We all use lenses every day. But most of us don’t really know much about them. In fact, most of us don’t even know what it is we don’t know. Here are a few facts that you may find interesting, useful, or amusing. They probably won’t make you a better photographer, but may help you avoid some bad shots or other frustrations.
Wide-Aperture Lenses May Have Focus Shift
A lot of you already know this one, but if you don’t, read on! Wide-aperture lenses, particularly prime lenses of f/1.4 or f/1.8 may focus very accurately wide open, but if you stop down slightly, they front−or back focus. It’s not something that can be fixed, it’s the way the lens was designed—the designers accepted the focus shift as a trade-off for more sharpness or less distortion. By f/4 or so, the depth of field of has increased enough that the focus shift isn’t noticeable. The end result then is the lens focuses fine wide open, and fine again at f/2.8 or f/4, but in-between doesn’t focus correctly.
Unfortunately you won’t find “has focus shift” in the manufacturer’s description. If you think your lens may have it, take a series of images focused on the same object, stopping the lens down a bit with each. If the lens has focus shift, you will notice the focus has ‘shifted’ in front of, or in back of, the object as you stop down. The experiment will also show you how far you have to stop down to overcome the focus shift (and this may be different at infinity or close up). The simplest work around is to shoot the lens either wide open or at the aperture at which focus shift is no longer apparent.
Floating Elements Don’t Float
Most lenses have poor performance at their closest focusing distance. Center sharpness may be good, but the aberrations that cause edge and corner softness increase when you’re shooting close-ups. Floating elements are lens elements outside of the primary focus group that move when the lens is focused on a close object, correcting aberrations and improving close-up performance. If you like close up work a lens with a floating element is usually worth the extra cost.
Fixed Apertures Aren’t Always Fixed
At least they aren’t with macro lenses. Focusing on very close objects reduces the effective aperture of the lens, so even if the lens is theoretically f/2.8, it may not be when you focus very closely. This is a bit of a simplification—the size of the aperture hasn’t really changed but the amount of light reaching the sensor is reduced. Your camera may, or may not, tell you about it. Nikon cameras, for example, will show the ‘effective aperture’ so your f/2.8 may actually show as f/4 when shooting at macro distances. Canon cameras will still read f/2.8, but if you look carefully you’ll notice shutter speeds or ISO is being adjusted to compensate for the reduced ‘effective aperture.’
Almost Every Lens is Sharper Stopped Down Just a Bit
Almost everyone knows that most lenses are at their sharpest with the aperture about two stops less than wide open. But few people realize how much sharper a wide aperture lens gets when stopped down just a bit. For example, the MTF 50 (a measurement of sharpness) for the Canon 35mm f/1.4 lens at f/1.4 is 625/550 line pairs /image height (first number is center MTF, second is average MTF across the whole lens). If you stop down to f/2.8 it increases to 905/760. That’s roughly a 40% increase in resolution. Obviously there are times that a narrow depth of field or getting the most light you possibly can into the camera are far more important than the sharpness of the image. But when those aren’t critical, stopping down, even just 1⁄2 or one stop, can really improve sharpness. There are limits, though, because stopping down too far causes diffraction softening. Most cameras with high pixel density are showing some diffraction softening by f/8, so stopping down an f/5.6 lens to f/11 may actually make it softer.
Variable Apertures Vary More Than You Might Expect
You know your f/3.5 to f/5.6 zoom goes from f/3.5 at the wide end to f/5.6 at the long end. But do you know at what focal length the aperture actually changes? You can look at the aperture readout on the LCD or in the viewfinder when you’re shooting to find out. Knowing that information before you shoot can help you decide what lens to mount on your camera, or sometimes even which lens to buy.
