Light Falloff ?'s

[Lemures-Ex]

I'm curious, how much of a low light advantage at a given aperture does a RF have over and SLR due to the distance of the lens to the film plane and the inverse square law? Is there any difference or does something account for this?

Jared

 

[rogue_designer]

the fractional difference of 1 - 2 cm shouldn't have any visible effect. Besides which, the lens designs are built such that the distance light travels to reach the film is accounted for - but that probably has something to do with why RF lenses can often be smaller for similar aperture performance.

 

[Bryce]

RF lenses are at a disadvantage compared to slr lenses in terms of light falloff. The nearer the lens assembly is to the film, the greater difference there is in distance from lens to film at the center and corners.
This of course only becomes significant with very wide lenses. And RF wides have advantages too- smaller, less elements (and less flare, so more contrast).

 

[Lemures-Ex]

RF lenses are at a disadvantage compared to slr lenses in terms of light falloff. The nearer the lens assembly is to the film, the greater difference there is in distance from lens to film at the center and corners.
This of course only becomes significant with very wide lenses. And RF wides have advantages too- smaller, less elements (and less flare, so more contrast).

I don't mean vignetting, I mean light falloff from the lens to film. Though I do understand that the light has to bend more and travel proportionally farther from the light in the center of the image when the lens is wider/closer to the film.

 

[Bryce]

Light falloff usually refers to something similar to vignetting- i.e. a gradual darkening near the corners. It is caused not by something obstructing the lens, like vignetting, but by the inverse square law.
Assuming we're talking about the same thing, slr lenses actually work better in this regard than the 'true' or non retrofocus wide lenses used on RF cameras.

 

[Lemures-Ex]

Light falloff usually refers to something similar to vignetting- i.e. a gradual darkening near the corners. It is caused not by something obstructing the lens, like vignetting, but by the inverse square law.

Light falloff is just that, light losing intensity over a given distance. I wasn't referring to vignetting as an effect of lens obstruction, rather I was reffering to it as an effect of lens design. Vignetting caused by the lens design is due to the fact that the light hitting the edges of the film has to travel proportionally further than the light hitting the center of the film. You are correct that this effect is minimised on and SLR because of the registration distance, but that was not my question. My question was asking what effect the inverse square law has, if any, on the overall intensity of the light hitting the film plane from an SLR lens verses that of a RF lens. I think rogue_designer hit the nail on the head when he said it was accounted for in lens design: A given aperture on an SLR lens is larger than the same aperture on a RF lens. This makes perfect sense to me in retrospect since the aperture is not a fixed light value but rather a fraction based on focal length and maximum allowable light through a lens (size of lens barrel).

Jared

 

[Bryce]

Ah, got it. Yes, it does sound like rogue_designer got that right.
I didn't follow the question.