Focal Length and Perspective

[Don Hoey]

Christine,

It appears that because you have moved, and changed the focal length, you have changed the perspective.

A really good set of images to show the effect of doing this though.

I still find considering one thing in isolation a bit hard to get to grips with, hence my last nights post where I got so confused, I binned my drafted post replying to Johns post 10.

I think as long as we understand that we can change how the background impacts on the subject, as you have here, we will be all right.

Don

[robski]

Christine there is no problem to using an empirical approach in everyday photography. You adjust something to get the sort of effect you require. It's when you need to exploit some of these features a deeper understanding of the science is required.

John's question was is it focal lenght or distance ?

Hence all the techno babble to explain it is distance.

I bumped into a similar issue when trying to see if I could increase DOF for a given f-stop by using a shorter focal lenght lens. The problem is if you want to maintain the subject size in the frame you need to move closer and then you end up with exactly the same DOF as before.

So distance and magnification factors come into play with both DOF and perspective.

[hollis_f]

Originally Posted by Saphire

I have just run some tests with the 100-400 lens @ 100mm, 150mm, 200mm, 250mm. All were at the same F stop and speed. I can see how the background is affected by the changes in distance and focal length. I am still finding this very hard to take in.

Excellent shots to show how the distances between camera, subject and background affect the perspective. That's the first half of the experiment.

The second half is to take the same scene at a variety of focal lengths - without moving the camera. Then take the resulting images and crop them all so that they show the same portion of the image. Then resize them all to the same dimensions. You should end up with a series of shots that look identical (apart from the obvious degradation in quality of the resized ones).

You then have two series of photos.

One where Focal Length and Distances have been changed - and where perspective changes.

T'other series, where Focal Length and Magnification have changed, will show no change in perspective.

[John]

What this thread has empahasised to me is that photography, like painting, is an art as well as a science. As long as one understands that the relative size of objects in a scene depends solely on where the camera is positioned and focal length is chosen to fill the frame with the subject then everything will be fine. From the responses everyone seems to understand that anyway.

[John]

OK then here is the first set of images, all taken from the same spot.

Image 1 focal length = 24mm
Image 2 focal length = 50 mm
Image 3 is image 1 cropped to match image 2

Notice how all images have the same perspective.

[Don Hoey]

Amusing the neighbours I see John.

Don

[snapper]

Just a note to thank Don for the link - some very good material there.