antoine_clappier
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Posts posted by antoine_clappier
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Hi all!
I have just ordered an EF-S 17-55 f2.8 for a 40D. I wish to protect it with a
filter without degrading the lens quality.
Traditionally peoples buy UV filters for that purpose. But, does it make sense
for a modern digital camera? Is the censor sensitive to UV?
If not, what are you using instead?
Thanks,
Antoine
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Steven,
No, I don't know. The article talks also of the 24x36 format ("Canon's designated circle of confusion is 0.035mm for the 24x36mm format". So it may also apply to fullframe censors.
Antoine
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Steven,
My original post was about the 40D and then APS-C censors. You can find detailed information there:
http://mkoehler.de/index.php?option=com_content&task=view&id=15&Itemid=9
I cannot say if the explanations are entirely correct but they definitely make sense. I found the article an interesting read.
Antoine
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Bruce,
I did read Photozone.de reviews before purchase. From there conclusion, the 17-85, while not a high-end lens, appears as a good choice. I was very tempted by the 17-55 which had a definitely much better review. But this lens is simple two time more expensive here in France.
While re-reading the reviews today, I noticed also that they announce that they will post an updated review. We will see what is their second take on the subject.
From the various comments I gathered, it seems that I have been unlucky and got a lens which appears to have a fault. I will return it tomorrow.
The only remaining question is whether I try again a 17-85 or extend my budget to get a 17-55!
I have nothing again Canon. That is the contrary! I have bought numerous Canon cameras for me, my company and as gifts to my family.
This is the first time my expectations with a Canon product were not met. This is why I have been quite careful in my "investigation" of the problem.
I take the opportunity of this last post on that subject to thanks you and the Photo.net community for the useful comments and advices.
Antoine
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Bruce,
I do not understand your answer. I am just trying to solve a potential issue and get feedback from knowledgeable people. I am not giving any judgment on the lens and/or body in general. I show test images made with my own 40D and its lens.
This is the only way for others to tell me if something is wrong with my camera.
About comparing my results, I cannot do that since I do not have access to another 40D. If it is was the case, I would not need to post on the forum to get an opinion about the quality I get.
Antoine
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Puppy Face,
EFS 17-55 is an entirely different beast. It is a way better than my 17-85 in any case. This said, I agree that I have probably with the lens.
I am actually thinking about returning the 40D + EFS 17-85 kit and get the 40D with the EFS 17-55. This lens has excellent reviews and what you are saying about it is pushing me to pay more and get a better lens.
Antoine
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Steven,
I was just talking about that in a different thread! Here is a partial copy of my post:
---
By reading recent posts on this forum, I have found a very interesting link (Technical overview of Canon AF): (link)
The infos found at that page could explain some of my focus issues. As you noted, I was using a small aperture. This combined with the fact that the AF is calibrated for a circle of confusion of 0.02mm may explain that focus stops way short. On a 10mp APS-C censor, 0.02mm is 3.45 pixel wide (3888*0.02/(36/1.6)). This is a large number! If AF is satisfied with a 0.02 circle of confusion, the resulting photo will have pixels blurred on a width of 3.45 pixels (defect clearly visible at 1:1 resolution).
I have found a way to prove/disprove indirectly this theory. If the AF stops as soon as it finds a 0.02mm circle of confusion, the prior lens focus distance will have a measurable effect on the focus plan distance. A simple experiment just shows that:
- Set manually the focus distance to the minimum. Aim to an object at say 3m. Turn AF-ON. Note the focus distance.
- Set manually the focus distance to infinity. Aim to same object. Turn AF-ON. The focus distance is now noticeably longer. Try this at home it is interesting!
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Based on the that, I think it is safe to say that prior focusing distance does have an effect on how AF will work next. This could also explain the difficulty to reproduce AF calibration tests.
Antoine
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Hi all!
This post is a follow-up on my previous thread on focus issues with a 40D. You
can read the initial discussion there:
http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00NVwg
The weather finally permitted more in depth tests. I have been able to take
several photos with a tripod under similar lighting conditions to compare manual
focus and AF.
Here is what I have obtained:
http://www.flickr.com/photo_zoom.gne?id=2086960672&size=o
Both images have the same settings (apart Focus mode obviously): 1/60Sec, F8.0
ISO Speed 400, Focal Length 17,0 mm.
