New idea...maybe?

yzmotoxer807

yzmotoxer807

Well-known member
I just had a thought. When using a 35mm adapter, there are three ways to adjust depth of field: 1) Aperture, 2) Focal length, and 3) Camera to subject distance.

Aperture is the easiest but can be hard to get right if you want a long dof because you need more light.


Focal length can be changed by using a different lens (or zoom lens but I heard that these don't work as well with a 35mm adapter) but this can be a problem if you only have one lens.

Camera to subject distance is not always a possibility for framing and other obvious reasons.

So, the only other way thing that changes dof is the imaging surface (ccd, film, etc). What if you made a 35mm adapter with an adjustable ground glass. If you could adjust/move the GG forward or backward (and focus the lenses on the new position) wouldn't this give you a smaller or larger projected image on the GG?

So for example: You are using your 35mm adapter and are loving the shallow dof. Then, you come across a shot that you need a very long dof. This is fine if you are shooting a city or mountains, etc because you are far away. But what if you want to shoot the inside of a large building with a long dof like the inside of an airport or auditorium? One way to make this happen is to stop down a little and make sure the camera is as far back as possible. But what if this is not enough? You can only go so high with the f-stop because you cant modify/add more lights. Well, if you could adjust the GG then you could make the projected image smaller and focus it all up. The dof would then be much longer would it not?

This is just an idea I had. If it has already been brought up or talked about and I am completely oblivious and ignorant (it happens a lot :-Laugh(DBG)-1) then please let me know.

:dankk2:
 
Problem #1
The distance between the 35mm lens and the ground glass has to be perfect. Moving the GG would prevent getting anything in focus.

I'll let somebody handle the more technical ones...
 
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I don't have a 35mm adapter so I the only thing that I can base my idea on is what I have read or seen, but why does the distance have to be perfect? Can't you just put the GG where you want it, focus the 35mm lens on it, then backfocus the camera to the new position? If this is wrong, please let me know. I would like to know how this works.
 
yzmotoxer807 said:
What if you made a 35mm adapter with an adjustable ground glass. If you could adjust/move the GG forward or backward (and focus the lenses on the new position) wouldn't this give you a smaller or larger projected image on the GG?


:dankk2:
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What you are suggesting has the same DOF effect as leaving the GG where it is and zooming in or out to the GG with the camera lens.
 
Each lens manufacturer has a given flange to film distance, which must be maintained correctly. The Brevis adapter uses a micro-collimation (simpler than it sounds) system so that users can adjust this to a high degree of accuracy, while maintaing the correct Z axis inclination of the lens. On wider angle lenses, if this distance is not correct, you may not be able to achieve focus at all, and on cinema lenses, it becomes even more critical. So basically, you should not change this distance, unless you're attempting macro photograpy, where extension tubes achieve a closer focal distance than the stock lens is able to achieve.

Our approach is to allow users to swap in different imaging elements (call it ground glass if you like), to achieve the effect you are looking for...that is, the appearance of varying levels of DOF.
 
Two things:
1) Dan I don't understand what you are saying. By zooming in further on the GG all you are doing is magnifying the projected image. I am talking about making the projected image be smaller or larger on the GG itself. The size of the image on the imaging surface (GG) is what changes dof.

2) Dennis from what you said, now I see that my idea would not work. So basically the only other way to change the dof is to use a more or less diffuse imaging element correct?
 
yzmotoxer807 said:
Two things:
1) Dan I don't understand what you are saying. By zooming in further on the GG all you are doing is magnifying the projected image. I am talking about making the projected image be smaller or larger on the GG itself. The size of the image on the imaging surface (GG) is what changes dof.
Click to expand...

By zooming in on the GG you ARE magnifying the depth of field :happy:


Neutral Density (ND) filters can also manipulate depth of field via allowing different apertures to be shot without changing lighting.
 
