50 1.2L : thanks to joe mama and the others who explained this lens

Started Apr 21, 2008 | Discussions
Doc Tonic Veteran Member • Posts: 3,215
50 1.2L : thanks to joe mama and the others who explained this lens

Just picked up my 50 1.2L and sadly I didn't really research it very thoroughly first. I just wanted to complement my 35 1.4L and 85 1.2L MII set. I often just carry a prime with me when I travel and during the day just use that single lens.

After my first shot with the 50 1.2L, I went WFT?!?!? It was seriously backfocusing... and I never experience that with my famed 85 1.2L lens.

But after reading all of the reviews and descriptions here on dpreview, I learned that the solution is to basically never use the center AF point. Which is unusual for me as that is what I typically do (the center AF is "supposed" to be better). After doing a few tests, I found the focus fairly decent if I just used a different AF point...no back focusing issues. Also, at f1.2 using the center point and MANUALLY focusing also seemed to produce decent results. f1.2 and using an off center point also produced decent results. f2.8 and center focus was horrible. f 2.8 and off center focus was fine.

Thanks to those who figured this out for me before I lost my mind. I feel so bad for those amateurs who buy this lens only to be sorely disappointed with the focusing issues. Canon really needs to include a user manual explaining this lens' flaws.

canerino Contributing Member • Posts: 800
Re: 50 1.2L : thanks to joe mama and the others who explained this lens

I am sorry, but I just do not get why anyone would keep a lens that the user has to change the way he/she has to shoot.

Don, with all due respect, why would you keep this lens? You are saying that the focus is "fairly decent" if you use a different focus point. And that it is 'decent' if you manual focus. You have the 35L and 85L...have you ever had to do anything different to make them work properly? Why would you accept a lens that needs some tweaking from the user to get only 'decent' results?

You say in the end that you feel bad for the amatuers who buy this lens and that they would be disappointed. Aren't you disappointed that the lens does not work properly? And why would Canon need to include a manual of the lens' flaws when there are plenty of people who will apologize for them.

I just do not get why users continue to apologize for a substandard lens.

Doc Tonic wrote:

Just picked up my 50 1.2L and sadly I didn't really research it very
thoroughly first. I just wanted to complement my 35 1.4L and 85 1.2L
MII set. I often just carry a prime with me when I travel and during
the day just use that single lens.

After my first shot with the 50 1.2L, I went WFT?!?!? It was
seriously backfocusing... and I never experience that with my famed
85 1.2L lens.

But after reading all of the reviews and descriptions here on
dpreview, I learned that the solution is to basically never use the
center AF point. Which is unusual for me as that is what I typically
do (the center AF is "supposed" to be better). After doing a few
tests, I found the focus fairly decent if I just used a different AF
point...no back focusing issues. Also, at f1.2 using the center point
and MANUALLY focusing also seemed to produce decent results. f1.2 and
using an off center point also produced decent results. f2.8 and
center focus was horrible. f 2.8 and off center focus was fine.

Thanks to those who figured this out for me before I lost my mind. I
feel so bad for those amateurs who buy this lens only to be sorely
disappointed with the focusing issues. Canon really needs to include
a user manual explaining this lens' flaws.

-- hide signature --
blackhawk13 Veteran Member • Posts: 4,742
Re: 50 1.2L : thanks to joe mama and the others who explained this lens

canerino wrote:
I am sorry, but I just do not get why anyone would keep a lens that
the user has to change the way he/she has to shoot.

Don, with all due respect, why would you keep this lens? You are
saying that the focus is "fairly decent" if you use a different focus
point. And that it is 'decent' if you manual focus. You have the
35L and 85L...have you ever had to do anything different to make them
work properly? Why would you accept a lens that needs some tweaking
from the user to get only 'decent' results?

You say in the end that you feel bad for the amatuers who buy this
lens and that they would be disappointed. Aren't you disappointed
that the lens does not work properly? And why would Canon need to
include a manual of the lens' flaws when there are plenty of people
who will apologize for them.

I just do not get why users continue to apologize for a substandard
lens.

Doc Tonic wrote:

Just picked up my 50 1.2L and sadly I didn't really research it very
thoroughly first. I just wanted to complement my 35 1.4L and 85 1.2L
MII set. I often just carry a prime with me when I travel and during
the day just use that single lens.

