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Author Topic: 24-70L plagued with some fluorescents?  (Read 3007 times)
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« on: July 23, 2005, 07:20:51 AM »
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I've seen this hysterical thread as well.

Ask yourself this...

This lens has been out for years.

CFL lighting has been out for years.

Many hundreds of thousands photographs have likely been taken by many thousands of photographers with this lens under those lights, and this is the first time that this issue has been raised.

Think as well about the issue itself. A lens misfocusing because of the spectral characteristics of the light source. Humm. The autofocus sensor and mechanism is in the camera, not the lens. The lens simply has a motor to move the elements. Can the light source be affecting the motor?

Perhaps? But I kind of doubt it.

Typical DPReview forum hysteria, if you ask me.

Michael
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Jonathan Wienke
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« Reply #1 on: July 23, 2005, 11:47:39 AM »
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I am planning to buy a Canon 24-70L lens and saw this topic in dpreview forums:

http://forums.dpreview.com/forums....4296547
The DPReview forums can be safely dismissed as the National Enquirer or Weekly World News of photography forums. The person talking about the "flicker" of incandescent bulbs as if that was a significant factor in focusing with that sort of lighting is typical, as is the people talking about the "autofocus mechanism in 24-70L". There is no such thing, the autofocus mechanism is in the camera except for the motor that moves the lens elements. The components in the body are responsible for judging focus accuracy and deciding which way the lens should move, and how far. The lens motor simply does what the body tells it to do, and plays no role whatsoever in the decision-making process. And the fact that those people never once thought to take into account the DOF difference between f/2.8 and f/4-5.6 when comparing focus results from the 24-70 and the 28-105 is worth noting as well. If there is a hardware problem, it is most likely the 24-70 and the particular body being slightly mis-focus-calibrated in the same direction, which will be more noticeable in the low light levels associated with compact fluorescent lights than other, brighter light sources. But it could very well be a simple case of attempting to use the 10D AF in light levels below what it is designed to autofocus accurately.

To those who think that the fluorescent flicker is a factor in the problem, I'd like to point out that the "focus assist light" of the 10D/20D is nothing more than the flash being pulsed, which in spite of being far more "flickery" than fluorescent lighting manages to work quite well as long as the subject is within flash range. The 10D and 20D AF systems are designed to AF using transient pulsed light sources as well as continuous lighting and still work effectively.

In addition , I have a 24-70/2.8L and have made tens of thousands of images with it with a combination of 10D, 1D-MkII, and 1Ds bodies (mostly the latter two), in a wide variety of light sources and brightness levels and have never observed a link between AF performance and a particular light source, only between brightness and AF accuracy.
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framah
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« Reply #2 on: July 23, 2005, 03:16:00 PM »
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So... Jani... Where are the sandpaper test shots??? I"m waiting!!!  Interesting that you conveniently "forgot" to perform that test.     :p
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« Reply #3 on: July 23, 2005, 09:35:41 PM »
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Thank you jani for spending so much time on this subject. As I see from your shots, they are not tack sharp.
The area in focus is pretty sharp, given the lighting conditions, and that two are at maximum aperture. Apparent sharpness really suffers from the direct and indirect light, this is why the mantra goes "use a lens hood - at all times".

Also keep in mind that I haven't done any sharpening (or used similar techniques), anywhere.

I've taken the same shot with a lens hood, just to show how different this can look, at f/2.8:



If you're thinking of the softness, then yes, I expect it to be slightly soft at its widest aperture.

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Have you compared those shots with some taken with a conventional bulb? Does 24-70 behave differently then?
I dug out a 60W bulb from the closet to check. Though it's 33% or so brighter than the other one, in one case I had apparent back-focusing at f/2.8 when the light was aimed at the camera. When the light was aimed at the bookshelf, it appeared slightly better. But the results are not consistent enough. Perhaps I should blame the light in the ceiling, that's fluorescent ...

In other words, the images doesn't show anything particularly different with or without CFL.

I also tested with my EOS 650 (yes, that first of the series film bodies). The brighter and larger viewfinder makes it easier to check focus. What I saw was that at the point I chose to focus, it was easy to provoke a slight nervousness in the camera's autofocus by moving the camera slightly sideways. This is also detectable by watching the focus indicator on the lens. I re-tested this with the 20D, and the same effect is apparent.

So maybe I didn't tighten the panning screw enough, for instance. If you look closely at my 100% crops, some movement clearly has happened between shots (the 100% crops are not perfectly aligned, though they should be because I used save/load selection).

Another error source is that when the lamp points at the camera, it also points at the eyes that are trying to verify the focus.

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My main concern is whether other cheaper lenses take sharper pictures under CFLs as claimed there.
Well, those cheaper lenses won't give you f/2.8.

Here's a similar crop from an image taken with the 28-135 at f/5:



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michael, I am not really concerned whether the spectrum of CFLs causes the problem or their flickering. And It doesn't matter how the AF mechanism operates. What is important to me is confirming or refuting what is claimed in that forum. If confirmed, I will know that 24-70 has some limitation not discussed yet. And if refuted, I will be happy that it has not that limitation. I think that in both cases I will buy one. As I said, it has proved to be a decent lens.
I think it's safe to consider it refuted that this lens model has some special problem. I certainly don't see this problem of apparent back-focusing crop up often. As I have just demonstrated through further testing, it may just as well be inaccuracy in the center focus point of the camera or inaccuracy on part of the photographer, though not inaccuracy in measuring the distance.