Here’s one example. A lot of people upgraded from the Sigma 50-500 f/4-6.3 HSM lens to the new Sigma 50-500 f/4.5−6.3 OS HSM lens. Just looking at the lens name tells you that its maximum aperture of the new version at 50mm is a bit narrower than the old version (f/4.5 instead of f/4). But unless you checked you wouldn’t realize that the old lens was f/5.6 all the way out to 420mm, while the new lens is at f/6.3 at 220mm. That may not make a difference to you, but it can for some people whose cameras don’t autofocus as accurately at f/6.3.
Knowing where the aperture changes can also make a difference with selecting a lens from your camera bag. Let’s say I want to take a shot at about 70mm focal length. I have a Canon 15-85 IS f/3.5-5.6 zoom, and also a Canon 70-300 IS f/4-5.6. The 15-85 is at f/5.6 at 70mm, while the 70-300 is at f/4. If I need that extra stop of light, the 70-300 may be the better choice.
Third-Party Lenses are Different in Different Mounts
Every camera manufacturer has a set of electronic algorithms built into their cameras that communicates with electronic chips in the lens during autofocus. Third-party lens makers have to reverse engineer these algorithms and then make different electronic chips to communicate with each brand of camera.
Sometimes the lens maker does a better job reverse engineering one type of camera than another. For example, the Sigma 120-300 f/2.8 wouldn’t autofocus properly with the Nikon D3x when that camera first came out. The Tamron 17-50 VC and 60 Macro lenses could only use the center autofocus point on Canon 50D and 60D cameras, although they worked fine with other cameras. Usually though, the difference is not this dramatic. A given third party lens may autofocus very accurately on one brand of camera, but have a tendency to hunt or miss focus on another. My point is, when you consider a third party lens, make sure you ask someone who shoots it on your brand of camera. Autofocus accuracy and speed may be different with dif ferent cameras.
That Rattle in Your Lens Means You Didn’t Turn the Stabilizer Off
If your Image Stabilized (Vibration Reduction, etc.) lens rattles, try turning the IS system off before turning your camera off. When active, the stabilizing element’s position is controlled by electromagnets that are active only when the lens is receiving voltage from the camera.
Most, but not all, stabilizing systems have a mechanism to lock the element in place when the IS system is turned off. If you remove the lens from the camera without doing so, the element is no longer held by the electro- magnets and rattles loosely. I don’t know if the rattling IS element causes any real problems, but I can’t think of anything good that’s going to happen because that element is free to bounce around in there. So turn your lens stabilizer off before removing your IS, VR, VC, Mega OIS (the terms differ with each manufacturer).
Your Lens has Dust in It
If you haven’t seen dust in your lens, you just haven’t looked carefully enough. Does it matter? Not unless you have so much dust that it affects light transmission, which takes a lot of dust. If you shoot through a chain link fence a few inches in front of your lens, your pictures barely show the fence. It’s too far out of the plane of focus. For the same reason, you aren’t going to see a dust speck inside the lens itself. The very rare exceptions involve large chunks of dust, usually with macro or wide-angle lenses.
Focal Length Numbers Aren’t Completely Accurate
The focal lengths printed on lenses are approximations. It’s not uncom- mon for there to be a variation of 5% between the focal length stated on the barrel and the actual focal length of the lens. So a 50-500mm zoom may actually be 53-475mm. Its not a big deal but it is noticeable sometimes. If you want to demonstrate it, take a shot at 70mm with your 24-70 lens, then change lenses and take another shot with your 70- 200 or 70-300 at 70mm. The difference will be noticeable, although it may or may not be important to you. What might be important is the focal-length change that occurs with some newer zoom lenses. When focused at infinity they are (nearly) the focal length they should be, but when focusing close the maximum focal length is quite a bit less than that. For example, one 70-200 f/2.8 lens is 200mm when focused at infinity, but only reaches to 140mm when focused on closer objects.
Does it Matter?
You certainly can take good photographs without knowing what’s going on inside your lenses. But a little knowledge may prevent panic when you see some dust or hear a rattle inside your lens, explain why they sometimes behave the way the do, or simply keep you from feeling left out when other photographers start tossing technical terms around. It also just might help you take better photographs.