Somebody PLEASE explaing this to me... - Focal Length

On 28/08/2015 16:09, Floyd L. Davidson wrote:
Bill W wrote:
Note that at the same focusing distance and the same actual
focal length, the "magnification" will be identical. Just be
aware that magnification means the comparison of the projected
image to the actualy object's size. It has nothing to do with
the size of the image you get!

Yes, and magnification is probably not the ideal term for what I mean.

It might be! If you are using the same camera for two different
lenses, the difference you see actually is magnification.

You might post a couple images, one from each camera/lens, to a
webpage or to alt.binaries.photos.original. If we could
download the image and look at Exif data it would help.

I'll try to do that, unless the whole issue turns out to be what
nospam said - focus breathing. This seems a bit extreme for that,
though. OTOH, I was focusing on something 10' away, so maybe. I'll
get back to this tomorrow.

If you are focusing at 10' there is no question but that you'll
be seeing the effects of focus breathing. Almost all modern
zoom lenses use an "Internal Focus" design, which has a fixed
sensor to lens distance (older lenses moved the lens away from
the sensor to focus closer), and focuses closer by moving
elements internally in relation each other. That does allow
better compensation for various aberrations at closer distances,
but it also changes the focal length. Often it is a significant
change. And the change might be very different for different
lens designs.

Some 70-200mm zooms are about 125mm when set to 200mm and
focused at the closest focusing distance. Others are nearer to
160mm. The Nikkor 105mm f/2.8 macro lens is actually about 76mm
when focused close enough to get 1:1 magnification.

It would be very surprising in two brands with different maximum
focal lengths were anywhere near the same!

Yes. Add in that with zooms, the focal length markings on the zoom ring
are usually kind of approximate, and the zoom (and focus distance)
setting recorded in the exif data come from an electrical brush sweeping
an arc of contacts, the position of which may or may not correspond to
what's marked on the ring, neither of which may correspond to what the
actual focal length at infinity is, let alone taking into account the
amount of focus-breathing at closer distance. So comparing two zoom
lenses' field of view at intermediate focal lengths in the zoom range
can get very problematical. Hope that made sense. I'd suggest that
unless you had a specific technical reason why you needed to know, then
ignore it - point the damned lens in the general direction and frame the
scene using the zoom - without worrying about what the actual focal
length may be.

"Bill W" wrote in message
...
On Fri, 28 Aug 2015 00:38:08 -0400, nospam
wrote:

In article , Bill W
wrote:

Well again, I was thinking of selling the 18-250, and getting the
18-300, but I'm not sure it's worth it. And I use long focal lengths
almost invariably for distant subjects, so I'm rarely concerned with
the macro capabilities.

the difference is minor. crop instead.

Probably good advice. I can use the $600 for better purposes.

For a superzoom, the 18-250mm Sigma is a good lens. Spending the cash
to get an extra 50mm is not worth it, IMO. The only benefit for
upgrading to the 18-300mm that I see is the ability to use the Sigma USB
dock to adjust the lens.

On 8/27/2015 10:59 PM, Floyd L. Davidson wrote:
Bill W wrote:
I am going crazy with this. Two lenses, side by side. A Pentax DA L
70-300 lens, and Sigma 18-250 DC lens. The DA in Pentax means it's
designed for APS-C sensors, and the same with the DC on the Sigma
lens. Why do they have nothing close to the same field of view &
magnification at the same focal length? To get the same photo with
both lenses, I need to set the Pentax to 170mm, and the Sigma to
250mm. When I do that, and then look at the two identical photos in
LR, the exif data shows the Pentax set at 170, with a 255mm 35mm
equivalent, and the Sigma at 250, with a 375mm 35mm equivalent, which
is correct, I guess, but the Pentax has much greater magnification at
*equal* focal lengths.

Can someone explain this to me? I am thinking of buying the Sigma
18-300 - also a DC lens, but it now appears that the Pentax would
still have greater magnification (375mm equivalent). How can one
compare the actual magnification of different lenses on paper? It's
obvious in this case that one needs to multiply the Pentax focal
length by 1.5, which is expected, but not the Sigma. How can I know
that going forward, and looking at other lenses?

Which cameras are you using?

Note that at the same focusing distance and the same actual
focal length, the "magnification" will be identical. Just be
aware that magnification means the comparison of the projected
image to the actualy object's size. It has nothing to do with
the size of the image you get!

You might post a couple images, one from each camera/lens, to a
webpage or to alt.binaries.photos.original. If we could
download the image and look at Exif data it would help.


