Author Topic: 8mm rokinon lens  (Read 4034 times)

hbryana

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8mm rokinon lens
« on: April 16, 2014, 06:18:32 PM »
Does anyone know how an 8mm rokinon lens would work on a 5dmiii using crop sensor mode. I don't have ML installed yet. So currently I get a circular image where you can see the lens shade at the outer edge of the image. Will the different crop modes allow this lens to be more useful on this camera. Thanks in advance

Bryan

dmilligan

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Re: 8mm rokinon lens
« Reply #1 on: April 16, 2014, 06:35:18 PM »
1. take a still photo in full quality
2. crop that photo with a photo editor to whatever resolution you plan on shooting at (e.g. 1920x1080) (center the crop)
3. this is what it will look like

hbryana

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Re: 8mm rokinon lens
« Reply #2 on: April 16, 2014, 08:34:20 PM »
Thanks dmilligan,

I have cropped my pics shoot with this lens. I was wondering if anyone has shoot in crop mode with this lens and what there thoughts were. Do you know the resolution of each crop mode (I read somewhere there are three modes). Is the resolution the same for all three modes or are they different?

dmilligan

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Re: 8mm rokinon lens
« Reply #3 on: April 16, 2014, 10:16:01 PM »
The three modes I think you are referring to are:
1. "Normal" mode (sensor does line skipping by 3, i.e. only every third row/column is read, max res is sensor / 3)
2. "Crop" mode (you get by using the 5x Digital Zoom, actual raw data is 3x, you get pixels that are 1:1, no line skipping, max res varies by camera)
3. "60p" mode (you get when recording high speed using 50/60p in Canon menu, max res varies by camera)

as far as max resolutions go, usually #2 > #1 > #3

The instructions I gave you would be for option #2. If you want to figure out what it would look like with #1, take the still image and reduce the resolution by 3x (rescale by dividing the width and height by 3), then crop to the resolution you'll be shooting at (Hint: on 5D3, 1080p is going to be the full FOV b/c sensor size = 1080p * 3). #3 is strange and the image gets 'squeezed' and it's different per camera and I don't have a 5d3, so I'm not sure how to tell you how to figure it out, but do you actually plan on doing high speed/slo mo? If not you won't use it (it's also buggier AFAIK).

I'm just guessing, but I think this is going to be a really good lens for wide angle crop mode shots. Crop mode has less moire and aliasing and you can get a higher max resolution (though you are limited by card speed).

hbryana

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Re: 8mm rokinon lens
« Reply #4 on: April 17, 2014, 07:12:09 AM »
Thanks,

That's exactly the info i was wondering about. Hopefully I'll be trying it out soon.

Bryan

Callas

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Re: 8mm rokinon lens
« Reply #5 on: April 23, 2014, 09:24:56 PM »
Hi guys!  I want to ask a question regarding lenses.

For example, I'm shooting raw in 3xcrop mode on a cropped sensor. So how I should figure out a focal lenght of my lens- if i shoot 10mm, it'll be 10X1.6X3 or just 10X3=30mm.

Thank you!

dmilligan

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Re: 8mm rokinon lens
« Reply #6 on: April 23, 2014, 10:33:14 PM »
Just to clarify, you would like to figure out the effective 35mm focal length equivalent of your lens, as the actual focal length is a function of the radius of curvature and does not change.

The problem is that it is going to vary, depending on the resolution you use, as that changes the size of the 'effective' sensor area. Since we are in crop mode and the pixels are 1:1 you can use the pixel pitch of the camera to figure out the 'effective' size of your sensor, and then use that to convert to 35mm equivalent.

This calculation boils down to:

EFL35mm = FLlens * 43.3 / sqrt((h*p)2 + (w*p)2)

where:
h = horizontal resolution you record at (in pixels)
w = vertical resolution you record at (in pixels)
p = pixel pitch of the camera

someone feel free to correct my Maths if I messed something up

EDIT: I'm not sure if pixel pitch is typically measured as the diagonal of a pixel or the width, if it's the diagonal, the formula would be:

EFL35mm = FLlens * 43.3 / (p * sqrt((h)2 + (w)2))