Linked Questions

6 votes
3 answers
9k views

Does focal length change when I adjust the focus? [duplicate]

When I adjust the focus of my camera, in general, does this affect the focal length? I have heard that it does.
MM.'s user avatar
  • 165
26 votes
3 answers
6k views

Why isn't this portrait taken at f/29 sharp?

I am confused as to why this picture is not sharp. This is a portrait shot at F/29 ISO100 1/250 with a 17-85mm lens focused at 38mm. The subject distance was 1.2M. My DOF app tells me that my DOF ...
Corrupted MyStack's user avatar
22 votes
5 answers
24k views

How can a lens with a single focal length focus on more than one plane?

By definition, a prime lens is a fixed lens system with a fixed focal length. Then, simple physics tells us that it should be able to focus only on one plane (at a fixed distance) in front of it. But ...
Lazer's user avatar
  • 4,049
21 votes
6 answers
23k views

Why is depth of field affected by focal length?

As the focal length decreases, the depth of field increases as well. Why is this? I'm not so much interested in a physics lesson as I am interested in a simple, down-to-earth explanation.
Daniel T.'s user avatar
  • 1,827
20 votes
3 answers
21k views

What is the technical difference between focus and zooming?

This may look like a silly question but I don't know what the difference between zooming and focusing is. Zooming in will usually let me have fewer subjects in the scene while zooming out will allow ...
K'''s user avatar
  • 6,497
28 votes
3 answers
12k views

How can a 18-55mm lens focus on objects beyond 55mm?

I guess this is a total noob question, but doesn't 18-55mm mean that it can focus only up to 55mm?
Lazer's user avatar
  • 4,049
16 votes
4 answers
4k views

What is a parfocal lens and how beneficial is it to photography?

I heard someone say that after using a parfocal lens for the first time, they would never switch back. What is a parfocal lens, how is it different from any other lens, and what are the advantages and ...
dpollitt's user avatar
  • 46.5k
9 votes
2 answers
4k views

Is it normal for focal length to change while focusing?

When I change the focus distance on my Pentax 55-300mm lens, the focal length appears to change. More specifically, the lens appears to zoom in as I focus towards the minimum focusing distance. This ...
bwDraco's user avatar
  • 5,936
-1 votes
3 answers
3k views

At what distance from the lens will the image be in focus given a subject distance and focal length? [closed]

I've been trying to figure this one out for more than an hour now and I have no idea how to calculate this... You capture a picture of a tree which is standing 3m away from the camera. The focal ...
Dep's user avatar
  • 1
4 votes
2 answers
593 views

How does changing focus affect focal length?

I know when I change focal length of my zoom lens the physical length of it is changed. It's understandable: the optical parameters of the lens were changed. But what is going on when focus is changed?...
Andriy Kryvtsun's user avatar
0 votes
1 answer
1k views

How can I calculate sensor size (width and height) from focus length, vertical, and horizontal angle of view?

How to calculate sensor size (width/height) when knowing focus length, vertical & horizontal angle view (FOV)?
user25's user avatar
  • 105