No, the focal lengths do not change. But a 24-70 lens on 1.6x will
give the exact same framing as a 38-112 lens on FF. Then lenses
are not the same, but the images the two lenses take on the two
different formats will have the same framing.
To get the same DOF, you will also have to use 1.6x the f-ratio for
the FF lens as well.
In other words, if I took a pic at 24mm, f / 2.8 on the 1.6x DSLR
and a pic a 38mm, f / 4.5 on the FF DSLR, of the same scene from
the same position , the two pics would look the same, in terms of
perspective, framining, and DOF.
Assuming the framing and DOF are as you describe, how does that
affect the light gathering ability of the f/2.8 on any camera, crop
or FF?
I'm glad you asked. : )
The light gathering ability of a camera for a given scene depends
only on the size of the aperture and how much of that light is
captured by the sensor. The aperture is determined by the FL
divided by the f-ratio.
For example, let's say we're at 24mm, f / 2.8 on a 1.6x DSLR. Most
of the light from the image circle from the lens falls outside the
sensor. The amount of light that does fall on the sensor is the
same amount of light as would fall on a FF sensor with a FL of 38mm
and an f-ratio of f / 4.5.
Thus, both sensors collect the same amount of light. However, the
FF sensor has 2.56 times the area as the 1.6x sensor, thus the
intensity of that light is 2.56 times less. So, to get the same
exposure, you either have to reduce the shutter speed by a factor
of 1.6 or multiply the ISO by a factor of 2.56, or some equivalent
inbetween combination.
But since both sensors receive the same amount of light, the noise
will be the same for both images.
So, to get an equivalent image for FF as for a 1.6x camera, use
1.6x the FL, 1.6x the f-ratio, and 2.56x the ISO (or divide the
shutter speed by 1.6).
The big bonus of FF is that, for the times that you can use the
lower shutter speed, and don't need to up the ISO since the shutter
speed is easily fast enough anyway, you get a cleaner and more
detailed image. Only in the worst case scenario when you have to
up the ISO accordingly do you get an equivalent image. But, even
then, you have the choice of having a more detailed image with more
noise, or an image with the same detail and same noise.
The reason you have those options is that the FF sensor has more
pixels than the 1.6x sensor. If the number of pixels were the
same, you'd always get the same detail, just with 1 1/3 stops less
noise. But, since the 5D has 50% more pixels, you only get a 2/3
stop noise improvement for the same ISO. When you up the f-ratio
and ISO to get the same DOF, shutter speed, and exposure, the 5D
image will actually have 2/3 stops more noise, along with its 50%
more pixels. However, if you then resample that image to the same
dimensions as the 1.6x image, you will have lost the detail
advantage, but regained the noise equivalency.
Does that make sense?
--
--joe
http://www.josephjamesphotography.com
http://www.pbase.com/joemama/
Please feel free to criticize, make suggestions, and edit my
photos. If you wish to use any of my photos for any purpose other
than editing in these forums, please ask.
