sl7346 Loc: Minnesota

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......

Orphoto Loc: Oregon

Your 50mm performs the same as an 80mm lense would on a full frame body. All exposure settings are unaffected. No one here can tell you the "best" shutter speed for any given shot. The mild increase in magnification does mean a somewhat faster speed is needed to overcome any hand movement in holding it still.

Longshadow Loc: Audubon, PA, United States

mwsilvers Loc: Central New Jersey

Focal length of a lens does not change. Your 50 mm lens is still 50 mm no matter what type of camera it's mounted on. However, when you put that lens on a crop sensor camera which has a smaller sensor area, as a result of the smaller lens image circle it gives you the impression that your camera has zoomed in a bit compared to a full-frame camera and the focal length has changed. But in fact it is not changed. 50 mm is always 50 mm. What has happened is that the smaller sensor has changed the angle of view and is giving you a full-frame cameras view equivalent to 80 mm. That is often referred to as the crop factor, and for Canon cameras the crop factor is 1.6. When you multiply 50 mm times 1.6 you get the equivalent full frame angle of view of 80 mm.

Longshadow Loc: Audubon, PA, United States

RichardTaylor Loc: Sydney, Australia

The focal length doesn't change. However the equivalent (in 35mm camera terms) field of view does change. So in your case multiply the focal length by 1.6.

As far as proper exposure goes, see this old tutorial of mine here on UHH.

https://www.uglyhedgehog.com/t-573456-1.html

Rab-Eye Loc: Indiana

Well said.

mwsilvers Loc: Central New Jersey

I assume you meant 1.6, not 1.4.

RichardTaylor Loc: Sydney, Australia

Thanks. My error.

FreddB Loc: PA - Delaware County

As I understand it:
Exposure is determined by aperture + shutter speed + iso. Focal length is not part of the equation.
Focal length is “fixed” by the physical properties of the lens. 50mm is 50mm.
Using the lens on a DX camera (“crop factor”), reduces the angle of view, capturing a smaller area of the scene. The resulting “cropped” photo gives you the illusion of increased reach.

If that’s not the case, someone please feel free to correct me.

I guess I have to learn to type faster.
Several others responded while I was diddling along.

mwsilvers Loc: Central New Jersey

In my first post I failed to answer the second part of your query regarding the shutter speed. The convention wisdom to prevent hand held blur of stationary subjects is 1/focal length x crop factor which in this case would be 1/80 second.

However don't use that guideline as a standard. First, it is only intended for static images. For moving subjects the shutter speed will have to be much faster to capture an image without motion blur. Second, lenses with image stabilization will allow you to capture sharp static images hand held at much lower shutter speeds. Cameras that also have in-body image stabilization (IBIS) will allow you to capture those same images at significantly slower shutter speeds.

In addition your skills as a photographer, how you stand, how you breathe, how you hold and brace your camera, and how you depress the shutter button will all affect how slow a shutter speed can be and still give you sharp results. However all of these techniques will have less impact on subjects that are moving.

Shutter speed is part of the exposure triangle with aperture and ISO. Selecting a shutter speed in relation to the ISO and aperture to get you the desired results is part of the creative process.

Slower shutter speeds can allow you to use creative blur to, for example, give a sense of motion by blurring a ball that's being pitched to a batter in a baseball game. It can also be used to give a milky or cotton like quality to moving water that many people like.

mwsilvers Loc: Central New Jersey

You are correct.

CHG_CANON Loc: the Windy City

Most of the issues have been covered already in the responses above.

You might also ask, what is the difference between an EF (FX) and an EF-S (DX) lens when they're both set to 50mm?

As said in the earlier responses, the lens focal length does not change. A lens at 50mm is 50mm no matter what camera / sensor type the lens is attached. The smaller EF-S (Nikon DX) lenses are designed to throw a smaller circle onto the smaller sensor size. But, that smaller circle of light is still passing through a lens with a 50mm focal length. Focal length being defined as the distance between the center of a lens (or curved mirror) and its point of focus.

The equivalent focal length mentioned above is based on comparing results from a full-frame lens with the results from a cropped sensor camera. That comparison is based on the 'field of view', not the magnification nor depth of field attributes of the images being compared.

Try this at home, look straight ahead and hold both arms out straight in front of your body, extended in front of your eyes. While continuing to look straight ahead, and keeping your arm extended, slowly move your hands to your sides, held at the same height as your eyes. At the point you're still looking forward and can also just still see your hands in your peripheral vision (the sides of your eyes), that is the field of view of your eyes. For most of us, this field of view is about 200 degrees.

If you look directly forward and focus your eyes on something specific in your view, you see about 40 degrees in sharp focus. This is the field of view (FOV) of a full frame 50mm lens. If you mount a 50mm lens to a full-frame camera and raise and lower the camera from your eye looking at the same subject, the view of what you see in the view finder is the same as what you see with your eyes. If you put an 80mm lens on an EOS full frame camera and a 50mm lens on a cropped EOS camera, and look at something and then look through each of the two cameras, the 80mm 'telephoto' aspect of the lenses differs from your naked eye, but the two cameras and respective lenses will seem equivalent to each other.

The crop factor for EOS cameras is typically 1.6x, for Nikon and Sony it is 1.5x.

CHG_CANON Loc: the Windy City

Emphasizing the impact of Image Stabilization, it really throws the 1/focal-length "rule" out the window. Although, that rule is worth remembering when considering the impact or 'support' of the IS. If you find you need about 1/100 sec to reliable get sharp (steady) focus with an unstabilized 50mm lens on a crop sensor, you should test your IS-enabled lens at 1- and 2- and 3-stops slower to get your own sense of your abilities using the IS. 1-stop down from 1/100sec is 1/50sec, 2-stop is 1/25sec and 3-stop 1/12sec. Even with IS-support and a wider lens than 50mm, I personally can barely / rarely hold a camera / lens steady below 1/15sec.

amfoto1 Loc: San Jose, Calif. USA

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).