After my first shot with the 50 1.2L, I went WFT?!?!? It was
seriously backfocusing... and I never experience that with my famed
85 1.2L lens.

But after reading all of the reviews and descriptions here on
dpreview, I learned that the solution is to basically never use the
center AF point. Which is unusual for me as that is what I typically
do (the center AF is "supposed" to be better). After doing a few
tests, I found the focus fairly decent if I just used a different AF
point...no back focusing issues. Also, at f1.2 using the center point
and MANUALLY focusing also seemed to produce decent results. f1.2 and
using an off center point also produced decent results. f2.8 and
center focus was horrible. f 2.8 and off center focus was fine.

Thanks to those who figured this out for me before I lost my mind. I
feel so bad for those amateurs who buy this lens only to be sorely
disappointed with the focusing issues. Canon really needs to include
a user manual explaining this lens' flaws.

-- hide signature --

ALL lenses need user adaptation to get the most out of them. The 50L is a superb street shooter that can nail shots other lenses miss. It's flare control is excellent. Ha-ha, it reminds you not to use the center focus point...

-Fortune favors the bold-

xtoph Veteran Member • Posts: 9,871
if this is what is happening...

Doc Tonic wrote:

Also, at f1.2 using the center point
and MANUALLY focusing also seemed to produce decent results.

IF you are getting a different result from manually focusing and autofocus using the center sensor (which is what it sounds like you are describing here), THEN you need to have the lens, or the camera, or both adjusted.

there is no optical 'flaw' in the lens design that could explain why a lens that is capable of focusing properly (manual focus works) would not focus properly using autofocus. the autofocus doesn't do anything other than to look through the lens, just like you do with your eye, and adjust focus. the only problem arises under specific circumstances, when something changes in between setting focus (whether manual or auto) and taking the picture (ie, stopping the lens down, which can cause the point of sharpest focus to shift).

this is the problem with many of the threads about this lens: there is rampant confusion about what, exactly, is behavior intrinsic to the optical design, and what is due to other (fixable) problems.

no working copy of this lens should backfocus wide open, period. if yours does, then get it adjusted or replaced by canon under warranty.

OP Doc Tonic Veteran Member • Posts: 3,215
Re: 50 1.2L : thanks to joe mama and the others who explained this lens

canerino wrote:

I just do not get why users continue to apologize for a substandard
lens.

It is true...I do wish it would actually focus like the other L lenses. And, I do already own the 50 1.4 lens...so I am wondering why I want to put up with this. Though I can say when you nail the f1.2 focus, it is quite a remarkable lens. Manual focusing for me is really iffy as I wear glasses.

I don't think canon will release a 50 1.2L MII anytime soon, so my only choice is to live with this off center shooting technique or throw in the towel and return the lens. I think I will give it a shot, and if it doesn't work out it will end up on ebay. I might send it in for service, but that doesn't sound promising either.

I would not recommend this lens to just anyone though. For me it completes my set of lenses as the 50 1.4 didn't cut it sometimes. This lens is wonderful when it hits the focus, which for me now is 1/5 shots...hoping to bring that number down.

OP Doc Tonic Veteran Member • Posts: 3,215
Re: if this is what is happening...

I think others have had a slight back focus even at 1.2 using center point...not as bad as f2.8 but still there. I think the reason why I get better "results" manually focusing at f1.2 with center is that I am probably accidently focusing too far in front and therefore getting a sharper image.

brianric Veteran Member • Posts: 8,413
Re: 50 1.2L : thanks to joe mama and the others who explained this lens

I usually have that problem when shooting with any fast lens wide open unless I put the focus point over the point of interest.

canerino wrote:

I am sorry, but I just do not get why anyone would keep a lens that
the user has to change the way he/she has to shoot.

Don, with all due respect, why would you keep this lens? You are
saying that the focus is "fairly decent" if you use a different focus
point. And that it is 'decent' if you manual focus. You have the
35L and 85L...have you ever had to do anything different to make them
work properly? Why would you accept a lens that needs some tweaking
from the user to get only 'decent' results?

