A good lens will focus about a foot away. Sometimes, though, that isn’t enough. If you’re shooting insects, product shots of small objects, etc., you’ll need to focus a lot closer.
For this reason we have adapters that extend the distance between the sensor and the back element of the lens. These are called Extension Tubes.
Extension tubes come in various lengths, depending on the effect you want. They are also designed to be stacked together for greater length. These tubes are a cheap way to make a lens a macro lens.
When in doubt, buy the extension tube made by the manufacturer of the lens you are using. Canon, Nikon and Sony all supply extension tubes for their mounts, and there’s usually no reason to opt for third-party systems. Third-party manufacturers known to make cheap but reliable extension tubes are Kenko, Fotodiox, and Zeikos.
If you’re doing a lot of macro work, then it is much better to get a macro lens instead. Increasing the distance after a certain point reduces the light falling on the sensor. Also remember what I said about the image circle.
Bottom line: Don’t go crazy with extension tubes. Use them sparingly, and with full intent.
I don’t consider any lens with a maximum f-number greater than 2.8 (f/2.8) a professional lens for video.
Whoa! It’s just my preference. I’m sure there are many who disagree with me. I’m not talking about image quality!
There might a shot where you’re absolutely certain you’ll have enough light to shoot at f/5.6, say, but due to unforeseen factors you are forced to wait till dusk to shoot the same scene. If you brought along a lens with a maximum aperture of f/4, and if that isn’t enough for the light you have available, then you’re in trouble.
Similar problems can happen in controlled lighting scenarios as well. Contrary to popular belief, you can exhaust all your lights in a truck and still have insufficient light for the exposure you want.
Tough luck? No, plain unprofessionalism. Ever heard the phrase: Failing to plan is planning to fail?
I can understand cash-starved indie filmmakers making such compromises. I’ve done it, too. But a professional can never afford to make such a compromise.
It doesn’t mean the other lenses can’t get the job done, if you have enough light available. A professional has to take into consideration any contingency. What do you think Greg Toland and Orson Welles would have done if they hadn’t been able to achieve deep photography? Would they have abandoned Citizen Kane? Of course not, they would have shot at a larger aperture with shallow DOF.
Video production is too expensive to subject it to whims and ‘lucky breaks’. Directors and Cinematographers don’t have that kind of luxury.
Many modern-day sensors are rated at about ISO 800. Some DSLRs are rated at ISO 100 or ISO 200, and will behave differently when compared to the Arri Alexa, which is rated at ISO 800. Here’s a list of cameras and their full range of ISOs:
We all pine for the day when we’ll have full control over our depth of field, and don’t have to stop up or stop down due to lighting issues.
However, at the time of this writing, ISO 1600 is still the limit for usable professional videography. This makes having a lens with an f-stop of at least f/2.8 almost mandatory for professionals.
If exposures and f-stops and ISOs don’t make sense to you, now would be a good time to read Driving Miss Digital.