Something I am not getting about lenses/getting enough light.

B

budha

Active member
I am still learning the math/art of photography and how focal lengths etc work.

I understand that the wider the aperture, the shallower the DOF.

I know if you are in a really bright place you can use an ND filter on your lens so you can still keep the aperture wide open. But lets say you are in a more medium lit place, and you want a specific DOF look. You set your aperture at the right spot but your picture is either too dark or too light. I know you can adjust the shutter speed, but everyone tells me when you shoot 24p you need to be shooting at a 48th shutter.

On a camera like a 7d that has a huge sensor compared to a 1/3 camera, how do you achieve the desired effect while still getting enough light to film? Do you have to raise or lower the ISO or is it something else.

I really appreciate the help.
 
Lowering the shutter speed will make the image brighter, but it will look "smeared". There will be lots of motion blur.
Conversely, increasing the shutter speed will make the image darker, but will make it appear "jittery" - action scenes are sometimes shot with a higher shutter speed for this effect.

Raising ISO will make the image appear brighter, but will also add noise (to an unacceptable degree at some point).
Lowering ISO will make the image appear darker, but may cause you to lose some detail.

Using a faster lens (say a f/1.2 or f1.4) will give you a brighter image as well. Faster lenses wide open may have issues in terms of image quality. Of course, you'll change your DOF here...

The best solution, which is not always available: Light the scene for your camera.

You can also mix and match: lower the shutter speed a tad, raise the ISO a bit, and use a slightly faster lens. Taken together, the effects may be enough - they may not. You mileage will vary. :beer:
 
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You only have so many things to work with in camera: Iris, Shutter, and ISO. There is a limit to these on the low and high end. If you want to go beyond that you need outside factors to do so: ND Filters and lighting. That's pretty much it.

If you want the shutter to always be at 1/48 and the f stop to always be at f5 (for example) you only have so far in the ISO before you need to use ND filters or extra lighting. The ISO on these cameras can be pushed pretty far but it introduces noise (not even close to as much noise as 1/4" sensor cameras but still some).
 
Michael Olsen said:
Using a faster lens (say a f/1.2 or f1.4) will give you a brighter image as well. Faster lenses wide open may have issues in terms of image quality. Of course, you'll change your DOF here...
Click to expand...

When you say faster lenses that are like 1.2 or 1.4, are they brighter because they are designed differently, or just because the aperture can open wider?

Is a 2.8 lens less bright than a 1.4 lens ( while at a 2.8 aperture)?
 
budha said:
When you say faster lenses that are like 1.2 or 1.4, are they brighter because they are designed differently, or just because the aperture can open wider?

Is a 2.8 lens less bright than a 1.4 lens ( while at a 2.8 aperture)?
Click to expand...

Lenses are complicated buggers, and I really don't know much about them. There are some people on the REDuser board who regularly blow my mind. However, I think I've got this much down.

Fast lenses generally require greater optical precision than slower lenses. They may use different coatings on the lenses, different grouping strategies, different shaped lenses, and different production methods yielding a finer "grind" on the lens.

A 50mm f/2.8 lens at f/2.8 and a 50mm f/1.4 lens at f/2.8 should, in theory, transmit the same amount of light.

This is where T-stops (which act as a better measure of the actual transmission of light of a lens as opposed to a theoretical number based on focal length) come into play. Since the optics of the f/1.4 lens are finer (and thereby better at transmitting light), the f/1.4 lens may actually transmit slightly more light at f/2.8 than the f/2.8 lens. So the f/1.4 lens may be something like a T2.6 when set at f/2.8.

I think.
 
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budha said:
Do you have to raise or lower the ISO
Click to expand...

Yes.

Adjust the aperture to give you the amount of DOF you want. Set the shutter speed to approx half your frame rate (i.e. 1/48th or 1/50th for 24p, 1/60th for 30p) unless you want to create a "special" look. Use NDs to get in the ballpark. Use ISO to fine tune the exposure.

-X
 
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