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sharper image with correct aperture


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#1 jason duncan

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Posted 25 March 2008 - 07:39 PM

Hello all,

I read a comment by Rick Palidwor about having an aperture setting of f/5.4 will give a much shaper image than that of f/1.4. I'm confused as I thought the more light being exposed to the film the better?
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#2 Jim Keller

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Posted 25 March 2008 - 07:45 PM

I read a comment by Rick Palidwor about having an aperture setting of f/5.4 will give a much shaper image than that of f/1.4. I'm confused as I thought the more light being exposed to the film the better?


More light on the film will give you a better image density (reducing grain for many color films), but doesn't affect the sharpness of the image.

Light diffracts more through a larger opening than a smaller opening. Therefore, a larger aperture (f/1.4) gives you a more diffracted -- and hence less sharp -- image than a smaller aperture (f/5.4).

That said, for the portion of the image that is in focus, most end viewers won't notice. Where the difference becomes more evident is in the depth of field the different apertures give you. It's really only an issue when sharpness is important, such as in scientific applications.
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#3 Jim Keller

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Posted 25 March 2008 - 08:05 PM

Oh, and I should also add that shooting wide-open can exacerbate chromatic aberration for the same reason.

Edited by Jim Keller, 25 March 2008 - 08:06 PM.

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#4 Rick Palidwor

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Posted 25 March 2008 - 10:22 PM

I've done some tests with many different cameras and most lenses are a little soft at 1.4, even when in perfect focus, and they get noticeably sharper as you stop down, for the reasons Jim gives above.

I am also led to believe that wide apertures are less sharp because they are drawing light from the edges of the lens and the smaller apertures are drawing mainly from the centre of the lens, and the centre of the lens is always sharper than the edges.

Apparently some reverse principle kicks in at the smaller apertures (e.g. f16 may be less sharp than f11) but I don't know the reason or the threshold when this kicks in. Maybe the enlightened folk here can elaborate on that. It may be related to drawing light exclusively from the very centre of the lens.

Jason Duncan wrote: "...I thought the more light being exposed to the film the better?"

Jason, I am assuming that a shot at f4 and a shot at f5.6 (which is a little sharper, based on optical principles) are both correctly exposed, the f5.6 shot having twice as much light available as the f4 shot.

Rick
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#5 Antti Näyhä

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Posted 26 March 2008 - 03:01 AM

Apparently some reverse principle kicks in at the smaller apertures (e.g. f16 may be less sharp than f11) but I don't know the reason or the threshold when this kicks in. Maybe the enlightened folk here can elaborate on that. It may be related to drawing light exclusively from the very centre of the lens.

From a Wikipedia article:

Picture sharpness also varies with f-number. The optimal f-stop varies with the lens characteristics. For modern standard lenses having 6 or 7 elements, the sharpest image is often obtained around f/5.6?f/8, while for older standard lenses having only 4 elements (Tessar formula) stopping to f/11 will give the sharpest image. The reason the sharpness is best at medium f-numbers is that the sharpness at high f-numbers is constrained by diffraction, whereas at low f-numbers limitations of the lens design known as aberrations will dominate. The larger number of elements in modern lenses allow the designer to compensate for aberrations, allowing the lens to give better pictures at lower f-stops. Light falloff is also sensitive to f-stop. Many wide-angle lenses will show a significant light falloff (vignetting) at the edges for large apertures.

See also http://en.wikipedia....iki/Diffraction
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#6 Bengt Freden

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Posted 26 March 2008 - 03:16 PM

Re. optimal lens resolution/performance:

I should think that in relatively modern (we are talking the late 70s or 80s here) and fast (f=1.2, 1.4 or 1.8) zoom lens designs in your average 'high-end' Super 8 camera - let´s say Beaulieu, Leicina, Nizo, Canon or Nikon - it would probably be quite sufficient to stop down to around f=5.6, if the shutter angle, light and film stock permits it, to obtain the maximum performance from your lens.

