DOF calculator for HVX + 35mm adapters

[daybreak]

I apologize if this has been answered many times, but I searched and could not find the answer.

Using an HVX + 35mm adapter, would I calculate DOF using the 35mm setting on a DOF Calculator, or do I need to factor in the HVX lens as well? I'm asking because i just added a DOF calculator to my iphone

 

[Luis Caffesse]

You would caluculate it using the 35mm setting.
The HVX lens has no effect on the apparent DOF of the image when using an adapter.

 

[daybreak]

Thank you, Luis!

 

[Barry_Green]

But keep in mind that it's all dependent on the frame size you're using. Many of these adapters have no indication of what the actual frame size is, and many times people just zoom in to whatever degree and start shooting. That will have a massive effect on your actual depth of field.

If you want to have a prayer of having an accurate DOF calculator reading, you'll have to first set the field of view to match an existing format -- whether you choose 35mm still frame or 35mm cine, you still have to know what your actual frame size is before you can attempt to have an understanding of what your DOF will be.

Further compounding the problem will be if you're using an adapter with a more-transparent ground glass, because then the DOF characteristics won't match up *at all*.

So -- good luck, but overall it's like using a light meter with video: don't expect much better than a vague approximation of usable results.

 

[Luis Caffesse]

But keep in mind that it's all dependent on the frame size you're using. Many of these adapters have no indication of what the actual frame size is, and many times people just zoom in to whatever degree and start shooting. That will have a massive effect on your actual depth of field.

You know I thought about that before my initial posting - but then I wasn't so sure.
I mean, you're obviousy right in that you have to be zoomed in the proper amount to have true 35mm field of view - but does the amount you zoom in on the GG of the adapter really have an effect on the depth of field of the image that you're getting on the GG?

As far as I understand it the only factors affecting depth of field are distance to the subject, field of view, and iris. But all of those factors are being dictated by the taking lens - not the HVX lens, isn't that right? I mean, if I change my iris on the HVX, it will affect the DOF of the HVX which is focusing on the GG - but it's not going to change the DOF of the image that is on the ground glass. Seems like the same would follow for field of view.

If the taking lens is set properly (ie. proper distance from focal plane to lens) then it seems as though the depth of field is going to be what it is with a given adapter/lens combo. I don't see how zooming in closer or farther is going to change what is and is not out of focus on the GG.

But maybe I'm missing something very obvious here.



EDITED TO ADD:
I just reread your post and I think maybe you simply meant that to be able to accurately use a DOF chart for 35mm you need to make sure you are shooting with a 35mm field of view - hence zooming in the proper amount. Is that right?

 

[Barry_Green]

but does the amount you zoom in on the GG of the adapter really have an effect on the depth of field of the image that you're getting on the GG?

No -- but it affects how close you'll have to get to the object, which will affect where you set the focus distance on the lens, which will affect the depth of field. The more you're zoomed out on the GG, the shallower your net DOF will be.

As far as I understand it the only factors affecting depth of field are distance to the subject, field of view, and iris. But all of those factors are being dictated by the taking lens - not the HVX lens, isn't that right?

Correct. But distance to the subject is the one that's going to change, because for equivalent framing the further you're zoomed in on the GG, the farther back you'll have to be with the actual lens.

And, unfortunately, this goes somewhat out the window with adapters depending on which adapter you're using. If you're using a Brevis with a CF1, you can't count on the DOF matching 35mm at all because you're not actually truly seeing the image that's projected on the ground glass as solely a product of what the lens is projecting, but you're also getting some see-through out the actual lens. That throws all the calculations out the window.

If the taking lens is set properly (ie. proper distance from focal plane to lens) then it seems as though the depth of field is going to be what it is with a given adapter/lens combo.

Basically, yes. On an adapter that simulates the 35mm field of view and uses a GG that adequately traps the image, yes. But the more transmissive the ground glass, the less this actually applies.

I don't see how zooming in closer or farther is going to change what is and is not out of focus on the GG.

Again, it doesn't -- but it changes how close you're going to get to the subject, and that's what changes the DOF.

EDITED TO ADD:
I just reread your post and I think maybe you simply meant that to be able to accurately use a DOF chart for 35mm you need to make sure you are shooting with a 35mm field of view - hence zooming in the proper amount. Is that right?

Indeed, although sometimes it seems like Kholi and I are the only guys who care.

 

[Luis Caffesse]

No -- but it affects how close you'll have to get to the object, which will affect where you set the focus distance on the lens, which will affect the depth of field. The more you're zoomed out on the GG, the shallower your net DOF will be.

AHA!
Got it.

I was getting stuck in seeing the direct effect of a given thing...not looking forward to the additional consequences.
Remind me never to play chess with you.

Barry Green... always thinking three moves ahead.
:thumbsup:

...sometimes it seems like Kholi and I are the only guys who care.

Definitely not - though you may be the only ones who understand.