Lars Johnsson wrote:
The 50L like many other lenses have focus shift when using the center AF. And Diglloyd also show this. He have a lot of tests and articles about it. But most of them are in the paid section.
Lars and Dwweiche,
After playing with SLRGEAR 50L MTF loader. I found there is a case that causes the optical shift at the center when the lens stopping down. Diglloyd might have a lemon 50L at wide open when he used for his focus shifting test. Whatever he found can be illustrated like the below sample: http://slrgear.com/reviews/zproducts/canon50f12/ff/tloader.htm
Light object was flattered to the bottom when the lens stopping down which causes the focus shifting there. Anyway, we will never see this senario if the 50L has been optimized at wide open. Another word, the focal plane at center to be the same postion when the lens stopping down. Which is the case of my two well calibrated 50L lenses.
Both cases (Diglloyd and mine) have exactly the same optical phenomenon that shifts the light object during recording the image which can't be keep up by some AF system. 1Dseries AF firmware might has its own EF fast lenses database on-board to offset this optical shift. I believed the focus shift mystery has been resolved.
Imo, this optical phenomenon can be happen to any fast lenses from 14mm to 50mm and just not the 50L by itself. Sometime the shifting is too small compared to image dof so not many has noticed it.
Canon, we need smarter AF system in your lesser AF body. It is pain in the butt to deal with this ghost things
Nov 30, 2012 at 03:08 AM
Lars Johnsson Offline • • • • • • • Upload & Sell: Off
Why would Diglloyd have a lemon? He say the 50L is a very sharp lens. And this has nothing to do with focus shift. Both a lemon or a very sharp 50L have focus shift. And he does not test it for focus shift wide open either I don't belive you really understand what "focus shift" is when you write this:
"Diglloyd might have a lemon 50L at wide open when he used for his focus shifting test"
Maybe "lemon" is not a right word...maybe "not well calibrated at wideopen"
That is the only reason the center point is being shifted. Yes, i do understand this optical phenomenon very well. I have used 50L day in day out for last three years and i have not ran into this focus shift thing with my shots when using center AF sensor with 5D2 and 1Ds2. Not only just the AF, i am also able to manual focus the 50L using the view finder and Eg-S screen with any aperture at any subject distance and the confirmation of center sensor. This process can not be done with VF if this optical phenomenon/focus shift does exists with my 50L lenses.
mttran wrote:
Light object was flattered to the bottom when the lens stopping down which causes the focus shifting there. Anyway, we will never see this senario if the 50L has been optimized at wide open. Another word, the focal plane at center to be the same postion when the lens stopping down. Which is the case of my two well calibrated 50L lenses.
You can't calibrate your 50L so the Focus Shift disappear.
Nov 30, 2012 at 04:22 AM
Lars Johnsson Offline • • • • • • • Upload & Sell: Off
Maybe "lemon" is not a right word...maybe "not well calibrated at wideopen"
That is the only reason the center point is being shifted. Yes, i do understand this optical phenomenon very well. I have used 50L day in day out for last three years and i have not ran into this focus shift thing with my shots when using center AF sensor with 5D2 and 1Ds2. Not only just the AF, i am also able to manual focus the 50L using the view finder and Eg-S screen with any aperture at any subject distance and the confirmation of center sensor. This process can not be done with VF if this optical phenomenon/focus shift does exists with my 50L lenses. ...Show more →
No, the reason for focus shift is NOT that the lens is "not well calibrated at wideopen"
You should read about focus shift at diglloyd or other sites.
I have also used my 50L without problem since the lens was released. But this has nothing to do with "if the lens have focus shift or not" All 50L lenses have it. Including your lens
A lot of other lenses also have it.
I see the focus shift when using the farthest 5D2 AF border sensor and lens stopping down in such distance but i have never seen one with center AF sensor. Even though the shift is too small to mention here
"Here are some options, none of which are entirely satisfactory.
Stop down — unfortunately, the entire zone of focus is shifted, and most of these lenses require f/8 or f/11 to regain “lost ground”. If your zone of sharpness needs are precise, stopping down isn’t much of a solution. Given the random variability of focus error, stopping down “kills two birds with one stone”, improving the odds. But didn’t you get that “fast” lens to shoot at wider apertures?
Compensate — deliberately focus slightly in front of the desired point (most lenses). With practice this is feasible, but it’s a skill that takes time to acquire.
Shoot wide open — wide open what you see is what you get.
Focus at the shooting aperture — focus and shoot at f/1.4, focus and shoot at f/2, focus at f/2.8 to shoot at f/2.8, f/4, etc (by f/2.8 spherical aberration is all but eliminated).""
Sven Jeppesen wrote:
"Here are some options, none of which are entirely satisfactory.
Stop down — unfortunately, the entire zone of focus is shifted, and most of these lenses require f/8 or f/11 to regain “lost ground”. If your zone of sharpness needs are precise, stopping down isn’t much of a solution. Given the random variability of focus error, stopping down “kills two birds with one stone”, improving the odds. But didn’t you get that “fast” lens to shoot at wider apertures?
Compensate — deliberately focus slightly in front of the desired point (most lenses). With practice this is feasible, but it’s a skill that takes time to acquire.
