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  #1  
Old 09-10-2005, 05:56 AM
ngythanh ngythanh is offline
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Default Help me, please!

A friend asked me why his Canon EF 28-135mm IS lens produced pictures with more clarity and depth-of-field than Canon EF 24-70mm L lens?
Out of the advantage of shooting under low light, he said he experienced problem [like out-of-focus most of the time] with the expensive lens while the "cheap" 28-135mm gave him better result.

I am not a technician to have the answer. Can you help me, please?
Thanks.

ngythanh
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  #2  
Old 09-10-2005, 07:03 AM
Darren Darren is offline
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Default Re: Help me, please!

Hi. YOu don't mention what camera he is using, and that could be important.

The DOF would only have to do with what aperture is being used, assuming the same subject is framed in the same way. As for other reasons the 28-135 might be working better, my guess is that there might be focus issues. If you read in the Canon forums, ever since the D60 (maybe before), there have been complaints of certain Canon bodies backfocusing with certain lenses. I believe the 24-70L is very, very often one of the lenses to draw the complaints. I know when i met Philip and Thien in Myanmar last year, Thien's 24-70 did indeed focus very poorly on Philip's 10D. At the time, Thien said that it was not uncommon with Canon's consumer level DSLRs (not 1 series). I believe with fast lenses, focus errors are often magnified, because we tend to shoot them more wide open, leading to an even smaller plane of focus.

If your friend's camera and lens are still under warranty, I believe Canon will calibrate them free of charge.
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Old 09-10-2005, 01:00 PM
ngythanh ngythanh is offline
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Default Re: Help me, please!

Thanks, Darren.

My friends tested both lenses on his Canon 20D Digital. I only heard about the result of his verifivation without seeing it for a comparison, but I understood his situation.

When I first bought the Canon EF 24-70mmL and used during my first "assigment" without experience, most (about 95%) of my photos taken at Sa Pa and Bac Ha in Vietnam have been in the same poor condition regarding plane of focus (while the ones at Lang Co were not). I remembered that at Bac Ha, I tried to catch people in movement. Film photos didn't store "exif" data but you sounded correct when saying "because we tend to shoot them more wide open". Yes, I may set the camera (EOS-3) at quickier speed and got the problem...

Because you mentioned an experience exchanged with Thien, I am going to email him this discussion. He is now very busy, but I hope he will contribute some help to me, and to my friend.

Thanks, and have a good weekend, Darren.

Thanh
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  #4  
Old 09-12-2005, 05:56 AM
thien thien is offline
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Default Re: Help me, please!

Allo chu Thanh,

Although my copy of 28-135mm is no slouch, what it can produce cannot compare with my current 24-70mm. I find that it is strange that your friend have found that it focus both slower and less contrasty (I am guessing that the clarity that you mentioned is due to contrast).

Does he mentioned to you how he did the tests? One of the thing that I have discover is that the UV filter (or any filter that is) has a negative impact on the performance of the lens. My friend has a problem with blurry pictures until we removed his supposedly pretty good Marumi filter. He is now using the B&W Pro as I am.

About the DOF, I could not quite understand if you meant the 28-135mm has more DOF than the 24-70mm. Presumably that he shoots at the same aperture and focal length, it should not make any difference between them. My 28-135 had the tendency of front focusing just a bit. The 24-70mm never had this problem.

Low light focusing, this one is a very interesting topic. In absolute term, the 24-70mm kills the 28-135mm except for these little tibits.

a) The focus travel of the 24-70mm is longer than the 28-135mm (i.e.: it takes longer for the focus ring of the 24-70mm to make a complete traversal from one end of the focus range to the other end).

b) If the flash focusing aid is turned on, in a low light situation, both may have the same focusing speed. The reason is quite simple, Canon flash focusing integration blows. It takes Canon quite sometimes to switch to the flash focus and the algorithm to decide how to switch is horrible. Turn on AI-Servo (which disable the flash focus aid) in a low-light situation and you will see that the 24-70mm will almost grab on anything with the central + sensor while the 28-135mm will hunt forever. This is because the central + sensor is hyper active when a F2.8 lens or better is attached.

I haven't use a 20D so I do not know how it behaves but I use the AI-Servo exclusily while moving the focusing trigger to the exposure lock (*) button with CF-4. This allows me to control the autofocus when I want and a side effect is the autofocus is a lot more nervous too (at least on a 1D mkII)

Hope this helps your friend, chu Thanh
Thien
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  #5  
Old 09-28-2005, 01:16 AM
NgocSon NgocSon is offline
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Default Re: Help me, please!

A. Thien,

I am a friend that run into the problems Chu' Tha`nh is mention about. I have spend a few days to do "research" on the filter B&W pro as you state above, but can't find such as B+W Pro for a 77mm. Please help me here, I will more than happy if you specify the filter for me in BH web site.

I am currently using a 24-70L with a Canon 77mm Haze UV-1 Glass Filter (L-39 Sharp Cut) together with a Canon 20D. Recently, I was playing with my old 28-135mm IS and the 24-70mm, both is at 70mm focal lens, F 5.6, and exposure time 1 second; both is under autofocus. I was especting the image came out from 24-70mm is better, but the 28-135mm's image gave me a sharper and more constrast. A. Thien, please help here or point out any thing that I am doing wrong.

Thank a bunch.
SN
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  #6  
Old 09-28-2005, 03:14 PM
AdrianW AdrianW is offline
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Default Re: Help me, please!

How are you conducting the tests? If IS is on, then I'd suggest camera shake would be the obvious reason behind the 28-135 being sharper...

If you have more DOF in the 28-135 shots, then I'd suspect you're shooting both wide open - remember that the 24-70 is f2.8 throughout, the 28-135 is f3.5-f5.6. Check the EXIF if you're using digital. Shoot both at f5.6 on a sturdy tripod, using mirror lockup and a remote release. Try focussing manually too, just in case there is an AF issue (some camera/lens combinations don't seem to focus particularly well - may need to go back to Canon for recalibration)

Also check the cleanliness of the front element, and look through both lenses against a flat white surface - it's possible that you have a mould problem inside one of them.

Just some quick thoughts, although I'm sure you've already covered all that :)
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