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Old March 20th, 2008, 10:53 PM   #1
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will the F stop in camera affect DOF while using an adapter?

i don't mean to sound ignorant, but just wanted to make sure that changing my F-stop in the XHA1 isn't going to affect the DOF while using my adapter.

it makes sense to me that since the focus is on the GG, changing the Fstop in camera (not attached lense) should only affect the light allowed into the camera. please correct me if i am wrong, but isn't the Fstop on the attached lens the only one that affects your DOF?

i had a look around the forum and couldn't find the answers for this. i am new to this and thankyou for any input.

aloha

jai
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Old March 21st, 2008, 12:36 AM   #2
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The still-camera lens depth-of-field is controlled by the iris setting of that lens as you suggest.

The field-of-view presented by that lens onto the groundglass is controlled by the area of the groundglass the camera is allowed to see by its zoom setting or in the case of detachable lens (ENG) style cameras, the choice of direct relay lens.

Like all good things there can be exceptions but this is anecdotal and not proven by technical tests as far as I know. - If the camcorder aperture is set to narrow, f16 or tighter, the inherent deep depth-of-field of the camcorder lens can be furthur deepened even with a close-up lens or dioptre on front.

A groundglass of fine texture under some conditions may pass some aerial image which would normally be out of focus and not resolved by the camcorder but might be perceived as slight flare if one searches hard enough.

The deep depth-of-field of the camcorder with tighter iris setting may then allow passthrough of bright pinpoint highlights or sharply defined high contrast edges to be resolved sharply within the areas the still-camera lens has resolved softly to the groundglass by the operator's focus choices.
This undesirable artifact has become known as "ghosting."

The camcorder iris sweet spot seems to be generally accepted as in the ballpark of f5.6 to f6.3. A wider camcorder aperture may be needed in low lighting conditions.

Depending on how good the camcorder's own lens is, the image may become a little softer with the camcorder iris opened wide which makes the camcorder depth-of-field shallower.

The circumstances in which this might happen most often co-incide with over-bright pinpoint highlights from artificial lighting so the pass-through problem is self-solving to some degree.

Outdoors in strong daylight, the situation is reversed. Shiny points on automobile bodies may get through if the camcorder iris is closed right up to control light.

Best practice is to use ND filters in a mattebox in front of the still-camera lens to enable camcorder iris and shutter speeds within the sweet regions. The side-benefit is that this helps to reduce flare propagation within the groundglass and any flare effect within the still-camera lens itself and is already a film-camera best practice of long standing.

If you are doing surf footage or more critically, scenes with early to midmorning sparkling ocean backdrop (east-coast circumstance), the mattebox and ND filters will be desirable.

I tend to be a bit wordy and pedantic so do not be too concerned about understanding my comments. Others may contribute in more precisely and succinctly so pay heed to them.

Last edited by Bob Hart; March 21st, 2008 at 12:54 AM. Reason: errors
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Old March 21st, 2008, 05:33 AM   #3
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To help sum up Bob:

Your cameras F-stop isn't going to impact DOF at all, but an f-stop that is too wide may cause some image softening as Bob suggests. Basically make sure the camera is in focus with the GG, as the softening would come from being slightly out of focus between the camera and GG.

All of your DOF will be from your "attached lens".

Also,

try to understand what Bob said, as there is some rather practical information there.
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Old March 21st, 2008, 09:10 PM   #4
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thank you both very much.
bob, cheers for the in depth answer to my question. i was wondering what that little "flare or ghosting" was. as well as that, i am going to be shooting some surf footage soon (you read my mind bob, did my location give me away?).
matte box huh? i have a 70-200mm L 1:2.8 IS USM canon lens.
ok, tripod mount, turn off the IS and AF, find some sort of lens support to the rails,(very heavy lens)any DIY tips for support? would some sort of sand bag work?
any thing else i may need to think about? is it difficult to accurately focus on the surfer with the adapter and lens attached?
i also have a 2X adapter for my lens which would boost it to 140-400mm while losing about 2.5 stops of light. would it be worth the weight and light loss to use?
i just recieved the brevis flip module (thank you dennis and crew!) and am using that with 60cm rails system with the XHA1.

thanks everyone! HAPPY EASTER!

aloha

jai

ps.. bob, i like your wordy and pedantic ways. better more info than not enough. cheers!
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Old March 22nd, 2008, 12:20 AM   #5
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IN the interests of hurry up and get it done quick I have copied your questions.