Your theory is spot on. However, some lenses have what I would call
optimistically labeled focal lengths, especially at closer distances. My
Nikon old 18-200 is an example. At close distances, I would estimate
the longer end as being closer to 160 than 200.

--
PeterN

On 8/28/2015 10:45 AM, PeterN wrote:
On 8/27/2015 10:59 PM, Floyd L. Davidson wrote:
Bill W wrote:
I am going crazy with this. Two lenses, side by side. A Pentax DA L
70-300 lens, and Sigma 18-250 DC lens. The DA in Pentax means it's
designed for APS-C sensors, and the same with the DC on the Sigma
lens. Why do they have nothing close to the same field of view &
magnification at the same focal length? To get the same photo with
both lenses, I need to set the Pentax to 170mm, and the Sigma to
250mm. When I do that, and then look at the two identical photos in
LR, the exif data shows the Pentax set at 170, with a 255mm 35mm
equivalent, and the Sigma at 250, with a 375mm 35mm equivalent, which
is correct, I guess, but the Pentax has much greater magnification at
*equal* focal lengths.

Can someone explain this to me? I am thinking of buying the Sigma
18-300 - also a DC lens, but it now appears that the Pentax would
still have greater magnification (375mm equivalent). How can one
compare the actual magnification of different lenses on paper? It's
obvious in this case that one needs to multiply the Pentax focal
length by 1.5, which is expected, but not the Sigma. How can I know
that going forward, and looking at other lenses?

Which cameras are you using?

Note that at the same focusing distance and the same actual
focal length, the "magnification" will be identical. Just be
aware that magnification means the comparison of the projected
image to the actualy object's size. It has nothing to do with
the size of the image you get!

You might post a couple images, one from each camera/lens, to a
webpage or to alt.binaries.photos.original. If we could
download the image and look at Exif data it would help.


Your theory is spot on. However, some lenses have what I would call
optimistically labeled focal lengths, especially at closer distances. My
Nikon old 18-200 is an example. At close distances, I would estimate
the longer end as being closer to 160 than 200.


I shold clarify, that issue exists with zoom lenses. I have not noticed
it with prime lenses.



--
PeterN

In article , PeterN
wrote:

Your theory is spot on. However, some lenses have what I would call
optimistically labeled focal lengths, especially at closer distances. My
Nikon old 18-200 is an example. At close distances, I would estimate
the longer end as being closer to 160 than 200.

the focal length markings are only valid at infinity and have a 5%
tolerance.

On Thu, 27 Aug 2015 23:02:25 -0400, nospam
wrote:

In article , Bill W
wrote:

I am going crazy with this. Two lenses, side by side. A Pentax DA L
70-300 lens, and Sigma 18-250 DC lens. The DA in Pentax means it's
designed for APS-C sensors, and the same with the DC on the Sigma
lens. Why do they have nothing close to the same field of view &
magnification at the same focal length? To get the same photo with
both lenses, I need to set the Pentax to 170mm, and the Sigma to
250mm. When I do that, and then look at the two identical photos in
LR, the exif data shows the Pentax set at 170, with a 255mm 35mm
equivalent, and the Sigma at 250, with a 375mm 35mm equivalent, which
is correct, I guess, but the Pentax has much greater magnification at
*equal* focal lengths.

that's called focus breathing, where the actual focal length changes as
you focus on closer subjects.

lots of zoom lenses do it, especially superzooms like the sigma.

try comparing with it focused at infinity.

This was indeed the issue. The field of view at 250mm is now nearly
identical when focused at, or near, infinity.

On Fri, 28 Aug 2015 09:23:50 -0400, "PAS"
wrote:

"Bill W" wrote in message
.. .
On Fri, 28 Aug 2015 00:38:08 -0400, nospam
wrote:

In article , Bill W
wrote:

Well again, I was thinking of selling the 18-250, and getting the
18-300, but I'm not sure it's worth it. And I use long focal lengths
almost invariably for distant subjects, so I'm rarely concerned with
the macro capabilities.

the difference is minor. crop instead.

Probably good advice. I can use the $600 for better purposes.

For a superzoom, the 18-250mm Sigma is a good lens. Spending the cash
to get an extra 50mm is not worth it, IMO. The only benefit for
upgrading to the 18-300mm that I see is the ability to use the Sigma USB
dock to adjust the lens.

Everyone seems to be in agreement that it would be a waste of money.
That now includes me.

