How Camera mirror image is sharp when lens was focused on film ?

[Mustafa Umut Sarac]

How cameras with mirrors shows images sharp when they are standing half way between film and lens ? I am thinking a 6x7 film camera with Ektar and I thought best way is to put a mirror on its way with spring to cut the time before exposure.

Umut

 

[cliveh]

Mustafa, I'm not sure I follow what you are saying, but the distance between the mirror and the ground glass screen should equal the distance between the the mirror and the focal plane.

 

[Sirius Glass]

What cliveh said.

 

[Trail Images]

I am thinking a 6x7 film camera with Ektar and I thought best way is to put a mirror on its way with spring to cut the time before exposure.

Not sure I understand this fully, but are you referring to a mirror up operation before shutter release like I use my RB67 all the time?

 

[Newt_on_Swings]

Yup, with a mirror at a 45 degree angle, the distance from the mirror to the focusing screen is the same from that mirror to the film plane.

 

[Dan Fromm]

Yup, with a mirror at a 45 degree angle, the distance from the mirror to the focusing screen must be the same from that mirror to the film plane if an object seen as in focus on the screen is to be in focus on the film.

The mirror, focusing screen, and film plane all have to be in the right positions. This is why replacing a Graflex SLR's Graflex back with a Graflok requires the camera's ground glass to be shimmed to the new right distance from the mirror.

 

[David Lyga]

If the mirror is only a TINY bit too high or low, that will provide a discrepancy between 'apparent' focus (what you see in the viewfinder) with 'real' focus (what gets recorded on the actual film). Test, wide open and focus PRECISELY upon a detailed object that is 45 degrees from the camera's front (I use an LP record album with much large text held at 45 degrees). - David Lyga

 

[RalphLambrecht]

How cameras with mirrors shows images sharp when they are standing half way between film and lens ? I am thinking a 6x7 film camera with Ektar and I thought best way is to put a mirror on its way with spring to cut the time before exposure.

Umut

because,what you're seeing through the viewfinder is not an image on the mirror but an image on the view screen,which is at the same distance from the lensas the film is.In many cameras,the view screen is adjustable to make it soand manualfocusing error are often due to poorly adjusted view screens.you can test the alignment yourself with a self-made focusing target email me for more detailed instructions

 

[AgX]

If the mirror is only a TINY bit too high or low, that will provide a discrepancy between 'apparent' focus (what you see in the viewfinder) with 'real' focus (what gets recorded on the actual film). Test, wide open and focus PRECISELY upon a detailed object that is 45 degrees from the camera's front (I use an LP record album with much large text held at 45 degrees). - David Lyga

The height of the mirror is of no interest.
What is important is that there is no deviance from the 45° resp 90° angle of the mirror plane, otherwise the orientation of the plane of focus will change regarding the groundglass.

 

[David Lyga]

Yes, AgX, that is what I meant but you made it clearer (and all the way from Germany, too!)

That 45 degree angle is crucial and most people do not know that. That 'height', which I talked about previously, is fixed with the hinge. - David Lyga

 

[Dan Fromm]

That 'height', which I talked about previously, is fixed with the hinge.

Not always. See, e.g., the Corfield Periflex and Peckham-Wray cameras, which use retractable focusing periscopes and most Beaulieu cine cameras, which use reciprocating shutters with the mirror on the shutter. No hinges in any of them. See also the Canon 310XL, which has a fixed focusing periscope. The Periflex and Peckham-Wray are sort-of SLRs (the finder screen doesn't show very much), Beaulieus and the little Canon are SLRs (the finder shows all that the film will see).

 

[David Lyga]

Sufficiently esoteric I am not, Dan. This is a rather exceptional exception. I was talking about common SLR design. But thank you for the interesting and informative input. - David Lyga