Light fall-off on Sigma 50-500

Since Heather reviewed this lens here on NSN, I thought I'd chime in with my comments...

The one real drawback I've got with the lens is that there appears to be light fall off at the corners (more gradual and covering a larger area than plain vignetting). It's not always obvious--in fact, unless the picture has a nice plain light-coloured background (like one might get shooting a bird in the water) I don't even notice it.

Is light fall-off a problem with an individual lens, or is it more related to optical design? Does anyone else with a 50-500 notice this?

-Paul

Light falloff in the corners is a very common problem with longer telephotos. Usually stopping down two stops from its wide open aperture eliminates it. Often Teleconverters makes it worse and extension tubes always makes it worse. Digital shooters with crop factor sensors do not need to worry about it though.

Thanks EJ!

I normally shoot at f/8 (2/3 stop down from wide open) with that lens. I guess I need to try f/11 and sacrifice a stop of speed if the light permits it... (But my lask of sharpness is far more likely due to motion--I need all the speed I can get!)

Is this in any way related to the asymmetric vignetting that I get with this lens. (That problem only affects the viewfinder, and in no way affects the image on film.)

-Paul

I noticed this a few times, like you said mostly against a light colored sky or something similar. It seemed to be most noticable when I was shooting film, but so infrequently for a long time I thought it was caused by the lens hood. It happened even less so with the D1's 1.5x magnification.

Paul Skoczylas wrote:Thanks EJ!
Is this in any way related to the asymmetric vignetting that I get with this lens. (That problem only affects the viewfinder, and in no way affects the image on film.)

Asymmetric vignetting that occurs in the viewfinder and not at the film plane is due to an interaction between the mirror and the optics of the lens. Basically the way the image is hitting the mirror has part of the light path missing the mirror so it isn't translated to the viewing prism. This is due to the light path coming in at a very big angle of incidence to the film plane and since the mirror is out in front of that, some of the light is missing the mirror. It's hard to explain, hope you get my drift.

E.J. Peiker wrote:

Paul Skoczylas wrote:Thanks EJ!
Is this in any way related to the asymmetric vignetting that I get with this lens. (That problem only affects the viewfinder, and in no way affects the image on film.)

Asymmetric vignetting that occurs in the viewfinder and not at the film plane is due to an interaction between the mirror and the optics of the lens. Basically the way the image is hitting the mirror has part of the light path missing the mirror so it isn't translated to the viewing prism. This is due to the light path coming in at a very big angle of incidence to the film plane and since the mirror is out in front of that, some of the light is missing the mirror. It's hard to explain, hope you get my drift.

Yeah, what E.J. said :roll:

Nikon Capture can compensate for that now if you are using RAW. Doesn't help you with film though.

Thanks again EJ and Greg. Actually, I already understood why the asymmetric vignetting occurred--I was trying (obviously not very well) to ask if the light fall-off was in some way related; i.e., is it somehow due to the position of the focal node not being where the camera expects it to be for most lenses?

I don't have any crop factor, unfortunately (yet--we'll see how the *ist-D performs if it turns out to more than vapourware) nor do I have Nikon Capture, Tony, since I shoot Pentax.

-Paul

Paul Skoczylas wrote:I was trying (obviously not very well) to ask if the light fall-off was in some way related; i.e., is it somehow due to the position of the focal node not being where the camera expects it to be for most lenses?

It is possible that the lens design that is causing the asymmetric vignetting is also what is causing the vignetting at the film plane but it could also be something else. One of the problems with third party lenses is that it is one basic optical design that they just put different mounts and camera interconnects on so by definition it is an optical compromise for any camera its used on. In the fast majority of cases that's not a problem but sometimes it is.

Digital shooters with crop factor sensors do not need to worry about it though.[/quote]

Hate to ask but as a new D100 user I would like to understand and I know EJ can explain. Is it the crop factor?

Since the digital sensors on DSLR's (except the Kodak 14n and Canon EOS 1Ds which are full frame) do not utilize the full 35mm frame area, any light falloff that you see in the corners with a film camera won't be present on the DSLR's simply because the sensor doesn't cover that area of the frame. A D100 has a 1.5x crop factor (effective focal length multiplier) it is only using a 16x24mm frame size while a 35mm piece of film or a full frame sensor uses a 24x36mm frame size, thus, anything that is in the corners on a 35mm frame isn't even in the frame on a D100.