On Thu, 27 Aug 2015 22:29:21 -0700, Savageduck
wrote:

On 2015-08-28 04:35:14 +0000, Bill W said:

On Thu, 27 Aug 2015 20:59:42 -0700, Savageduck
wrote:

On 2015-08-28 02:23:30 +0000, Bill W said:

I am going crazy with this. Two lenses, side by side. A Pentax DA L
70-300 lens, and Sigma 18-250 DC lens. The DA in Pentax means it's
designed for APS-C sensors, and the same with the DC on the Sigma
lens. Why do they have nothing close to the same field of view &
magnification at the same focal length? To get the same photo with
both lenses, I need to set the Pentax to 170mm, and the Sigma to
250mm. When I do that, and then look at the two identical photos in
LR, the exif data shows the Pentax set at 170, with a 255mm 35mm
equivalent, and the Sigma at 250, with a 375mm 35mm equivalent, which
is correct, I guess, but the Pentax has much greater magnification at
*equal* focal lengths.

Can someone explain this to me? I am thinking of buying the Sigma
18-300 - also a DC lens, but it now appears that the Pentax would
still have greater magnification (375mm equivalent). How can one
compare the actual magnification of different lenses on paper? It's
obvious in this case that one needs to multiply the Pentax focal
length by 1.5, which is expected, but not the Sigma. How can I know
that going forward, and looking at other lenses?

Some clarification is need:

What camera are you using?

K5.

To what purpose are you going to put these lenses. An occasional longer
range lens, or wider range walk-around lens?

Yes...

Focal length is not magnification.

A typical APS-C sensored camera is going to have a x1.5 35mm
equivalence (x1.3 for Canons).

Did you mean Sigma 18-250mm or Sigma 18-300mm?

I have the 18-250, but I'm thinking about the 18-300.

Both are general purpose
"super zooms" and as such have to make some compromises. Both have a
macro feature and have a corresponding macro magnification ratio 1:3
for both of them.
There is a Pentax equivalent in their Pentax DA 18-270mm f/3.5-6.3 @
$447 from B&H.

Pentax doesn't seem to have a 70-300mm, they have a 55-300mm. It is not
a macro lens and thus has a max magnification of 0.28x.

Yes, I meant 55-300.

You can't compare the two brands for magnification based on total
specs, one with a macro feature and one without.

Personally unless I needed/wanted a macro feature I would go with the
Pentax 55-300mm, the HD Pentax DA 55-300mm f/4-5.8 ED WA is also
weather sealed.
...and for $290 from B&H it looks like bargain.

The issue with the Pentax 55-300 lenses is slow, noisy, hunting AF. It
is almost entirely useless at air shows, and even auto racing at
times. And using it with video is hopeless because of all the noise.
Even the Sigma walkaround stuff is much, much better with AF. The
Pentax IQ is pretty good, though, especially since I think I got it as
a kit lens.

I am unfamiliar with the pentax lenses and current Sigmas. All I can
say is the performance of my Nikkor 70-300mm VR has not dissapointed
when paired with my D300S.


So, what lenses are you actually looking at, and what do you want to
use them for, and what sort of budget constraints do you have?

Well again, I was thinking of selling the 18-250, and getting the
18-300, but I'm not sure it's worth it. And I use long focal lengths
almost invariably for distant subjects, so I'm rarely concerned with
the macro capabilities.

I really wanted to fill out my lenses with a 150-600 type lens, but no
one makes newer ones for Pentax, except a Pentax full frame 150-450,
and I don't really want to pay their prices ($2500). The most recent
Sigma available is about 5 years old, and it's still $1600. I don't
mind the price, but I always want the latest thing. There are usually
improvements over time, but it looks like Sigma will not be putting a
Pentax mount on their new 150-600. It's too bad - the lens gets very
good reviews.


Perhaps it might be time to look at other systems.

That's certainly the only way out, but I think I'm going to just live
with the limitations. A whole new system is a very big hit, and for my
purposes, not nearly worth it. If I could earn some money with this
stuff, that would be different, but that's not going to happen.

On Fri, 28 Aug 2015 23:58:05 +1200, Me wrote:

On 28/08/2015 16:09, Floyd L. Davidson wrote:
Bill W wrote:
Note that at the same focusing distance and the same actual
focal length, the "magnification" will be identical. Just be
aware that magnification means the comparison of the projected
image to the actualy object's size. It has nothing to do with
the size of the image you get!

Yes, and magnification is probably not the ideal term for what I mean.

