Testing for mirror clearance

Mirror clearance is less with a full frame sensor camera

Many lenses that work fine on an EF-S mount cam­era will not work on a full frame sen­sor cam­era because they extend too far into the camera.

As the camera’s mir­ror swings upward, the dis­tance between the edge of the mir­ror and the lens mount decreases. When the mir­ror is down on an EF or EF-S mount cam­era, the edge of the mir­ror is 16mm behind the lens mount. When the mir­ror is up on an EF-S mount camera, there’s 12mm of space between the mir­ror and the lens mount but there’s only 6.5mm of clearance on a 5D Mk II.

Mir­ror clear­ances are not an issue with mirrorless cam­eras which allow you to use a larger selection of lenses because of their low FFD.

Testing for mirror clearance

An old Canon film cam­era is a good test cam­era if you have the 5D Mk II. Focus the lens at infinity, where the rear ele­ment is clos­est to the mir­ror, and trip the shut­ter. If the mir­ror hangs up on the lens, remove the lens, let the mir­ror fall back into place and don’t use the lens on your full frame sensor digital camera.

A test cam­era like this will cost you around $15.

A test cam­era like this will cost you around $15.

A rear lens cap with a hole cut in the bot­tom can also be used to check the mirror clearance for a particular lens. Set your lens to infin­ity and mea­sure from the inside of the lens cap to the high­est point on the lens. Sub­tract that mea­sure­ment from the the depth of the cap and you’ll know how far the lens extends into the camera.

This Voigtlän­der 55mm f/1.4 Color Ultron (Rollei QBM mount) extends 7.5mm into the cam­era at the lock­ing tab and 6.2 mm every­where else (the lens cap is 13.5mm deep). Because the tab sits in the 6 o’clock posi­tion, where there’s 16mm of clear­ance, the lens works fine on my 5D Mk II.

This Voigtlän­der 55mm f/1.4 Color Ultron (Rollei QBM mount) extends 7.5mm into the cam­era at the lock­ing tab and 6.2 mm every­where else (the lens cap is 13.5mm deep). Because the tab sits in the 6 o’clock posi­tion, where there’s 16mm of clear­ance, the lens works fine on my 5D Mk II.

This $20 dig­i­tal caliper can be use­ful on occasions.

This $20 dig­i­tal caliper can be use­ful on occasions.