Bart_van_der_Wolf
pro member
As my first post here I'd like to share my findings on the vignetting/light fall-off that I've quantified for two of my lenses (others may follow) on the EOS-1Ds Mark II. I've not done a comparison with a film image of the same size, but I've no a priori reason to assume it's much different. To me it seems that the microlenses do a good job at decreasing the angle of incidence. I've also chosen to post here instead of in a more lens related forum, because the results can/will differ between sensors, and I wanted to avoid a 1.6 crop versus FF sensor debate as much as possible.
I used Imatest to take the tedium out of the testing procedure, and summarized the results in a graph. To provide a uniform 'flat'-field for exposure, I used a piece of opaline glass held flush to the front of lens, which was pointed at the sky (overcast), neither filter nor hood was used. The exposure was set to +1.0 stop on Aperture priority. The Raw file was processed as a linear 16-b/ch RGB, and the darkest corner was plotted in the summary (as a worst case scenario).
Here is an example of one of the Imatest output options for an f/8 aperture, and this is the summary for the full range of apertures of my EF 50mm f/1.4 .
What may be interesting for those seeking to reduce vignetting/light fall-off is, that stopping down beyond f/4.0 serves no purpose (other than for DOF and diffraction), it won't get any better stopping down further.
As a comparison, I've also verified my findings with another lens of somewhat similar focal length, the T/S-E 45mm f/2.8 in an unshifted state. It also won't get any better beyond a given aperture, f/4.5 in this case (which happens to be where the lens also peeks in resolution). Another noteworthy observation, the wider image circle won't change the source of the phenomenon.
What happens is, that at the widest apertures the cone of light at the exit pupil shows signs of the barrel construction partially cutting-off the most oblique rays and the cone going from an elliptical diameter to a more round one at smaller apertures. Beyond that, it's mostly the larger distance to/magnification at the corner that causes the loss of intensity. That law of physics is a given, it will be darker at the corner (with a rectilinear projection that is).
Also, These 'worst corners' are indeed that, the most extreme corners of the sensor, and things already improve when you move in a millimetre, so don't despair ..., but be prepared for the additional fall-off that the shift will bring, but that's something for another subject (and forum I guess).
Bart
I used Imatest to take the tedium out of the testing procedure, and summarized the results in a graph. To provide a uniform 'flat'-field for exposure, I used a piece of opaline glass held flush to the front of lens, which was pointed at the sky (overcast), neither filter nor hood was used. The exposure was set to +1.0 stop on Aperture priority. The Raw file was processed as a linear 16-b/ch RGB, and the darkest corner was plotted in the summary (as a worst case scenario).
Here is an example of one of the Imatest output options for an f/8 aperture, and this is the summary for the full range of apertures of my EF 50mm f/1.4 .
What may be interesting for those seeking to reduce vignetting/light fall-off is, that stopping down beyond f/4.0 serves no purpose (other than for DOF and diffraction), it won't get any better stopping down further.
As a comparison, I've also verified my findings with another lens of somewhat similar focal length, the T/S-E 45mm f/2.8 in an unshifted state. It also won't get any better beyond a given aperture, f/4.5 in this case (which happens to be where the lens also peeks in resolution). Another noteworthy observation, the wider image circle won't change the source of the phenomenon.
What happens is, that at the widest apertures the cone of light at the exit pupil shows signs of the barrel construction partially cutting-off the most oblique rays and the cone going from an elliptical diameter to a more round one at smaller apertures. Beyond that, it's mostly the larger distance to/magnification at the corner that causes the loss of intensity. That law of physics is a given, it will be darker at the corner (with a rectilinear projection that is).
Also, These 'worst corners' are indeed that, the most extreme corners of the sensor, and things already improve when you move in a millimetre, so don't despair ..., but be prepared for the additional fall-off that the shift will bring, but that's something for another subject (and forum I guess).
Bart