** Process:
- Top image was shot using Central AF Point only. This is the best image I have
obtained. To be fair, I have shot several images using the AF and kept only the
best one.
- Bottom image was shot using best possible manual focus.
- Both images were taken using a tripod (camera was not moved between AF and
manual shots).
- Both images were taken in RAW, developed with Canon's Digital Photo
Professional with all settings to zero (no enhancement, no correction), exported
to TIFF and cropped in Photoshop (no other edit).
- The images shows the exact center of the photographies at 1:1 resolution.
** Conclusion and questions:
- Bottom image is clearly sharper than top image. AF was wrong.
- At focal 17mm the AF tends to set focus distance around 3m whether subject
distance is 3m, 10m, 50m or more (from my repeated tests).
- Problem disappear for focal lengths greater than 35mm. AF is then as good as
manual focus.
- Additional issue: you can clearly see Chromatic Aberrations on the border of
the background white fence. Visible CA this close to the center of the lens
seems unusual.
- Corner CA seems very, very strong. I know that CA is not exactly the strong
point of the EF-S 17-85mm but this seems a bit excessive. See a top left crop of
the same photo:
http://www.flickr.com/photo_zoom.gne?id=2086972998&size=o
** What is you opinion?
- is the focus issue a limitation of the 40D AF or is it abnormal?
- do you experience such bad Chromatic Aberrations with your EF-S 17-85?
- Whether AF or manual, I don't find the images especially sharp. I was
expecting a much better result from the 40D and this lens. Do you agree?
Thanks,
Antoine
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Bruce,
The sun was back on Monday morning and I had the possibility to do systematic tests on a tripod at various focal lengths. I will prepare a complete follow up post today with my findings. Let's just say for the moment that I think they prove that my AF is not working correctly for short focal lengths.
I will post under a new thread as the current one is getting a bit too long.
Thanks,
Antoine
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Joshua,
I thought like you that Live View will not be precise enough for manual focus. I was surprised to find I was wrong.
The quality is very impressive. At full zoom, the Live View gives a 1:1 resolution. One censor pixel maps to one LCD pixel. The image is extremely sharp and is superior to the viewfinder (at least tothe 40D one!) for very accurate manual focus (on a tripod of course).
A surprising thing is the fact that the picture is much sharper during Live View than during review of the taken picture. I don't know how Canon achieves that.
I think anyway LiveView is more useful in a controlled environment (studio work).
Your A-DEP suggestion is interesting. I will try that too.
Thanks,
Antoine
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Bruce,
As I wrote in my today post, I do believe that it is probably due to a defect with the lens or body (which probably increase the "FCCoC").
In any case, it has forced me to look in more details how a modern AF works. The AF history phenomenon is interesting. It could be actually used to help the AF in some situations.
I will continue to post if I find an answer to my problems.
Thanks,
Antoine
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Many thanks for your useful comments.
By reading recent posts on this forum, I have found a very interesting link (Technical overview of Canon AF):
http://mkoehler.de/index.php?option=com_content&task=view&id=15&Itemid=9
The infos found at that page could explain some of my focus issues. As you noted, I was using a small aperture. This combined with the fact that the AF is calibrated for a circle of confusion of 0.02mm may explain that focus stops way short. On a 10mp APS-C censor, 0.02mm is 3.45 pixel wide (3888*0.02/(36/1.6)). This is a large number! If AF is satisfied with a 0.02 circle of confusion, the resulting photo will have pixels blurred on a width of 3.45 pixels (defect clearly visible at 1:1 resolution).
I have found a way to prove/disprove indirectly this theory. If the AF stops as soon as it finds a 0.02mm circle of confusion, the prior lens focus distance will have a measurable effect on the focus plan distance. A simple experiment just shows that:
- Set manually the focus distance to the minimum. Aim to an object at say 3m. Turn AF-ON. Note the focus distance.
- Set manually the focus distance to infinity. Aim to same object. Turn AF-ON. The focus distance is now noticeably longer.
Try this at home it is interesting!
