Zoom versus distance away from object

How exactly will a photo change if say we:

• Stood 1 foot away from an object and shot with the zoom lens set to 100mm
  
• Stood 1/2 foot away from an object and shot with the zoom lens set to 50mm

Why would exposure change based on camera location?

One of the things that changes is the perspective of objects relative to each other. For example, if I were to shoot a subject that was 10 feet in front of me, and they were 100 feet in front of a large building with a 24mm lens I could get all of the subject in the shot as well as all the building and maybe even some of the area surrounding the building.

But, shoot the same shot with a 200mm at 30 feet away and now maybe you get all of the subject but only part of the building in the background and none of the surrounding area.

Sometimes if I am using a small-ish background in the studio and the subjects I am shooting make it so I can't quite make it so I don't see the edges of my background i choose to use a longer focal length and backup, this makes it so I capture less of the background in the shot but still can get the subjects.

So one of the things it does for you is change perspective of subjects spaced at differing depths in your image.

But you also have to be careful about lens distortion. If you get 3 inches away with a 15mm fisheye it will have quite a different look than 30 feet with a 200mm. This is an extreme example but distortions happen with other lenses as well. Most people like to shoot portrait/people shots between 50mm and 105mm because these ranges offer the least amount of lens distortion.

bobbyz wrote in post #5508620
Why would exposure change based on camera location?

If the lighting is the same and the FOV is the same, exposure will be the same regardless of camera position and lens choice.

The crop should work that way, but the images will be very different due to the change in perspective. Your face looks a lot differently from 3 feet than from 6 feet.
Other than the perspective issue the longer lens is better for sensitive subjects. Most animals would prefer that you keep your distance for example.

big_apple_ken wrote in post #5508558
Is it because you can use a lower f-stop when you are closer or maybe there is less noise?

No. All that changes is the perspective.

Another reason you may choose one focal length over another that a lot of people don't think of is when you are shooting with flash on camera. Sometimes you are pushing the limits of how effective your flash is at a certain distance. By getting close to your subject with a wider lens you also bring the flash closer which might make the difference of the flash being useful or not. This can be especially helpful if all you have is a popup flash on camera.

Here's a video to watch, the man sitting on a chair at 0:07 is a pretty good example:

http://www.youtube.com​/watch?v=Y48R6-iIYHs

I think I need to test it out for myself since I can't seem to grasp the perspective idea. I just seems to me if I shoot with a 100mm lens at 10 feet away if I moved 5 feet closer to the subject and shot with 50mm shouldn't it yield the same perspective? Maybe I need to reread these responses.

As for the subject sensitivity or proximity limitations (can't get closer) that I totally understand.

If you double your distance and double your focal length (or halve and halve), your subject remains the same size in your image, but the size of the foreground and background will change, roughly put. That's the perspective change.

In the video I linked, you can see examples of increasing the focal length (zooming in) while increasing the distance simultaneously. (or the other way around)

Ahhh...I think I get what you are saying. Will watch the video when I get home. Can't really watch it with sound here at work. Thanks!

big_apple_ken wrote in post #5509752
Ahhh...I think I get what you are saying. Will watch the video when I get home. Can't really watch it with sound here at work. Thanks!

The change in perspective works without a camera: take an object like a coffee cup and set it at say a foot in front of your computer monitor. Look at it from a foot away. Then, lean down and look at it from 3" away. The size of the cup relative to the monitor grows rather significantly, right?

The same will happen when you get closer to a 3-dimensional subject/scene.

tonylong wrote in post #5510002
The change in perspective works without a camera: take an object like a coffee cup and set it at say a foot in front of your computer monitor. Look at it from a foot away. Then, lean down and look at it from 3" away. The size of the cup relative to the monitor grows rather significantly, right?

The same will happen when you get closer to a 3-dimensional subject/scene.

That's a little different as it does not take into account the different FOV you would get by changing to a wider angle lens and getting closer.

Ken, Once you try it, you will see it at work and know that it happens. Holloywood uses this all the time They shoot with an ultra long zoom lens some guy running away from a car that suddenly explodes in the background and it looks like he was like 20 feet away, when in reality he could be like 200 yards away. Long focal length compresses depth. It makes things look closer together.

We've all done this.... look through binoculars through the wrong end and what happens? Everything looks so far away, further than we know it is. Well long focal length lenses do the opposite, they make things look closer. So far away things look closer. So combine that with close and far objects in the same frame and the background obljects appear closer to the subject than they really are... convex rear view side mirrors warn us "objects in mirror are closer than they appear", opposite effect of the long focal length lens.

It all messes with perspective.

big_apple_ken wrote in post #5509592
I think I need to test it out for myself since I can't seem to grasp the perspective idea. I just seems to me if I shoot with a 100mm lens at 10 feet away if I moved 5 feet closer to the subject and shot with 50mm shouldn't it yield the same perspective? Maybe I need to reread these responses.

As for the subject sensitivity or proximity limitations (can't get closer) that I totally understand.

'Perspective' is the relative relationship between a subject and its surrcoundings. When you change camera position and also change FL, you are keeping 'framing' of the subject the same, but now you are altering its relationship to other objects, so you are altering 'perspective'. For example...

Assume camera a 0' with 100mm lens, subject at 5' and background object at 20'. From this position, the background item is 4 times as far away (from the subject) as the subject is from the camera. (20':5')
Now move to 2.5' and mount a 50mm lens...the subject fills the frame the same as before (same 'framing'), but the background is now 7x as far (17.5':2.5'), so the background item is now smaller in the frame than it was before. Different 'perspective'.

Wilt wrote in post #5510178
'Perspective' is the relative relationship between a subject and its surrcoundings. When you change camera position and also change FL, you are keeping 'framing' of the subject the same, but now you are altering its relationship to other objects, so you are altering 'perspective'. For example...

Assume camera a 0' with 100mm lens, subject at 5' and background object at 20'. From this position, the background item is 4 times as far away (from the subject) as the subject is from the camera. (20':5')
Now move to 2.5' and mount a 50mm lens...the subject fills the frame the same as before (same 'framing'), but the background is now 7x as far (17.5':2.5'), so the background item is now smaller in the frame than it was before. Different 'perspective'.

Although I really should play with it to get a firsthand understanding this mathematical explaination makes perfect sense to me. I can't believe I didn't see this before. I really want to thank everyone that helped out today.