I picked up an Oly21 this week and did a test against a Sigma 12-24 at f11. The center of the Oly is significantly better but the corners are a toss up between the 2 lenses with perhaps a slight edge to the Sigma. Is that possible? I will post the pics as soon as my acct gets updated.
Could the Oly need adjustment and if so where is the best place to send it?
Edited by jonboring on Jun 20, 2006 at 02:32 PM GMT (Reason: pics now posted)
Just set the focus at 6 feet. According to my dof calculator (www.dofmaster.com, Circle of Confusion .19), everything should be in focus from about 3' to 50'. Same process with the Sigma.
That procedure doesn't work well for the Oly 21. Do a search and you should be able to find previous discussions on this subject. Using film hyperfocal distance doesn't work when you have a sensor instead of film. Do a test with the same scene where you move the focus point a little closer each time to infinity. You'll find the spot you need for your CCD at F/11. It should be much nearer to the infinity mark than what you calculated.
pdmphoto wrote:
That procedure doesn't work well for the Oly 21. Do a search and you should be able to find previous discussions on this subject. Using film hyperfocal distance doesn't work when you have a sensor instead of film. Do a test with the same scene where you move the focus point a little closer each time to infinity. You'll find the spot you need for your CCD at F/11. It should be much nearer to the infinity mark than what you calculated.
Paul
Why different for the sensor...other than the sensor size?
Maybe I remember it wrong but I thought what I have read here about moving the focus closer to infinity had to do with the adaptor not being the ideal thickness and also had to do was focusing at infinity. I was focusing at 6 feet. Besides the center is sharp on the Oly ... the hyperfocal distance wouldn't just effect the corners. Also the DOF calcuator I use takes into consideration what size sensor you are using.
Yep the Sigma 12-24 is not supposed to be the sharpest knife in the drawer (see www.16-9.net/ultrawides) which is why I was surprised its corners were slightly better than this Oly 21.
Its hard to discuss results when you can't see them ... sorry but my image upload is still not on so I can't post the files yet.
There was some discussion here on FM, but I don't think there was any conclusion. Just some talk about sensor flat vs, film flat, retro-focus optics, microlenses, light paths, things being more apparent with sensor resolution...
I've done the test myself and am convinced of this technique for wide angles.
sorry still can't post the pics ... i bought the upload capability 23 hours ago so maybe in an hour ...
paul yes I tried it. i bumped the focus ring slightly. everything from about 20'+ is now sharper and now the Oly is slightly outperforming the Sigma on the corners and still significantly outperforming the sigma center. but at 12feet the image is softer. i am not sure what is happening but maybe the hyperfocal pt starts farther away than what this calculator is saying ... more later when i can post the pics
still can't post my pics ... but i shot these with a 20D ... on my DOF calculator do i say I have a 21mm or 33mm lens? the docs say 21mm but the lens is behaving closer to the math if i say i am using a 33mm lens (21 x 1.6 ). i need to redo the test on my 5D when my brother returns it ... and will post the 20d pics as soon as upload is turned on.
From my experience with both lenses, I would have expected them to perform very similarly at f16. A healthy 21/3.5 should be sharper across the frame at f8/11. At f5.6, the Olympus will probably be better in the centre, but softer in the corners.
It is imperative that the focal plane is at exactly the same angle for both shots, and placed at exactly the same distance from the sensor. The only way of doing this, other than by actual-pixel trial and error, is with a split screen. Fractionally different focus completely nullifies the test.
It is a 21mm lens. But does your DOF calculator let you specify circle of confusion ? It may well be that on a crop camera you should specify a smaller CoC as you will be magnifying the image more. I may well be wrong though; others here know this stuff better than me.
still can't upload ... fred must be backlogged ...
yep you can mod the CoC with www.dofmaster.com.
>Fractionally different focus completely nullifies the test.<
with manual focused lenses it is next to impossible to be 100% identical on the focus distance ... so does that mean you can't compare lenses? i am just placing my tripod in a specific spot and changing the lenses while the body remains on the tripod.
I think one of the points made above translates to "when the adapter is on the lens, the distance scale is no longer accurate" i.e. if you set it to x feet based on the scale on the lens barrel, it will focus at some other distance. What that distance is, depends on the absolute thickness of the adapter or how closely it is able to position the lens with respect to the sensor compared to its distance from the film on the camera it was designed for!
FWIW, my copy of the Sigma 12-24 in Pentax mount is quite sharp at f8-f11, but i know that quality is all over the place with Sigma lenses. it's probably almost as good as my Nikkor 12-24 DX or the Nikon mount Tokina 12-24 that i have tried. two major drawbacks with the Sigma is that it flares much more so than either the Nikkor or the Tokina and doesn't accept filters on the front of the lens without severe vignetting.
still can't upload ... reshot the lens test ... did the best I could to make sure the Sigma and Oly21 where the focused at the same distance (6 meters). The Oly21 has a 6m mark on the focus ring; the Sigma doesn't. To set the Sigma, I had my wife stand 6 meters, turned autofocus on and focused on her, then turned autofocus off and was careful not to bump the focus ring.
the Oly has slightly sharper centers and corners f8-16 but it is slight. i had to look at 100% crops very carefully to see the difference the first time, then it wasn't so hard to spot. however at f16 the oly's center is much much sharper. might be vibration, was shot at 30th and the wind was blowing a little.
What you haven't considered in your comparison is resistance to flare -
the 500gm Sig(monster) WA has 16 elements in 12 groups,
the little 180gm Oly has 7 elements in 7 groups -
I have shot with this Oly lens at F16 directly into the sun on snow covered mountain slopes - it is virtually flare resistant - the Sigma (as any wide angle zoom) would flare like mad in these situations.
