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Lens focal lengths and fields of view.
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Sep 20, 2022 01:20:33   #
Ozychatie Loc: Melbourne Australia
 
Hi Guys,
I have a queer issue concerning lenses for my Canon 700D. A couple of years ago I bought a Tamron 18-270mm EFS mount zoom figuring it would cover my existing 18-55 EFS and 55 to 250 EFS Canon lenses; making life easier.

Recently I thought I'd compare the field of view of the Tamron at full zoom against the Canon at full zoom. The Tamron will naturally have a slightly narrower field of view being marginally longer won't it?

Not so! At least not in my case.

I was shocked to find that the Canon's field of view was substantially narrower - suggesting greater magnification than the Tamron's. Has anybody else experienced this phenomena? This does not make sense to me so I was wondering if anyone in the community might have an explanation of what's going on. Comparison shots were taken of the same subject from the same position with the same camera body.

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Sep 20, 2022 02:17:49   #
Orphoto Loc: Oregon
 
This is fairly common when anywhere near the close focus limits of zooms. Not so much when closer to infinity.

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Sep 20, 2022 06:21:51   #
Wallen Loc: Middle Earth
 
Ozychatie wrote:
Hi Guys,
I have a queer issue concerning lenses for my Canon 700D. A couple of years ago I bought a Tamron 18-270mm EFS mount zoom figuring it would cover my existing 18-55 EFS and 55 to 250 EFS Canon lenses; making life easier.

Recently I thought I'd compare the field of view of the Tamron at full zoom against the Canon at full zoom. The Tamron will naturally have a slightly narrower field of view being marginally longer won't it?

Not so! At least not in my case.

I was shocked to find that the Canon's field of view was substantially narrower - suggesting greater magnification than the Tamron's. Has anybody else experienced this phenomena? This does not make sense to me so I was wondering if anyone in the community might have an explanation of what's going on. Comparison shots were taken of the same subject from the same position with the same camera body.
Hi Guys, br I have a queer issue concerning lenses... (show quote)


Long zoom lenses usually change field of view when focusing at near objects. Some change more than the others, hence, if your shooting at something near, some lenses would seem to have more magnification (narrower FOV).

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Sep 20, 2022 06:27:37   #
Architect1776 Loc: In my mind
 
Ozychatie wrote:
Hi Guys,
I have a queer issue concerning lenses for my Canon 700D. A couple of years ago I bought a Tamron 18-270mm EFS mount zoom figuring it would cover my existing 18-55 EFS and 55 to 250 EFS Canon lenses; making life easier.

Recently I thought I'd compare the field of view of the Tamron at full zoom against the Canon at full zoom. The Tamron will naturally have a slightly narrower field of view being marginally longer won't it?

Not so! At least not in my case.

I was shocked to find that the Canon's field of view was substantially narrower - suggesting greater magnification than the Tamron's. Has anybody else experienced this phenomena? This does not make sense to me so I was wondering if anyone in the community might have an explanation of what's going on. Comparison shots were taken of the same subject from the same position with the same camera body.
Hi Guys, br I have a queer issue concerning lenses... (show quote)


Were both at infinity?
Canon lenses hold the marked focal length at the tele end better than most other makes as one focuses closer.

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Sep 20, 2022 07:54:37   #
BebuLamar
 
You need to compare at infinity. Some 200mm shorten its focal length to something like 135mm at closet focusing distance. It's much more common with new lens and AF lenses because they tend to have internal focusing.

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Sep 20, 2022 07:58:07   #
Longshadow Loc: Audubon, PA, United States
 
Could it be caused by the different lens designs?
Are there field of view figures in the specs for each lens?

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Sep 20, 2022 08:36:55   #
Bayou
 
Try your experiment again focusing quite closely, at midrange, and at infinity at max focal length. I bet the closer you are, the more difference you'll see in the field of view of these two lenses. At infinity they should be about the same. Chalk it up to design difference. Zooms are sometimes not exactly as represented.

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Sep 20, 2022 12:58:36   #
BebuLamar
 
Longshadow wrote:
Could it be caused by the different lens designs?
Are there field of view figures in the specs for each lens?


