# EF-S lens & Mirror Lock-up Questions



## cnardo (Jul 2, 2012)

Two quick questions:

1)	If you buy a EF-S lens… say the EF-S 15-85 lens; Does the 15-85 mm focal length designation refer to a full frame sensor (so I would really have a wide angle lens of 24-136mm) or to a cropped sensor thereby having a true 15-85mm lens?
2)	If you use Live View, then your mirror is “in the UP position”. Then if you take a photo while in Live View… is this equivalent to taking that same picture with the “Mirror Lock-up” option enabled? 

Thanks.


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## neuroanatomist (Jul 2, 2012)

Focal length is an intrinsic property of the lens. EF-S lens focal length markings are *not* 'corrected' for the smaller image circle. Your example, the 15-85mm lens, gives the FF equivalent AoV of a 24-136mm lens used on a FF camera because of the smaller sensor. 

Yes, the mirror is up during Live View, and so that's the same as MLU. In some ways, it's actually better - Live View uses an electronic first curtain, meaning not only is the mirror vibration eliminated, so is the vibration caused by the first shutter curtain opening.


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## preppyak (Jul 2, 2012)

cnardo said:


> 1)	If you buy a EF-S lens… say the EF-S 15-85 lens; Does the 15-85 mm focal length designation refer to a full frame sensor (so I would really have a wide angle lens of 24-136mm) or to a cropped sensor thereby having a true 15-85mm lens?


In a way it's both. The focal length of your lens is 15-85mm...if you were to mount it on a full-frame camera, it would have that field of view (though there would be a black area because the image circle isn't large enough). But, when mounted on an APS-C camera, it adds the crop factor, which makes it perform like a 24mm-135mm lens on full-frame.

The focal length of a lens is universal, it's how the camera's sensor handles it that impacts the field of view


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## cnardo (Jul 2, 2012)

Thank you for your prompt replies: preppyak & neuroanatomist.

The comment about 1st curtain shutter was especially interesting!!


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## revup67 (Jul 7, 2012)

PS - you can also try this test..enable Mirror Lock up on your camera. Then use the view finder (not live view) and you will note the shutter release must be pressed twice to take the photo (one is the mirror lock up and the other is the shutter).

Now put your camera in Live View with mirror Lock Up still enabled. You will note it only takes one press of the shutter to take the photo. This is because the mirror lock up feature is already enabled when in Live View.


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## cnardo (Jul 7, 2012)

Excellent.... Just tried that and it worked as you described .

Thanks for the PS. !!!


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