[HarryLally]

I tend to agree with you2....i.e. I don't understand either. Why would changing a physical aperture on a lens change the point of focus? Does anyone know the mechanism for this?

[mst]

Why would changing a physical aperture on a lens change the point of focus? Does anyone know the mechanism for this?

http://toothwalker.org/optics/spherical.html

-- Markus

[popo]

You could of course suggest instead that the camera refocuses when you finally press the shutter button. However, that would probably lead to some sort of shutter lag.

There already is a lag time, due to setting the aperture. I once tried to measure the aperture movement speed of the Canon 50mm f/1.8 and concluded it was a little too slow for the full fps of the 7D. I don't know how much aperture settling time they allow, but possibly the minor focus nudge could be done in that time with minimal impact.

[HarryLally]

Markus, thanks for that link. Very interesting. So it seems to happen to all lenses to some degree. I guess the designers assume that any focus shift that occurs as you stop down will be masked by the increased DoF so the photographer won't notice. Still, it's quite an oversight on Canon's part.

[kirbysdl]

From the review: 
 

The macro performance is not all that hot at 70mm @ f/4

 
The lens is designed so that you extend the lens PAST 70mm when attempting close focus shots.  I wonder if the softness at f/4, 70mm, and close focus would be solved by turning the zoom ring further toward the "Macro" mark.  This seems too obvious to be the case, but I thought I'd bring it up in case something was overlooked.
 
The complexities of including the Macro feature probably account for some of the asking price, and make Macro somewhat of a headline feature for this lens.  Therefore it seems odd that there were so many problems with the Macro mode.  Did you take any "real life" sample shots that really exercise the Macro mode?  It would be nice to see how the curiosities seen in the lab translate into real life experience.  
 
EDIT: The Tamron 24-70 seems similarly soft wide open, but this wasn't really mentioned as a problem in that review.  Is there a functional difference between the performance of the Tamron at 2.8 and the Canon at 4?  Is it because the Canon is labeled a Macro lens?  I'm trying to decide between the on-paper flexibility and compact nature of the Canon and the wider aperture of the Tamron, and the RSA/LoCA sections of the two reviews show very similar performance characteristics.

[soLong]

for my point of view, to try to make this lens usable in autofocus i will use the 3 available camera user settings to enter 3 different micro-focus settings that i hope might cover the necessary focus compensation in autofocus, but using this rough'n ready lash up only one of these will allow initial TV operation (in my case the 0 setting will be close enough from 50-70mm) otherwise they will have to be Av 

 

however i don't see why Canon can't put in compensation no matter which mode (Tv, Av) the camera is in, if the camera and therefore lens knows what the correct exposure is, it can then put the comp in for the aperture, no? ... but to do this the manufacturer would have to admit that my lens has a problem...

 

.... and for the intelligent, lovely and knowledgable k who suggested other lens choices as a better option, yes i agree, but my days of playing this pick-a-lens-and-hope-for-the-best-copy game are now over

[Klaus]

From the review: 
 
 
 
The lens is designed so that you extend the lens PAST 70mm when attempting close focus shots.  I wonder if the softness at f/4, 70mm, and close focus would be solved by turning the zoom ring further toward the "Macro" mark.  This seems too obvious to be the case, but I thought I'd bring it up in case something was overlooked.
 
The complexities of including the Macro feature probably account for some of the asking price, and make Macro somewhat of a headline feature for this lens.  Therefore it seems odd that there were so many problems with the Macro mode.  Did you take any "real life" sample shots that really exercise the Macro mode?  It would be nice to see how the curiosities seen in the lab translate into real life experience.  
 
EDIT: The Tamron 24-70 seems similarly soft wide open, but this wasn't really mentioned as a problem in that review.  Is there a functional difference between the performance of the Tamron at 2.8 and the Canon at 4?  Is it because the Canon is labeled a Macro lens?  I'm trying to decide between the on-paper flexibility and compact nature of the Canon and the wider aperture of the Tamron, and the RSA/LoCA sections of the two reviews show very similar performance characteristics.

 

Regarding the Tamron - it is basically a standard zoom lens like any other. Most them are soft at close focus distances.

However, the Canon goes officially down to 1:1.4. This is a different marketing statement and as such you have different expectations here. Consequently there is an explicit statement about this here. 

To be fair - the results are obviously very sharp here at f/8 - assuming that you hit the focus point.

