Focus question with fish eye lens...

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martyp

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Marty
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Evening all,


At work I'm processing photos taken with a Sigma 8mm fisheye lens on a Canon 1000D.

Now these shots were taken indoors on a tripod with a panoramic head, however some shots are OOF making stitching a pain and the end result also suffers a little.

Looking at the EXIF data the camera is choosing f/3.5 as the aperture for every shot - the guys are using the camera in full auto mode. Some come out okay, others are OOF.

Would I be correct in thinking if the aperture was set to f/11 or higher there would be a better chance in the shots coming out sharp even if the lens is slightly out? Or would it need to be a manual focus job? :shrug:

Thanks in advance!!
 
didnt know you had to focus these things
they are really quite prone to distortion....
manual focus...but where
 
Out of focus with a fisheye lens? Amazing. Your hyperfocal distance is about 3 feet. Are you sure it's a focus problem? It might help if you posted an example.
 
Thanks for the input!

Can't be certain its a focus problem, but it certainly looks that way. Some are nice and sharp and the odd few are blurred as if OOF.

It's a circular fish-eye if that makes a difference?

I'll try and post a shot at lunch time tomorrow...
 
martyp said:
Thanks for the input!

Can't be certain its a focus problem, but it certainly looks that way. Some are nice and sharp and the odd few are blurred as if OOF.

It's a circular fish-eye if that makes a difference?

I'll try and post a shot at lunch time tomorrow...
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try setting up and moving the manual focus from up close to about half way out...taking some shots at each setting
the info i googled for circular fish eye gave dof as 0.5m to inf..but some of the
that may only occur at one lens setting...the lens itself i dont think needs to be focussed...just aligned
 
StewartR said:
Out of focus with a fisheye lens? Amazing. Your hyperfocal distance is about 3 feet.....
Click to expand...

With the EF15/2.8, that seems to be pretty much it....set the focus to 1m and the job's done.

Bob
 
Sorry for the delay, I was meaning to do this earlier but I didn't have time...

I don't actually have the lens handy to play/experiment with since it's being used on a job.

Here is one photo and a 100% crop version.

1_1.jpg

1_2.jpg
 
OK, here's my guess. The lens was focussed manually at infinity. Then at f/3.5 you should have everything from about 3 feet distance in focus, but from the look of the picture I'd say that equipment cabinet could well be closer than 3 feet.

At f/11, the DOF would extend from 1 foot to infinity, so yes, if my diagnosis is correct, using f/11 would have helped. But if my diagnosis is correct, there was a much easier remedy available without worrying about the aperture!
 
PS I assume the fact that you say some shots are OOF (not all) means you've ruled out the possibility of the lens being faulty?
 
I use the 15mm Sigma, Aperture usually set to f8: Focus set to about 1/3 into image.. No problem
 
StewartR said:
OK, here's my guess. The lens was focussed manually at infinity. Then at f/3.5 you should have everything from about 3 feet distance in focus, but from the look of the picture I'd say that equipment cabinet could well be closer than 3 feet.

At f/11, the DOF would extend from 1 foot to infinity, so yes, if my diagnosis is correct, using f/11 would have helped. But if my diagnosis is correct, there was a much easier remedy available without worrying about the aperture!
Click to expand...

I think it may be the other way round since even the BG is OOF.

May have focussed real close if that's possible.
 
petersmart said:
I think it may be the other way round since even the BG is OOF.

May have focussed real close if that's possible.
Click to expand...
That's definitely a possibility. The minimum focussing distance of the Sigma 8mm is about 5½ inches, and at that distance the DOF is surprisingly shallow.

But (just for completeness, though it's not strictly necessary to your argument) I'd say your observation that the background is OOF is erroneous, because there isn't really any background.

You have to remember that lenses focus on a plane, not on a sphere. If the focus is at a distance of, say, 1 foot, it doesn't follow that everything at a distance of 1 foot is in focus. You need to visualise a flat plane, perpendicular to the axis of the lens and 1 foot away from it. Then everything in that plane will be in focus, even if it's much further than a foot away from the lens. (That's how it's possible to take a picture of a brick wall and get the whole wall in focus.) OK?

Now in the picture, it looks like the fronts of the equipment cabinets are pretty much all in a plane, and the axis of the lens is roughly perpendicular to that plane. (We can tell that because the edge of the closest cabinet looks straight.) So if one of them is in focus, they all should be, side to side and top to bottom.
 
Thanks for the very informative replies!

I spoke to the guy with the camera yesterday and talked him through using aperture priority and setting it to f/11. Hopefully I'll have better luck second time round. I'll post back on Monday when I see the second batch. :)

I can definitely confirm the lens was on AF, the guy using it is not experienced with cameras at all... The lens itself is definitely not faulty either as the outdoor shots are sharp so it must be a AF error.

Chaz Photos said:
A fish eye is the last lens I use for stitching best to use as long as poss
Click to expand...

Unfortunately it is mandatory this particular job and the camera can't be moved around to keep 3ft away from objects as it needs to be in particular spots.
 
martyp said:
I spoke to the guy with the camera yesterday and talked him through using aperture priority and setting it to f/11.
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If you are stitching images together you would be better off with manual exposure, to maintain consistency from frame to frame. You really don't want to be stitching together frames with exposure differences.

EDIT : p.s. I bought a Sigma 10mm fisheye last year and the focus calibration was abysmal. It needed a +15 adjustment on my 50D and was unusable on my 40D. I returned it and didn't bother to replace it.
 
the stitching software i use...a freebie...introduces a bit of unsharpness and warping

time to rethink
 
mrcrow said:
the stitching software i use...a freebie...introduces a bit of unsharpness and warping

time to rethink
Click to expand...

I use PTGui for the stitching process, excellent software IMO.

tdodd said:
If you are stitching images together you would be better off with manual exposure, to maintain consistency from frame to frame. You really don't want to be stitching together frames with exposure differences.

EDIT : p.s. I bought a Sigma 10mm fisheye last year and the focus calibration was abysmal. It needed a +15 adjustment on my 50D and was unusable on my 40D. I returned it and didn't bother to replace it.
Click to expand...

Can't agree more with taking them in manual and fixing the shutter/aperture to give matched exposures although this would be a step to far for the guys using the camera and the results would probably be over or under exposed. At least CameraRAW can process JPEG files even though they're not as flexible as RAW...

The sigma lens being out doesn't surprise me much to be honest, however the 15mm Canon fisheye would need a full frame body.
 
I have the same problem with my fisheye, it focuses on the the lens or the bits that come out past the lens around the edge.
 
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