you really care about is how much difference you see between the two. Let's say the viewfinder at f/3.5 shows roughly the same DoF as the picture on the laptop that you took at f/8. That gives a difference of roughly 2.5 stops. From then on, you can use DoF preview to adjust the zone of sharpness as you see fit, and know that you'll need to open the aperture ~2.5 stops from there to get about the same DoF when you take a picture.

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Wow, very thorough answer. Thanks very much for that! –  Damovisa Nov 23 '10 at 6:34
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It should be noted that these days, with most DSLR's (if not all) having a live-view capability, it is pretty essential to use the DOF preview button with live view enabled. Unlike the physical viewfinder, previewing DOF with the LCD screen is MUCH more accurate. The larger size of the LCD screen makes it much easier to see what your DOF will look like in the final picture. –  jrista Nov 23 '10 at 7:04
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+1 for the focus screen info, I didn't realise it's full effect on DOF until now. Good explanation. –  Guffa Nov 23 '10 at 7:34
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Depth of Field preview is supposed to show you exactly that. The depth of field.

The easiest way to check, is to not use Live-View, and look through the view finder. If you stop the lens down to f8 or higher, and hit the DoF Preview button; the view finder will get noticeably darker.

I'm not 100% certain about live view. In the 40D; DOF did not show in Live View because the sensor was not able to sufficiently show the image because the amount of light is so minimal. Based on your post, I would wager that something similar is going on here.

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Thanks for that. I must have misread the manual; I got the impression it was only used for Live View. I'll give it a go with the viewfinder. –  Damovisa Nov 23 '10 at 1:54
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It's not the most useful button in the world — any significant change in DOF will also significantly darken the viewfinder making it hard to actually see if something's in focus or not... I generally just take a few shots at different apertures to "preview" DOF. –  drfrogsplat Nov 23 '10 at 3:25
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Almost all SLRs for the last several decades do metering and allow focusing, either manual or phase detection AF, with the lens at its widest aperture setting. This allows focusing to be more accurate and the viewfinder to be as bright as possible. But this also means the DoF when viewing the scene through the viewfinder is not an accurate indication of the DoF when the picture is actually taken with the lens stopped down.

The original intent of the Depth of Field (DoF) Preview button was to allow the user to stop down the lens to the set aperture while still viewing the scene seen through the lens via the optical viewfinder. With older film cameras this worked very well. The viewfinders were typically larger and brighter. The focusing screens were also designed to accommodate manual focusing more than the focusing screens on modern digital SLRs. Our modern DSLRs don't do DoF preview near as well as the old film cameras did because of these differences in the viewfinders and focusing screens. For more about this, please see Why is the depth-of-field preview in the optical viewfinder of my Canon 500D inaccurate? But there is still some, albeit less, usefulness to the feature when using the optical viewfinder. And like other features that have existed on top end cameras for several decades, a Depth of Field Preview button may still be seen as an essential feature by many buyers of high end cameras. With the advent of Live View, the DoF Preview button offers one way to preview the actual DoF via the rear LCD screen. Depending on the camera model and the settings selected, the screen can even boost the brightness to compensate for the reduced amount of light reaching the sensor when the lens is stopped down so that the preview includes DoF and brightness simulation.

Beyond the basic function of the Depth of Field (DoF) button, most cameras that include a DoF button also allow the user to remap the button to a different function via the custom functions menu. I sometimes, for example, use it to switch between One Shot AF and AI Servo AF when shooting action. The button is easily pressed by my left thumb while keeping my eye to the viewfinder. In a typical scenario while shooting American football from the sideline I might use One Shot AF to pre-focus on a spot (that I expect the action to come to during the play) before the play begins. Once the play begins and the planned shot has been taken I can then quickly shift to AI Servo AF and continue following the action as it continues onto another part of the field.

With Canon EOS cameras there is an additional function of the Depth of Field (DoF) Preview button: By selecting an aperture setting (using M or Av) and holding down the DoF Preview button while turning off the camera before removing the lens from the camera, the lens remains at the selected aperture value, rather than returning to wide open as is normally the case with EF mount lenses. This is useful if you want to use the lens stopped down while reversed, while 'free lensing', or with an adapter mount or extension tube that doesn't allow the camera and lens to communicate properly.

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