You say in the end that you feel bad for the amatuers who buy this
lens and that they would be disappointed. Aren't you disappointed
that the lens does not work properly? And why would Canon need to
include a manual of the lens' flaws when there are plenty of people
who will apologize for them.

No one is twisting your arm to buy/use the lens. I look at at my equipment, and work around its weakness to get the results I want. If it means using something different than center point focus, so be it.

I just do not get why users continue to apologize for a substandard
lens.

I'm not apologizing, I know its flaws, and willing to work with it. I don't unterstand why non users of the lens get themselves worked up over this.

 brianric's gear list:brianric's gear list
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garyw1 Regular Member • Posts: 386
Re: if this is what is happening...

Hi Doc. Just got mine a short timke ago and had the exact prob. Sent it back to Canon to get calibrated and now its perfect. Love thye lens and would have kept it anyway. At 1.2 though you still have to be very carefull.

Here is my link to my situation:

http://forums.dpreview.com/forums/read.asp?forum=1029&message=27269829
--
'The moment you think your great is the moment you quit learning.'
http://www.gawalters.com

DavidSvensson Senior Member • Posts: 1,374
How manual focus works optically

Doc Tonic wrote:

I think others have had a slight back focus even at 1.2 using center
point...not as bad as f2.8 but still there. I think the reason why I
get better "results" manually focusing at f1.2 with center is that I
am probably accidently focusing too far in front and therefore
getting a sharper image.

Doc,

I want to chime in with xtoph, it does seem the lens needs to get calibrated, and preferrably matched to your camera.

The 50/1.2 is a phantastic lens, once you get it calibrated, and learn how to best use it.

There is an optical explaination why you will get better focusing results at f/2.8 with manual focus with this lens.

The focusing screen will only transmit light that comes from the central part of the angles in which light is transmitted by the lens. The angles that correspond roughly to f-numbers 1:2.5 and smaller (more stopped down).

The reason this lens has a problem at f:2.8 and wider is that it has a lot of spherical abberation, meaning that light coming from the periphery of the lens will not focus in the exact same plane as light coming from the central part.

The AF system is designed such that irrespective of what f-number is used, the lens is focused the same. It is also designed to give a maximally sharp image at wide open aperture.

This is quite different from the manual focus, which by the described nature of the focusing screen, effectively will let you focus such that the lens is maximally sharp at f:2.5 or thereabout, irrespective of if you use a wider aperture.

Anyway, to summarize, if the lens is not 100% sharp at f:1.2 using the centarl AF point, the lens will benefit from calibration of the lens. (As you are using an 85/1.2, I am assuming your camera must be already in proper calibration.)

It is perfectly normal for this lens to backfocus when using the central AF point and f-numbers 1:2 to 1:4, and a shorter shooting distance than 10ft.

At f-numbers 1.2 to 1.8 and 1:5.6-1:16 it should be 100% sharp at all shooting distances.

It should also be 100% sharp at all apertures at shooting distances above 10ft.

(BTW, the reason the 85/1.2 is such a darling and will focus properly all the time is because it has a so called floating rear element, which moves slightly while focusing the lens. This adjusts for the changes in spherical aberration while focusing on closer subjects.

Same goes for the 35/1.4.

On the 50/1.2 Canon decided to save a few bucks and over-simplified the mechanics of the focusing mechanism.)

David

booggerg Senior Member • Posts: 1,378
it is an absurd notion that...

... you'd be contempt with NOT using the center AF point with this lens. The outside AF points on the 5D are WEAK, especially in low light situations.

You need to have your lens send to for calibration.. end of story.

xtoph Veteran Member • Posts: 9,871
wide open, there should be no difference

whether you autofocus, use manual focus, or use live view magnification and focus manually (assuming the lens and camera are properly calibrated). they are all different ways of doing the same thing: just making the target appear as sharp as possible. however you achieve this, as long as you then change nothing (ie, don't move the camera, and don't stop down the lens), then you will have identical focus. if you don't get identical focus in all three situations, then something is out of calibration, and can be fixed. this principle applies to every lens, including the 50/1.2.

