You do in my place.
I set my lens at 100mm, I measured an object as two finger spreads, which are about 8 inhes each, and I looked through the camera.
But that was yesterday.
Today I found a 15 inch ruler, and I put it on top of a bookcase.
Horizontally.
I had my 55mm lens on the camera, so I looked through that.
Not particularly inconvenient.
Reminded me of an 85mm lens I used to own, years ago, that I used on a Canon manual focus 35mm camera.
Then I put on a zoom lens that reached to 100mm and set it there.
And I looked through the camera, held horizontally, at the 15 inch ruler, resting horizontally on top of the bookshelf.
And lo and behold, I had to back up a lot.
Far enough that it would be a stroll over to the ruler, to change its angle, say, like product photographers do.
So I'm right.
And I put my stuff away.
BUT THEN, I THOUGHT,
What if the 16 inch subject the original poster was interested in was square?.
So I went back to his original message.
Bingo.
He wrote > I am photographing products that are 16 inches by 16 inches or smaller
SO... I got out the camera again. And I put the telephoto zoom on again,. And I set it at 100mm again.
And I walked over to the ruler and put it vertically into a pencil jar, and then I walked back to my original 100mm horizontal shooting spot and looked through the camera, still held horizontally.
The ruler did not fit in, top to bottom, when the camera was horizontal.
So I backed up further and further.
Good thing it's a fair-sized room.
ALL FORUM MEMBERS with 100mm focal lengths on their lenses are welcome to try the same experiment, and see just how convenient it would be to take your 100mm advice.
Or see if they think, as I do, that a 100mm lens on a T2i is too long a lens for convenient product photography of objects 16x16 and smaller.
And the original poster can experiemnt too, with his telephoto lens set at 100mm.
BAK