EOS M6 What would cause the preview to look better than the shot? (both on the Camera's LCD Screen)

[robertkjr3d]

So I have a EOS M6.  And I have an 'EF Adapter'... Attached to that is a 'Canon EF 24mm 1.4 L Ultrasonic'.  This lens has some age, and I hope your not going to tell me that it may have gone bad.  However, I would get a decent picture on the LCD screen... then click the screen to take the shot.  Then it would show me the shot:  The resulting shot would then come out, not nearly has good (and I don't mean I took it to computer monitor to compare, I mean right there on the same M6 LCD screen).  I was like huh?!  I just saw a great image.  How come you can show me a great image, but can't save one?  Sometimes I was shooting 'M'.... sometimes I was shooting fully 'Auto'.  But unfortunately the best shots I was getting were not with this lens.  My other thinking is, that I'm missing some, or have changed some setting on the M6 camera body that is ruining the resulting picture.  This older still rather expensive EF lens is supposed to be still nice for shooting at night, and darker indoors.  The other item that could be to blame is the 'Adapter', but people have not had trouble with it, in the reviews.  I have the brand marked 'Xuerebs'... Yes a Chinese brand to be sure.     Help....

[ebiggs1]

"Needing to lean tricks to hold the camera better or rest on wall or something if the 'shutter speed' must be so high."

The higher the SS the less you need a steady hold. The opposite is true. The slower the SS the more steady hold is required. 

"... the simple 'shutter speed' being too high causing double exposures."

Huh?🤔

[robertkjr3d]

Ok... let me rephrase... I meant 'slow'.   I meant high as in a lot of time?!?.  wow talk about zero-in on technicalities in my wording.  What you said was correct (although wouldn't you usually use the word 'fast' not high?).  Thank you. 
Example 0.6 is 'high'-er number than 1/60th.  This was my meaning of high and low.  

[ebiggs1]

It will help if you use normal accepted photography nomenclature.  That way we can more quickly zero in on your concern.