The clearer the photo, the more accurate the diagnosis. Keep these points in mind when you are standing in the field.
Good light, but not direct sun
Take photos in the morning or evening. At midday leaves shine and spots wash out.
One leaf, up close
Instead of fitting the whole field into one picture, photograph one damaged leaf or fruit from about an arm's length. Spots, holes or mould should look big in the frame.
Show the underside of the leaf too
Aphids, whitefly and some fungi sit under the leaf. Send both sides if you can.
Add a little information
The crop, how many days since sowing and any spray you already used. Two lines make the answer much more precise.
Retake blurry photos
It is hard to identify a disease from a blurred photo. Hold the phone steady and wait for it to focus.
If the damage is spreading fast, please also speak to your agriculture assistant or the nearest Krishi Vigyan Kendra.