Depth of Field Question

[Sam Roberts]

Sorry, Sam -- I erased your message when I typed my answer, by hitting Edit instead of Quote, an option that Moderators have. The two buttons are right next to each other. David Mullen, ASC. The post below is by me, not Sam. His question was why does a smaller sensor produce more depth of field.

Sensor or film size affects field of view and degree of magnification (which affect the circle of confusion figure used), and field of view affects focal length choices.

You go to a sensor that is half the size and you have to use a lens that is half the focal length to get the matching field of view, and while a focal length that is half the size would show a four-stop increase in depth of field according to charts (at the same f-stop and distance focused), the fact that the smaller image has to be doubled in viewing size to match the image from the larger sensor means that the circle of confusion figure you chose is cut in half, made twice as critical, so a four-stop difference becomes a two-stop difference.

The 35mm frame is 2.5X larger than a 2/3" sensor, so the effective difference in depth of field is 2.5 stops, i.e. once you match field of view by using a focal length that is 2.5X shorter on a 2/3" camera, and you use the same f-stop at the same distance to subject, you have gained 2.5 stops more depth of field, as if you were shooting in 35mm but had stopped down the lens by 2.5 stops.

 

[Brandon Fraley]

sensor size dictates depth of feild characteristics. The larger the image "window" the shower the depth of field. that's why when you close up a lens you get more depth. you're making the "window" smaller. the lenses don't dictate dof, they're just made to fit that size sensor.

 

[Seth Larney]

sensor size dictates depth of feild characteristics. The larger the image "window" the shower the depth of field. that's why when you close up a lens you get more depth. you're making the "window" smaller. the lenses don't dictate dof, they're just made to fit that size sensor.

A good way to think about it is :

Your DOF doesn't actually change with different sensor (or film) sizes, your FOV does. A 50mm lens is a 50mm lens..

So, a 50mm lens has exactly the same DOF no matter whether it's on a 16mm film camera, or a 35mm film camera, or the RED. It's the portion of the lenses' image that you see that changes, effectively making it seem as though the DOF is shallower or deeper.

For instance, a 50mm lens on a 35mm camera may have a 'similar' FOV as a 35mm lens on a 16mm camera (guesstimations only) because you are seeing a smaller part of the lenses' image (This is why tiny chip camcorders have lenses in the vacinity of 4mm etc) and because the 35mm lens has shallower DOF, the 35mm captured image will 'look' like it has shallower DOF for the SAME PERCEIVED FOV as the 16mm image, because the 50mm lens has shallower DOF than the 35mm. If you put the same 35mm lens on the 35mm camera, you would see the SAME DOF as the 16mm camera, you would just be seeing farther out to the edges of the lens (and if it was a 16mm lens you'd probably get vignetting because of this) and thus think you had shallower DOF.

Hope this helps.
S.

 

[Brandon Fraley]

very true, i was just trying to use the simplest language possible

 

[Sam Roberts]

OK that makes sense. So for someone moving up to RED from a 2/3 inch HD cam
and working without a focus puller, it will be more of a challenge to stay in focus
when the lens is wide open or close to it? Especially if the object is moving?

Keeping this in mind will the ability to maintain focus be difficult using just the LCD monitor or would getting and using the EVF be a better idea?

 

[Brandon Fraley]

more difficult, yes. but its seem most people aren't having too much of an issue. of course EVF is better, but most reports seem to be that focusing from the LCD is very doable.

 

[Nathan Garofalos]

The 5 inch is really small. Its hard to pull focus off of something that small but defiantly doable by all means, but I would go with the 7 inch plus the EVF. You NEED both. The EVF has more resolution than the 7 inch LCD, yet both the 5 and 7 inch LCD have the same resolution.

 

[Sam Roberts]

When shooting interiors, say in a situation where you have to use available light,
and you're shooting a moving object, what is the best way to coax a little more focus depth in the shot- increase the camera's ASA rating, open the shutter, or both?

 

[Brandon Fraley]

When shooting interiors, say in a situation where you have to use available light,
and you're shooting a moving object, what is the best way to coax a little more focus depth in the shot- increase the camera's ASA rating, open the shutter, or both?

best thing to do would be add more light, but sure, add gain, use a slower shutter

 

[Brandon Fraley]

WAIT! shoot 2K, use 16mm lenses if you can

 

[Alexis Hanawalt]

For the sake of it, here is the text of Sam's original post:

In another thread Mr. Luis Bustamante wrote:

"The greatest benefit to the cinematographer is the fact that the sensor on a RED One is Super 35 sized at 24.4mm x 13.7mm, offering 35mm Depth of Field."

I take it he's speaking about a shallower depth of field at any given f/T stop compared to 2/3 inch HD cam's?

How does sensor size effect depth of field?

How much to 35mm lenses contribute to this?

Thanks.

(Original post text always lives on in the RSS feed, even deleted posts, FYI.)

 

[Sam Roberts]

I'm probably going to have to use a zoom most of the time which means not that fast. I do however, have my eye on two very nice Zeiss Planar T* prime lenses both f1.4...they should come in handy in those interior available light situations. Thanks for the replies.