[Brightcolours]

however i don't see why Canon can't put in compensation no matter which mode (Tv, Av) the camera is in, if the camera and therefore lens knows what the correct exposure is, it can then put the comp in for the aperture, no? ... but to do this the manufacturer would have to admit that my lens has a problem...

Wow, the overhauled forum software is not without its bugs... Took me 6 tries to get this part quote correctly (well, I have not posted it yet, so it might still be wrong.. and then I can't edit the post anymore...).

 

soLong, in theory you can only compensate for a focus shift when the aperture is known,/"set" at the same time the AF takes place. When you separate the AF and metering in Tv or M it will not work.

There are many situations where that occurs. When you have it in Tv, and focus on the subject, then recompose, and do not hold the metering, the f-value used can change. When you separate the AF from the metering (not both with the shutter button) the f-value used for the final image will always be unknown at AF time.

 

Also, it is unknown which (the lens, or the camera body?) has to take care of the focus shift compensation. If it is the body, only the newer bodies will know what a new lens needs. 

 

How bad is the focus shift in "normal" use? And how bad is the focus shift in macro use? And does it occur over the entire focal range?

[soLong]

hey Bc, thought you'd gone into hibernation it's been so long - 

 

um, on my camera this lens works like no other, and i've looked at two of these things and they both appear to be very similar, so Qed: it's the lens - but i could be wrong - THAT company USA say 'we know nothing' = i might have had two bad copies

 

- so re. my lenses - autofocus does it's stuff at f/4, the expert designer/programmer knows the focus shift numbers for all the f/'s and mm's for this lens, the camera wants to expose at f/6.3@35mm, it tells the internal lens chip this fact, the chip has been set to microfocus shift +4 (say) for these settings and yea the subjects eyes are in focus .... if my hands aren't shaking already wondering about all this

 

... well it makes sense to me ... but not sure about my brain now -

 

take a look at my beautiful beach scene in the 'lenses coming' thread for where to shift the focus at some various settings - it was at Wamberal, why the beach? - it's easy to see if sand is in or out of focus, anyhow it's quite nice and uncrowded there so they didn't mind a silly person pointing his camera at a white board and writing down numbers when he should have been enjoying the day

 

why haven't i done f/4.5,5.6,6.3,8,11@ 24,30,35,40,45,50mm's? - see the last line last para.

[soLong]

...... How bad is the focus shift in "normal" use? And how bad is the focus shift in macro use? And does it occur over the entire focal range?

 

.... oops sorry Bc i've only now read your last line, and i can't seem to be able to edit this extra answer into my last post to include the quote above, so another answer - 

 

.... well i checked the first lens more thoroughly than i have so far with this one, but this one does look to be very much the same - 

 

so the focus shift for lens#1 on my 7D appeared to be a 0 micro-focus correction at 50-70mm from f/4-f/8 and then from 50-24mm the correction required for best focus increased to a setting of +8 m-f correction at 24mm & f/4 - it appeared to require (at 24-50mm & f/8) an m-f correction of 0 (no doubt the extra depth at f/8 helps) - this current lens might vary that range from -1 to +7 but you can see for yourself by looking at the movement of focus on the sand in my gallery shots, the white board being the focus point, the shots id indicate the mm(24 or 70), the f# (4 or 8) and the plus or minus m-f, the shots are 100% crops which you can make full size  - this smugmug gallery is at -  <a class="bbc_url" href="http://www.smugmug.com/gallery/28208048_CvkBjw#!i=2385876358&k=K76g6wC" title="External link">http://www.smugmug.c...76358&k=K76g6wC</a>

- i haven't done 50mm again yet because: (1) no time, (2) weather and (3) i'm bored with all this, but i will and will update any difference from lens#1, just looking at it i expect not (but that's not very scientific and precise is it)

 

re. macro: i always focus manually and at 70-80mm should require no micro-focus correction anyway, but f/4 is a tad soft 

 

.... anyhow for me all this is insane, so as i have said already enough of these silly lens games for me, no more, sorry

 

 

.... edit (8 march) ...

 

ok for what it's worth, here now my 7D micro-focus settings for this thing that appear to find auto-focus - 

 

- taking f/8 as the constant zero point -

 

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

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

at 35mm: f/5.6 = 0(close enough as i have only 3 quick picks via the C knob), f/4 = +6

at 24mm: f/5.6 = +3, f/4 = +6

 

definitely an Av lens, definitely a lens tragic