Lense Compatiblity

[Stumann]

I have an old Canon Rebel XTi that has a 17-85mm IS lens on it.  I recently purchased a Rebel T6s and tried to use that lene on the T6s.  It appears to be the same, (the same number of brass pin connections in exactly the same place), but it is not quite compatable.  The old one is USM, the new one is STM, if that matters.  It functions well where autofocus is concerned, but it doesn't seem to read light correctly.  I have to manual set speed and aperture to get the right exposure through trial and error.  I do have a new lense, the same as the old one, although outside appearance is quite different, it is still a 17-85 IS lense.  It works fine on the T6s, but the old lens acutally takes much clearer pictures so I would like to use the older one.  is there some way I can make this work?

[Waddizzle · Accepted solution]

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@Stumann wrote:

I have an old Canon Rebel XTi that has a 17-85mm IS lens on it.  I recently purchased a Rebel T6s and tried to use that lene on the T6s.  It appears to be the same, (the same number of brass pin connections in exactly the same place), but it is not quite compatable.  The old one is USM, the new one is STM, if that matters. 

It functions well where autofocus is concerned, but it doesn't seem to read light correctly.  I have to manual set speed and aperture to get the right exposure through trial and error.  I do have a new lense, the same as the old one, although outside appearance is quite different, it is still a 17-85 IS lense.  It works fine on the T6s, but the old lens acutally takes much clearer pictures so I would like to use the older one.  is there some way I can make this work?

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Hmm.  It sounds like a possible aperture issue, but you say it works on the XTi.  Plus, you say the new lens works fine.  Try resetting the T6s back to factory defaults using the menu option.  

What shooting mode are you using on each camera?  Yes, I know you have to manually set it to make it work, but what mode are you using when it will not adjust automatically?  Are ISO settings the same on each camera?

Set the camera to Av mode.  Mount the older lens on the T6s and use the Depth of Field preview button to see it the aperture is stopping down at various aperture settings from wide open to fully stopped down.  If the STi has a DoF preview button, then try the same tests on that body, too.  It should have one, too.

[EDIT]. Do not bother with trying to clean the contacts.  Just inspect them for any large pieces of debris or foreign objects.  The friction across the contacts when you are mounting or unmounting a lens is sufficient to keep contacts clean.  But, sometimes the contacts can take a beating over time.  Seeing how you seem to have counted them, I suspect they are nice and clean.

[Stumann]

Thanks, Waddizzle, for all that information.  I tried the depth of field preview and the aperature did adjust.  I did a reset as you suggested and I think it might have fixed the problem.  I'll do some more testing tomorrow and let you know for sure.

[ebiggs1]

This is very simple to solve.  First out of curiosity, both lenses are Canon brand?  OK, first reset both cameras to factory.  menu, tools, clear all settings and all custom settings. Now set both cameras to P mode.  ISO 200, Average WB, Set the lens switch to AF. On a nice sunny day go outside and take some shots using each lens on both cameras.  Do not change the settings.  All should be as it should be.  If not.......well if not that's bad. Something is broken, if it works everything is OK.

It is not uncommon for different lenses even with exactly the same focal range to take different pictures.  USM  and/or STM in itself does not effect the picture quality or IQ if you will.