Smal question about aperture

[tomas_p]

@ROUBAL: Your chart shows the angle of vision and doesn't uses the film plate dimensions. It is probably for 35mm format (24x36mm) only, but I'm not sure that it is really related to something usable within Octane.

I think, the values angle and focal length are the base dimensions for camera type assesment (example: 50 mm length and 47 degres angle for 35mm camera). The base film plate in this example is fullframe. If you need this with cropfactor, the DOF must be larger (lesser blured). And if you need other crop of format, you must calculate it.

[ROUBAL]

I think, due to this is in Octane separate F-stop (darking efect of F-stop in real lens) and aperture (the aperture radius like in real lens in cm).

That makes sense. Thanks !

[SamPage]

For my own, my current theory is that in actual photo cameras, DOF is function of the Focal length, the F Stop value, the distance from the object in focus and the dimensions of the film plate or the sensor.

Film gate / sensor size has nothing to do with depth of field. Where people get confused is that with an APS-C, then lens has to be shorter to get the same field of view of a full frame camera (24mm x 36mm). So on the APS-C, you might need something like a 30mm to replicate the field of view of a 50mm on full frame. The 50mm will have a shallower depth of field than the 30mm. If you take a 50mm on a full frame, then take that same 50mm on an APS-C, the depth of field will be the same as the mechanics of the lens don't change, you are just capturing a smaller area of the imaging circle.

[ROUBAL]

I would rather say that DOF has nothing to do with Focal length, but as in practical the optics are built according to the dimensions of the film plate, the Focal length has to be taken in account for the chosen sensor. That means that when building a digital camera you have to chose the couple film plate/focal length.

[SamPage]

I would rather say that DOF has nothing to do with Focal length, but as in practical the optics are built according to the dimensions of the film plate, the Focal length has to be taken in account for the chosen sensor. That means that when building a digital camera you have to chose the couple film plate/focal length.

I don't think I follow. I can take the same EF 85mm f/1.2 lens, mount it on a 5D (full frame), or on a 50D (1.6x crop). Between the two bodies the imaging circle and the depth of field doesn't change at all (because it is the same lens), just the area captured changes.

Conversely if I have a 16mm f/2.8 focused at 10 feet on a 5D and a 200mm f/2.8 also focused at 10 feet on a 5D, they are going to have vastly different depth of fields.

[ROUBAL]

I don't think I follow. I can take the same EF 85mm f/1.2 lens, mount it on a 5D (full frame), or on a 50D (1.6x crop). Between the two bodies the imaging circle and the depth of field doesn't change at all (because it is the same lens), just the area captured changes.

This is true, but comparisons must be done for the same framing of the image, so it appears obvious that to get the same visual result on the captured image, you have to use a shorter focal length when using a smaller sensor !

As an example, I built a super long focal camera (for astronomy) by mounting a 300 mm Tamron teleobjective on a black box in which I put a very small 320x240pixels CMOS sensor taken from a webcam. I used a small part of the imaging circle, in the center. That way I was in the almost planar region of the lens, with very few aberrations. The apparent focal length was around 4000mm ! I tested it at home, aiming a mountain at 10 km, and I could see someone skying almost full frame ! Obviously, the camera must'nt move at all !

[tomas_p]

Hi

I think, the more are the lens closer to senzor or film and the farther from focused oject, the deeper is DOF (more sharper). Is simple. With cropfactor must be the focal length shorter and in this case ist DOF deeper.