Cropping "Rules of Thumb"?

[krs]

Are there any rules of thumb as to how much you crop a given picture given:

1. Megapixal of original picture (like K10D's 10M)
2. Target picture size (4X6, 5X7, 8X10, etc)

and still have a "crisp" picture?

[rburgoss]

It all boils down to actual printing resolution for a given print size.

I work in graphic arts and have stated some basic rules for printing (lab or home printing with a good photo printer).

Remember that the bigger the print is, the "farther" you'll look at it. For example, a 4x6 print is usually looked at 10 to 20 inches away, but a 24 x 36 inch poster, will be looked at several feet away.

That means that actual printing resolution, will "increase" as the print size "reduces".

According to some eye doctor sources, the human eye without the help of magnifying devices, is capable of resolving up to 200 dots per inch (dpi), at 20 cms (about 8 inches) away. That is for a 10 year old kid with perfect vision.

Human eye resolving power diminishes with age, being at around 160 dpi at age 25 and as low as 125 dpi at age 40.

Based on that, you can crop and resize your photo files for printing purposes, according to the following basic table. Remember, resolution given is at actual print size.

For 4x6 use a 200 dpi resolution. That is, a file with at least 800 x 1200 pixels.

For 5x7, use a 170 dpi, file size at least 850 x 1190 pixels

For 8 x 10, use a 150 dpi, file size at least 1200 x 1500 pixels.

For 11 x 14, use about the same as 8 x 10.

For 12 x 18, use 130 dpi, file size 1560 x 2340 pixels

For 17 x 22, use 120 dpi, file size 2040 x 2640 pixels.

Remember, those resolutions are AT LEAST, meaning that anything below will be apparent in lack of resolution. Anything above is just waisting hard drive (cd, dvd, etc) space. As for me, I always shoot in highest resolution given by the camera, then crop and resize as needed and save the file with a different file name and keep the original file "untouched". I can't remember how may times I've had a nice 5 x 7 print, and some time later, wanted a 12 x 18 or bigger print.

Of course, this is my personal experience. Anyone can have different experiences.

Anyway, now in the gigabyte era, file size is becoming less of a concern (when big files are used). It all depends on your hard drive space and your back up methods.

Hope this helps.

RB

[krs]

Some very interesting insight that I will try to apply!

Giving a little background, I frequently find myself shooting in low light situations (gyms, churches, etc). I recently picked up a 50mm, F/1.4 lens, and at 1.4 to 1.7, can get the shutter speed I need for a decent shot of things like Tae Kwon Doe competitions. The challenge is sometimes I am not as close as I would like to be, so I need to crop to get to what I really want.

I also have a 135mm, F/2.8 manual lens that I can sometimes use, but sometimes the light will not allow for a decent shutter speed with it (I try to not go above ISO 800). I was in Chicago for an international competition a week ago where the light was so poor that I had to go to a F/1.7 minimum to get a decent shot!