View Finder

I know that conventional lenses on a digital camera take a hit, about a 1.5 crop factor. In practical terms that means my viewing area is less wide. Reviewing the pictures I took at EPCOT, last week, it was obvious.

I decided to run a practical experiment this afternoon (Sunday). I wanted to see two things. (1) How wide is my vision? and (2) How much more than the viewfinder focus box is included in the picture?

My home faces West, so the setting sun hits my front door in the late afternoon. The sun would be at my back and provide great lighting. The house has a low wall, a bit over 5 foot high, in front of the house.

I took some oversized yardsticks (4 foot) and laid them on the wall. Then I took index cards, drew a line down the center of the card, and then folded the card (line inside). I positioned a card every 1 foot, using the yard stick to hold down the card, i.e. slipping the bent half under the yardstick.

I got my 50 foot tape out and fixed one end at the wall. Then I stretched the tape out toward the sidewalk, essentially at a right angle to the wall.

I got some quad ruled paper and recreated the viewfinder focus box.

With the camera mounted on the monopod, I’d take the camera out to a 5 foot mark and look through the lens zooming between 35mm and 80mm. At the two extremes I would note the index cards visible relative to some of the focus marks, and then snap a picture. (Here's a cropped example )

I’d record on my grid paper how wide I could see in the focus box, not the total viewfinder.

Then I would repeat the process at the next 5 foot interval, i.e. 5, 10, 15, 20, and 30 foot from the wall. The monopod was always at the same height. So, even though my lawn slopes down a bit toward the curb, all the pictures were roughly of the same scene and the same relative center. The index cards (i.e. the horizontal) were always visible.

Took the pictures with medium resolution. Then I took a few with small resolution, and a couple in large format. “Size” is really a designation for pixels not image dimensions.

After cleaning up my props, I downloaded the test pictures to my computer. Definitely cheaper than 1-hour photo processing. Then it is mostly a question of trigonometry.

The calculator function in Windows is a bit limited. It will do square roots but not sine or cosine. Not to worry, I have my CRC Math tables and a slide-rule for backup. (The slide-rule doesn’t have a sine scale but does have a T (tangent) scale.)

It looks like the 35mm opens up about 36.52 degrees. That works out to 6.6 ft from 10 feet out, between the focus box brackets. The resulting image adds about 1.5 feet outside the focus box, on both sides. (Distance from target x .33 = one half of the focus box view.)

The 80mm setting is a little less than half that. At 20 feet, 6.32 feet are in the focus brackets, about 18 degrees of view. The image adds about a foot to the edges, on both sides.. (Distance from target x .158 = one half the focus box view.)

The images are identical for S, M, L images. So the number of pixels (resolution) changes but not the image width.

I think my next excursion into physics will be to lay out the tape and place my markers every foot straight out, stretching in front of me like a path. Then if I focus on one marker, I should see the depth of field impact to other markers.