I believe the previous experiment shows that the AF indeed stops at a given circle of confusion and does not go further. Then, the correct focus distance is between the two extremes of the Depth of Focus starting and ending with a circle of confusion of 0.02mm (probably the values determined by the previous experiment).
This said. I do not have the same issues with other Canon bodies (including the simple Canon G7 that I am still using). Also, why the problem suddenly stops happening after 35mm?
I have the feeling that there are two problems: it is possible that the 40D favors AF speed over accuracy (a much longer investigation is needed to prove that) and that I have a calibration issue with the lens/body that increases the AF problem.
Find below personal answers to your comments,
Antoine
Bruce:
how I have determined that the focus should have been infinity? Easy! The 40D features a very capable Live View that allows you to manually set the focus with almost perfect accuracy. From there it is easy to check what is the focus distance by reading the marking on the lens itself.
I know this is not a reliable method and gives only a ballpark number. But, in my case, manual focus gives a focus distance "around infinity" and auto-focus sets a focus distance around 3m. The difference is large enough to show there is indeed a problem.
About my experience: I am not a professional photographer but I believe I have a strong background in optics, computer graphics, etc. I have written several software in these fields and founded several companies. I am currently developing a Computational Photography software (HRDI, Synthetic Relighting, etc). I have bought the 40D to test my algorithms. In any case, I am here to learn. Your comments are welcomed!
Bruce, Joshua:
I agree with your comments about the photos. The problem is not glaring but that is just because I have used a fairly large DOF (aperture was respectively f10 and f9). The photos are not totally ruined and could be saved by using a fairly strong sharpening in post-process. Anyway, that does not change the fact that the camera has focused in the foreground instead of the background as wanted.
Rainer T:
Very interesting post. I will investigate this simple method for back/front focus test. I tried quickly with a simple book yesterday night and did not find an obvious issue. But I will try again with the provided chart.
Baby Face, Joshua:
I cannot do further tests today. The weather is absolutely awful today in the Loire Valley! I will continue on Monday if I can.
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Another example:
Focus was on the house not on the iced lavender in the foreground. Same comment as previous post about the strange exposure!
Antoine
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Posted an example on Flickr:
Image developed in LightRoom 1.3 from RAW file.
Exposure is a bit unusual because this photo was taken for future HDRI treatment and was underexposed on purpose. I have corrected in part the underexposure in LightRoom.
Antoine
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Joshua,
I am fairly new to this forum. Is there a place where I can post full resolution photos? Or do I have to post on an other web site?
Thanks,
Antoine
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Hi all!
I have lately acquired a 40D with the EF-S 17-85 IS kit lens and am experiencing
severe focus issues.
After taking a few hundred photos today, I came home to discover that about 90%
where badly focused. My first thought: user error! Sadly, it does not seem to be
the case. Here is what I have found:
1- This is not a speed issue. The photos are not fully blurred. The focus
distance is simply wrong. Besides, the speed was largely in range for the focal
length/aperture/ISO used (even without taking into account the IS which was
turned on).
2- This is not a focus lag issue. I was not shooting Formula 1 racing cars but a
peaceful landscape!
3- The issue appears only for focal lengths between 27 and 35mm (35mm equivalent.)
4- Focus distance is always way too short. Most of my pictures were faraway
trees over a clean lawn taken at 27mm. Focus distance should have been infinity.
Photos show a focus distance of about 3 to 4m.
From there, I assumed another user error: bad selection of the AF point! Not so!
I went again outside and took a close look at what was going on:
1- First thing first, I activated only the central AF point. Aimed to a tree (I
love trees!) and shoot. Same problem: Tree blurred, the lawn in front of my
feet: crisp.
2- I then switched to tripod, LiveView and manual focus (BTW manual focus with
LiveView is a pleasure to use for landscapes). No problem: focus distance
perfect, picture so crisp it hurts!
3- Third test, LiveView, manual focus, then AF-ON: photo blurred, bad focus
distance. AF has actually ruined the focus previously done manually.
4- I will skip a few other tests, the conclusion is surprising: my 40D + EF-S
17-85 IS almost always focus at about 3m or less when using the low end of the
focal length. Prayers, threats will not change its behavior. My 40D thinks
infinity starts at 3m!
Questions:
- am I still doing something wrong (I just started using the 40D after all)?
- am I experiencing some kind of hardware failure?