Oh, okay I get it now. It is the same thing as moving the camera way back and zooming in. The dof is technically and mathematically close to the same thing but the effect of it is magnified by zooming in.
And yes I knew about using an ND filter for this purpose but I was thinking about what to do to give longer dof. I guess you got what you got and if you need it longer, add more light and stop down. Thanks
 
yzmotoxer807 said:
Oh, okay I get it now. It is the same thing as moving the camera way back and zooming in. The dof is technically and mathematically close to the same thing but the effect of it is magnified by zooming in.
And yes I knew about using an ND filter for this purpose but I was thinking about what to do to give longer dof. I guess you got what you got and if you need it longer, add more light and stop down. Thanks
Click to expand...

No.

Zooming in on the GG does not change the DoF at all. It would require you to use wider lenses for the same effective field of view which will reduce your DoF but the act of zooming on the GG by itself has nothing to do with DoF.

And what the image is projected on has nothing to do with the DoF either. DoF is determined by aperture, focal length, and distance to subject. That's it.it doesn't change when projected on a strip of film, a CCD, a CMOS, or frosted glass. Different GG elements can make the bokeh appear different and make it look as if it is effecting the DoF but mathematically this isn't possible.
 
However cinevate boasts shallower dof according to which cinefuse or GG is used. Maybe someone can explain how this works?
 
William I agree with what you are saying about it mathematically being the same dof when you zoom in on the GG, BUT because when zooming, the effect of the dof bokeh that is already there is magnified and this makes it seem like there is a narrower dof.
 
William,

Mathematical measurements of DOF are accurate with regard to film, but "apparent DOF "( my phrase, but all that matters I'm afraid) is what matters with GG adapter screens and there is undoubtedly a difference with these screens. No question - well documented . Denser screen = more scattering of the circle confusion = lower DOF. Argue about it all you want, but the pictures don't lie. This is well documented.

_ Lenny Levy
 
Correct me if I'm wrong, the way I see it seems like for example the CF3 on the Brevis may have a shallower DOF because of overall sharpness that is transmitted to the backprojected image giving a more faithful dof to the lens used. Logically then, could the CF2 and CF1 suffer on overall sharpness to mistake for more DOF? (though with less vignetting + other problems)
 
No - CF1 is probably sharper though I'm not sure, and I have not seen vignetting issues on CF1 or the reputed "other problems".

This is not a scientific explanation - just mine - COF is just a measurement of the size of the out of focus circle made by a point of light that is of course out of focus. What seems to happen is that denser more diffuse screens seem to scatter the out of focus light more than less diffuse screens. The in-focus areas are is still in focus but the out of focus area is more spread, also tends to be softer and not as bright which makes perfect sense and might tend to be pleasing as well unless it spreads too much.

Lenny Levy
 
What I don't get is that the circle of confusion is a pretty subjective idea, even to the human eye. so I don't understand how a manufactured surface can differentiate what's truly in or out or in between. But I ain't challenging anyone here, if that's what happens then they've done a good job at it.
 
Nobody is manufacturing the COC tolerances on GG's, its just a way to describe what's happening when the DOF changes with a denser screen. DOF film charts are made by optically measuring DOF at the film plane, That is a precise optical measurement.
 
On a last note, doesn't that lead us though to the similar discussion if the Brevis is actually gaining light, or just not using enough of it / or is the Brevis reducing DOF or not actually imitating enough a lens' DOF on the less diffused elements? Id like to hear comparisons in the sharpness of CF1/2/3.
 
yzmotoxer807 said:
1) Dan I don't understand what you are saying. By zooming in further on the GG all you are doing is magnifying the projected image. I am talking about making the projected image be smaller or larger on the GG itself. The size of the image on the imaging surface (GG) is what changes dof.
Click to expand...

Yeah, I had misunderstood what you were saying. Dennis cleared it up--back focal distance has to be precisely maintained.


Regarding the differences in DOF with different diffusion, that's "apparent" DOF, but then, ALL DOF is "apparent." Even the hard equations for DOF are based on the subjective look of the image. So it's no surprise that diffusion would affect the look of the DOF.
 
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