All things modern.
There are loads of opinions on the internet about what happens when you use full frame lenses on crop sensor camera. The reason why people would want to do this is that they want to invest in full frame lenses but only use them on a crop sensor body for the moment. When they do finally upgrade to a full frame body, they would already have suitable lenses.
In theory, it should work just fine. In fact using a full frame lens on a crop sensor body should give very good results as it only requires the centre portion of the image circle to be used and this is traditionally the ‘best’ part of the image. But does this work in practice?
I happen to have both a full frame and APS-C body. Both use the same Sony E mount so I can use the same lenses on either. Also I happen to three zoom lenses, one made for APS-C and two designed for full frame. I can therefore test the various combination as follows:
Sony 7R with 16-50 APS-C lens, 24-70 FF lens and 16-35 FF lens
Sony a5000 with 16-50 APS-C lens, 24-70 FF lens and 16-35 FF lens.
To keep as much constant as possible:
The results are presented first with the whole image, and then followed by 100% crops, they are crops of the same portion of the image and so are different sizes. If you would like to see for yourself you can download the full size images by licking on them below. So first the full images:
…and now the 100% crops:
*It may seem strange that the image from the a5000 is larger here but it’s because when using an APS-C lens, the a5000 can use its full 20 mPx but the a7r is limited to 15 mPx in crop mode.
You can probably come to your own conclusions by looking at these photos. From my point of view there is no difference in exposure. However I do notice a very slightly elevated noise level when using the FF lenses on the APS-C sensor. This could be just down to sensor technology as the sensor in the A7r is by all counts a much better sensor as can be seen by direct comparison on DXOMARK.
Sensor comparison for a5000 and a7r
While we are on the DXOMark website, lets see what they say about using the full frame on crop sensors. Looking at the 24-70mm, DXOMark rates it at 15 pMpx on the a7r but only 7 pMpx on the a5000. Similarly the 16-35 rates at 16 pMpx on the a7r but only 8pMpx on the a5000. The 16-50mm APS-C lens rates as 7 pMpx on the a5000. This would explain why the photos taken with the a5000 with the kit 16-50 and the much more expensive full frame lenses look so similar. Comparing the images from the a5000 and the a7r using the same lens however, the increase in sharpness is clear.
From these results it seems that if you want to build your camera kit slowly, it is perfectly fine to buy a cheaper crop sensor body now and future proof by buying compatible FF lenses. The only drawback doing this would be that you will need to carry around larger lenses for the time being.