FF lens on APS-C camera - better or not?

[xpatUSA]

Sorry if this a beaten-to-death subject. I did try a search but couldn't find the right keywords.

I had read that, if you use a FX or a DG lens on a camera which has an APS-C sensor, the image is "better" because the center area of the lens is more accurately made and there is less vignetting and less CA compared to a DX or DC lens. And it has been my own habit on both my SD10 and my D50 to use FF lenses for macro work and the kit lenses for outside snaps (I'm not a Pro).

Then, over on another forum, I read:

This is the classic case of re-using a lens designed for a larger format system. FF and APS-C get this all the time (lenses that perform well on FF often lack sharpness and contrast when mounted on APS-C: the 24L on Canon is a pretty stark example). You are magnifying the image more on the smaller format, which puts more "stress" on the lens, enlarging the aberrations. Unless the lens falls off into pea-soup territory near the edges of the frame with vignetting destroying all light collecting advantage, you will always get better results on the native mount.

queried by:

I would have thought that since an aps-c designed lens on m4/3 or FF lens on aps-c would be just as good since it essentially would be using the center of lens which typically tends to be a lens sweet spot in terms of contrast, sharpness, and overall IQ.

Rebutted by:

This argument would be correct if you are comparing how many lines can be resolved per mm of sensor, which nobody does in real life. People compare resolution per entire image (width or height). With a larger sensor the lens can bring a larger area into play so the total number of lines that you can distinguish across the entire image height (or width) should be larger.

An interesting set of posts which could determine the fate of my Sigma 50mm macro DG lens currently attached to a D50 APS-C camera.

Unfortunately, the first and third quoteés are not really specific in their claims. So, lets go to extremes:

You have a FF camera and a 4/3" and just one mount-compatible FF lens. 24mm sensor height vs 13mm. Both sensors have 5um pixel pitch and the shot is of a 40mm object at 1:4 mag which is typical of what I do. Within these bounds, what do you think?

My thought is that there is no difference in image quality but the FF image would need a bit of cropping ;-)

So, are there other factors involved? Shooting distance? DOF?

[Inkanyezi]

It's too easy to extrapolate a general conclusion to a specific situation, and often people do.

If, as in your example, the pixel pitch is equal, and the distance too, whereas you'll crop the FF image, there will probably be no difference, provided both sensors are equal also in other respects. Effectively, you're then using the FF camera as were it a 4/3 camera, cropping off the unwanted parts of the image. Then everything else is also equal DOF as well.

On the other hand, taking the same scene with both cameras and suitable lenses, i.e. for example using a 50 mm macro lens on the 4/3 camera and a 70 mm lens at the FF camera, from a similar distance with a larger image on the FF sensor, then most aspects of image quality would certainly be superior on the larger sensor, while at the same f-stop, the 4/3 image would show less bokeh and larger DOF.

So it is also a matter of how comparison is made. What I have seen from several FF lenses that I have, is that lens sharpness of the FF lenses is adapted to the larger format, and they generally don't produce as sharp results. But that's a general statement, it doesn't hold water for a few of those lenses. So my Carl Zeiss lenses are different. The Planar 50/1.4 is less sharp than all the lenses I have that are made for smaller formats, while the Distagon 35 mm and Sonnar 85 mm competes well with original µ4/3 lenses, producing just as crisp images.

So I'd say that you can use lenses made for a larger format, but some of them might not have the MTF required for the small format, while others might be just as good as lenses made for the small format. When i want bokeh and a 50 mm lens, I prefer the Fujinon 55/1.8 before the Planar 50/1.4, because it is sharper, which in fact accentuates the bokeh of the less sharp background, and with the definition of DOF being the region where the sharpness is not unacceptably less sharp than the sharpest part of the image, at f/1.8 that lens has shallower DOF than the 50/1.4 Planar. That said, my preferred lenses for macro are the Distagon 35 with a macro teleconverter and a Rodenstock Trinar enlarging lens 50/3.5. Both of those rival µ4/3 lenses for sharpness.

So a general rule is not always applicable to a particular situation.

[Glenn NK]

I use my 24-105 on my APS-C body with as good or better results than on my FF body (in fact I bought the 30D with the 24-105).

Both bodies have very similar pixel density with the advantage that the APS-C uses the central portion of the lens.

This first result is on an 8 MP body:

http://www.photozone.de/canon-eos/18...review?start=1

This second result is on a 21 MP body:

http://www.photozone.de/canon_eos_ff...5_4_5d?start=1

Note the massive barrel distortion and the horrible corner vignetting at 24 mm FL on the FF body - not nearly as bad as on the APS-C body.

So as Urban points out so validly - it all depends on the specific application or combination.

Glenn

[xpatUSA]

Thanks to you both, Gentlemen,

Excellent and clear comments and I do agree. In the OP there was a bit of what I call "photo-babble" and I found it hard to understand at my age :-(

Thanks also for the links to that test site Glenn. Graphs and figures tell the story, IMHO.

Happy Holidays and . .