Tilt/Shift Lenses or
Everything you ever wanted to know about some guy named Scheimpflug but were afraid to ask.
Here's a photo made with the Canon 5D and the Canon 35mm Tilt/Shift lens. The lens has been modified to fit the 5D body. It was made for FD cameras about 30 years ago, long before Canon made EOS cameras.
This is a bicycle wheel sitting in a wheel building stand. There's no tire on the wheel. You can see on the right side of the image that the rim is in sharp focus. It could be sharper but I didn't process the image at all in Photoshop. All I did was adjust the levels a bit and reduce the pixel count. That right edge of the rim is about 7.5 inches from the front element of the lens. In other words it's 7.5 inches from the glass. The far left side of the image also shows the rim in almost sharp focus. OK so I missed a bit but you can easily read the brand name on the rim's label. That left side of the rim is 29 inches from the front of the lens.
Notice the hub in the left center of the image. Of course it's right in the center of the wheel. The spokes go to each side of the hub. The spokes attach to the part of the hub called the flanges. There's a flange on the right and a flange on the left. This image was shot at f/2.8, the widest aperture of this particular lens. If you look carefully you'll see that the spokes aren't in sharp focus at the flanges.
Also, if you look up on the shelf behind the wheel you'll see that the shelf on the left is in sharp focus but as you look to the right the shelf gets progressively out of focus.
Now let's look at another image.
I didn't change the position of the camera for this shot. All I changed was the exposure. Now the aperture is set to f/8. Notice that the rim on the right side as well as on the left side is in sharp focus. In fact it's slightly sharper than in the image above. That's because the lens can resolve a bit more detail at a smaller aperture than it can at a larger aperture. But notice something else. Notice the hub, and the spokes as they connect to the hub. They are now much sharper. And notice the shelf behind the wheel. It's now sharper over to the right than it is in the photo above. At f/8 the depth of field is much wider than it is at f/2.8.
But where normally we think of depth of field giving us sharper photos from close to far away, here we're getting a sharper photo from left to right, sort of. Why?
Oh No! Somebody bent my lens! Waaaaah!
It's because the lens has been tilted to the right. The plane of sharp focus is not parallel to the sensor plane, but it's now angled so that it passes from just to the right of the lens and extends forward and to the left of the lens, moving away from the lens. If you were looking down at the camera body (like in that picture right there) the lens, and the invisible plane of focus, you would see the camera body facing forward, the lens angled to the right, and the focus plane rotated around to clockwise. The lens was angled exactly the same in both images. All that changed was the f stop and shutter speed to keep the total exposure the same. As you look at my camera setup from above, the plane of sharp focus is passing through the bicycle wheel. In fact the plane of the bicycle wheel and the plane of sharp focus are one and the same. So that plane of sharp focus extends along the wheel and passes the camera about 5 inches from the hand grip on the right side!
At f/2.8, the plane of sharp focus is very narrow. Those hub flanges are exactly 60mm (just over two inches) apart. And in the first image above, they are not sharp. So the plane of sharp focus is narrower than 60mm at that distance, about 18 inches away from the lens. But at f/8, the plane of sharp focus is wider and you can now see the flanges much more clearly.
But the key issue is revealed in the top photo. Here we have a 35mm lens set at f/2.8, an aperture which doesn't give you much depth of field. And yet we have something 7.5 inches from the lens as well as something 29 inches from the lens in sharp focus! And if we set up a similar shot outdoors we could have something 8 inches or so from the lens in focus, as well as the moon, and we don't have to be Neil Armstrong to do it. All we have to do is place the camera and lens such that our subject 8 inches away and the moon are on the same geometric plane. And that plane needs to bisect two other planes, the sensor plane, and the plane defining the optical center of the lens. To better understand the principles behind all this, do a google search on the "Scheimpflug rule". You can also go to the Large Format Photography web site where you'll find a wealth of information. Don't be put off by the large format aspect. The principles are all the same, regardless of the format you're shooting. Large format cameras typically have more adjustments available than my Canon, and you've got a nice big ground g;lass to focus on, but the laws of physics are identical.
For your information, the amount of tilt you see in the photo from above is the maximum amount of tilt available with this lens. I'm showing you the most extreme example of focal plane tilt possible with this lens. Normally you wouldn't use this much tilt in order to make your landscape or architectural image with the maximum about of sharp detail possible. That's simply because you normally don't need to get something 8 inches from the lens in focus at the same time something at infinity is also in focus. But as you can see it is certainly possible to do just that.
Canon now makes three lenses that tilt and shift. They are designated TS-E and there are three focal lengths, 24mm, 45mm and 90mm. And of course the old 35mm can be converted to EF mount. And just because it's old that doesn't mean it's no good. My own tests show this lens is every bit as good as the new ones. With this lens you adjust aperture with a ring on the lens rather than using a dial on the camera body as you do with the new TS-E lenses and the other EOS system lenses. The filter size is 58mm.
Here are a few more images that may help show where your depth of field is when you tilt the lens.
Here the front of the lens is about two feet from the near end of the yard stick and about 14 inches above the bench surface. It's the same 35mm T/S lens and it's still tilted all the way to the right. Aperture is f/2.8. As you can see the plane of focus is right down the yardstick. The entire yardstick is in sharp focus. The wrench with the dark red handle is exactly 10 inches long. If you want to see this image at full size, click here. In the next image, below, everything's the same, except I've stopped down to f/8.
Now at f/8 the rulers at both ends show a lot more depth of field. Much more of each ruler is in focus. And the wrench in the middle is much more in focus over its length. If you want to see this image at its full size, click here. By the way, these last two images are pretty noisy. That's because I've been doing some testing of the 5D at ISO3200 and had forgotten. So the full size images show lots of noise. But it's not too bad, considering the speed. The Canon 5D produces extremely low noise images at high ISOs. This image quality would have been impossible with film at this ISO speed
I'll have more on this page shortly to show how the shift function works, along with some real world images, in case you aren't enthralled by my ruler pics.
And here's an interesting thing, I think. Go back up to the image looking down at the camera. See how the top of the rim is in sharp focus along with my feet, and along with everything else in the image? How'd I do that? Huh?
I took that picture with a Canon Powershot Pro 1, a camera with a tiny sensor, and an extremely short focal length lens. The result is that the camera has extremely wide depth of field. That can be a very good thing. The main limitation to cameras like the Pro 1 and all the little shirt-pocket digicams is that the tiny sensor produces very noisy images at relatively low ISOs. So while this shot looks fine, at ISO 200, the Pro 1 has more noise than my 5D camera does at ISO 3200!