think through the way that the focus shift manifests. any lens-design-specific shift in focus has to shift from some apparent achieved focus to another one. there is no way that this can happen if nothing changes; looking through the viewfinder and taking the picture (wide open) have to be the same . we don't get 'focus shift' wide open; it is categorically impossible. what would it be shifting from ? (in fact, if we wanted to focus while the lens was stopped down to the shooting aperture all the time, this focus shift thing would never be an issue.)

as other posters have helpfully suggested, use of a precision focus screen (cheap! and useful) can help you to evaluate exactly where the focal plane is, especially for off-center subjects. it can also make it simple to make a manual focus adjustment (really, an apparent de-focusing) of the lens to counteract the effects of the stopped-down focus shift. i don't want to over-emphasize this technique, however; it really isn't a very significant part of how i use the lens, as i've explained elsewhere.

joe mama Forum Pro • Posts: 12,623
Actually, I have *no* problems with center AF...

But after reading all of the reviews and descriptions here on
dpreview, I learned that the solution is to basically never use the
center AF point. Which is unusual for me as that is what I typically
do (the center AF is "supposed" to be better). After doing a few
tests, I found the focus fairly decent if I just used a different AF
point...no back focusing issues. Also, at f1.2 using the center point
and MANUALLY focusing also seemed to produce decent results. f1.2 and
using an off center point also produced decent results. f2.8 and
center focus was horrible. f 2.8 and off center focus was fine.

...or any AF point, for that matter. It is only when focusing close and stopped down that the focus shift occurs, but for that type of shooting, I'm rarely have a need for center AF, anyway.

But at f/1.2 (where I am 99% of the time), focus is spot-on at any focus distance with center AF. It's just that the DOF is so small that even the slightest motion of the camera or subject from the time that AF lock is achieved to the time the image is captured will cause an OOF pic. You also have to take care that the camera did indeed focus on the intended target (the eye, for example, rather than an eyebrow, an ear, a stray hair, etc.).

My point is that AF with ultra shallow DOF requires a certain level of experience to master, particularly with making sure the camera focuses on the intended target and not something else within the AF sensor's coverage area.

Thanks to those who figured this out for me before I lost my mind. I
feel so bad for those amateurs who buy this lens only to be sorely
disappointed with the focusing issues. Canon really needs to include
a user manual explaining this lens' flaws.

Try some shots on a tripod, or a stable base, and see what's happening. In my opinion, a great deal of the AF issues wide open, if not the vast majority, are due to ultra shallow DOF and slightly missed AF, rather than the any issues with the lens itself.

For example, consider the pic below. Just how easy do you think it would have been for the camera to lock focus on the sunglasses, the hairline, the fingers, or any other "reasonable" place within the range of the selected AF point, rather than the eye?

Canon 5D + 50 / 1.2L @ f / 1.2, 1/250, ISO 100

http://www.pbase.com/joemama/image/95757482

DavidSvensson Senior Member • Posts: 1,374
Re: wide open, there should be no difference

xtoph wrote:

whether you autofocus, use manual focus, or use live view
magnification and focus manually (assuming the lens and camera are
properly calibrated). they are all different ways of doing the same
thing: just making the target appear as sharp as possible. however
you achieve this, as long as you then change nothing (ie, don't move
the camera, and don't stop down the lens), then you will have
identical focus.

Those methods are quite different. The only truly accurate method is Live view at 10x magnification. It will collect light from all of the lens, in the exact same proportions as each pixel does when taking the picture.

When you stop the lens down while focusing with Live view, you will see the exact effect of focus shift due to shutting off the peripherial light rays.

The AF systems works on a different principle. The AF sensors will only collect light from a narrow cone looking into the lens. The width of the cone is roughly equivalent to the cone of light from the lens when stopped down to f/5.6 or f/2.8
dependent on which AF sensor it is.

The AF sensor does not collect any of the peripherial light. To compensate for the focus shift between the centrallight and the peripherial light there is a calibration table stored in the lens. This table basically says: when focusing this lens add this much lens focus movment to the AF reading, after the AF sensor has found the best focus, such that the image formed by the combination of central light rays (which are sensed by the AF sensor), and peripherial lightrays (which are invisible to the AF sensor, but visible to the image sensor) is as sharp as possible.

The drawback is that the lens calibration table only has four entries in a prime lens.

