Related Items
Equipment
Musings
Improving Manual Focus
by Russell J.T. DyerIf you’re considering purchasing a lens which can be focused manually only (e.g., Zeiss-Ikon 21mm f/2.8 ZE lens) or if you want to improve your ability to focus manually with any lens, you may be able to enhance your camera to help you with manual focusing. You may be able to purchase a more precise focusing screen for your SLR camera. For the examples in this article, I used a Canon EOS 5D, Mark II digital SLR camera. Canon makes three focusing screens for this camera model: the standard focusing screen (Eg-A); one with grid lines (Eg-D) to assist in aligning the frame of a shot; and a third one for super precision focusing (Eg-S). The camera is shipped with the Eg-A screen. For testing and comparison, I purchased the Canon Eg-S focusing screen. The cost is only about £30 in England and $40 in the U.S. There are other companies that may be found on the web which make focusing screens for Canon and other cameras.
Canon Focusing Screen Comparison
This is the set of the shots taken for testing the two Canon focusing screens in this article.
Switching & Other Considerations
Although you may be timid about changing the focusing screen in your camera on your own, after you’ve done it once you’ll realized that it’s simple and that there’s very little about which to be concerned. When installing it, though, do not touch the old screen or the new one with your fingers. If you have clean cotton gloves, wear them while changing the screens. Even a minor touch of your finger will cause oil to get on the screen. You can’t easily wipe off a finger print smudge without damaging the screen while wiping it clean. Any oil or faint scratches will disrupt the camera’s ability to focus. If you do wipe the screen with a cloth, you will easily scratch it and ruin it, permanently. Then you’ll have to buy a new screen. To be safe, you have to use the tongs provided with the new focusing screen to hold each screen as you remove or install them. You’ll have to rely on the instructions that come with the focusing screen for specific information on changing the screen in your particular camera model.
Even if you buy a precision focusing screen with the intention of always using it, keep your old one. You may want to put it back one day—that very likely will happen soon after first using it. Images in the view finder will be darker with the Eg-S focusing screen. Therefore, you may only want to use it when you know you will be using your manually focused lenses and then only when the aperture is near its maximum. So, keep the old focusing screen and be careful with it.
Since the precision focusing screen causes the viewfinder to be so dark, you will need to change one of the custom functions in the camera’s menu so as to tell the camera that you’re using a different screen. This is necessary so that it can adjust accordingly the light meter readings through the focusing screen. Again, refer to the instructions that comes with the new focusing screen or the manual for your camera.
With the Canon focusing screens, there aren’t two half-circles that one lines up and bring into focus as you might think. This was standard in SLR film cameras long ago, but no longer used by Canon. There is very little noticable difference between standard and the precise screens when you first compare them. In fact, at first you may think that you were inadvertently sold an Eg-A model. The difference is in the finish: the precision focusing screen is matted and more opaque. When side by side, you can see that the Eg-A is much clearer.
Understanding & Using
Because of the opaque finish, although it still allows the focus points provided with the camera to be used for auto-focusing, only objects which are very much in focus appear clear in the view finder. The standard Eg-A focusing screen can deceive you into thinking objects are in focus when they aren’t. The engineering concept is that most of the time you will use a greater depth of field, an aperture of f/8. However, when you use a lens with the aperture set to greater than f/4, the clear focusing screen gives no indication that objects are out of focus, at least none which are easily discernible. The tight range of clarity of the Eg-S focusing screen, though, will indicate what’s in focus and what’s out of focus. This is what it’s best at indicating, which is especially useful when focusing manually. Conversely, if you use the Eg-S focusing screen with an aperture of f/8 or smaller, you may think that objects which are within the depth of field are very much out of focus when they’re not. So, use the proper focusing screen for your needs. Incidentally, a precise focusing screen has no effect on the automatic focus feature. It doesn’t diminish its abilities or improve them. It’s only for improving focusing using your eyes.
As suggested just above, you may want to switch the focusing screen back to the Eg-A when you’re not using a lens with a large aperture. Even though it’s easy to swap them, you shouldn’t do this when you’re out and about. If you drop the screen, you’ll probably ruin it.
Canon Eg-A Focusing Screen
This shot was taken with the Canon Eg-A, the standard focusing screen in the Canon EOS 5D, Mark II camera and a Canon EOS 50mm f/1.2 L lens.
When you first attempt to use the precision focusing screen, you may question its value. You may not see a difference even. To illustrate the potential difference, I set up my studio lights and all to take a couple of pictures to compare the two focusing screens. Above is a shot showing the basic set up. It’s fairly uncropped, so that you can see what I photographed. It’s of a Canon Canonet rangefinder camera and a folded map of Milan. These shots were taken with a Canon EOS 5D, Mark II camera and a Canon EOS 50mm f/1.2 L lens on a tripod with two external flashes—the same set up for both focusing screens. The center focus for each shot is on the letter C of the word Canon on the subject.
Eg-A Focusing Screen
On the right is basically the same shot as above, but I’ve cropped on and enlarged the image by the left end of the camera—or rather, I didn’t reduce this image as I did above. This image was taken with the lens set to manual focus. When using manual focus, if you hold the shutter release button half way, or if you hold down the AF button on a Canon EOS camera as you turn the focusing ring on the lens, the focus light will blink in the bottom right corner of the viewfinder. If you have beeping enabled, the camera will also beep to indicate that the image is in focus. Plus, the focusing point box will light red. The camera doesn’t focus for you when the lens is set to Manual focusing, but it will let you know when it thinks you’re in focus. Incidentally, this feature will function with Zeiss-Ikon ZE manually focused lenses on a Canon EOS camera. You can click on these photos to see a larger view of them, so as to better see the clarity differences.
Canon Eg-S Focusing Screen
This shot was taken with the Canon Eg-S, the standard focusing screen in the Canon EOS 5D, Mark II camera and a Canon EOS 50mm f/1.2 L lens.
Eg-S Focusing Screen
The shot on the left is the same close-up, but with the Eg-S super precision focusing screen. Notice that the Canon name and other text plates aren’t as hazy. Look at the text on the map, too. It’s much clearer in this shot that the previous one. Also, the ridges and crevasses of the simulated leather covering are clearer. Although the focus confirmations (the beeping and all) are helpful for getting the image focused, they aren’t necessarily the most focused position of the lens. At first I tried waiting for the beep and then taking the shot. However, with the Eg-S focusing screen I noticed that the camera beeped when I was well in focus, but that I was able to do better when I didn’t rely on the focus confirmation. I did better when I used my eyes (even with my glasses).
Summary
As you can see, a greater level of precision is possible when focusing the lens, manually. Canon’s automatic focusing is very precise, but when you want shots which are even more in focus, a precision focusing screen and some practice and patience can improve the results. The lens, particularly a top of the line lens like one of the Canon L-series lenses, can often be fine tuned even more than what is capable by the camera’s on-board focusing system. However, without a more precise focusing screen, it will be difficult for most photographers to out perform the camera’s auto-focus system. A precision focusing screen is particularly useful if you use a lens that doesn’t have an automatic focus feature. It may be necessary with such lenses.
Copyright 2010 FotoCapito & Russell J.T. Dyer. All Rights Reserved.