150 ppi at print size is a good rule of thumb, but you can get away with less with images that don't have a lot of high contrast edges or where the intended viewing distance is large. See Distance-Resolution Formula
But if you are shooting for 150 at print size, that means you need 300 ppi EFFECTIVE resolution in the doc that is scaled to 50%.
Thanks, Peter. This is helpful.
My problem is that it's been difficult to find suitable photos. And, when I do, they would be great for (say) a print magazine but still don't have the resolution to handle such a large banner size.
Maybe some of the stock agencies have even higher res photos available, if I were to phone or ask them?
Yep, finding good photos is hard.
I have a several clients who do banners, and I do things like reduce opacity of large background images or add gradient backgrounds so I don't need to fill the whole banner with one image, or if I do it doesn't need to be sharp. Generally, that background is not what you want the customer to focus on, anyway, so a bit of blur is OK. And I'll drop to as low as 120 ppi without worrying most of the time, and even to 100 if I don't have a choice. Most of these are meant to be viewed from at least 6 feet.
When I worked in a large format service bureau we once printed wallpaper for a shoe store that was photos of models wearing the shoes at about 120% of life size using images that would only give us 75 ppi, and it looked great.
Thanks, Peter -- again very helpful.
I'm hoping we can go as low as 75 dpi at 100%. Maybe I could increase the photos' resolution in Photoship in small increments by resampling the photo with the Bicubic Smoother option selected?
Now, my broker/printer is saying that this 50% size and 150dpi minimum is too low and the photos will pixelate.
That sounds like a potentially erroneous assertion from a broker, rather than the printer. (Which is it?) Many "brokers" (phone-call makers) will parrot the silly catch-all 300 ppi rule of thumb without ever asking someone who knows, whereas a knowledgeable large-format printer should certainly know better first-hand.
Peter has given you good information. In my experience, it would be unlike a purveyor of stock photos to tuck away a larger size of a given image (which would bring a higher price/profit margin) in reserve only for those who call to ask for it.
Thanks, John. I'm waiting for my broker/printer to supply some proofs, which will help us. Yes, I suspect that he needs to clarify specs with his supplier.
I just checked with one of my stock photo sources and they don't have anything higher-res, other that what's on their website. Still worth checking with others though.
If the image is good quality, up-sampling to to twice the current size with a good algorithm like that used in Genuine Fractals or the more recent versions of Photoshop will probably give acceptable results, but it's highly image dependent. Using the formula from the link, 100 ppi is great at 6 feet. By the time you get to 7 feet you'd barely see a problem at 75 ppi.
This must be a miscommunication. Pro DSLR cameras were in the 24 mpixel range until fairly recently (in one major brand's case last month), which, as you say, barely fills the 50% size @ 150 ppi. It's time to talk directly to the tech people.
Thanks, John. I'll be meeting with our supplier tomorrow or Tuesday.
In other words, there aren't cameras that are capable of taking photos at the size that is needed. So the only options are to go down to 100dpi or lower.
I just want to thank everyone for their advice here. Our banner proofs arrived today and they were excellent! This project was crucial as this was a new client.
Basically, it seems that (as above) you can get away with going down to 100dpi or even 75dpi for large banners -- you almost have no choice. In the end, our supplier did offer good direction and we are pleased with the result. Thanks again for your help.