ghosh wrote in post #5366258
I am sorry, I typed wrong.
Let me make it simple.
at 200mm F4, I make a frame of the subject. Lets assume X frame.
Now I take another lens say a 50mm lens. Stand in appropriate distance from the subject so that I get the same X size frame as in the previous case. and use aperture F4.
Will the blur be the same in both frame? In other words, keeping the aperture same. Keeping the frame same. Using different focal length lens. Standing away from the subject accordingly. How would it effect the blur?
This question came to my mind because I was thinking of buying a portrait lens. Now if the result from the above are same then why buy a long lens and stand far way? Use a small lens, stand close and get the same blur, isn't it? When I am only concern with DOF (as of now).
If you use the DOFMaster calculator, you can work the DOF numbers....
200mm at 20 feet, f/4: 19.8 ft to 20.2 ft, .45 ft DOF
50mm at 5 feet, f/4: 4.79 ft to 5.23 ft, .45 ft DOF
While the calculated depth of field at the subject's plane is the same, there are some VERY significant differences in what you will see in the images.
As related to depth of field alone, the background will probably be more out of focus with the longer lens.
Because of the difference in distance between the camera and subject, there will be a significant difference in perspective. Perspective, in this sense, is defined as the relative sizes of foreground and background elements of the subject in the image. An experienced photographer will choose the distance between camera and subject as well as the focal length to control perspective.
Here's a simple example of how perspective control can work for you:
Let's assume that you are taking a photo of some friends in a scene that has mountains in the background. You stand 20 feet from the people and view the scene. A 50mm lens will let you fill the frame with the group of people and some of the background quite nicely, so you take a shot. Then you realize that the mountains are rather small in the background.
Back up to to 40 feet (twice the distance) from the group of people and view the scene, you will see that the mountains are now larger relative to the people - twice the size they were before, in fact. However, the people are smaller in your viewfinder. You now need a 100mm lens to keep the people the same size as in the first image, but the mountains now appear twice the size that they were in the first shot.
Why is this? It's because the additional twenty feet that you put between yourself and the people is insignificant relative to the fifteen miles between your viewing spot and the mountains.
When you are closer to subjects, perspective still comes into play. If you shoot a portrait from a location very close to the subject, the nearest objects (a nose, for example) will be larger relative to more distant objects (such as an ear) than they would appear from a greater distance. That is why experienced portrait photographers like to use a little more distance - and thus a little longer lens – than some beginning photographers would choose when shooting conventional portraits. The subject will usually be happier with the perspective achieved by the greater distance.
In other words, you don't normally want to be really close to a portrait subject if you have the option to back off a bit.
