Forums > Back > Canon 24-70 f/4L IS - Residual Spherical Aberrations?

07-28-2013, 11:51 PM

Hi,

Please can anyone explain how residual spherical aberrations adversely affect the performance of the Canon 24-70 f/4L IS? I understand that the focus point shifts backwards when the lens is stopped down but I am not sure how this affects focusing in close up photography which is this lens' main party trick (i.e. the 0.7x macro mode).

Does this mean that if the camera is in Program mode and meters, say, for f/5.6 at 70mm, the camera has focused on an object near minimum distance and the image appears in focus in the viewfinder the focusing point would shift backwards causing loss of focus when the shutter is released?

I hired a 24-70 f/4L IS recently for use with my 650D and found the close up focusing performance to be unreliable for flowers and other small objects. The tiny working distance precluded the use of flash, unless you like arc shaped shadows. I am not sure whether this was caused by wind and / or not selecting AI Focus. I used either auto or manual focus during the 3 day hire. When the lens did focus correctly the results were markedly better than my old 24-105 f/4L with less distortion and chromatic aberration. I want to like this lens but the idea of buying a lens with an apparent design fault (even if common to lenses without a floating focusing mechanism) is a potential deal breaker.

I am thinking of getting a 6D soon either with a 24-70 or a Canon 35mm f/2 / Sigma 35mm f/1.4 plus a macro.

Many thanks,

Martyn

07-29-2013, 08:59 AM

Hello Martyn,



Welcome,



Hmmm….I've been enjoying sitting back here, looking in and making no comments, and I was hoping that everyone here who are all more knowable than I, would answer this question first, so that I could remain invisible, but I have this lens so suppose that I should say something. Although I wish I didn't feel so inclined, and if you were to search my previous comments, it's all back there in ages past.



Also it might possibly be a waste of your time reading this stuff below if your camera doesn't have micro-focus. And as a disclaimer, not wanting to tread on any persons or manufacturers toes, this is my experience with my lens on my camera, a 7D. Having tried two copies roughly the same - I feel that this lens was designed to help stop my brain from ossifying out in the field and never coming back home with me.



Anyhow, quickly: I actually quite like the pictures from this lens when it's in focus….in focus exactly on what I focused on in the scene :-)



So my micro-focus settings for accurate auto-focus with this lens on my 7D are -



With f/8 being a constant 0 :



at 24mm: f/5.6 = +3. f/4 = +6 (or a tad more)

at 35mm: f/5.6 = 0, f/4 = +6;

at 50mm: f/5.6 = 0, f/4 = +3;

at 70mm: f/5.6 = 0, f/4 = 0;



Macro heads off in the other direction so that roughly 78mm and f/4 it appears to require a micro-focus setting of -6 (or more) for accurate auto focus on my camera.



So your description 'floating focus' is nice….for my lens…..using floating micro-focus.

And on a separate matter you mention, I like the Sigma, but don't take my word for it, try it yourself.



…..Why give the short answer when a long one will do to.

07-29-2013, 06:57 PM

Hi soLong,

Thanks for the detailed reply. My 650D doesn't have micro focus adjustment but the forthcoming 6D will have.

Just to confirm, with your settings can your 7D adequately compensate for the RSA problem affecting the 24-70 f/4L IS USM?

Here's my mini review of the 24-70 f/4L IS USM: http://www.dpreview.com/forums/post/51779111. I think its better than the 24-105 at 24mm wide open but I keep returning to the RSA issue.

I'm an amateur so I'm not sure if I have the dedication to be constantly making micro adjustments. I just wish Canon didn't include RSAs in this lens.

Kind regards,

Martyn

08-07-2013, 09:39 PM

Quote:Hi soLong,

Thanks for the detailed reply. My 650D doesn't have micro focus adjustment but the forthcoming 6D will have.

Just to confirm, with your settings can your 7D adequately compensate for the RSA problem affecting the 24-70 f/4L IS USM?

Here's my mini review of the 24-70 f/4L IS USM: http://www.dpreview.com/forums/post/51779111. I think its better than the 24-105 at 24mm wide open but I keep returning to the RSA issue.

I'm an amateur so I'm not sure if I have the dedication to be constantly making micro adjustments. I just wish Canon didn't include RSAs in this lens.

Kind regards,

Martyn

hello again Martyn,

very sorry for this very late reply, I haven't been looking in here very much lately, I should set my preferences to inform me of replies I think.

Yes with the 7D I have 3 quick settings +6, +3 and 0 which just about cover everything except macro which I set as I go.

But more importantly: I visited Canon repair here at North Ryde yesterday for another matter and they said that they could reset the lens internally for focal length and aperture differences as shown in my macro settings to correct any errors. So they have the lens + camera now for the fix and I will post a message here if this repair works - hopefully it will.

I do like the pictures thru this lens so will be happy if it becomes easier to use.

Sorry everyone to have been a bother but I've not been doing any photography for some time now - maybe later.

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