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Tim Polster October 1st, 2012 01:57 PM

Lens Experts, I Need Your Opinion
 
1 Attachment(s)
Hello,

I wanted to ask the community about an image issue I am seeing with a Varicam "H" with a broadcast lens. I do not want to name the lens company as I am waiting to hear back from them on their opinion. So I will just post an image.

Please click on the image an give your opinion on if the lens is functioning as intended, properly aligned etc... The aperature as around f-2.8

Thanks for your help.

Shaun Roemich October 2nd, 2012 11:13 PM

Re: Lens Experts, I Need Your Opinion
 
I assume you are talking about the "sharp in the centre, blurry toward the left and right edges on the same focal plane"...

Nope. Not normal.

Bob Hart October 2nd, 2012 11:15 PM

Re: Lens Experts, I Need Your Opinion
 
Tim.


As no one has responded ( EDIT - Shaun beat me by two minutes. ) I'll give it my best but I am no expert. What follows may be stuff you well know so please forgive any redundent suggestions.

Firstly, it seems to be a zoom lens with deep range, maybe with an optical doubler, which is an optic which swings into the optical path within the lens body. The deeper the range a lens is built for the greater difficulty there is in keeping soft areas or distortions to a mnimum.


The soft area may just be a characteristic of the lens one has to learn to live with.

The lens likely has a "backfocus" adjustment at the rear of the lens. This adjustment may need resetting.

The lens may also have a "macro" function, also at the rear of the lens. This may have been inadvertantly set or not in its locked postion.

The "backfocus" and "macro" locks may be broken or worn, allowing the rearmost element to float during transport and setups. The operator may be compensating without knowing.

If either is not correct, compensatory refocussing by the operator with the lens's main focus itself might introduce an area of soft focus around the frame periphery. Does the lens hold focus through the zoom movement? If the focus shifts through the zoom movement, then the backfocus need to be reset.

Typically, the "backfocus" and "macro" functions use the same helicoid movement on the rearmost optical element. The "backfocus" locks off to a "permanent" position suited to the particular camera the lens is fitted to at the time. The "macro" temporarily disrupts the "backfocus" setting to allow the operator to focus upon close objects.

If the lens has a doubler, then image defects due incorrect backfocus may be magnified.

I don't know that the mount is on a varicam. I am guessing it will be B4. If there is a third party adaptor between your lens and the camera's own mount, there could be another factor in the mix. Then, if near to the full available range of backfocus adjustment is needed to make the lens hold focus through the zoom movement, a peripheral softness effect like you are seeing may be the compromise result.

To my eye, the lens may have had a bit of a smack at some point in its life and the mount might be very slightly bent to the left as the left edge of the image ( right side of camera sensor ) seems to have a broader zone of softness. Otherwise there may be a bit of dirt or worn metal on the mating surface of the mount. It may also be natural variation. It does not take much.

Except for operator error on the day, this will be a matter for the technicians to deal with.

Tim Polster October 3rd, 2012 06:23 AM

Re: Lens Experts, I Need Your Opinion
 
Thanks for your replies. I was the camera operator so the chance of operator error is very high!

But, the lens does not have an extender, I set the backfocus which seems to be in check and the macro knob was locked (I am pretty sure).

This is a used setup that I purchased, Varicam & lens (18x) for a very nice price. Low enough to compensate for repair if needed.

My thought was to the mount or lens adjustment. I inspected the mount and to my eyes it looks as it should. No signs of trauma. The Varicam is built like a tank and it seems like it is o.k. But maybe a small difference would not show to the eye.

Why I posted is the company lens reps are saying it is a depth of field issue. I told them I disagreed. I do not want to call them out because they are being very nice and I suggested I bring the lens in for them to look. It will be interesting to find out the cause.

