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 Post subject: How to test 24-105
PostPosted: Mon Sep 24, 2007 10:38 pm 
Hi
I recieved my 24-105 today.
First impression, it was solidly built and about as heavy as my 70-200 f/4 (non IS).

I thought it was made of metal just like my white lens, but it feels like hard plastic(correct me if Im wrong).


What is the proper way to test its sharpness against the 70-200?


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PostPosted: Mon Sep 24, 2007 10:43 pm 
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Hi, our testing methodology is here:

http://www.cameralabs.com/about/testing.shtml

But if I were you, try finding a nice scene with lots of details in the corners which doesn't change much over the course of a few minutes and take photos with each lens - on a tripod with IS switched OFF - at identical focal lengths - and if you're being very thorough, at every aperture setting too.

Then zoom into each image and look closely, especially towards the edges and corners.


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 Post subject:
PostPosted: Tue Sep 25, 2007 6:40 am 
Hi Gordon
I am actually interested in testing the sharpness especially in the middle ( but Ill also check the corners). Can I print an A4 size paper full of Fonts? If thats possible, how big fonts should they be, and how far must I be from the subject?
Also I will be testing indoors with tungsten bulbs.
Thank you


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PostPosted: Tue Sep 25, 2007 6:55 am 
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Photographing charts is all very well, but you'll be testing the performance at close range - that's why I also always include outdoor tests taken at infinity - it's much better.

But sure, print out, say, five pages of A4 with various sized fonts on them and stick them on a wall arranged as an X - maybe with a gap between each so the total area would measure maybe 1.5m by 1m. Try to keep the shape of your chart the same shape as the camera frame, which on most DSLRs is 3:2 aspect ratio - so 1.5 times wider than it is tall.


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 Post subject:
PostPosted: Tue Sep 25, 2007 10:08 pm 
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Why print? Just shoot your monitor :wink:
See my article here: http://www.cameralabs.com/forum/viewtopic.php?t=1283
At 50-100xfocal length i.e. 2.4m-5m your pretty safe to capture the sharpness of the lens and not the monitor...
Just watch out to focus well!

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 Post subject:
PostPosted: Thu Sep 27, 2007 10:09 am 
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eL wrote:
I have a 17 inch flat monitor with 1024x768 resolution.
Can you please tell me how big fonts do I put on the screen and just how far I need to be for the following: 24mm, 50mm, 70mm, 105mm


Hi eL!

If you use the standard windows "Editor" or Excel and don't fiddle with font sizes, the letters are displayed at a 1-pixel width. That is the sharpest you can get on a monitor. So just fill up your screen with "X"s and "H"s and set your monitor to a brightness which you find "bright" but not "glaring". The black letters should still look just that: black. As an aside my monitor is not too bright, so I turn brightness all the way up to 100%. A bright monitor also makes it simpler to shoot at shorter shutter-speeds and avoid shake if you're shooting handheld (for best results though use a tripod or at least some suport for your cam).

Now on to taking the shots:
For starters just take pictures at your favorite focal lengths where the monitor just fills the viewfinder. In this case the magnification is about 1:15 meaning that the distance you need to have from your monitor is about 15xfocal-length: so for testing at 70mm your camera has to be around 1m away from the screen. Just go back and forth until the screen totally fills your view-finder. Watch-out to be perfectly perpendicular to the centre of the monitor (that's where the tripod helps, too) which you can easily controll by checking that the pic in your view-finder is as rectangular as can be - otherwise, you're slightly off-centre and have your cam tilted non-parallel to your monitor.
Remark 1: As your monitor has an aspect-ratio of 4:3 and your cam an aspect-ratio of 3:2 you will not see the upper/lower borders of your monitor, when you have the left/right-borders of the monitor aligned with the left/right borders of your view-finder. But that's ok.
Remark 2: It may happen, that at 24mm focal length you cannot get close enough to your monitor to fill the viewfinder (24mmx15=36cm distance). That's also not critical, just go as close as the AF of your camera allows.

Next have a close look at those pictures on your monitor at 100% magnification. Typically you should see that at the centre the pics are sharpest and sharpness deteriorates to the corners. At least in my tests I found this revealing although at this magnification there are still approx. 13 sensor-pixels peeping at one (full-color) monitor pixel - so you're far away from the limits of sensor-resolution. Following is a 100% crop from the centre of such a pic:
Image
You can clearly see the single pixels on the monitor. If not, check focus or alignment.
If you make two such shots, one with say f3.5 and one with f11 you should be able to see that the f11-shot is sharper than the f3.5 shot.

If the centre is as sharp as the corners and there is no discernible difference between a f3.5 vs. f8 shot, your lens is certainly quite good and you have to tighten the test-procedure a bit. This is done by going further away from the monitor and repeating the shots.
How far?
Well in the first sequence of tests the distance should have been approximately 36cm for 24mm focal length, 75cm for 50mm focal length, 105cm for 70mm focal length and 160cm for 105mm focal length.
Now try for 4x the distance, that is 1,4m for 24mm, 3m for 50mm, 4m for 70mm and 6m for 105mm. The exact distance is not critical, but if you compare two lenses at the same focal length (say your 24-105mm at 70mm and a 70-200mm at 70mm) use the same distance so that magnification is comparable between the two lenses!
At that distance you're close to 1 sensor-pixel peeping at one (full color) monitor-pixel, that being the resolution-limit of your cam. So you will certainly see a degradation in sharpness in the centre. Now that your monitor fills only 1/16 of your pic, you can only judge centre-sharpness but that can then easily be compared to the same shot from another lens.
Then you know which lens is sharper.

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 Post subject:
PostPosted: Sat Oct 06, 2007 9:32 pm 
Hi all

I have done some handheld test shots of big wind chimes using 24-105 here:
http://www.flickr.com/photos/14637386@N02/?saved=1

The IS is very beneficial for me as my hands shake a lot.
I have also done some test shots with my 70-200 f4 non IS but I did not upload them as they were all blurry. Though my amateur eyes tell me that the images taken with the 70-200 were betterand nicely saturated, I cant compare the sharpness as they were blurry ( i dont have tripod ring).

Could you please take a look at the pics posted above and tell me if the sharpness is what I should expect of the 24-105 lens, or do I have to do some more testing?

I cant do tom's test because my computer is in a small room so there's no space.

Lastly, I just sold my 70-200 and now planning to get a zoom with IS.


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 Post subject:
PostPosted: Sat Oct 06, 2007 9:39 pm 
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The pics look pretty good to my eye, but only at 1024 x 683!
This is a fair bit below full-resolution, so we cannot really judge sharpness from your flickr-uploads.
But you can do on your monitor at 50-100% magnification. If you're content with the f4-shots, everything's fine :)

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 Post subject:
PostPosted: Sat Oct 06, 2007 9:49 pm 
:?

That means I got to do monitor shots. :oops:

Ill print your testing guide and read it tonight and try to relocate my PC.
:evil: grrrrr!!! :twisted:


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 Post subject:
PostPosted: Sat Oct 06, 2007 9:53 pm 
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No, no! Just look at your monitor at the pics you already have made at 50-100% magnification. You can view them in full glory=resolution. We (=internet) can only view them in very limited res, so cannot really judge their quality!

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