- is there a known issue with the 40D AF (based on several forums it seems that
I may not be alone)?
Can someone on the forum with a 40D + EF-S 17-85 shoots at 17mm a few faraway
trees (or something else ;-) ) and see if he/she can reproduce the issue.
Sorry for the long post. I am highly interested to get your comments or tests,
Antoine
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Tim: Yes, this is quite strange. LCD Monitors are probably one of the few fields in computer hardware where there is still a very large difference in quality between entry level and high end.
I don't understand why the monitor industry is communicating so little. It is trully difficult to find good review sites.
I hope that Marketers at Eizo, Samsung, Nec and Lacie are reading this forum and will react. These brands carry truly superior models but if you are not an expert there is noway to know that!
Antoine
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Hi all!
A few days ago I discovered the Photo.net forums and asked questions about
Monitors. Since then I have collected a few interesting links with good reviews.
<BR>
<BR>
As a small contribution to the forum, here is the list:<BR>
<BR>
** Reviews of monitor sorted by color accuracy:<BR>
<a
Not a huge list but quite interesting.<BR>
<BR>
** User reviews with relatively good information:<BR>
<a
href="http://www.trustedreviews.com/displays/p1">http://www.trustedreviews.com/displays/p1</a><BR>
<BR>
** A huge post on Anandtech:<BR>
<a
Very detailed LCD buyer guide. Interesting.<BR>
<BR>
** A good review site specialized in LCD monitors:<BR>
<a
href="http://www.prad.de/en/monitore/reviews.html">http://www.prad.de/en/monitore/reviews.html</a><BR>
<BR>
** More mainstream web sites but interesting anyway:<BR>
<a
<a
<a
href="http://www.pcworld.com/ic/monitors/">http://www.pcworld.com/ic/monitors/</a><BR>
<BR>
I have found the reviews on CNet / ZDnet of mediocre quality. Their conclusions
are often very different from other reputed sites. Why?<BR>
<BR>
<BR>
I hope this short list will help others to find the LCD Monitor of their dreams!<BR>
<BR>
If I have missed major review sites please reply to this post to complete my
list.<BR>
<BR>
Antoine Clappier<BR>
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Many thanks for your interesting answers.
Paul P: thanks for your offer but I would rather buy a new monitor (besides I live in Paris, France so it may be a bit complicated to ship it anyway!).
Antoine Clappier
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Thank you all for your answers.
I just discovered the Photo.net forums. I like very much the tone of the forum both serious and funny at the same time.
This is getting a rarity on internet forums!
Antoine Clappier
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Hi all!
I have a second question about buying a new LCD Monitor.
My new laptop comes with an HDMI output. Problem: most if not all the
professional Monitors come with a DVI input!
I have found HDMI to DVI cables but I have no idea of what happens inside this
converter cable. Does the signal is downgraded? Is it converted to analog video?
Remain stricly the same? Does it have an impact on color accuracy?
I am greatly interested to get your feedback on this question.
Thanks,
Antoine Clappier
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Hi all!
I am looking for a new LCD monitor for a "peculiar" purpose!
A bit about myself, and what I am planning to do with the monitor! I am a
professional software developer with a strong interest in photography and
computer graphics.
I am currently developing a new standalone software for high end digital
photography edition (HDRI, Tone Mapping, synthetic relighting, depth of field
re-rendering, etc). The software will be a commercial product and will be used
by people like you (at least I hope so)!
In other words, I am looking for the typical monitor for digital photography
edition (if such a beast exist): not super high-ends, not low end. I want to
find a monitor that is close to what my future users are using.
So the question is: what do you think is the middle range monitor for the
serious photographer (pro or enthusiast)?
Let me know what you think,
Antoine Clappier
PS: I was stunned to discover that monitor reviews for photographer do not exist
on the web (or are really hard to find)! Do you have any recommendation of
serious monitor review sites?
Does a UV filter make sense for digital sensors?
in Canon EOS Mount
Posted
Hi all!
This forum is truly incredible! Six hours after posting what I thought was a minor question, I get no less than 21 answers!
Many thanks for your useful advices. I will add to my arsenal a 77mm filter (I am bit concerned by the "dust problem" that was also noted on others forums).
Antoine