(matte box huh? i have a 70-200mm L 1:2.8 IS USM canon lens.)

A mattebox is somewhat of a luxury for those up us with worn seats to our trousers and ruined shirtcollars and maybe overkill for an outdoors thing where you want light and agile.

For the big lens, a bridgeplate and rods is desirable. Once you have that, a mattebox can be had from India for fairly cheap.

For what you are doing, enquire at a camera shop to see if they have ND filters which will screw into the front of your Canon lens. Say to the salesman you want to drop two f-stops.

You can play with your in-camera ND filter and iris for more subtle adjustments. It is just about getting rid of some of that flare.

Dennis Wood's Brevis is apparently pretty good in this regard, depending on what CF screen you have had put in. With the one optimised for telephoto lenses there may be some aerial image passthrough but at the long focal lengths this is much much less of a problem. Hopefully he will chime in here and put you right and me wrong if I am misinforming you.

With cheap NDs, you may end up with a bit of ochre red in your blacks if you bring them up in colour or brightness/contrast correction later on.


(ok, tripod mount, turn off the IS and AF, find some sort of lens support to the rails,(very heavy lens)any DIY tips for support? would some sort of sand bag work?)

The Cavision kit for the older Letus had a sort of strap which went under the adaptor body. In the plant nurseries and gardening centres you can find rolls of doublesided velcro tomato tie or some of the hardwares or dressmakers carry a more robust two piece velcro which is stick-on or sew on.

A hammock woven across the rods with tomato ties would be quickest and dirtiest until something more professional can be made, with a spacer block or a rolled up pizza carton cut down and shoved in between the tomatoe ties and undersurface of the lens.

You can buy lens supports which bridge the 15mm rods of 60mm between centres, which mount to the lens support and have a vertically adjustable centre post. You would probably need to contact a pro AV vendor for that. Itmay be available as an accessory to the rods kit you are already buying.

Again Dennis might be able to guide you here.

(any thing else i may need to think about? is it difficult to accurately focus on the surfer with the adapter and lens attached? )

I chase aeroplanes with a 50mm - 500m zoom. I aquire wide and zoom in. It takes a bit of practice but try to learn to use your right eye for the eyepiece viewfinder and left eye for sighting. If you are lucky, you may a discover a point near the Canon wide-end of the zoom range where you can actually get stereo vision with your left eye au natural and your right eye from the camera image.

This is really, really handy if you can get it to happen. The angular position of the viewfinder and any flex or droop of the Canon lens and your Brevis will be critical in keeping both views harmonised.

Otherwise, you can make a crude oversized gunsight (vee and pin) and fasten it on to the camera to be in a position where your left eye is correct for the gunsight and your right eye for the viewfinder eyepiece.

To avoid overshoot when tracking a subject, I have an old Miller fluidhead tripod and I set the tripod high. The tilt-pan handle I set about 30 degrees upward and a little outwards.

I use either the rods or the lens itself as a second tilt-pan handle and stand in under the camera with a right-hand palm-inward hanging grip (think chin-ups on the monkey bar) on the tripod handle and an overhand downward grip on the lens or rods.

There is a natural see-saw motion across the tripod centre in this position and you use whole of upper body and shoulder movement which is easier to control with the tripod fiction only set enough to dampen. Because I have the lens firmly supported, this practice does not do damage.

(i also have a 2X adapter for my lens which would boost it to 140-400mm while losing about 2.5 stops of light. would it be worth the weight and light loss to use? )

You may find this is a dog with a tendency towards a softer image, maybe vignetting and for bits of fluff or dust showing up in the image as dark patches but try it. It may perform differently to mine. It certainly is an issue if you shoot aerial image with the groundglass removed.

The doubler will also make aquiring and following more difficult and the gunsight may be more necessary.

An alternative to a sighting device is to make a strong flat bar to fit across the tripod and mount two camera on the one tripod, one with Brevis one without, harmonise the two cameras to a common distant centre target.

