The depth-of-field preview button
Video: The depth-of-field preview buttonHaving all this depth of field theory at your disposal is great, but unless you have got a really good eye for distance, it can be hard to judge what might be in focus when you are shooting with deep depth of field. This means it can be hard to determine when you have chosen an aperture that's small enough to get you the deep depth of field that you want. Remember too, that ideally you don't want to close your aperture down any further than you have to, because if you close down, sometimes refer to stopping down, if you close down too far, your image might suffer a sharpness loss due to diffraction artifacts inside your lens.
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Arriving at the best exposure for a photo is part science and part art. In Foundations of Photography: Exposure, Ben Long helps photographers expand their artistic options by giving them a deep understanding of shutter speed, aperture, ISO, and all other critical exposure practices. This course covers the basic exposure controls provided by all digital SLR cameras, as well as most advanced point-and-shoot models. Learn how to master a camera's metering modes, how to use exposure compensation and bracketing, and much more. By the end of the course, you'll know how to develop an "exposure strategy" that will allow you to effectively employ your exposure knowledge in any shooting situation.
- What is exposure?
- Exploring camera modes
- Light metering
- Shooting sharp images
- Controlling shutter speed
- Understanding f-stops
- Controlling motion
- Working with a shallow depth of field
- Measuring aperture
- Shooting in low light conditions
- Performing manual light balance
- Working with the histogram
- Using fill flash
- Understanding reciprocity
The depth-of-field preview button
Having all this depth of field theory at your disposal is great, but unless you have got a really good eye for distance, it can be hard to judge what might be in focus when you are shooting with deep depth of field. This means it can be hard to determine when you have chosen an aperture that's small enough to get you the deep depth of field that you want. Remember too, that ideally you don't want to close your aperture down any further than you have to, because if you close down, sometimes refer to stopping down, if you close down too far, your image might suffer a sharpness loss due to diffraction artifacts inside your lens.
You can of course shoot a picture and then look at it on the back of the screen to try to determine your depth of field. But as you may have already noticed the screen on your camera's LCD tends to render things in focus even when they are little soft. This is simply because it's so small. So images sharpen up when reduced to fit on the screen. Even if you zoom in, you won't necessarily get an accurate view of focus. Also, if you are shooting in bright daylight, you might have trouble seeing your screen at all. Another option, of course, is to wheel around the giant plasma monitor though, that's not necessarily practical because these things are like grocery cards: there is always one wheel that's just a drag.
A better choice is to use your camera's depth of field preview button if it has one. On this camera, it's a button that's right under here. Now here is how it works. If you remember back to the Aperture video, we showed you a lens, and you saw the iris inside, closing. Now when your lens is on your camera, the iris is always open all the way, because if it's closed down at all, when you look through the viewfinder, there won't be enough light, and you won't be able to see. So the camera leaves the iris opened all the way all the time, to let as much light as possible through the viewfinder.
You dial in an Aperture setting, it still doesn't change the size of the iris. It's not until you press the shutter button to take the shot that it closes the iris down to your chosen setting, and takes the picture. So when I am looking through here, even if I dialed in f22, which should give me a real depth of field, what I am getting is a wide-open aperture. So I am always having small or shallow depth of field. In fact, I am in an aperture priority mode, I am just going to dial right on up to f16, something that may not be the best thing in terms of overall sharpness of the image, but we are going to risk it anyway.
So I am at f16. That's a tiny, little aperture. That should be very deep depth of field. Everything in that shot should be in focus. But as I looked through the view- finder--and what we are seeing on the screen here is what I would see in my optical viewfinder-- as I look through the viewfinder, only the middle camera is in focus. Now remember, I have focused on the middle camera, so that's where my depth of field is centered around. So plainly, it's looking like my depth of field is not deep enough to get all three cameras, even though I am at f16. Now if I press the depth of field preview button, what's going to happen is the iris is going to close down to my chosen setting, which in this case is f16, and when I do that, pay attention to the front or rear camera.
When I press the depth of field button now, they snap into focus. Because when I press the depth of field button, my iris closes down and I get to see the true depth of field in my image. I am going to let go and I pop back up. Here it is again, and here I am back out. Let's go open to our wider aperture and see what happens. I am going to go to f11, hit the depth of field Preview button, take a look at the first camera. It sharper, but it's not as sharp as it was at f 16. So I am seeing a change in my depth of field as I press the depth of field preview button.
Now what we are seeing here in live view is not entirely an accurate reproduction of what would you see if you were looking through your optical view- finder, because when you look through the optical viewfinder, when that iris closes down, as you press the depth of field preview button, your viewfinder is actually going to get darker, because that's stopping down. It's going to cut out a lot of light. So if you are out in the sunlight, a lot of times what happens is you press the deep depth of field or the depth of field preview button, and you go, "Well, now I can't see." Keep looking through the viewfinder. Try to block out the area around the viewfinder. In other words, you are trying to get your eye to adjust to the new darkness.
Wait a minute, and you should then be able to see depth of field. Then you can take your shot. It's still not quite as accurate a representation as dragging around a giant plasma display, but it's much easier to do. In the end, if your depth of field preview button is too hard to see in your current live, or your camera doesn't have one, then your best bet will be to do what's called bracketing. Shoot at your chosen small exposure, then try shooting at a smaller one, just in case. And also, remember what you saw earlier about depth of field and focus.
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