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Hard Mask smaller than Lens Diameter


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#1 Johnathan Holmes

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Posted 17 June 2008 - 02:51 AM

Wanted to know if this was absolutely true or not:

One should never use a hard mask on a matte box with an opening smaller than the diameter of the front element of the lens, as it affects exposure.

As a general rule, I never do anymore but I'm curious if anybody has actually tested this and proven that doing so does in fact reduce exposure, and what the maximum reduction in exposure is.

I used to think that any stray light hitting the lens that wasn't part of the image wouldn't be wanted, even if it wasn't flaring the lens, as it would reduce contrast and saturation. I imagine the light coming into the lens in a cone-shape spreading out from the lens, and anything outside that cone is not part of the image that the lens sees and should therefore be matted out. Then again I can also see the the hard mask acting as an aperture, and maybe even a pinhole effect. So I'm confused about this issue.

As well, a DP told me that when punching in on tighter lenses for closeups, it is necessary to open up by 1/3 to 1/2 a stop as going on a longer lens doesn't let in as much light. I'm confused about this as well since lenses are rated in T-stops, and theoretically all lenses should compensate for that exposure difference in the glass. My feeling is if a lens is properly calibrated, a T/2.8 on an 18mm should look exactly the same exposure-wise as a T/2.8 on a 300mm. After all, isn't that why we have T stops?

Thoughts? Proof?
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#2 Stephen Williams

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Posted 17 June 2008 - 04:12 AM

Wanted to know if this was absolutely true or not:

One should never use a hard mask on a matte box with an opening smaller than the diameter of the front element of the lens, as it affects exposure.

As a general rule, I never do anymore but I'm curious if anybody has actually tested this and proven that doing so does in fact reduce exposure, and what the maximum reduction in exposure is.

I used to think that any stray light hitting the lens that wasn't part of the image wouldn't be wanted, even if it wasn't flaring the lens, as it would reduce contrast and saturation. I imagine the light coming into the lens in a cone-shape spreading out from the lens, and anything outside that cone is not part of the image that the lens sees and should therefore be matted out. Then again I can also see the the hard mask acting as an aperture, and maybe even a pinhole effect. So I'm confused about this issue.

As well, a DP told me that when punching in on tighter lenses for closeups, it is necessary to open up by 1/3 to 1/2 a stop as going on a longer lens doesn't let in as much light. I'm confused about this as well since lenses are rated in T-stops, and theoretically all lenses should compensate for that exposure difference in the glass. My feeling is if a lens is properly calibrated, a T/2.8 on an 18mm should look exactly the same exposure-wise as a T/2.8 on a 300mm. After all, isn't that why we have T stops?

Thoughts? Proof?


Hi,

If you look through the eyepiece you will see the image get darker when you plave a smaller apeture in front of the lens. Also the bokeh in the out of focus highlights will be the shape of your mask.

A proper cine calibtated prime lens will be accurately marked in T stops, the same can not be said for older zoom lenses, or many compact modern ones. There is a very good reason why good cone zooms are large, heavy & very expensive.

Stephen
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#3 Johnathan Holmes

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Posted 18 June 2008 - 01:46 AM

I'm still confused.

Seeing as this is something that has been bothering me for a while, I decided to go all out and explain my logic here.

Take a look at the image that I drew up in Illustrator instead of sleep tonight.
"Please excuse the crudity of this model as I didn't have time to build it to scale or paint it!"

Posted Image

Figure 1 shows a cross sectional of an awkward looking long lens and how it sees a typical scene. We have the frame with a subject, and the light coming from the frame shown in blue. The light within the frame (blue) hits the lens and is refracted onto the film plane (brown). This is pretty simple, and again all theoretical and illustrative. Now consider the red light rays that represents "everything else" in the world not within the frame. Logically any light that is emitted from a source outside the frame is not going to hit the film plane (otherwise it would appear as part of the image on the negative). This can only mean that the light hitting the lens (red) is somehow refracted in such a way that it bounces around within the lens and within the camera. As I understand, lens manufacturers take great care to eliminate the amount of reflections within the lens resulting from this "red" light bouncing around. I'm not sure what the technical term for this light bouncing around is (ghosting? glare?) but logically it would seem that it could only affect the exposure negatively by raising the base black level (see exposure curve) and reducing contrast and saturation. I always thought that a lens flare was an extreme case of this where a light aimed at the camera is positioned so close to the edge of frame that some of this "red" light bleeds into the "blue" light and fogs the image. Am I correct here or am I missing something?

Now look at Figure 2 which shows what (I think) happens when you use a matte box with a hard mask smaller than the diameter of the lens, yet just small enough to not vignette. The blue light is still transmitted to the negative just fine, but now all of the "red" light originating from outside the frame does not hit the lens. Logically I can see how this would appear to lower exposure, as I would assume it lowers the base black level and increases contrast and saturation. Less stray light means a cleaner and deeper black, no? So according to this logic I can see how it would appear to make the image darker, where really what is happening is the blacks are becoming deeper and this creates the illusion of the image becoming darker.

Now I'm certainly no authority on lens theory and optics, so I'm just using my own logic and experience to come up with this theory. I certainly hope anyone out there can tell me if I'm on the right track, or if there's some huge flaw in my logic that I'm not seeing. Looking forward to some discussion here!

Aaaand I have no life.

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#4 Chris Keth

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Posted 20 June 2008 - 11:54 PM

I don't think there is any harm to close-fitting hard masks as long as they don't cut into the frame. Any light entering from out of that pyramid of light is non-imageforming and only raises the black levels of the image. Sometimes that is a good thing but most of the time you want the contrast. As long as the matte doesn't cut into the image area, the entirety of the iris is working and you won't affect exposure and your out of focus points will be the shape of the iris, not the shape of the matte.
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#5 Chris Keth

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Posted 21 June 2008 - 12:02 AM

an addendum, since I can't edit any longer:

Your theory is generally correct. The one catch is that it's the entrance pupil of the lens that matters in the way you're thinking of, not the diameter is the barrel. The entrance pupil is usually much smaller than the barrel of the lens, especially at small stops.
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#6 Bruce Greene

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Posted 21 June 2008 - 11:30 PM

Wanted to know if this was absolutely true or not:

One should never use a hard mask on a matte box with an opening smaller than the diameter of the front element of the lens, as it affects exposure.


I recently shot a film with a foreign crew on location. I asked the camera assistant, who had 38 years experience, not to use the hard mattes with the longer lenses (actually with any lens). He thought I was nuts and generally ignored my request because we were using prime lenses with small front elements I guess.

One night I looked though the lens and all the out of focus street lights in the distance were rectangular. I couldn't understand it and thought it was because we were photographing them through a fence. I liked the effect of the out of focus lights though and added a couple of pen sized flashlights to the background, but these were not through the fence and they were rectangular as well. Then I saw the front of the camera. They had put the 60mm hard matte on for shooting with the 85mm lens I think. They assumed this was more than safe. I pulled the hard matte off and all the highlights became round as expected.

The hard mattes to me do very little with a wide lens, and often impact the exposure of long lenses. I really only will use them for special occasions as they are usually just too close to the lens to be valuable.

It's just my opinion, so don't yell at me:)
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#7 Max Jacoby

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Posted 22 June 2008 - 03:07 AM

A proper cine calibtated prime lens will be accurately marked in T stops, the same can not be said for older zoom lenses, or many compact modern ones. There is a very good reason why good cone zooms are large, heavy & very expensive.

Hey Stephen

What modern zooms have you noticed this with?
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