Crop lens on a FF camera.

Hey guys, I had something confusing happen to me today, and I'm hoping someone could shed some light on what's going on. From what I understand most of the time manufacturers will make different lens mounts for their FF, and CS cameras( EF=FF, EFS CS) ). I got the Sigma Canon EF to L mount adapter, and tried out some of my older Canon glass. I still have my Kit 18-55 from my T2i, but that is an EFS lens. I didn't expect it to mount, but it did (Awesome!) I figured it would have some crazy vignetting, but it didn't. Is there any reason why it's working when the crop lens isn't supposed to cover FF? It even reads the mm correctly.
Is my reasoning correct, or am I mistaken, and been thinking about this wrong for 8 years?


I'm still getting used to the Panasonic S1R, but is there a function like the Sony A series where it automatically goes into a crop mode if a crop lens is placed on the camera? I didn't see any indication on the screen, but you never know. If it does do that, I'm surprised the Sigma adapter would 1. know it's a crop lens, and 2. Tell the camera to change mode.

Thank a you for reading, and any help figuring this out.

No surprise 'the mm' is read correctly. Focal length of a lens does not change. I'm not talking about zooming. Rather, an 18-55 is always an 18-55.

I'm surprised you are not seeing vignetting. Have you tried the 4 ends?
ie, 18mm, close to minimum focus distance, 18mm at infinity. 55mm, close to minimum focus distance, 55mm at infinity.

There's a simple way to see if the Lumix is using a crop mode. Just check the image dimensions.
At 3:2 apect ration, full size image is 8368 x 5584 pixels. If it is cropping, it will be about 5500 pixels wide (instead of 8368).

Djones09@gmail.com wrote in post #19055745
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Crop lens on a FF camera. ..... I figured it would have some crazy vignetting, but it didn't.
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Are you sure the corners of the photos aren't darker than the center? . How 'bout at the widest aperture and the shortest end of the zoom range? . Are the corners dark then?


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As a generalization, usually the image circle of any lens is
A) largest at the longest FL, and
B) largest at its smallest aperture (e.g. f/22), and
C) is largest at its MFD.

So if you want to check for vignetting, set your lens to shortest FL, max aperture, and focus at Infinity. Then check the pixel dimensions of the image to determine if your camera was in FF mode or in crop mode.

You were absolutely right, I just didn't know where to look. When I go to the Image Size settings It shows appropriate MP for the crop lens, and when I put my L-Mount FF lens on it shows the original MP sizes. It's crazy that the Sigma adapter can communicate that information to the camera and make those adjustments. Definitely well worth the money to be able to use any (EF) lens I want. It also still lets me record video in 4k which is cool.

Thanks for your help clearing that up!

By now, you've already got your answer. I got busy with some other video work until now.

It took a little bit of finagling but I mounted an EF-S 18-135 lens on a full-frame body. The effect is about 12mm of extension. I didn't include aperture as a variable, but here's what happens at the different focal lengths and focus settings...

That's what I expected to happen. Thanks for the illustration!!

It's much much simpler hooking up EFS lenses to the R and RP, since the adapter does that acknowledgment thing to the camera and it crops the image automatically as well.