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01-29-2015, 08:34 PM   #1
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Fisheye Focus Issue - What's going on?

I recently picked up an old Accura 12mm Fisheye f8. (<Link is to forum review for this lens.) It's a T-mount, and I'm using a T > K bayonet mount adapter.

I wasn't expecting stellar results, and I figured the softness was just due to the nature of a budget fisheye. I was getting better results at f16 than f8, but it still was quite soft.

At f16 and 12mm on a K-30, according to my DOF calculator, at 3' to subject I should have focus from about 1' to infinity. It's for this kind of DOF that there is no focus ring on the lens. But I thought maybe I had been off somehow, and so I unscrewed the lens (ie, the T < K adapter part) a few turns which effectively moved the lens further away from the sensor about 1mm or so. And... now the pic was much, much sharper. See the mouthwash examples below which were about 3' to subject. (center crop)



Thinking it might just be that it was close, I tried again, and the giraffe figurine pics are taken from about 15 feet away. Same deal. (extreme center crop)


So is this a fault of the lens? Of the adapter?
I suppose I could devise some kind of washer to put between the lens and the adapter so that I can tighten the lens so it isn't loose. Or is there something else I can do?
Thanks for your suggestions!

01-29-2015, 09:42 PM   #2
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Fisheye lenses are notoriously hard to focus and regardless of DOF, focus them you must if you want your subject to be sharp. At f/16, the hyperfocal is at 24" with near focus at 12". This assumes that the lens focus is set at 24". I would have expected that the makers would have set the focus point at about 48", but your experience seems to indicate that the focus point is somewhat further out. A spacer might be a good idea. Alternatively there might a way to do an internal adjustment.


Steve
01-30-2015, 12:09 AM   #3
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Most DOF calculators assume by default that you will print 10x15 cm (4x6") picture and look at it. Default CoC for most calculators is 0.02 mm, which equals to 1200x800 px image (about 1 Mpix) from 24x16 mm sensor.

No need to say that is not really what we want to give out of our xx MPix sensors.

I found that CoC of about 0.005 mm is adequate for 1.5 crop 15-30 MPix sensors. And if you want to know what will be brilliantly sharp- take 0.0025

This will give hyperfocal of 1.81 m and DoF of about 0.91 m- inf (multiply by 3 to get feet) with 12 mm @ F/16 and hyperfocal 3.61 m (so DoF 1.8 m - inf) at F/8.

You should also consider that all objects within DoF are not equally sharp. Subjects are really sharp only in focusing plane, everything farer and nearer are just acceptibly unsharp. And degree of this "acceptibility" drops considerably towards DoF borders.

That explains why you get those blurry images when shooting objects that are nearer than 3 feet without proper focusing.
01-30-2015, 07:47 AM   #4
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It's a fixed focus T mount setup, that's how your supposed to "focus" this lens!
The 12mm Sigma goes under various names and is interesting because it uses waterhouse stops - actual round apertures, not an iris. That's why it's only 8 - 11 - 16.
I found best performance at 11, with the center being almost "sharp enough". Off-center you'll have to apply lots of extra sharpening if you want usable results.
This lens has three things going for it (assuming you got it for real cheap):
1. It's effectively a full frame fisheye on APSC sensors.
2. The round aperture opening means that the sun appears round with no star points!
3. Mine was remarkably resistant to flare and rendered the sun and sky very smoothly.

The negatives:
It's slow!
It's not sharp.
The edges look like those taken with those cheap fisheye converters (very soft).

It was a great day when I resold mine (at 2x profit!).


FYI .. you'll get beter results with the IPIX fisheye converter (see the Q threads).

01-30-2015, 08:04 AM   #5
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QuoteOriginally posted by mgvh Quote
At f16 and 12mm on a K-30, according to my DOF calculator, at 3' to subject I should have focus from about 1' to infinity. It's for this kind of DOF that there is no focus ring on the lens. But I thought maybe I had been off somehow, and so I unscrewed the lens (ie, the T < K adapter part) a few turns which effectively moved the lens further away from the sensor about 1mm or so. And... now the pic was much, much sharper. See the mouthwash examples below which were about 3' to subject. (center crop)
Are you using a DoF calculator for a rectilinear (ordinary) lens? If so, the numbers won't apply to fish eye lenses. A DoF calculator for a rectilinear lens tells you the nearest and farthest planes that will be in focus. This works because rectilinear lenses map planes in space to planes in the camera. A fish eye maps planes to curved surfaces. Thus, the region of focus is not a plane but rather surface of nonzero curvature.

The calculations needed to determine this region of focus for a fish eye require detailed knowledge about the curvature of the lens. For a rectilinear lens, the distortion is not considered and is generally negligible (<3% even for the poor lenses); if you had huge barrel distortion and tried to take a shallow DoF picture, you'd find the center in focus and the edges out of focus because the lens sees them as farther away than the deepest plane of focus. You can think of a fish eye as a case of extreme barrel distortion.

The closer you are to the object, the more pronounced the distortion, so it becomes more important to nail the focus because only the center point will be in focus. If you back up, you'll be able to miss a bit more because the sensor will see a less extreme curvature of the lens, meaning more the surface will seem to "flatten out" a little bit. That will give you more in focus.
01-30-2015, 08:37 AM   #6
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They can fit together differently/wrongly quite easily from what I remember. (There are a couple of versions.) The top part of mine was screwed on too far or not far enough, I can't remember, but I had a similar issue that I fixed adjusting something else other than the mount. But basically, do what you can to get it sharper. You will never get a sharp image at f/8 (or ever, really, by modern standards of sharpness) -- f/11 or f/16 you can get something acceptable. I had some luck taking multiple shots and then using stacking software to get rid of the heavy aberrations and increase detail. But that's not always an option. That lens is more or less a fun novelty at this point, don't expect too much. It really belongs on a full-frame to get the full effect...
01-30-2015, 08:51 AM   #7
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Thanks for your responses!
A little more testing and a bit more explanation...

With the lens tightened as normal to the T mount adapter, everything--center and edges--is soft, regardless of how far away the subject is. I just went outside, and an object 50 yards away is very soft. I can get much better sharpness simply by loosening the lens (ie, turning it so that it is farther away from the sensor) a few turns. Then the whole image is sharper, both center and edges.

Remember there is no focus adjustment on the lens, so @digital029art has to be right in saying that the way to focus this lens is by loosening it. (I do have another T-mount lens, but it does have a focus ring, so I don't have this problem.) Since it is T-mount, there are 3 screws that hold the inner ring in place. (And these were meant to be adjustable so you could adjust where the aperture settings end up relative to the camera.) There is some ability to move that ring back and forth, but I had too much trouble trying to move out that ring and keep it straight.

I've ended up cutting out a 'washer' from the cardboard on a cereal box, and placing it between the lens and T-mount adapter. (It's ~.75mm wide.) This seems to do the trick, and I'll just accept that it is the nature of this lens. I'm much happier with the results of this lens now. Still not in the ballpark of my Sigma 10-20, but certainly a fun way to play with fisheye and get acceptable results.
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