If they specify the field of view (or focal length for that matter) it's only for when focused at infinity.
Lenses of older designed which simply moves all the elements in and out to focus would not change the focal length or actually the effective focal length is a bit longer at close focusing range.
Newer lenses especially AF lenses move only some of the elements to focus and shorten the focal length at close focus distance. These lenses have the advantages that not as much weigh needs to be moved and thus the AF works better. Also the center of gravity of the lens doesn't shift as much.

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Sep 20, 2022 13:30:13   #
Longshadow Loc: Audubon, PA, United States
 
BebuLamar wrote:
If they specify the field of view (or focal length for that matter) it's only for when focused at infinity.
Lenses of older designed which simply moves all the elements in and out to focus would not change the focal length or actually the effective focal length is a bit longer at close focusing range.
Newer lenses especially AF lenses move only some of the elements to focus and shorten the focal length at close focus distance. These lenses have the advantages that not as much weigh needs to be moved and thus the AF works better. Also the center of gravity of the lens doesn't shift as much.
If they specify the field of view (or focal length... (show quote)

Thanks.

(I just shoot and get what I get for each lens.)

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Sep 20, 2022 23:21:36   #
Ozychatie Loc: Melbourne Australia
 
Hi Guys,
Thanks for your replies. I didn't know this "quirk" with zoom lenses but it is very useful to know. It may be beneficial when trying to maximise a close subject. There is always something new to learn in photography and this is one topic I have never seen covered before.

I will try the lens comparisons again with a distant subject and see how they compare. I expect that when I do this, the Tamron will have the narrower field of view.

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Sep 21, 2022 17:45:35   #
profbowman Loc: Harrisonburg, VA, USA
 
These answers have either all made the same assumption or have ignored a very important fact. The depth of field depends upon the aperture. The smaller the aperture, the longer the depth of field.

Since Ozy did not tell us the aperture at whatever zoom value and distance he was using, all of the above answers do not make sense. If the apertures are the same at the conditions of experimentation, then the above answers can be correct. --Richard

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Sep 21, 2022 18:45:16   #
BebuLamar
 
profbowman wrote:
These answers have either all made the same assumption or have ignored a very important fact. The depth of field depends upon the aperture. The smaller the aperture, the longer the depth of field.

Since Ozy did not tell us the aperture at whatever zoom value and distance he was using, all of the above answers do not make sense. If the apertures are the same at the conditions of experimentation, then the above answers can be correct. --Richard


Ozy said nothing about depth of field. He talked about field of view.

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Sep 21, 2022 22:07:23   #
Grahame Loc: Fiji
 
profbowman wrote:
These answers have either all made the same assumption or have ignored a very important fact. The depth of field depends upon the aperture. The smaller the aperture, the longer the depth of field.

Since Ozy did not tell us the aperture at whatever zoom value and distance he was using, all of the above answers do not make sense. If the apertures are the same at the conditions of experimentation, then the above answers can be correct. --Richard


Wrong, DOF does not affect the dimension of the FOV.

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Sep 21, 2022 23:35:46   #
Ozychatie Loc: Melbourne Australia
 
Wow! So many potential variables. It makes one realise that when buying a lens it could be useful to try it out first and not simply rely on the figures.
I did do a re-shoot at full extension and infinity with both lenses and this time the FOV was in keeping with expectations. The Tamron had a noticeable narrower FOV compared to the Canon lens.
The Canon is significantly older than the Tamron lens and lacks the IBS the Tamron has so these could be factors as BebuLamar indicated in his reply.

Many thanks to all who have responded.

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Sep 22, 2022 00:17:49   #
profbowman Loc: Harrisonburg, VA, USA
 
Grahame wrote:
Wrong, DOF does not affect the dimension of the FOV.


Graham, you are so right. I misread the original question. Please, all, accept my apologies.

The field of view, or angle of view when one is calculating it, is defined by

angle (radians)=2 x inv-tan(d/2/f), where d (size of sensor along the appropriate direction) and f (focal length) must be in the same units.

So, since camera designers want the technical field of view from the lens to be enough larger than the sensor size so that vignetting is not obvious, any two cameras and any two lenses will differ in the field of view seen on the sensor. --Richard

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