What's happening with my lens?

B

Benjamin Corwin

New member
Hello everyone. I have recently been dealing with a lens problem that I can't figure out. The lens that is giving me trouble is the Canon Telephoto EF 100mm f/2.8 USM Macro Autofocus Lens. I am shooting video on a Canon 7D and have been wanting to get some nice macro rack focus shots. The problem I am dealing with is that when I adjust the focus the aperture also adjusts. I understand that if I were zooming or focusing with a variable aperture lens as opposed to a constant aperture lens I would get an aperture change, but here I have a fixed lens with a constant aperture of 2.8 and adjusting the focus is what is causing the change in aperture. Is there anyone that can explain to me what is happening? Thanks!
 
Benjamin Corwin said:
No nothing changes on the display.
Click to expand...

If it indicates the same f-stop, what indication do you have that the iris is changing?

I have noted in looking at some reviews of the lens that the iris may open wider on it's own when changing focus, even past the f2.8 maximum, without indicating anything on the display. This might be a product of the wide focal range available in a macro lens. You might want to research that, and see if there's a fix. Make sure you're in full manual mode. (Edit: you are.) If the lens does an automatic compensation for focus by adjusting the iris, in a way that's visible, you might have to return it (or sell it) if you're doing a lot rack focussing.

Edit 2: note that this issue wouldn't come up in a lot of reviews, because it wouldn't be noticeable by a stills photog. Looking at Ken Rockwell's review, he does mention in passing that the lens opens wider than its 2.8 maximum during focussing operations:

"Maximum Aperture:

f/2.8 indicated at all distances. This Canon does get slower, unfortunately the camera doesn't indicate the true f/stop as you focus more closely. You'll have to do the compensation in your head if you're using an external meter! "

If I read that as I think he means, it might be a fatal flaw with this lens for video.
 
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The iris visibly changes while viewing the footage on video playback. Sorry for the confusion.

Also, thanks for looking that up! That might just be the problem! Next time I play with the camera I will find out for sure. I appreciate your time and effort to help me.
 
Which version of the firmware do you have? I seem to remember this being reported as a problem for the 5Dm2 and 7D, and a firmware update was the solution.
 
Oh really? I am not sure on that one. It's my boss's camera. I will let him know that might be a possible solution. Thanks for the tip.
 
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