When you do this you need to be aware that some cameras optics are not correctly centered to centre of imager and the centrepoint of image will move sideways as you zoom in.

It is not enough to throw you right off but is a distraction you must learn to adjust to.

You may find that the camera without the Brevis at about 90% zoom-in and your Brevis with the Canon zoom at its wide end will present near to identical fields of view.

So you initially aquire with the normal cam and zoom in, centre the image and it should appear in the Brevis cam with Canon on wide as long as you have both harmonised to the same centre.

You may also be dissapointed to discover that your zoom lens on the Brevis may not be all that more powerful to the wider 35mm format than the camcorder's own zoom direct to its 8mm (+ or -) imager and that except for the groundglass image aesthetic, there may be little advantage.

Again Dennis may be able to advise you here. As I mentioned above he has a CF screen optimised for telephotos where shallow depth-of-field is inherent in the lens and need not be enhanced by an opaque groundglass.

I am not salesmanning Dennis Wood's product here, only making suggestions in the knowledge you have the Brevis.

Hope this helps.

Last edited by Bob Hart; March 22nd, 2008 at 12:39 AM. Reason: error
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Old March 22nd, 2008, 06:33 PM   #6
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And after a detailed response like that Bob all I can suggest is that Chris Hurd archive all of your posts like this into the "Bob Hart Adapter University" thread :-)

Jai, I'm glad that the Brevis rig arrived OK after our last minute drama :-) That 70-200 L f2.8 will work amazingly well on the adapter..I've used it myself. I've also had a professional mount up his EOS f2.8 300mm prime, and f4 500mm. The rig with these lenses is almost ridiculous, however the image is amazing. Patrick and the crew over at Stillmotion.ca use the EOS 70-200 f2.8 in much of their adapter head shot type work. The bokeh from that lens is nice and creamy and gets better at f4 or so.

We have a nice little lens tuning video that uses the XH-A1, Brevis flip, and a 50mm f1.4 lens. There's a 100 lines or so of resolution at the corners to be gained by using the XH-A1 at about f4, and with that lense, likely f4 or so. The video we shot on XH-A1 chromatic aberration testing graphically demonstrates. With the right presets loaded in and good light (which you'll have plenty of), your setup will yield excellent footage.

Cheers,
Dennis.
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Old March 22nd, 2008, 09:30 PM   #7
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Furthur to above. Here is a link to a shot of an aircraft in flight with harmonised cameras.


I had forgotten I had this clip up.

http://www.youtube.com/watch?v=o4fEmLg8M2k


The advantage of two cameras if you can afford it is to have the wide shot to fall back on if you lose the telephoto momentarily which with fast moving objects like aircraft happens often.

Surfers will be a lot more gentle with you.

It is not writ in stone that the cameras have to be perfectly harmonised to a common centre.

When shooting with ground in view in the wide camera, it looks better if you set the wide camera centre high as viewed to get a more natural ground-sky composition.

For dynamic shots where the subject's intended movements are known, it is helpful to set the wide camera centre to right or left for more "noseroom' in front of the subject's direction movement in the wide shot and to frame the telephoto shot normally.

With practice, knowing just where your wide frame will be becomes intuitive.

Another side benefit of using two cams is that you may decide to use your wide shot only because of subject and framing considerations, eg., a group of surfers pelting down the same wave.

In that event, providing you are vigilent on the wide view or even have an assistant maintain the frame with the zoom, by using the telephoto view as a guide, you can achieve these incredibly honey-smooth camera follows in the wides.

Of course with a wide shot, it is often better to hold the camera position and let the subject do the moving through the frame

This smoothness happens because your ability with the telephoto lens to observe small deviations from frame is magnified and you compensate sooner than if you rely on the wide view.

Last edited by Bob Hart; March 22nd, 2008 at 09:36 PM. Reason: error
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Old March 27th, 2008, 06:12 PM   #8
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thanks again bob,
i have a friend with the same camera, so we might have to try out the two camera wide/zoomed shot.
do you have a picture of your two camera setup? i am not very handy around the shed, so it would be interesting to see a working DIY rig in action.

thanks again bob. i enjoyed your movies.

aloha

jai
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