ND filter and Depth of field question.

G

Gene Sung

Veteran
From footage I've seen, I noticed that with the Sigma f 1.4 you can get a really shallow focus. I'm assuming they're shooting f 1.4 to achieve this.

Let's say you have the Tamron 17-50 lens that only goes to f 2.8 (1 stop higher). If I put an ND 3 (cuts it down 1 full stop), then the f2.8 lens would essentially be an f1.4?

By cutting down an f 2.8 lens 1 full stop to an f 1.4, in theory, I would be getting a shallower depth of field? Something similar to DOF of the the Sigma f1.4 correct?

The more (or higher) ND filters stacked on, the shallower the DOF? Correct?

Thanks for any insight.
 
no . the nd filter will help get to the max opening of f stop of your lens lowest f stop without being over exposed. most video cameras on auto will ,in bright light, close up the irus to 5.6 or smaller . the mechanics of lens construction to create what is called -CONE OF CONFUSION - this is what gives the dramatic separation of objects in the frame. the wider and steeper the cone of confusion the more towards a 1.2 or 1.4 lens. the longer taper cone of confusion the less seperation of objects.(F5.6 up to like f11 ) this is expensive to achieve the longer the lens to mantian a steep cone .( hence the huge cost of the glass with fast f stops. ) hope this helps.
 
DOF is determined by the F-stop, not by how much light is hitting the sensor.

Deciding what f-stop to use is determined by you based on a number of factors including how much light is available.

Reducing the amount of light that hits the sensor ALLOWS you to open up to a wider f-stop and still achive proper exposure. By opening to a wider f-stop you get shallower DOF. But just making something dark does not give you shallow DOF. It makes your image dark. The key to proper exposure is a combination of f-stop selection and amount of light. The key to DOF is in the f-stop you choose, not in the amount of light.
 
Also, the difference between f/1.4 an f/2.8 is two stops, not one.

The scale is: 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11.0, 16.0, 22.0, 32.0
 
Don´t confuse the OP with these answers....they seem complicated ;-)

So here´s my attempt (and I am not English speaking, so that might cause confusion in my answer by itself):
Basically if you are at 2.8 and your exposure looks correct then adding an ND filter will only darken your image (and destroy the shot). The DOF will stay the same.

If you are at for instance f8.0 and your exposure looks correct and you then WANT shallower DOF, that´s when you use an ND filter (I assume you are already at 1/50th and ISO160).
Because now when the ND makes the image darker you can open the iris ideally to 2.8. But that´s when you are done again. You can´t go faster than that with this lens.....
unless you have a 2.0/1.4/1.2 lens......that´s why people like them...
 
alpi69 said:
Don´t confuse the OP with these answers....they seem complicated ;-)

So here´s my attempt (and I am not English speaking, so that might cause confusion in my answer by itself):
Basically if you are at 2.8 and your exposure looks correct then adding an ND filter will only darken your image (and destroy the shot). The DOF will stay the same.

If you are at for instance f8.0 and your exposure looks correct and you then WANT shallower DOF, that´s when you use an ND filter (I assume you are already at 1/50th and ISO160).
Because now when the ND makes the image darker you can open the iris ideally to 2.8. But that´s when you are done again. You can´t go faster than that with this lens.....
unless you have a 2.0/1.4/1.2 lens......that´s why people like them...
Click to expand...
No, that still doesn't quite clear the OP's confusion...

OK, DOF is determined by three factors for any given image format: aperture, focal length, and subject to camera distance. So if you want shallower DOF:
1) move the camera much closer to the subject
2) use a longer lens
3) open the iris

Those are the only things that you can do. So something like ND won't work. ND is only useful for getting the exposure that you want. The only thing that ND has to do with DOF is that it allows you to obtain the correct exposure after you choose an aperture value that you want. The wider the aperture is open (i.e. the smaller the f-number), the more light you let into the camera, and thus the heavier ND you'll need to use to avoid overexposing.

Long story short: Putting ND on your f/2.8 lens won't magically make it f/1.4 or f/2.0. Just think about it for a moment. Why is f/1.4 great for low light? Why do fast lenses cost so much? If you could alter the maximum aperture of a lens simply by using ND, then nobody would buy fast lenses. The maximum aperture value that you can use on a lens is determined what's printed on the lens barrel. It seems like common sense to me. If your lens says f/2.8, then you can't do f/2.0 or f/1.4, unless it also has those numbers on it. :)
 
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