Filter i kameran bakom objektivet?

Det har nu kommit filter i kameran på flera kameror med stor sensor, Alexa Sony F5 och F55. Visst har vi haft filter i kameran men då gäller det 3CCD kameror 2/3” med prismor och tre sensorer. I dessa är filter och prismor en del av det optiska systemet, där objektivet är beräknat med tanke på de brytningsindex som tillkommer av dessa glasmassor.

Men när det gäller 35 mm objektiv är dessa beräknade för att inget skall finnas mellan objektiv och film/sensor. När jag studerade optik fick jag lära mig att man inte kunde ha glasfilter bakom objektivet. Tunna gelatinfilter kunde användas om kamerans fattning kollimerades med hänsyn till detta.

Clairmont Film & Digital var först med filter i Alexa-kameran, nu har även Arri kommit med filterhållare framför sensorn. Då jag känner Denny Clairmont sedan 1980-talet skrev jag ett mejl till honom och fick detta sva på mina frågor.

Dear Denny,

I want to hear your opinion. Filter behind the lens?

Both Alexa and Sony has now glass filter behind the lens. How does this affect the sharpness?

Putting a glass filter behind the lens produces undesirable effects in the form of a non-flat image field. Light behaves differently if it goes through the middle of the filter compare to the edges. This would provide a non-flat image field.

I know that you can use gelatin filters behind the lens, because these filters are thin. Cameras like the Bolex, Panavision, CP, and some modified Arri.

Regards Hans Hansson, FSF, Sweden

Hello Hans,

I am assuming that you are talking about the glass filters in front of the sensor or the video and the HD cameras. Starting at the closest to the lens is the Inferred cut filter to cut the inferred light going to the sensor. Then there is other filters before the sensor.

These filters have not been a problem for most lenses designed for film. The one lens we have to adjust it’s back focus on is the Zeiss Master Prime T 1.3. I have talked to Doctor Scherle at Zeiss about this and he says that the Master Prime was not designed to have anything between the lens and the film in a film camera.

When we test the Master Primes on an HD camera the focus shift is seen easily when we separate the different colors and the shift shows up in the blue channel a lot.

The other types and brands of lenses we have are not a problem as rule. Dr. Scherle says this is because they are not as fast usually and are not as sharp as the Master Primes. When I said the Zeiss High Speed lenses T 1.3 we have been using for years are not a problem his reply was they are not as sharp as the Master Primes so we do not see the focus shift.

We can not change the flange depth on the camera as then the zoom lenses will not work well.

Clairmont Camera saw what Jerome Sabourin in Montreal putting filters into the both the Alexa and the Sony F 35 cameras and with Jerome S blessing we improved on it. Clairmont Camera has licensed Schneider Optics to do what we have done. Schneider makes the filters that go into this filter holder. They have to be made very carefully and must have anti refection coating or there can be some problems. Tiffen is supplying some filters for this to Schneider for this when we get request for certain Tiffen filters.

You can go to Jon Fauer’s Film and Digital times and find an article about the Clairmont in the Alexa filter system and then in a latter issue find an article how we now can add a blue streak filter inside the camera with this filter system with some footage we shot. We can also provide in the camera net holders.

Now Arri is doing this. Of course they claim the did it even better and I have not see Arri’s filter system yet.

Panavision Toronto did the same for an Sony F 35.

Until recently lenses were not concerned in sending the image straight towards the film and the film lenses do not see to have an serious issue with this on he HD cameras.

Lens designed in the past few years now send the image straight back to the film or the imager. In theory this is the best way for video and HD cameras. Having said this we are not having trouble with normal film lenses and people are putting some very old lenses on the HD cameras.

I hope I answered your question. I have attached some interesting reading about optics.

It is 2 PM here in Sherman Oaks. Sunny and + 16.5 C.
Denny Clairmont

Här är två länkar till F&D Times med mer info om filter i Alexa-kameran.

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