Sigma 18-35mm f/1.8 DC HSM Art Lens QUESTION

[rob norton]

I have the edelkrone focus module and PD MOVIE LIVE AIR.

IDK much about lens control systems but would you see a difference if you used the tilta nucleus m?

If tilta handles the spinning issue better (again I have no idea), then you could sell the edelkrone and get a "free"-ish different set up rather than buying more lenses.

[thefilmaddict]

I'm stuck with that follow focus systems because it works with my slider.

I guess the reason I was confused about the crop factor is because:

When I put a full frame lens on a full frame camera, it's much wider than when I put it on my pocket 6k (which is not full frame). But I guess you are saying, that a 17mm EFS lens looks the same as a 17mm full frame lens on the pocket 6k? Just want to make sure I nail this concept.

[Tim Sewell]

Apparently the manual focus throw is 110deg, most of which is devoted to 0-2m distance. However - I did find this, which might be of interest: https://www.ebay.co.uk/itm/Sigma-18-...-/203143866943

[Run&Gun]

I'm stuck with that follow focus systems because it works with my slider.

I guess the reason I was confused about the crop factor is because:

When I put a full frame lens on a full frame camera, it's much wider than when I put it on my pocket 6k (which is not full frame). But I guess you are saying, that a 17mm EFS lens looks the same as a 17mm full frame lens on the pocket 6k? Just want to make sure I nail this concept.

The smaller the sensor, the narrower your field of view/angle of view is vs. a larger sensor with the same focal length lens. So, yes, if you take a 17mm lens designed for full frame and a 17mm lens designed for S35 or APS-C(very similar) and put them both on the s35 camera, the FoV/AoV would be the same.

[NorBro]

I'm stuck with that follow focus systems because it works with my slider.

I guess the reason I was confused about the crop factor is because:

When I put a full frame lens on a full frame camera, it's much wider than when I put it on my pocket 6k (which is not full frame). But I guess you are saying, that a 17mm EFS lens looks the same as a 17mm full frame lens on the pocket 6k? Just want to make sure I nail this concept.

When you see a lens as "EF-S" or "APS-C" or "full-frame", they are telling you this to let you know what sensor size it can successfully cover.

The full-frame camera is wider because it has a full-frame sensor, not because it has a full-frame lens. You can put an APS-C or EF-S lens on a full-frame camera and you'll have the same exact image as the full-frame lens but the sides will be black because the lens cannot successfully cover the sensor size.

The 17mm and any other EF-S or APS-C or full-frame lens on the planet will all look the same on the Pocket 6K because they can all successfully cover the smaller sensor.

There is no other math involved in your case.

[ahalpert]

But I guess you are saying, that a 17mm EFS lens looks the same as a 17mm full frame lens on the pocket 6k?

Yes, both a 17mm full-frame lens and a 17mm EF-S lens will look the same on the Pocket 6K.

A full-frame camera will record the entire image circle of a full-frame lens (or at least, a rectangular cut-out that extends to the sides)

A crop-sensor camera will record a smaller rectangular crop within that same image circle

In each case, the image circle being projected by the lens is the exact same size.

But EF-S and APS-C lenses do not project a large enough image circle to cover a full-frame sensor. If you put that lens on a full-frame camera, you will see a circular image in the middle of the frame surrounded by black.

When manufacturers report the "equivalent focal length" of a lens, they do so for the sake of photographers to make easy comparisons between lenses. If you already know what a 17mm looks like on a full-frame camera, then you know that a "17mm-equivalent" APS-C lens will produce the same field of view on the crop-sensor camera. The depth of field will be different, but you can frame the same shot.

[thefilmaddict]

That’s an excellent explanation.

Another question: will the cine lens help with follow focusing? For example, when I use my sliders focus module, I can set in and out points on my starting and stopping focus points for the start and end of the shot/move. It works ok with my photo lens at the start and stop points, but sometimes in the middle of the move, the focus is off. Would a cine lens fix this issue or help it?

[ahalpert]

That’s an excellent explanation.

Another question: will the cine lens help with follow focusing? For example, when I use my sliders focus module, I can set in and out points on my starting and stopping focus points for the start and end of the shot/move. It works ok with my photo lens at the start and stop points, but sometimes in the middle of the move, the focus is off. Would a cine lens fix this issue or help it?

I'm not sure I understand, but my guess is that you will have the same experience. Cine lenses have some advantages when working with a follow focus - namely, the built-in gears that mean you don't have to strap on an accessory gear, which could slip. The focus throw is typically much longer than a photo lens, enabling you to pull focus manually with greater precision. The resistance of the barrel should be firm but smooth. And cine lens manufacturers typically try to minimize focus breathing, which photo lens manufacturers needn't worry about.

But if I understand correctly, your problem may relate to the logarithmic scale of focus on the lens barrel. Due to the nature of optics, a lens requires logarithmically greater degrees of rotation to achieve the same absolute reduction in focus distance as you focus closer and closer to the sensor plane. Turning the lens a few degrees may move the focus from 15' to 10'. But the same amount of rotation may only move the focus from 10' to 8'. You would need to turn it more to reach 5'. All lenses are like this, both cine and photo. So, if you move the camera at a linear speed, and your focus module is moving at a linear speed, then it might happen that the focus gets ahead or falls behind in the middle but converges on the correct distance at the end. If that is the problem, then a cine lens won't help you.

[thefilmaddict]

You are correct about the issue. Starting focus point is good. Ending focus point is good. Middle sometimes out. Probably because linear does not represent the true focus distance. I think I’ll just have to close the lens down a little more to make it easier.

[thefilmaddict]

Thanks for all of your help, guys.