It might be! If you are using the same camera for two different
lenses, the difference you see actually is magnification.

You might post a couple images, one from each camera/lens, to a
webpage or to alt.binaries.photos.original. If we could
download the image and look at Exif data it would help.

I'll try to do that, unless the whole issue turns out to be what
nospam said - focus breathing. This seems a bit extreme for that,
though. OTOH, I was focusing on something 10' away, so maybe. I'll
get back to this tomorrow.

If you are focusing at 10' there is no question but that you'll
be seeing the effects of focus breathing. Almost all modern
zoom lenses use an "Internal Focus" design, which has a fixed
sensor to lens distance (older lenses moved the lens away from
the sensor to focus closer), and focuses closer by moving
elements internally in relation each other. That does allow
better compensation for various aberrations at closer distances,
but it also changes the focal length. Often it is a significant
change. And the change might be very different for different
lens designs.

Some 70-200mm zooms are about 125mm when set to 200mm and
focused at the closest focusing distance. Others are nearer to
160mm. The Nikkor 105mm f/2.8 macro lens is actually about 76mm
when focused close enough to get 1:1 magnification.

It would be very surprising in two brands with different maximum
focal lengths were anywhere near the same!

Yes. Add in that with zooms, the focal length markings on the zoom ring
are usually kind of approximate, and the zoom (and focus distance)
setting recorded in the exif data come from an electrical brush sweeping
an arc of contacts, the position of which may or may not correspond to
what's marked on the ring, neither of which may correspond to what the
actual focal length at infinity is, let alone taking into account the
amount of focus-breathing at closer distance. So comparing two zoom
lenses' field of view at intermediate focal lengths in the zoom range
can get very problematical. Hope that made sense. I'd suggest that
unless you had a specific technical reason why you needed to know,

It was going to help me decide on a new lens. It's a bit convoluted -
and trivial, but now that I see it was focus breathing, it did help me
to make a decision.

then
ignore it - point the damned lens in the general direction and frame the
scene using the zoom - without worrying about what the actual focal
length may be.

The actual number never mattered, it was just the difference in the 2
lenses.

PeterN wrote:
On 8/28/2015 10:45 AM, PeterN wrote:
On 8/27/2015 10:59 PM, Floyd L. Davidson wrote:
Bill W wrote:
I am going crazy with this. Two lenses, side by side. A Pentax DA L
70-300 lens, and Sigma 18-250 DC lens. The DA in Pentax means it's
designed for APS-C sensors, and the same with the DC on the Sigma
lens. Why do they have nothing close to the same field of view &
magnification at the same focal length? To get the same photo with
both lenses, I need to set the Pentax to 170mm, and the Sigma to
250mm. When I do that, and then look at the two identical photos in
LR, the exif data shows the Pentax set at 170, with a 255mm 35mm
equivalent, and the Sigma at 250, with a 375mm 35mm equivalent, which
is correct, I guess, but the Pentax has much greater magnification at
*equal* focal lengths.

Can someone explain this to me? I am thinking of buying the Sigma
18-300 - also a DC lens, but it now appears that the Pentax would
still have greater magnification (375mm equivalent). How can one
compare the actual magnification of different lenses on paper? It's
obvious in this case that one needs to multiply the Pentax focal
length by 1.5, which is expected, but not the Sigma. How can I know
that going forward, and looking at other lenses?

Which cameras are you using?

Note that at the same focusing distance and the same actual
focal length, the "magnification" will be identical. Just be
aware that magnification means the comparison of the projected
image to the actualy object's size. It has nothing to do with
the size of the image you get!

You might post a couple images, one from each camera/lens, to a
webpage or to alt.binaries.photos.original. If we could
download the image and look at Exif data it would help.


Your theory is spot on. However, some lenses have what I would call
optimistically labeled focal lengths, especially at closer distances. My
Nikon old 18-200 is an example. At close distances, I would estimate
the longer end as being closer to 160 than 200.


I shold clarify, that issue exists with zoom lenses. I
have not noticed it with prime lenses.

Zoom lenses have no lock on focus breathing. But don't
miss the point that the focal length at the minimum
focus distances make no difference at all! Who cares
what it is!

What you want is at least some minimum magnification.
If the lens provides that...

The fact is that focus breathing is what allows the lens
to focus that close with fewer aberrations. If the
design target required the focal length to remain
longer, the lens simply would not be able to get the
same quality at the same magnification!

--
Floyd L. Davidson http://www.apaflo.com/
Ukpeagvik (Barrow, Alaska)