Alister Chapman October 3rd, 2012 09:38 AM

Re: Lens Experts, I Need Your Opinion
 
It looks like a bit of field curvature which is not all that unusual in wide ratio zoom lenses. The best way to check for this is to shoot a flat test chart, or wall with wall paper or another fine pattern and see whether when focussed at the centre you get soft edges and corners and then try and focus the edges and corners and see if you get a soft centre. I see it on a lot of zooms to different degrees and it's one of those things thats very difficult to design out. It may be different at different focal lengths.

Bob Hart October 3rd, 2012 01:11 PM

Re: Lens Experts, I Need Your Opinion
 
Two votes so far seem to come down on the possibility of this being a characteristic of the lens.

At f2.8, the depth-of-field will be fairly shallow as the vendor has mentioned. In brighter lighting conditions at about f5.6, you may see considerable improvement.

Tim Polster October 3rd, 2012 07:55 PM

Re: Lens Experts, I Need Your Opinion
 
I am hoping this is not a characteristic of the lens. In my view, depth of field differences would be more vertical (distance away from the lens). I would think all of the players on the horizontal plane of focus would be sharp. The falloff seems very drastic from sharp to blurry as well.

More thoughts on the depth of field topic would be appreciated.

Bob Hart October 3rd, 2012 10:20 PM

Re: Lens Experts, I Need Your Opinion
 
Tim.


I am guessing that your lens may be f1.8 at its widest aperture. With such a wide zoom range, there are compromises lens engineers have to make. You may find the lens resolves better in the zone f5.6 to f8 or maybe f11. This would seem to be a doubtful guess at an aperture sweet spot on my part.

However, some zoom lenses with a wide zoom range become slower at the telephoto end of their zoom range. This trait is generally identified on the lens body or around the front rim of the lens with text. In the case of the Sigma 50mm - 500mm stills zoom, it reads f:4-6.3 or similar.

With wide open aperture, the lens is rated at f4. Zoomed right in, it becomes f6.3. Wide open at the telephoto end, it is much less sharp. Most B4-Mount "ENG" style lenses have a constant aperture performance through their zoom range.

When a doubler if fitted is swung in, the lens typically loses a stop or so. My guess is that your lens will be a constant aperture type. The model number should help confirm that but a data sheet might have to be looked at to know for sure.

Is the lens which came with your camera an original supplied with the camera or was it sourced separately? If you could advise the make and model number, this could be helpful.

The soft areas in your lens image are at the side edges of a 16:9 format 2/3" sensor frame. If your lens was originally engineered for a 4:3 format 2/3" sensor frame, those side edges may be aquiring from outside of the lens's original designed image footprint.

As well as Alistair's suggested chart test, maybe also try outdoors on a more distant subject, for instance a large brick wall with discernable texture or your sports field again if that is more convenient. Operating the lens with aperture set in the zone f5.6 - f11, have a look to see if the side edges have become sharper at the telephoto end of the zoom range.

If you are still getting soft edges, try using the backfocus adjuster and main lens focus interactively and maintain sharp focus on the same target in the image to see if the edges become sharper. There may only be a narrow band of backfocus movement before the image falls apart.

If you do find a compromise interactive setting which enables sharpeness across the frame, the focus numbers on the main focus may become incorrect. The lens may no longer hold focus through its zoom range. Edge softness may appear at other positions.

Without a model number and ata sheet, I am really only guessing and may be sending you off chasing a dead end.

Alistair likely will be able to elaborate better than I can on this topic as he knows his stuff. My own personal knowledge is to the point of self-endangerment.

Tim Polster October 5th, 2012 07:21 AM

Re: Lens Experts, I Need Your Opinion
 
Bob,

Thanks for your replies. Correct, the lens is a constant aperature 1.8 and it is an HD class lens made by one of the two makers.

I am travelling at the moment but when I return I will post a screen grab from another lens in the same shooting environment. In short, I am not seeing any of these issues with that lens which leads me to believe this is not a DOF issue or it would show up in any lens that was used.

I will have the lens looked at next week and report back. Thanks again.


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