photo dpi/size question

[Chad Fitzgerald]

In general question. assuming everything is good with the photo to be used. if i want the same photo two different sizes, very different, one to be lets say 1" x 2" and one to be 18"x 24".
would i use the same dpi for both OR low dpi for the small one and high dpi for the large one OR vice versa.
Basically i am wondering if there is a rule of thumb when it comes to the size of the pic to be engraved.
smaller the size you want less dpi, etc.
just curious.
thanks chad

[Dan Hintz]

Your question is somewhat ambiguous. If you are printing/engraving an image, you want them both to have the same dpi... but the larger image will have more pixels.

[Frank Corker]

I think that the best rule of sum is the higher the dpi and resolution of the image the better, however there are occasions when too high dpi will result on too much damage to the image. Wood will be obliterated quicker than acrylic. On the examples shown above, a 1" image against an 18" image, the extremes are too big to make such a comparison. The larger image will always hold more detail if the original was of a large size in the first place. You could not take a 1" photograph at 300 dpi and extract it to 18" because the original image is too small.

[Chad Fitzgerald]

i figured that might not be the best way to ask.
engraving at higher dpi is better, more detail.
my question should have been, is lowering the dpi a result due to what the material will handle only
OR
if i want to engrave a photo very small would that alone be a reason to lower it? everything else being the same.
Here, i will just ask this,
I you were going to engrave a picture of a person on to a 1 " square piece of material that will take high dpi. would you set dpi as high possible or would you lower it due to the small size of the end product?

[Glen Monaghan]

You want/need "enough" pixels to portray your image in the desired size and quality on the intended substrate. Any given substrate has an upper limit on how high the dpi can be, as does the imaging machine (your laser, due to its spot size and mechanics). All that aside, let's assume you have enough pixels to print or engrave your 18" image at 300 dpi with suitable quality. That's 18*300=5400 pixels across, for what it's worth. Now, if you want to engrave that same image on the same type of substrate but just 1" across, you'd normally want to maintain the same 300 dpi but reduce the image size from 5400 pixels across to just 300 pixels across. That's pretty severe reduction and, if there is very much fine detail in the original image, you may not find the quality acceptable at 1" size. You could try to increase the dpi, say to 600, which would allow you to double the pixels to 600 for your 1" image, but the substrate would have to be able to support that (for example, anodized aluminum would work at 600 dpi, but many wood species might not).

Generally speaking, you don't want to lower the dpi when printing/engraving a smaller version of an image. Instead, you want to reduce the size of the image, which decreases the number of pixels (losing some detail/quality in the process, whether it is perceptible or not), and then engrave at the same dpi. If the substrate supports the higher resolution, you may get better results by actually increasing the dpi and reducing the size of the original image proportionately less (so, in the example above, a 600 dpi image is twice the resolution of a 300 dpi image, so the reduction factor for the dimensions of the original image would be half as much for 600 dpi as for 300 dpi).

[Chad Fitzgerald]

that makes sense, well it will when i reread it a couple times but that is what i was wondering.
thanks you