Any high-end, modern lens usually benefits from an aperture 2-3 stops less than full, other than (as mentioned above) with the four-element Tessar/Elmar/Xenar/Heliar types and the lenses used for large format reproductions, like the f=9.0 Rodenstock Apo-Ronar, which has to be stopped down to f=22, to obtain the maximum resolution, However, that is also about three stops down from full opening on this rather 'slow' lens design.
Other symmetrical, large format lenses (like the Sironars or Symmars), with the common maximum opening of f=5.6, the best performance is usually obtained at f=16. Further stopping down only improves the depth of field. And even if diffraction 'kicks in' at f=45 or 64, it is usually not that noticeable in the larger formats, such as 4x5 or 8x10".

But in Super 8 cinematography, with it´s tiny 4x5,5mm frame, it makes very good sense to try to achieve an aperture setting of at least f=4, or even better f=5.6 or 8, which greatly improves the overall resolution and also impoves on any small fluctuations in the camera gate (in front of Kodak´s plastic pressure plate in the cartridge), as well as reduces the fall-off at the far corners of the image.
But then again, you might want to deliberately and creatively use a selective short depth of field, to direct the film viewer´s attention to something specific in the image - and you might have to use an ND filter to obtain a larger (next to full) opening. Which calls for a lens design with optimal resolution already at full opening, like some of the Angénieux zoom lenses (or the 16mm Cooke and Zeiss zoom lenses, if adapted properly, for longer focal lengths).

Severe corner fall-off, with superwide long telephoto lenses (or settings on the zoom), especially at full aperture and also with a widened camera gate, can be reduced or countered with a 'center filter' (for example by Schneider), which has it´s greatest ND density at the center, gradually lightening up towards the filter ring. To achive a smooth effect, without too much adjustment (corners too light!) you might have to experiment by using a slightly larger center filter, mounted in a step-up ring. Trial and error, as usual.

To be absolutely sure about your lense´s maximum performance, you would have to consult the manufacturer´s MTF curves for that particular lens construction - if you can obtain them, they are usually given both at full opening and stopped down to the best aperture setting.

Best,
Bengt Fredén, photographer
Stockholm
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#7 jason duncan

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Posted 26 March 2008 - 03:59 PM

I'm somewhat of a newbie, so I admit I don't understand all of the info given so far. Currently I'm concentrating on filming a band in a bar (with dim light of corse) so I figured I'd have to set the aperture to the largest setting, but I didn't know that f/1.4 causes lower res. I've done research on battery pack light kits, but so far it looks like a 100W is the largest that can be ran from the pack (remember the original 110VoltAC lights that are 625W and blinding). I want to use b/w and not 500T, and I'd rather use Plus-X and not Tri-X. I've been told that if I want to film in a bar, the only way is to use 500T. I know back in the day Kodak had 400ASA b/w.
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#8 Michael Nash

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Posted 26 March 2008 - 04:02 PM

And of course, extra exposure (density) on the film is a separate issue from the optical performance of the lens.
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#9 Chris Burke

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Posted 26 March 2008 - 04:48 PM

I'm somewhat of a newbie, so I admit I don't understand all of the info given so far. Currently I'm concentrating on filming a band in a bar (with dim light of corse) so I figured I'd have to set the aperture to the largest setting, but I didn't know that f/1.4 causes lower res. I've done research on battery pack light kits, but so far it looks like a 100W is the largest that can be ran from the pack (remember the original 110VoltAC lights that are 625W and blinding). I want to use b/w and not 500T, and I'd rather use Plus-X and not Tri-X. I've been told that if I want to film in a bar, the only way is to use 500T. I know back in the day Kodak had 400ASA b/w.



