How wold you test a sensor

I've just upgraded my 'carry out' from an EOS-M10 to the new M50
The M10 was rejected for PX by the dealer and returned to me with the reason 'There are 2 permanent marks on the sensor'
Strange - as I'm particular about camera hygiene when changing lenses on a a mirrorless as it leaves the sensor exposed.
On return I checked the sensor and found nothing visually.
To test it I took a massively overexposed picture of the sky with a long shutter speed moving the camera slightly to blur out any real content, so any sensor flat spots would show up
I did the same test, without moving taking a shot with the lens cap on.
I revealed no marks, no hot pixels, no dead pixels.

Any ideas?

Feeling apprehensive about trying to sell it in 'excellent condition', I bought the cheapest second hand EF-M 18-55 I could find to ship with it as a starter kit for someone on a low budget.
The lens is sharper than the 15-45 current kit lens, and delivers great IQ, The cameras performing great so I'm keeping it.

Canon User wrote in post #18671645
I've just upgraded my 'carry out' from an EOS-M10 to the new M50
The M10 was rejected for PX by the dealer and returned to me with the reason 'There are 2 permanent marks on the sensor'
Strange - as I'm particular about camera hygiene when changing lenses on a a mirrorless as it leaves the sensor exposed.
On return I checked the sensor and found nothing visually.
To test it I took a massively overexposed picture of the sky with a long shutter speed moving the camera slightly to blur out any real content, so any sensor flat spots would show up
I did the same test, without moving taking a shot with the lens cap on.
I revealed no marks, no hot pixels, no dead pixels.

Any ideas?

Feeling apprehensive about trying to sell it in 'excellent condition', I bought the cheapest second hand EF-M 18-55 I could find to ship with it as a starter kit for someone on a low budget.
The lens is sharper than the 15-45 current kit lens, and delivers great IQ, The cameras performing great so I'm keeping it.

How did the dealer test the sensor to know that there are two permanent marks on it?

I would just take a picture of a blue sky at f16, if nothing shows up I would be confident in selling it privately.

This may be effectively what you did, I'm just not sure about the over exposure - I haven't tried that.

Even if it somehow did have 'spots', if nothing shows up at f16, then whatever 'it' is is irrelevant.

not sure I'd bother trying to overexpose a shot, just shoot a clean background (clear blue sky is ideal) and set aperture to quite high.

e.g. f/22 1/40 @ ISO100

ejenner wrote in post #18671662
I would just take a picture of a blue sky at f16, if nothing shows up I would be confident in selling it privately.

joeseph wrote in post #18671685
not sure I'd bother trying to overexpose a shot, just shoot a clean background (clear blue sky is ideal) and set aperture to quite high.

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There's one more thing that will help to show sensor spots ........ take that clear blue sky shot and then, in whatever editing program you use, crank up the contrast or levels way way high. This will reveal anything that you don't see at normal contrast levels.


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Use a sheet of clean white paper evenly lit, often easier to find than a clear blue sky, switch to MF and shoot out of focus, works perfectly.

I suspect you have a computer monitor somewhere near. Just open a text editor document, set your focus to infinity (to assure the lens is out of focus) and shoot the screen.

Then, as mentioned earlier, view the image in a photo editor and crank the contrast way up.

-js

Thanks for your posts everyone,
The guy in my local photo shop had a look at it while I was buying a filter.
He confirmed he saw one spot on the sensor but without any signs on the image lets live and let live.
I've kept it on. Too sentimental!

Tom Reichner wrote in post #18671765
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There's one more thing that will help to show sensor spots ........ take that clear blue sky shot and then, in whatever editing program you use, crank up the contrast or levels way way high. This will reveal anything that you don't see at normal contrast levels.

.

Yes, I agree, especially as you will often increase sky contrast in landscapes. I quite often end up having to clean up images near the end of processing, rather than the beginning, because I didn't check for spots by cranking up the contrast.

OTOH at f22 this will almost always show up something, so one should probably not get too anal about every last small/diffuse spot.