sl7346 wrote:
Hi...everyone,
I still don't understand the Focal Lenght for crop sensor cameras. I have a canon 70d and use the 50mm lens. So, what is my focal lenght or the shutter speed for proper exposure for this lens?
Thanks advance for reply......
For a 50mm lens without image stabilization, while hand holding the camera in order to avoid possible "camera shake blur" in images, you should use a shutter speed 1/80 or faster. Note that this is just a general rule and an estimate.... some people can get away hand holding at slower shutter speeds... others require faster. Other factors also can affect this... for example the Canon 90D has same APS-C "crop" size sensor as your camera (70D, 20MP), but with a much higher 32.5MP resolution. Higher resolution makes it more difficult to get a steady shot free of shake-blur. So a 90D user might use an even fast shutter speed, just as a precaution. There's also a lot of variability among users and situations. Some people are just more skillful or more careful than others. Some shooting situations are very fast and frantic, while others are more relaxed allowing time to think and take care with your shots.
The "rule" here is that your shutter speed should be 1/focal length.... although in this case we adjusted for the crop factor. This "rule" was originally for 35mm film cameras, which are like "full frame"... not cropped. So for any of your lenses you should use shutter speed that's 1/focal length x 1.6. A 100mm lens would theoretically need a 1/160 shutter speed... a 200mm lens would call for 1/320... etc.
However, many lenses today have image stabilization. Canon pioneered putting this in lenses (optical) back in the 1990s and today roughly half their lenses have "IS". Other lens manufacturers have added it too... Sigma calls it "OS" (optical stabilization)... Tamron calls it "VC" (vibration control).... Nikon calls it "VR" (vibration reduction).... Sony calls it "OSS"... etc. It does the same thing, regardless of the name.
Image stabilization counteracts minor movements to reduce or prevent shake-blur in images... allowing users to get a high percentage of sharp shots even at shutter speeds slower than they can normally hand hold. This "assistance" is stated in stops.... older lenses gave 2 to 3 stops of assistance, while more modern ones give 3 to 4 stops worth. What this means is that a 100mm lens where you need 1/160 to have high likelihood of sharp shots without IS, two stops of assistance from IS means you can probably get by with 1/40... 3 stops means 1/20 is usable with that lens... 4 stops would mean you could expect pretty good success even at 1/10.
All this gives you some idea how slow you can go with your shutter and any given lens before you may need a tripod or at least a monopod to helps steady the gear.
Finally, shake-blur from too slow shutter speed isn't your only concern. If you're shooting moving subjects, depending upon their speed, distance from you and the direction they're moving you may need a faster shutter speed to freeze "subject motion blur". A person walking and some distance away might only need that 1/80 shutter speed to freeze most of their movement, but may not be sufficient to freeze some of the faster moving parts of their body like their feet. A hummingbird, on the other hand, requires extremely fast shutter speeds to fully freeze the movement of wings that beat 500 times per second!
I know this is a lot of info and you can hardly be expected to "do the math" for all your lenses and apply the 1/focal length "rule" all the time. One of the great things about digital photography is that you can get pretty immediate feedback... you can review an image right after you take it. Do that and just keep this info handy for answers when you see a problem in your image. Experiment with the lenses in your kit, explore your own limits and learn good techniques.
None of the above is about "exposure", per se. Shutter speed is just one part of exposure. Whatever shutter speed you set, to make a "correct" exposure you will need to adjust the lens aperture size and sensitivity of your camera's sensor (ISO) accordingly. Exactly how everything is set will depend upon the ambient light conditions and subject tonality.... as well the depth of field you desire in the image (lens aperture size or "f-stop") and tolerance for digital noise in the image (ISO).