Do I have a crop factor on iris

[Carl Epstein]

I am slightly confused.
A 50 mm FF lens gives the view of a 75 mm at 1.5 crop factor. Given
If it is a F2.0 then is it a F3.0 iris also at a 1.5 crop?

 

[Robino_J]

Aperture is not affected by crop factor.

 

[Jeffery Anderson]

might be thinking how some cameras have a native ISO... usually 400-2000ASA... it's the same as film, but the lenses are different consideration

 

[Nathan DuMoulin]

The DOF is also multiplied, but not the amount of light hitting the sensor.

Said differently, the f2 on FF35 or S35 will produce the same level of brightness, but the DOF will be different given that you'll need to use a wider lens on the S35 to get the same FOV as the FF35.

 

[Karim D. Ghantous]

Nathan is right, but I'd like to explain it a different way. Let's say you have a frame of 4-perf 35mm. Let's say the ASA is 50. You expose it correctly with a 25mm lens set to T5.6. You then view the footage and decide that you want to crop that frame. Does the exposure change?

 

[Carl Epstein]

ok that makes sense. I think I am confusing the iris function. For an analogy. If the lens was a projector projecting a FF size circle onto a FF sensor all the light would be used. If the sensor was half the size of the projected circle of light then it would only be using half the projected light? I will have to look into how iris works more. When you close down the iris circle it seems like the circle of light would get smaller but if that were true at f22 you would only have a picture in the center of your ff sensor. Duh. I guess the lens disperses the light across the whole size circle with less intensity at a lower iris. Does this sound correct?

 

[AndreeMarkefors]

ok that makes sense. I think I am confusing the iris function.

There are a couple of issues with what you're writing.

For a certain lens design, let's say Canon EF full frame lenses, the image circle that the lens "projects" remains constant, even if you change aperture. Otherwise selecting a smaller aperture (size) like f22 would create severe vignetting. Makes sense?

If you use a full frame lens on a full frame sensor, the lens illuminates the sensor evenly. If you crop the sensor (or have a smaller physical size to begin with), some light will indeed be wasted outside the crop or the smaller sensor since the projected image circle is still the same. BUT, the illumination on the lit part of the sensor remains the same between the full frame sensor and the crop sensor. Think of it as light per area unit. Or light per photo diode.

Ex. Light your palm with a flashlight. Move half of your hand outside the light beam. Does the half that is still lit get half as bright? No.

Then there is the question of "DOF crop". If you have decided on a certain framing with a 50mm lens at f2 in the 5K mode of a RED camera and then decide to shoot 4K, you will need to reposition the camera to get the same framing. Changing the distance between the subject and camera will give you a new DOF with the same lens, still at f2.

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If a lens manufacturer decides to design a lens for a smaller sensor size to begin with, in Canon's case their EF-S lenses for their 1.6x crop bodies, they can use smaller lens elements and a more compact design since the image circle will be smaller at the end. Benefits are lower weight and price.

 

[Carl Epstein]

Yes sounds good. Thanks.