你还在认为景深预览按钮没用吗？

本来对相机的景深预览功能很失望：拍风景的时候光圈比较小，按下景深预览按钮取景器很暗，景深看不大清楚，以为景深按钮用处不大。就在想把这个景深按钮指派给别的功能的时候，读了一篇非常有趣的文章（详见后文部分）现把这个按钮的妙用总结如下，详细内容请参见原文：

1. 帮助构图。在原文里边这个列在最后，但似乎是最有用的或最有趣的，所以我放在最前边。不管准备使用多大光圈拍摄，把光圈先打到16或是22。按下景深预览按钮，取景器中光线很暗。场面的细节看不清楚，只剩下场景的明亮和阴影部分的轮廓。细节丰富的三维场景变成了由形式、形状、光和阴影组成的简单抽象的二维图画，抽象出了构图所需的主要元素或骨架，大大方便了构图。原文的作者说这在拍摄主要由光、影、和流水构成的景象的时候最好用。

2. 检查背景中的亮点。背景中的亮点非常分散照片观赏者的注意力，而在最大光圈下这些亮点很不显眼。光圈收缩之后，场景变暗，亮点会更明显。

3. 检查前景中的黑点。前景中的黑点同样可以分散人的注意力。收小光圈的时候，暗的部分变得更暗。在光圈16和22也许很难看清暗部的情况。但是在光圈f8左右，暗的部分在一定程度上还可以观看。可以看一下暗部是否变成死黑使得相机无法记录下细节。

4. 检查不想要的元素。光圈全开的时候，不想要的元素也许在焦外，看不到。档光圈收小，不想要的元素可能变得清晰。发现了不想要的元素，重新构图。

5. 寻找最好的背景。当近摄或者照肖像的时候要背景虚化。按下景深预览按钮，调节光圈大小选择最佳的虚化背景。

6. 调整中灰渐变滤镜。光圈变小，调整中灰渐变滤镜效果更明显一些，方便调到正确位置。（估计调整偏振镜也一样，待验证。）

7. 优化主体聚焦范围。拍摄花朵和动物，用景深预览可以查看是否想清晰聚焦的部分都在聚焦范围内，比如花蕊、花瓣、动物的鼻子、眼睛、耳朵等等。

8. 查看欠曝。有时候需要一点欠曝，这个时候通过景深预览可以查看光圈缩小后欠曝是否合适或严重。

9. 检查镜头的耀斑效果。如果有耀斑，取景器变暗会看得更明显。

10. 检查滤镜和遮光罩可能带来的暗角。光线变暗，如果有暗角会更明显。（这个只有对100%覆盖范围的取景器才有用）

使用景深预览的一个问题是取景器变暗，眼睛看不大清楚。但眼睛有适应黑暗的能力，对着变暗的取景器看几秒钟，就适应了（瞳孔变大）。也可以装个目镜罩（eyecup），或用衣服盖住头和相机，减少进入取景器的杂光，也可以帮助看得清楚。

原文阅读

10 Practical uses for the Depth of Field Preview Button

Text and photography copyright © Rod Barbee. All rights reserved.

Editor's note - This article first appeared in the Sept. 2001 issue of Outdoor Photographer magazine.

One of the most overlooked tools at the photographer’s disposal is the depth of field preview button. I’ve been teaching photography for a number of years and what I usually hear is something like this: “This button just makes everything darker, it’s useless.” I’ve heard this many times from beginning as well as experienced photographers. The first part of what they say is true but the second isn’t; they’ve yet to discover the many uses of the depth of field preview button.

Not all cameras have one and if you’re new to photography, you may not even know what it is. When you look through your camera, what you see isn’t necessarily what you’ll get. As you probably know, as you look through the viewfinder of an SLR camera, you’re looking through the lens at its widest aperture setting, which may or may not be the aperture chosen for your picture. By pushing the preview button, the lens closes down to the shooting aperture. This way the photographer can view the depth of field of the final image. However, when the aperture closes down, less light comes through the lens and the viewfinder can get very dark. This makes details hard to see, especially if a small aperture opening is used. This is also where many photographers give up on using the DOF preview button. As you’ll see, you don’t always have to use the DOF preview button at f/22 nor do you always need to see every detail.

Beyond making “everything darker”, the DOF preview button has several practical uses;

Note - thumbnails are links to larger images with captions.

