[Closed] Tell me about projectors

can't really advise on brands but the figures that matter are:

the resolution - you want something at least vaguely close to the screen res of the laptop you're working from. Some of the cheapest one are are very low res - the computer has to re-size all its windows to suit the low pixel count and the result is a mess. Try seeing how low you can set the screen res on your laptop before it becomes a jumble then make sure anything you're looking at has an higher res than that.

brightness - measured in lumens - you'd want at least 2000 to be able to get a decent projection without having to black out the room. You can compensate for lower power by having a smaller image and placing the projector closer to the screen

contrast ratio - this is basically how black the black is. Projectors with a poor ratio leak a lot of light it areas of the picture that should be dark - your 'black' can only be as dark as the white surface you're projecting on so any light getting through the 'slide' reduces the contrast quite significantly and makes all the dark areas of you image grey. Look for at least 2000:1 if theres any ambient light - anything upward of that is better. 5000:1 is pretty decent

The cheapest projectors tend to be LCD based - effectively shining a light through a tiny LCD screen the same way as an old slide projector did. These tend to have poorer contrast as the screen can't be totally opaque to make blacks and dark colours and the brighter the light source the more this is a problem. If you're going to run the projector for hundreds of hours the screen can also bleach out and the contrast worsens - you loose the yellows and the image becomes bleached out and blue.

Better, newer, technology is DLP (digital light processors) - the pixels in these are arrays tiny articulated mirrors - light bounces off these and they only direct the light you want through the lens - meaning much better contrast and they also don't fade over time.

Lower spec DLPs have a single monochrome DLP chip and a spinning colour wheel and basically flash different colour images in quick succession to give a perceived full colour image, higher end ones have three chips - one for each colour. The cheaper single chip machines show their limitations if you have bright or fast moving imagery on screen which can reveal a lag and you'll notice fringes of colour around bright objects. (I'm not sure if everyone notices them - I do, I notice when I blink too)

Finally look at the lens ratio - the ratio between between image size and projection distance. Think about the likely distance you'll have between the screen and the projector in the circumstances you'll use it in then calculate how large the image will display. Typically budget machines are pretty wide angle - and give a large image from a short distance. You might want to project from the back of the room like a in a cinema, in which case you want a narrow angle lens otherwise the picture would be huge and dim. This is expressed as a 'throw ratio' - take the image size you want to see - multiply it by the ratio - thats how far the projector needs to be from the screen to get a picture that size. A ratio of 1.8:1 would mean for an image 1m across the projector needs to be 1.8m way - a 2m wide image means 3.6m away and so on

Holy smokes, maccruiskeen, that is hugely helpful. Thanks!

I've got a bit of an affliction for cheap end-of-life projectors - I own more of them than is probably sensible and use them for multi-projector artworks, but these never have to run for more than a few hours so longevity isn't a worry

If you'll not but using it an awful lot then a cheap way to get something better quality is to look for ones being decommissioned from schools and the like on ebay. A bulb maybe lasts 2-3000 hours, but replacements can be prohibitively expensive so some places will just replace the projector as it nears the limit of its expected bulb life rather than have it fail in service. I picked up four ex-school 2500 lumen DLP projectors for £50 each.