I need to pull a still frame from from our footage to be printed on a poster.
I have done this but the image is not great quality (looks slightly pixelated), especially when poster size.
I tried exporting the frame as TIFF, JPEG and a Photoshop documents.....all with the same result.
The exported still images were 1920x1080 pixels (same as footage)
The director insists there is a way to export these images that will be good quality for print......he says he's done it with standard def footage.
However I'm not so sure.
There were stills shot on the location which look much sharper (because they are approx 4000x2500 pixels)
Our footage is 1080p 29.97fps shot on a Cannon Mark V.
You could try increasing the pixel count in photoshop and adding a little unsharp mask, but I have a feeling that the "slightly pixelated" look may be compression artefacts (IIRC those Canons use a version of h264 that's quite heavily compressed for video), so the compromised quality is not just about pixel count. You could try making multiple gabs over a few frames to see if one is better than others (it's conceivable that exporting from an I-frame might be slightly better than from a B- or P-frame, but I'm only speculating about that).
Still images from video, even HD video, in print terms, are low res.
Its worth noting that an HD frame at 1920x1080 is only equivalent to a shade over 2 megapixels. So something poster sized may not look great.
SD video is less than 0.5 of a megapixel so again something poster sized is going to look poor.
Your 4000x2500 still images equate to 10 megapixels so as you have noted will look much sharper.
As a rule of thumb the maximum print size without suffering some quality reduction for a 2 megapixel image is reckoned to be something like a 6"x4" print.
Your 4000x2500 stills on the other hand should make a 16"x24" before they start to suffer.
Those maximum sizes are not absolute of course, much depends on the sharpness and quality of the original image but video grabs are not always optimal when compared to a still image of the same dimensions taken with a stills camera. So you can usually push stills to a larger size.
It helps if you pick frames where there was not much camera or subject movement.
I think you might struggle to get poster sized prints without some kind of quality loss being noticed from a frame grab though.