Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Canon Lenses

Reply
 
Thread Tools Search this Thread
Old Aug 26, 2005, 11:27 PM   #1
Member
 
Join Date: Jun 2005
Posts: 30
Default

Having owned the Sigma 18-200 for several months, and being a bit disappointed in the sharpness, I purchased the first piece of my planned two piece replacement. I just got the Canon 17-85 EFS, and when I can afford it I intend to get the 70-200 F4 L.

I took both lenses to my roof for a comaprison test. I set up a tripod and set the center AF point exactly where my face would be when I sat down on a ledge. The shot would encompass some of the roof in front of me, as well as my camera bag on the ledge, myself, and some buildings in the distance. I took a series of shots (all at Av) with the Sigma at wide open, going from the 17 to approximately 85 in around four steps. Then, I took the same series of shots at F9. After each shot I manually unfocused the lens, and all shots were triggered by a remote in my hand. I then repeated the same process with the Canon.

The results: Both lenses properly auto-focused on my face. Both lenses exposed at the same settings; occassionally, they were one stop different in shutter speed. The Canon is definitely sharper; although, not by much. Most surprising, the Canon photos appeared to be brighter, almost as if the exposure were different, even when the exposures were exactly the same. This was puzzling. If both lenses expose at the same focal length, F stop and shutter speed (the lighting did not change from one series of shots to the other, same overcast sky, and only minutes apart) why was one series of photos brighter.

I assume, what I saw was simply the difference between two lenses, and the reasons why people choose one over the other. I imagine that the same tests with L lenses would again produce different results. I expected that the only differences would be in sharpness.

Is there another test that any of you can suggest, which would be a valid and dramatic side-by-side test? It was not possible to perfectly match the focal lengths, but the differences were very slight (17-18, 44-48). Does the very slight difference in focal length skew the results?

If I get a good suggestion on how to compare these two lenses (through the similar focal lengths) I'll post examples in the forum.
GovtLawyer is offline   Reply With Quote
Sponsored Links
Old Aug 27, 2005, 5:59 AM   #2
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

Most tests should be done in a controlled environment to minimize variations:

1. Use the camera in manual (i.e fix aperture, fix shutter)
2. Possibly indoor (outdoor cloud cover may change)
3. Use a tripod (distance + target are fixed)
4. Use test charts so the subject is the same (and measureable)
http://www.stevesforums.com/forums/v...mp;forum_id=65


But IMO you can already determine which lens is sharper from the MTF chart :idea:
(ie the work is already done for you): http://www.photozone.de/3Technology/mtf.htm



EF 17-85 IS USM:


18-200 DC:

NHL is offline   Reply With Quote
Old Aug 27, 2005, 6:40 AM   #3
Senior Member
 
G35Guy's Avatar
 
Join Date: Apr 2005
Posts: 323
Default

As for brightness, each lens is definitely going to be different. It's a combination of things, the type and quality of the glass used and the various coatings on the lens elements. Certain types of glass and coatings create images that are more/less contrasty, brighter/darker, more/less color saturated. When you purchase a lens, you aren't just purchasing sharpness, you're purchasing brightness, contrast, color, bokeh, etc...as well.
G35Guy is offline   Reply With Quote
Old Aug 27, 2005, 6:45 AM   #4
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

He had the camera on auto... so the brightness should be set by the camera (but the exposure may vary depending on the AF points) - The Sigma 18-125 is a better lens to compare to the EF 17-85 because of their similarity in MTF's

That's what the MTF does - It's a measure of contrast in the various lenses: "In a MTF test a pattern of lines, black and white, are projected with the lens. The resulting contrast is measured..."
http://www.photozone.de/3Technology/mtf.htm


A lot of folks say an f/2.8 lens is not sharp wide open, but that's because they shoot in the dark - try to shoot with the same lens outdoor when the contrast comes into play:


NHL is offline   Reply With Quote
Old Aug 27, 2005, 10:01 AM   #5
Member
 
Join Date: Jun 2005
Posts: 30
Default

Thank you for your help. As much as I am technically oriented, and by no means a "nerd," the MTF charts and linked page, were too difficult to absorb. Suffice it to say, the other explanations were quite valuable. I believe the test was conducted under fairly good conditions. Although the camera was on Av, the AF point was always on the same point, regardless of lens or focal length, and the lighting was very consistent.

My understanding is that I should have expected results which differed in more than just the sharpness, and I did.My main concern was sharpness, and I believe the 17-85 is the winner. The end result is quite pleasing. I'm going to go out now, in brighter conditions, and conduct a further test onsome stationary subjects.

I assume I am correct in testing the lenses wide open, and at 9.0. All lenses perform better stopped down, so the wide open is the better test.

Comparing the 18-200as opposed to the 18-125,is the correct test, as I own the 18-200 and it is the lens I am ultimately replacing.
GovtLawyer is offline   Reply With Quote
Old Aug 27, 2005, 2:42 PM   #6
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

GovtLawyer

Unless you have selected the metering differently (i.e. centerweight or partial) the camera metering is still evaluative even with one AF sensor selected -> it evaluates the whole scene - What are the resulting pictures aperture/shutter values: Are they all the same? if they are (like if you were on manual) then and only then you can contribute the differences to the lenses

What I try to explain was even without any pictures taken the MTF already clearly shows that the EF 17-85 is sharper than the 18-200!
NHL is offline   Reply With Quote
Old Aug 27, 2005, 2:43 PM   #7
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

Now when the MTF differences are minor as in the case of the Sigma 18-125 DC - you can test the two lenses and notice that at one end (the wide) might be sharper than the EF 17-85 while the 17-85 is sharper than the 18-125 at the long: In between is anyone guess so a test there might be better suited... :O
NHL is offline   Reply With Quote
Old Aug 27, 2005, 3:29 PM   #8
Member
 
Join Date: Jun 2005
Posts: 30
Default

I made another roof test, and the results are interesting. First, I did not shoot in manaul as the lighting was changing, clouds moving and light changing. My main concern is focus and sharpness. The Sigma 18-200 focused on an object behind me at 18 and wide open. Very surprising as the AF point was directly on me. At longer lengths and more DOF, it got better. I guess some of what I had been attributing to my error has been equipment error. I think this may be the dreaded Backfocusing, and I am not sure how to prove this. Of course, at wide open and widest focal length, it was focusing on my face around 20 feet away. As I got closer with the zoom, the task was easier. Canon had no problem at any length.

Both lenses showed some strengths and weaknesses. The Canon had more strengths and less weaknesses. I shot everything at Parameter 2; all neutral, so to see a fair test. Overall, the Canon showed better contrast.

The Sigma is a fairly decent lens throughout, but as I had surmised since I got it, its focusing is not very reliable, and not quite as sharp as the Canon under similar focal lengths. I'm sure the 70-200 L, if and when I can afford it, will blow thw Sigma away on the rest of the focal length.
GovtLawyer is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 12:12 AM.