FX lenses on a DX body

[Summerstar]

This might sound like a silly question but bear with me - I'm good at asking them...

I'm using a Nikon D7000; that's a crop sensor camera. I have quite a few AF-D lenses from when I shot film with my F90X, and of course they are all FX. Now when I used them on the F90X the received wisdom was that any lens would perform better in terms of both sharpness and contrast when stopped down by a couple of stops. But is this true in the current situation using them on a DX body? Surely even if I shoot with the diaphragm wide open, since the sensor is only receiving light from the central part of the lens I am effectively stopped down in terms of quality? I appreciate that other factors like depth of field are unaffected.

 

[Nod]

Absolutely no problems with using FF lenses on crop bodies. Like yourself, I came from film to digital and so all my lenses (bar the kit lens that came with my first DSLR) have been full frame.

The sweet spot is still there - generally (as you say) when stopped down a couple of stops from wide open and this still applies to crop bodies using FF lenses. While you're only using the central part of the image circle on a crop body with an FF lens, the lens's whole surface and aperture are still involved. If we're lucky, a chap called Hoppy might explain why and if we're VERY lucky, he'll do so in words us mortals can understand! He'll probably also explain why DoF is actually affected - all to do with it actually being perceived acceptable sharpness rather than scientifically measurable (IIRC!)

IMO, FF lenses generally give better results than crop lenses on crop bodies since almost all lenses' performance drops off a little towards the outside of the image circle and with a crop sensor, that potentially soft part is discarded. There is a slight price to pay for using FF lenses rather than crop ones - they're heavier! Far outweighed by the extra IQ IMO, not to mention that I'm now an FF digital user so crop lenses are useless to me!

 

[Phil V]

Back to front, I'm afraid.
You're using the central part of the lens, which is generally optically better than the edge, so you will get slightly improved optical performance. However, all lenses perform better stopped down a little, that still holds. But if the lens is really poor

DoF will be increased at the same apertures from the smaller sensor, the larger the sensor (or film) the less apparent DoF. The caveat to this being DoF is a moving feast with many variables.