Focal length on lens

BazEP

BazEP

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Chris
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This might be a really dumb-ass question but should a 70mm focal length give exactly the same field of view on what ever lens? I am a bit confused as I thought it should but I now have two lenses that cover 70mm. 1 is a 70-300mm and the other is a 18-70. There is a fairly large difference between the fields of view (see pics). What's going on? both confirm 70mm as the focal length on the exif. or am I misunderstanding the physics (highly possible)?

#1 pic - at 70mm on 18-70mm



#2 pic - at 70mm on 70-300mm



anyone got any idea? :shrug:
 
In theory they should give the same focal length, but it does seem to vary dependant on the lens and manufacturer. However, there does seem to be a big difference in these two, which is a bit concerning.
 
something doesn't seem right :-/ I was going to point out if the 70-300 wasn't a DX lens but...seems they both are. Odd :shrug:
 
Well, if Ken says so... :shrug:

Doesn't sound right to me. Not that he actually explains it properly.
 
Thank you Martyn,
still bloomin confused (and not going to get the results I was after with the 18-70mm but heh ho).
 
confirmed at infinity focus - same FoV for both lenses
 
morsing said:
Well, if Ken says so... :shrug:

Doesn't sound right to me. Not that he actually explains it properly.
Click to expand...

It may not sound right, but is the nature of the physics involved in the design of the lens, there was a similar 'issue' with the Nikkor 70-200f2.8 when they brought out the new model.

He tries again to explain here

Thom Hogan probably makes a better job of it here

So you will see there is nothing odd in this phenomenon, and certainly not a fault of the lens :thumbs:
 
Quite simply you took the images from different positions..........

One was 1.5m away and the other is 1.68m away from the subject
 
Dogfish_magnet said:
Quite simply you took the images from different positions..........

One was 1.5m away and the other is 1.68m away from the subject
Click to expand...

not from what I understand (from Thom Hogan I might add), its to do with how the IF works
 
Dogfish_magnet said:
Quite simply you took the images from different positions..........

One was 1.5m away and the other is 1.68m away from the subject
Click to expand...

where are you seeing that? i can't see it in my exif profile from opanda or from ViewNX
 
strange?!?

oh.. hang on.. did you say it they were both nikon? :wave:
 
shot at different distancses :bonk: the end of the 70-300 is closer hth mike
 
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Dogfish_magnet said:
if you take a picture with the same lens from different positions the resulting image will be different - never mind using a different lens !
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ofc but I assumed the OP had taken both pictures from the same place..i.e. set up a tripod, took one picture, swapped the lens and set 70mm and taken another. Is this not the case? if not then :bonk:
 
timmygcsc2308 said:
ofc but I assumed the OP had taken both pictures from the same place..i.e. set up a tripod, took one picture, swapped the lens and set 70mm and taken another. Is this not the case? if not then :bonk:
Click to expand...

To be fair you are quite right there are differences between lens's when shooting from the same position and focal lengths - but i have never seen this amount of difference
 
timmygcsc2308 said:
ofc but I assumed the OP had taken both pictures from the same place..i.e. set up a tripod, took one picture, swapped the lens and set 70mm and taken another. Is this not the case? if not then :bonk:
Click to expand...

would,nt make a differance the front of the lens is where the fov starts, so its the front of the lens at the time of shooting that counts. hth mike.
 
This is known as "focus breathing" and is designed into the lens to allow it to focus closer. At infinity the different lenses should give the same field of view at a given focal length as the OP has observed.
Most macro lenses do this when focused at minimum focus distance. I believe the Tamron 90mm macro has a FOV of 60mm at 1:1 focus.

See Thom Hogans comparison shots between the mk1 and mk2 Nikon 70-200 2.8s
 
:shrug: are we getting crossed wires or am I just being silly?. The Thom Hogan link explains that the 200mm is 200mm (or thereabouts) on both the VR and the VRII when focussed to infinity, yet due to the internal focussing mechanism differences, when focussed closer on the VR its 182mm and on the VRII its 134mm.
Your explanation Mike sounds right but I don't think the fov from the front of the lens is what Thom Hogans link explains
 
timmygcsc2308 said:
ofc but I assumed the OP had taken both pictures from the same place..i.e. set up a tripod, took one picture, swapped the lens and set 70mm and taken another. Is this not the case? if not then :bonk:
Click to expand...

You assume correct - no tripod but sat in my chair in the same place.
 
PS - to be fair there might have been a slight movement but the pics were just to show what I had discovered on numerous other tests in the house. Problem now understood anyway.
 
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