Softness on the edge

Started Feb 22, 2021 | Questions thread
Andy01 Veteran Member • Posts: 4,849
Re: Softness on the edge
5

Kendunn wrote:

Getting a weird, overly soft edge on rare occasions, not sure whats up. Canon 5DS with a 17-40. The example posted shows the top 20% of the frame is way too soft, but the rest of the frame is reasonably sharp, including the bottom. It was shot at 35mm, f/7.1 off of a tripod, I think it was 1/20 at ISO 100. Most of the time images are fine. Loose element?

I assume that you are aware that the EF 17-40L is well known for it's soft edges - especially on a FF camera, and I would image that a 50Mp sensor would greatly magnify this. I chose not to buy a 17-40L when I tried it on a 600D APS-C in 2011 because I thought the edges & corners were not great - I can only image how a 5Ds would amplify this issue People talk about using it at f11 on a 20Mp 6D or 5D ii/iii to avoid softness.

While it could be a lens defect (decentreing) the defect would be there in every image (although some more obvious than others, but it probably wouldn't "come and go" unless something was very loose, in which case you would probably hear something moving), and it should be easy to pick by doing some checks such as a well set up brick wall test.

Have you checked the lens for decentreing ?

The bottom corners are fairly soft as well (especially bottom left), though not as bad as the top portion.

I am inclined to think that this is the inherent softness amplified by the DoF difference (perhaps with a touch of decentreing). While a DoF calculator might indicate that at f7.1 and 100', the DoF is huge, it does not mean that everything within the suggested DoF is in "perfect" focus - my understanding is that it would be in "reasonable" focus.

I would certainly NOT recommend a 17-40L for use on a 5Ds - I actually wouldn't recommend this lens at all unless budget is VERY tight and there is no other choice AND it was going to be used on a APS-C body. A EF 16-35L f4 is a vastly better choice that is likely to do some justice to the great sensor on the 5Ds.

Colin

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