FF lens on apsc, good or bad?

Started Nov 7, 2012 | Discussions thread
Gerry Winterbourne
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Re: FF lens on apsc, good or bad?
In reply to Lawnmower Shooter, Nov 7, 2012

Lawnmower Shooter wrote:

So I am in the midst of deciding on a portrait/street photog lens, sharpness being the most important attribute. I am wondering if anybody can help with a specific question regarding the sensor size difference. Since a FF lens on a crop camera only makes use of the center portion of the lens, do you get more resolution out of a top end APSC lens which you will use 100% of?

It depends a lot on (a) the individual lens and (b) the aperture you shoot at.  In the absence of any tests of Pentax lenses on FF I've looked at a couple of good Nikkor lenses as tested by Photozone on both DX and DX.

First the 50/1.4D - I've picked this because the FOV on APS-C is 75mm equivalent, which is quite good for portraits.  I've gone 1 stop from wide open: on FX the loss from centre to edge is 21% while on DX the loss from centre to edge is 11%.  So in this case the centre advantage of using a FX lens is about 10%.

Now the 85/1.8, also stopped down.  At f/2.8 the loss from centre to edge on FX is 9%, on DX it's 6% - not much difference.

At wide apertures resolution is dominated by aberrations; at smaller apertures diffraction is increasingly significant.  By f/8 there's hardly any difference in centre-to-edge loss between the formats.

But there's another factor to consider too: so far we've looked at the same lens on different formats.  As you will be using a crop sensor camera it's just as significant to compare how FA lenses tested on APS-C compare with equivalent DA lenses also tested on APS-C.

One pair of lenses that are roughly comparable is the FA77/1.8 and DA70/2.4 (both Limited lenses).  Their centre resolution at f/2.8 is almost identical, while the edge resolution of the DA70 is 25% higher than the FA77 even though the FA is designed for FF.  Again, the differences narrow as you stop down.

It's a general fact of lens design that centre-to-edge difference gets less as FL increases.  For the "portrait" lengths I've used as examples the difference isn't always significant; at the longer end of zoom lenses it will be less.

What it boils down to is this: there is no general rule that says a lens designed for FF will perform better or worse than an equivalent lens designed for APS-C.  What is much more important is how well a specific lens suits your needs.  For example, if you need a zoom then none of the lenses I've quoted is any use to you regardless of which is sharper.  If you want a prime in the 70-85 range you need to think of more than edge resolution when picking between the DA70 and FA77; and if 77 is too long it makes no difference if it is better or worse than the DA70.

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Gerry
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First camera 1953, first Pentax 1985, first DSLR 2006
http://www.pbase.com/gerrywinterbourne

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