1. Detecting hotspots. When doing close-up and portrait photography, one of the prime concerns is the background. Bright spots are very distracting but they can go unseen when viewing the image while the lens is wide open. When you push the preview button, the viewfinder will get dark. But that’s OK; it makes it easier to detect any distracting hot spots that may end up in the final image.
2. Detecting dark spots. Just as hot spots can be distracting, dark spots, especially in the foreground of landscape images, can be just as distracting. To get the maximum depth of field, many landscape photos are made at an aperture of f/16 or f/22. When previewing with the lens set to f/22, the viewfinder will get dark, maybe too dark to tell what’s really going on. By previewing at f/8 or so, the lighter areas stay relatively light, but the darker areas become readily apparent. You’ll be able to see where shadow areas in your foreground might be too dark for the film or sensor to record. You can then decide if fill-flash, a reflector, or perhaps a different composition is appropriate to eliminate these shadow areas.
3. Detecting unwanted elements (or “Intruder alert”). How many times has your film come back from the lab with some out of focus branch or blade of grass sticking part way into your frame? You can stop this from happening by using your preview button before you trip the shutter. When you preview the depth of field, run your eyes around the edges of the frame and look for intruders trying to poke their way in and ruin your image. Look for intruders trying to sneak into your backgrounds. Remember, you may not notice these things while looking through the lens while it’s wide open because of the shallow depth of field. Use your DOF preview button and catch these intruders. Again, the viewfinder will get dark but you’re not trying to see all the detail in the image, you’re just looking for intruders. Sure, you may be able to clone them out later, but it takes less time to remove the intruder while in the field than it does to clone it out in Photoshop.
4. Finding the best background. Especially when doing close-up and portrait photography, hold down the preview button while changing the aperture until you get the best combination/compromise for a pleasing background and the focus needed for your subject.
5. Placing a graduated neutral density filter. Grad ND filters are essential to successful landscape photography but they need correct placement so that their use isn’t obvious. Hold down the preview button while moving the filter in front of your lens. This will make placement easier and more precise.
6. Optimize the range of focus for your main subject. Whether photographing wildlife or wildflowers you can use the preview button to see if everything you need to have in focus will be in focus, from the stamens and stigmas on flowers to the nose and eye of an animal.
7. Visualize underexposure. By looking at a scene through your viewfinder and setting the lens one or two stops smaller than the widest aperture on your lens, you can use the DOF preview button to get an idea of what an image might look like if you underexpose it a little. You may want to underexpose to create a different mood and this will help you see what that might look like. Conversely, you can set your lens to f/8 or so and push the button, let your eyes adjust, and then start opening the lens to get an idea of what overexposing a scene might look like.
8. Detecting lens flare problems. This is similar to detecting intruders. When photographing into the sun, especially when the sun is low in the sky, you run the risk of lens flare, which results from direct light bouncing around the elements of your lens. By previewing the scene, lens flare will become more apparent. You can then decide if you want to do something to mitigate it, like using a wider aperture, changing composition or shading your lens. You may find that you like the effect. You can also preview the “star burst” effect that results from using small aperture openings with wide angle lenses while including the sun in the image.
9. Detecting vignetting from stacking filters and lens hoods. With wide-angle lenses, there’s a greater risk of vignetting, or cutting off the corners, while using filters, especially if you stack filters or put your lens hood on the filter. By previewing and paying special attention to the corners of the frames, you’ll be able to know ahead of time if there will be a problem. (Be aware that most viewfinders only show about 90-95% of what will end up on film. If in doubt, be sure to run tests with all combinations of filters you may wish to use. Photograph a blank wall or the blank sky and evaluate the resulting images.)
10. Use the DOF preview button as a compositional aid. This may just be the most valuable tip of the bunch. More and more I find myself using the DOF preview button to aid in my compositions.

As a compositional aid, and regardless of what f/stop you use for the final image, set your aperture to f/16 or f/22. This makes the dark areas darker and yet the bright areas stay relatively bright. It also eliminates much of the detail, leaving behind only forms and shapes, light and shadow. In other words, it reduces everything to graphics. You can then arrange these graphics in a pleasing manner. This works especially well when you’re working with light and shadow or moving water. It’s hard to predict what moving water will look like in the final image, especially if you’re shooting film. This technique gives you a better idea of how the white areas of moving water will flow through the image, allowing you to arrange lines, curves and other graphic elements.

As far as dealing with a darkened viewfinder, letting your eye adjust is probably the first thing you should try. There are several techniques to help with this. First, don’t start with your lens at its smallest opening; hold the preview button down and close down your lens gradually. On newer cameras, this may not be possible. Second, use an eyecup to help cut out stray light. Third, throw a jacket or dark cloth over your head like the users of view cameras do.

The depth of field preview button can do much more than just strain your eyes. Being a photographer who prefers to get the image the best I can while in the field, I find the DOF preview button an essential tool and a must have for any camera body I own. If your camera has this valuable feature, be sure to use it to its fullest.

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