FS7: Sony 28-135 soft in center crop mode

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dpryke

Active member
I was just using the Sony 28 - 135 on my FS7 in center crop mode. We all thought that it was noticeably softer than when not using center crop. Has anyone else noticed this? I wasn't sure if this is an optical issue when using full frame lenses. I was using the lense at around f8.

Thanks for the help.
 
Have you tried any other lenses in center crop mode - especially a good lens like a zeiss prime or something that you know is sharp? My understanding is that center crop is always going to be softer to some extent.
 
End footage after debayer will be less than HD in centre crop.. but with a high end, sharp lens in the first place the difference will be very slight .. I use it when I need the shot.. but generally avoid using it if possible.. it wasn't originally designed as a digital extender..
 
Of course it is noticeably softer than not using centre crop mode. That's just physics. The question is 'is it sharp enough for me to use?'


If I was doing a crappy event thing and just getting random coverage for internal use then absolutely yes.

If it was a commercial then absolutely not.

It is also, as others have pointed out, wholly dependant on how sharp your image was to begin with before cropping into it.
 
Usually I say any glass is sharp enough for any camera that does compressed video.

But in the crop mode Ive considered that this might not be the case with FF glass

Let us consider a lens that resolves 4k aka 4000 lines spread over a FF35 aka vistavision sensor 36mm wide

For simplicity lest us say it resolves 3.6k, aka 100 lines per mm.

How big is the crop sensor mode? about 15mm wide?

One would expect the resolution to be about 1500 wide.. which is less than 1920.

Using this ham fisted maths one can see that there is potentially an issue.
 
To be honest you can forget the maths. Plenty of vintage lenses might not technically resolve enough but still look great due to contrast or whatevers that all make up what we perceive as sharpness. Just look at it.

Does it look like mush? Don't use it. Can you stop the lens down to sharpen it up but not be underexposing? No? Don't use it.

Also be aware that noise will be amplified too so make sure you're not underexposed.
 
Ben Scott said:
To be honest you can forget the maths. Plenty of vintage lenses might not technically resolve enough but still look great due to contrast or whatevers that all make up what we perceive as sharpness. Just look at it.

Does it look like mush? Don't use it. Can you stop the lens down to sharpen it up but not be underexposing? No? Don't use it.

Also be aware that noise will be amplified too so make sure you're not underexposed.
Click to expand...

Well maths is a usniversal language.

But yep Im thinking some of my nikkors are not up to the task... a casual observation.. the maths drives me to investigate further when I have a moment.

Incedentally on that other thread about using 2/3 lenses I was advocating use of a doubler where possible to use the most pixels.. I feel this to be connected.
 
Maths might be universal but 'sharpness' isn't. Which is as much perception as it is resolving power and micro contrast.


But yeah, using all the pixels is always to be preferred where possible.
 
dpryke said:
I was just using the Sony 28 - 135 on my FS7 in center crop mode. We all thought that it was noticeably softer than when not using center crop. Has anyone else noticed this? I wasn't sure if this is an optical issue when using full frame lenses. I was using the lense at around f8.

I would consider this troubling as it been long known the sharpest part of the lens has always been in the middle and as you widen the aperture, you start using more of the lens surface. So while we would expect a bit softer image when shooting WFO, it should be sharper in the middle via the center crop mode.
I agree with the other posters about checking other lenses for this anomaly; and as part of this test, try a good quality still zoom for further comparison. Betting the still lenses will fare better in center crop.
Click to expand...
 
Even my very sharp Sigma Art 18 -35 is noticeably softer in CC mode. Is it still "usable"?... short answer is yes - but as Ben mentioned above, it might really depend on what you were shooting & where it was going to live.
 
allenfacemire said:
dpryke said:
I was just using the Sony 28 - 135 on my FS7 in center crop mode. We all thought that it was noticeably softer than when not using center crop. Has anyone else noticed this? I wasn't sure if this is an optical issue when using full frame lenses. I was using the lense at around f8.

I would consider this troubling as it been long known the sharpest part of the lens has always been in the middle and as you widen the aperture, you start using more of the lens surface. So while we would expect a bit softer image when shooting WFO, it should be sharper in the middle via the center crop mode.
I agree with the other posters about checking other lenses for this anomaly; and as part of this test, try a good quality still zoom for further comparison. Betting the still lenses will fare better in center crop.
Click to expand...


In an FS7 1080p image the camera is using the entire 4k sensor and downsampling that to 1080p, meaning each pixel (and grain of noise) is made a lot smaller in the process. Hence sharper images and less noise.

Centre crop basically uses a smaller than 1080p sliver of that sensor and then blows it up to 1080p, increasing the size of the pixel and noise.

Not really sure what you mean about using more of the lens as you open it up, and the more light to you let in from various angles, the softer an image is. That's why we squint with our eyes to see something sharply.

In summary, it's not an anomaly but the simple results of making a big thing smaller vs making a small thing bigger.
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The Lines resolved in lenses tends to drop of at the edges of the native frame - aka soft corners

Using just the centre of the lens avoids this drop off so some lenses 'flawed' on fullframe are great on S35.

But S35 designed lenses still tend to be better on S35 as they are designed to have an inherently higher res. Eg 4k over 26mm not 4k over 36mm

S
 
~I was trying to clarify this "I would consider this troubling as it been long known the sharpest part of the lens has always been in the middle'
 
Ben Scott said:
allenfacemire said:
Centre crop basically uses a smaller than 1080p sliver of that sensor and then blows it up to 1080p, increasing the size of the pixel and noise.
Click to expand...

There's no "blowing up" the picture with Center Scan. Nothing is being magnified or enlarged. The camera simply uses the middle 1920x1080 photosites and ignores all the rest around the perimeter. It's like taking scissors and cutting a 5"x7" image out of an 8"x10" print.
Click to expand...
 
I wonder if mild sharpening in the menu might help. We had it on for most older cameras all the time. Its only recently that those circuits have usually been turned off by users. I haven't experimented yet.
I think Ben's rule of thumb applies though. If image quality is not critical then its probably fine , but otherwise it is less than 1920 HD anyway.
 
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