And after all, worse things happen at sea, you know:




(70mm, f/5.6, 1/125s)

Source of problem: photographer (not that I had the time to refocus, it was a moment). The camera could only have saved this one if it was psychic.
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Jan
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« Reply #4 on: July 23, 2005, 01:58:19 AM »
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I am planning to buy a Canon 24-70L lens and saw this topic in dpreview forums:

http://forums.dpreview.com/forums....4296547

I know it is a decent lens. But I want to take many shots under CFL lighting. Almost all of the lamps in my house are CFL.
Has anyone here noticed the problem described in that forum? Or, can those owning this lens take some shots with CFL lighting and check whether the problem really exists?

Thanks
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« Reply #5 on: July 23, 2005, 08:44:44 AM »
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I can certainly see why this issue doesn't warrant taking a lot of time, and why people might call this a waste of time -- yours and mine.

Well, I took the time -- for once -- to read a bit more of the thread on that other site, and it seems that at least some posters thought of Michael's objections.

The response was, in brief, that it might have something to do with the coating of the lens elements. And that at least one person had a "good" sample.

But still, it's only speculation, and as Michael mentions, this lens isn't exactly rarely used among photographers.

I find the lack of information from some of them disturbing:

 - Which f stops did they use?
 - ISO?
 - Did they use a UV filter?
 - If so, of which quality?
 - And did they retest without a UV filter?
 - Did they take their lens out of the fridge before shooting at 70% relative humidity?
 - Have they scrubbed their lenses with sandpaper? Who knows.

Okay, the last two may be out of line.

So I decided to set up a test shot myself (and I certainly don't blame anybody else for not doing this):

 - EOS 20D
 - EF 24-70mm f/2.8L
 - Tripod
 - Compact fluorescent bulb (IKEA brand) in desk side lamp
 - Poor ambient lighting
 - Bookshelf at ~1m distance
 - AF set to single point center

To ensure that those ominous IR rays would have a chance at disturbing the autofocus, I didn't use a lens hood. No UV filter, either.

I took four shots with the self-timer and mirror-lockup at ISO 100, 70mm, whereof two at f/2.8 and two at f/5.6, and of course I refocused between each shot. Two of these were with the lamp aimed at the lens at an angle that didn't put the lamp itself in the view:



I used PS CS2 with the most recent ACR, camera white balance, all other adjustments zero (not auto), prophoto RGB. Resized in CS2, 100% crops from CS2. Converted to 8bit, then sRGB with relative colorimetric and dither. Converted to JPEG with save for web, JPEG maximum quality (the 300x200 minis are in medium quality). No, I didn't bother to do anything in CS2 except that.

Image 1 (f/2.8, lamp aimed at camera, 0.3s):





Image 2 (f/2.8, lamp aimed at bookshelf, 1/30s):





Oops, out of focus?



Yes, it appears to have back-focused slightly. The first f/2.8 shot was similar, but to a lesser degree.

Image 3 (f/5.6, lamp aimed at bookshelf, 1/8s):





Image 4 (f/5.6, lamp aimed at camera, 1.3s):





I can't really say that any of these images show any evidence of the problems that people claim in the DPReview forum. But this is my lens and my camera. *shrug* My guess is that it is the photographers having focusing problems.


That being said, I had an interesting experience with my 20D and 24-70L myself yesterday evening. I was out shooting around the abundant forest roads in Maridalen, Oslo, hoping to capture some scenes I saw while on a bicycle trip earlier in the evening. There had just been a rainfall, and I thought "cool, that will finally let me capture how greenery looks just after rain".

The 24-70L consistently back-focused. On subjects 2-3m away, it insisted on focusing at more than twice that distance (close to 10m according to the lens). Weird. Perhaps I'll see a hint when I look at the raw files later.

In other situations, the camera and lens have been behaving in a predictable manner.
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Jan
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« Reply #6 on: July 23, 2005, 02:54:10 PM »
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Thank you jani for spending so much time on this subject. As I see from your shots, they are not tack sharp. I mean they confirm what has been claimed on dpreview forum. Do you really expect 24-70 mounted on a 20D with tripod to behave as in the second shot, even at f2.8?

Have you compared those shots with some taken with a conventional bulb? Does 24-70 behave differently then?

My main concern is whether other cheaper lenses take sharper pictures under CFLs as claimed there. If so, I will keep my 28-135 IS happily without spending so much on a shorter range zoom. But I am kidding. as michael says, this lens has proved as a good one for a long time.

michael, I am not really concerned whether the spectrum of CFLs causes the problem or their flickering. And It doesn't matter how the AF mechanism operates. What is important to me is confirming or refuting what is claimed in that forum. If confirmed, I will know that 24-70 has some limitation not discussed yet. And if refuted, I will be happy that it has not that limitation. I think that in both cases I will buy one. As I said, it has proved to be a decent lens.

I really do not expect time consuming precise tests as jani has kindly done. If someone has a 24-70, he/she can take two shots under both types of lights. I mean CFL and conventional and compare the two shots. If any significant difference is noticed, please let me and others share your finding.

Thanks
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« Reply #7 on: July 23, 2005, 07:14:24 PM »
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So... Jani... Where are the sandpaper test shots??? I"m waiting!!!  Interesting that you conveniently "forgot" to perform that test.     
Hot dang!

I'll get right to it, I just have to check with chicken fat, first. :cool:
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Jan
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« Reply #8 on: July 24, 2005, 12:01:56 PM »
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Thank you jani for your patience in solving my problem. As I said before, I am going to buy this lens regardless of what has been said about it in that forum. You have convinced me not to worry about its speculated shortcomings and no one can ignore tons of data about its superiority over other lenses in its class.
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