I have shot many a band in bars on Super 8. Often, to even get a usable image, I would have to push Tri-X 2 stops. Most rock shows are really dark, its all about the sound and the mood. So, if you are really married to shooting with Plus-X, then you will have to light the stage a bit. The more light you can use with out ticking anyone off, the better. Many bands don't mind being back lit, with them in silhouette. You pretty much are going to have to shoot wide open or close to it. What kind of camera are you going to use? Is it an "XL" model? If you don't know what this is, it means the camera has a 220º shutter angle and can shoot in low light. If you can use such a camera, do so. Otherwise, you have to get creative in the way you light the show. Good luck and please post some stills when it is all said and done.
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#10 Michael Nash

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Posted 26 March 2008 - 05:30 PM

(I tried to post this before; I must have accidentally deleted it when editing)

Light diffracts more through a larger opening than a smaller opening. Therefore, a larger aperture (f/1.4) gives you a more diffracted -- and hence less sharp -- image than a smaller aperture (f/5.4).


If I'm not mistaken diffraction within the image is only an issue at higher apertures, not smaller ones. Light bending around the iris blades at f/16+ falls within the image area, essentially overlaying a blurry image on top of a sharp one. At wider apertures (f/1.4) most of that diffraction falls outside of the image area, and what's happening instead is that more light is going through more glass, compounding the defects in the glass onto the image area.

http://www.cambridge...photography.htm
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#11 jason duncan

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Posted 26 March 2008 - 09:12 PM

I have shot many a band in bars on Super 8. Often, to even get a usable image, I would have to push Tri-X 2 stops. Most rock shows are really dark, its all about the sound and the mood. So, if you are really married to shooting with Plus-X, then you will have to light the stage a bit. The more light you can use with out ticking anyone off, the better. Many bands don't mind being back lit, with them in silhouette. You pretty much are going to have to shoot wide open or close to it. What kind of camera are you going to use? Is it an "XL" model? If you don't know what this is, it means the camera has a 220º shutter angle and can shoot in low light. If you can use such a camera, do so. Otherwise, you have to get creative in the way you light the show. Good luck and please post some stills when it is all said and done.


Yes I do have a XL camera, Canon 1014. So I know I can use the "window" setting (220). I'm not necessarily married to Plus, but I do like the smooth 1950's b/w look more than Tri. When you say backlite, do you mean having a light in the back of the guys on stage as opossed to shinning a light in front of them?
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#12 Chris Burke

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Posted 26 March 2008 - 10:42 PM

Yes I do have a XL camera, Canon 1014. So I know I can use the "window" setting (220). I'm not necessarily married to Plus, but I do like the smooth 1950's b/w look more than Tri. When you say backlite, do you mean having a light in the back of the guys on stage as opossed to shinning a light in front of them?




Yes, light behind them as well as in front. It is just that you have to get light into the shot anyway you can and a lot of rock show are heavily back lit. Agreed, the Plus-X is a beautiful stock, but without a lot of light you really run the risk of serious underexposure. If you can, use any sort of practical on stage with them. Christmas lights, footlights, anything to get a descent ? stop.
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#13 Rick Palidwor

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Posted 26 March 2008 - 11:01 PM

Duncan
You have a Canon 1014XLS? Good camera.
I have shot many bands in bars and it's really a question of whether there's any light. I can usually get an image, but have refrained from shooting a few times because I knew there wasn't enough of it. Plus X would be tough. Trust your light meter. And remember, all you need are some highlights. Don't expect to light and see the whole stage. But if the singer has a light on their face (they usually do) you will see them. Drummers are notoriously difficult as they are usually in the back. Many musicians have an aversion to lights and tend to stay out when they can so (assuming they are cooperative with you) let them know they need to stand in the light. I go to a bar shoot early and talk to the techs and tell them I'd like as much light as possible. See what they have. Find out if you can adjust a few. Consider shooting fast film, or be prepared to push the Plus X, if the bar does not have enough light.

Rick
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#14 Rick Palidwor

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Posted 27 March 2008 - 12:03 AM

Oh, and assume you might be shooting wide open so don't worry about the apreture/sharpness issue, assuming that the band is the important thing and "you get what you get".
Rick
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