(In a zoom lens, there are 4 entries for each of 8 differnet zoom focal length settings)

Those for entries describe the correction data for each of the types of AF sensors used by Canon (horizontal and vertical sensors in 1-series and in other EOS cameras respectively).

There is no provision calibrating for different focusing distances or for different aperture settings, or for different AF points.

The influences of those is modeled roughly by the camera software, but not on an individual basis.

The net effect is that a well-behaved lens whith not-too-much spherical abberration (central andperipheriallight rays focus in the same plane), and with a floating rear element (that makes the lens stay well-behaved at all focus distances)

will work nicely, while a lens lacking this will exhibit a varying degree of focus shift depending on aperture setting and focus distance.

if you don't get identical focus in all three
situations, then something is out of calibration, and can be fixed.
this principle applies to every lens, including the 50/1.2.

think through the way that the focus shift manifests. any
lens-design-specific shift in focus has to shift from some apparent
achieved focus to another one. there is no way that this can happen
if nothing changes; looking through the viewfinder and taking the
picture (wide open) have to be the same . we don't get 'focus shift'
wide open; it is categorically impossible. what would it be shifting
from ?

The focus shift between when focusing with live view and when focusing using the viewfinder screen comes from live view seeing the full cone of light, and the viewscreen seeing only light from a cone as narrow as about 1:2.5

In the case of a very bright lens like the 85/1.2 there is four times more light (in terms of cone angluar coverage) seen by the image sensor than what the viewfinder focusing screen can transmit to the eye.

This means three quraters of the light forming the image will come from the peripherial rays, rather than from the central rays visible to the focusing screen (or to the Af sensors for that matter.)

The reason for this is that the design of a focusing screen sets how diffuse the screen is. If it is fully diffuse it will display the full cone of light, but it will be very dim.

(in fact, if we wanted to focus while the lens was stopped
down to the shooting aperture all the time, this focus shift thing
would never be an issue.)

True.

as other posters have helpfully suggested, use of a precision focus
screen (cheap! and useful) can help you to evaluate exactly where the
focal plane is, especially for off-center subjects. it can also make
it simple to make a manual focus adjustment (really, an apparent
de-focusing) of the lens to counteract the effects of the
stopped-down focus shift.

I think that the method of stopped down focusing using live view is superior, for the abovementioned reasons.

i don't want to over-emphasize this
technique, however; it really isn't a very significant part of how i
use the lens, as i've explained elsewhere.

David

brianric Veteran Member • Posts: 8,413
Re: it is an absurd notion that...

Not everyone uses a 5D with the 50/1.2.

booggerg wrote:

... you'd be contempt with NOT using the center AF point with this
lens. The outside AF points on the 5D are WEAK, especially in low
light situations.

You need to have your lens send to for calibration.. end of story.

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DavidSvensson Senior Member • Posts: 1,374
You need to have your lens send to for calibration.. end of story.

booggerg wrote:

You need to have your lens send to for calibration.. end of story.

Definitely true. Send it in with the camera so the lens can be matched. There is next to zero margin for calibration error with the 50/1.2, so they really need to have the camera too to get it perfect.

David

DavidSvensson Senior Member • Posts: 1,374
Joe Mama is right!

Have a look at his images. Artistically those images are in a class of their own, but if your images are not on that level of sharpness, something is not optimally calibrated, or there is a problem with the technique of holding the camera very, very steady.

IMHE Canon is very kind and helpful when it comes to calibration, if you have a good relationship with the technicians.

David

saynomore Veteran Member • Posts: 4,253
Not really a "defect" in my opinion.

brianric wrote:

I usually have that problem when shooting with any fast lens wide
open unless I put the focus point over the point of interest.

Actually that's not the "problem" with this lens. What you mention is not a lens thing, but a DOF and plane of focus thing when recomposing, as you may know. With this 50, when shooting very close (like 1.5m or less) there is some backfocus when shooting a bit closed down.

It is a bit unsettling at first, because, for example, you shoot at f/1.2 from a tripod, and it appears to focus OK. Switch to manual focus, don't move the ring and close down to something like f/2.8 and it would appear backfocused. I think it was concluded that there's some sort of spherical aberration going on at close focusing distances and the lens closed-up a bit, but others might have some updated information on that. I still don't understand fully why it doesn't seem to happen with the peripheral focus points though, but I myself have not tested this particular claim.

That said, it's not a lens "defect" per se, but more of a limitation. I guess they could have shifted the closest focusing distance to say 1.5m, but I rather prefer it this way, since it can be worked around. When shooting at more normal distances, I haven't found any problem at all. I don't shoot too closely with a 50 anyway, especially people. I have the 85 and 135 for closer shots.

-- hide signature --

Andy

saynomore Veteran Member • Posts: 4,253
This is essential to understand

joe mama wrote:

...or any AF point, for that matter. It is only when focusing close
and stopped down that the focus shift occurs, but for that type of
shooting, I'm rarely have a need for center AF, anyway.

This is exactly the situation here. It only happens at very short distances, which most won't be using a 50 for anyway. And if you're that close MF is probably better.

Another thing though, was this concluded to be a case of actual focus shift? I am partial to the spherical aberration hypothesis.

-- hide signature --

Andy

xtoph Veteran Member • Posts: 9,871
the crucial part is where it says

'assuming your lens and camera are properly calibrated' (which it did say). in which case, the sum result of what we are both saying is the same: that there should be no difference between focusing methods wide open.

of course live view focusing is the best option. but it isn't always available or possible.

now, beyond that, this has the potential to be a pretty interesting discussion. i am not sure that it is there yet, however. i already understood that the focus screen alters the apparent dof of the image, and i understand that the autofocus sensors don't see all of the light from the lens, though i hadn't thought all that much about it before your comments. the problem i am having is that you seem to be predicting that live view and manual focusing (with a viewfinder screen of one or another type) will give different results for a wide-open lens. i have done a fair amount of testing, and that does not appear to be the case: manual focusing wide open is accurate, and then focus will shift upon stopping down. that is the empirical data i am dealing with; your explanation of how things work would seem to predict something different. since i don't get anything different, i am inclined to believe that physically stopping down the lens has a different effect than the ersatz effects of the fresnel screen (and possibly of the autofocus sensors). but i don't have time to think more about it right now--i will be happy to hear any clarification, though, and try to work it through later when i have time.

DavidSvensson Senior Member • Posts: 1,374
More on focusing screens

xtoph wrote:

'assuming your lens and camera are properly calibrated' (which it did
say). in which case, the sum result of what we are both saying is the
same: that there should be no difference between focusing methods
wide open.

of course live view focusing is the best option. but it isn't
always available or possible.

now, beyond that, this has the potential to be a pretty interesting
discussion. i am not sure that it is there yet, however. i already
understood that the focus screen alters the apparent dof of the
image,

This is due to the inability of the focus screen (unless it is deigned to be extremely diffusive and thus very dark) to collect light from all over the lens.

The focusing screen will act as a secondary aperture to the lens. The effect is visible if you keep the DOF button pressed while changing the aperture setting. There will be little change visible between the aperture stops when the aperture is larger (lower f-number) than 2.5.

and i understand that the autofocus sensors don't see all of
the light from the lens, though i hadn't thought all that much about
it before your comments. the problem i am having is that you seem to
be predicting that live view and manual focusing (with a viewfinder
screen of one or another type) will give different results for a
wide-open lens.

If the lens has spherical abberration, and its maximum aperture is bigger than f/2.5, there will be a difference in focusing wide open between using the focus screen and using the Live-view.

i have done a fair amount of testing, and that does
not appear to be the case: manual focusing wide open is accurate, and
then focus will shift upon stopping down.

Perhaps on your camera there is an offset relative the image sensor and the focusing screen, such that by chance the sources of error cancel out when focusing a 50/1.2 manually on the focusing screen?

I don´t know, but there was an offset of 0.1 mm of the focusing screen on my 5D until I figured it out, and Canon service adjusted it. That tiny offset made manual focus next to useless.

that is the empirical data
i am dealing with; your explanation of how things work would seem to
predict something different. since i don't get anything different, i
am inclined to believe that physically stopping down the lens has a
different effect than the ersatz effects of the fresnel screen (and
possibly of the autofocus sensors). but i don't have time to think
more about it right now--i will be happy to hear any clarification,
though, and try to work it through later when i have time.

Optics is such fun, isn´t it!

David

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