Shoot wide open — wide open what you see is what you get.
Focus at the shooting aperture — focus and shoot at f/1.4, focus and shoot at f/2, focus at f/2.8 to shoot at f/2.8, f/4, etc (by f/2.8 spherical aberration is all but eliminated).""...Show more →
The last option is not available with the 50L though unless you focus with the DOF button pressed (will that work?) or using LV focusing.
Focus at the shooting aperture — focus and shoot at f/1.4, focus and shoot at f/2, focus at f/2.8 to shoot at f/2.8, f/4, etc (by f/2.8 spherical aberration is all but eliminated).""
Doesn't the actual auto focusing take place at the widest aperture of the lens? So, if you are at f/2 and focus, still the effects of focus shift will be present. Am I right?
Diglloyd article: Focus shift is a displacement of the sharp plane of focus when the lens is focused wide open, but the image is made with the lens stopped down.
This phenomenon has been showned as above in two cases in this threads, one at center and another at border of the lens. As explained at my earlier post, you will not see a focus shift from center of the lens (50L or any fat and short focal lenses) from a well calibrated lense at center point.
Click twice to simulate one during AF and other when aperture stopping down then flip flop between the two model windows to simulate your focus plane and light object plane shifting.
This focus shift phenomenon is nothing more than an optical bending effects and some of our AF system can not keep up the light object shifting when the lens is stopping down.
Send your lens in for optical shift calibration if you noticed any blurry object at center when the lens was stopping down.
mttran wrote:
Diglloyd article: Focus shift is a displacement of the sharp plane of focus when the lens is focused wide open, but the image is made with the lens stopped down.
This phenomenon has been showned as above in two cases in this threads, one at center and another at border of the lens. As explained at my earlier post, you will not see a focus shift from center of the lens (50L or any fat and short focal lenses) from a well calibrated lense at center point.
Either you don't really understand what Focus Shift is. Or you got it all wrong.
Yes, Focus shift is a displacement of the sharp plane of focus when the lens is focused wide open, but the image is made with the lens stopped down.
But it doesn't help to calibrate your lens so it's good wide open.
1. You don't have the Focus Shift wide open.
2.It's when you stop down the lens, that the Focus Shift appear. Because the focus moves away. How much depends on what aperture you use.
3. The focus will move away even with a perfect calibrated lens (wide open)
Focus shift is caused by spherical aberration. And you can read about that to understand the issue here. it's a very technical discussion.
Chumma wrote:
Doesn't the actual auto focusing take place at the widest aperture of the lens? So, if you are at f/2 and focus, still the effects of focus shift will be present. Am I right?
Yes normally the focusing take place at the widest aperture of the lens. But you can focus at the shooting aperture if you like that. (stopped down) It's not very practical to do it.
I stand with what i said since i am able to get the best sharpness at center focus point manually using 5D2 View Finder, Center AF sensor confirmation and Eg-S screen at any subject distance and any aperture setting. The process has been repeated with auto focus and i got the same results in both cases
mttran wrote:
I stand with what i said since i am able to get the best sharpness at center focus point manually using 5D2 View Finder, Center AF sensor confirmation and Eg-S screen at any subject distance and any aperture setting. The process has been repeated with auto focus and i got the same results in both cases
Yes I'm sure that you can get your best result with center focus point manually using 5D2 View Finder, Center AF sensor confirmation and Eg-S screen.
But this has nothing to do with the question if the 50L have Focus Shift or not. And you can't make the Focus shift disappear by doing it. Most of the time the Focus Shift will not be of any problem at all. Some people will maybe never see it because of the apertures and distance they shoot at.
Sven Jeppesen wrote:
Yes I'm sure that you can get your best result with center focus point manually using 5D2 View Finder, Center AF sensor confirmation and Eg-S screen.
But this has nothing to do with the question if the 50L have Focus Shift or not. And you can't make the Focus shift disappear by doing it. Most of the time the Focus Shift will not be of any problem at all. Some people will maybe never see it because of the apertures and distance they shoot at.
I did say center focus shifting can be fixed and disappeared by a well calibration but i did not say off center focus shift can be resolved the same way. Please, read again the whole sequence that i have presented here. I think you and Lars really misunderstood what i have said.
Gentlement, just think about a thin bowl sitting on the thin flat plate. The bowl represents the 50L light object @f1.2 and the plate represents the 50L light object @f8. Now, do you see any light shift at the bottom of the bowl or center of the plate when a well calibrated 50L is stopping down between f1.2 and f8. Your answer is a big "NO" ...so there is no light bending or shifting at that center point
This is exactly what i have tried to say since the bowl and the plate ilustrate how 50L light object works in real life. You will not have any center focus shift if you have a well calibared 50L but the border away from center point is a way off for another stories
I hope you all get it now.
Again: I have not found a single case of focus shifting when i am using my 50L with center AF sensor point. I pretty much rely on 1Ds2 AF and 5D2 AF plus Eg-S adjustment when focusing using off center AF sensor point
I know exactly what you mean but this is another level that applies to all lenses not just 50L...we only want to discuss what matter most for the 50L right now in this thread Hope this help
I don't belive you really understand what "focus shift" is when you write this: