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Question: Quality of "Digital Optics" Lenses

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Anonymous
July 19, 2005 3:41:58 PM

Archived from groups: rec.photo.digital (More info?)

Hi, all.

I'm looking at options for my new Rebel 300d.

I am considering Canon's 75-300 IS lens. I expect it would be a good
choice.

I see an inexpensive 2x telephoto out there, from a company called
Digital Optics. this would give me 900mm effective (yikes!).

Does anyone know about this crew's lenses? It's certainly a low price
point ($60 for the 2x), so I am suspicious of the quality. On the other
hand, it's cheap enough that it might just be worth getting for kicks.

But some precedent on Digital Optics' quality would be useful.

Any wisdom out there?

Thanks!

DW.
Anonymous
July 19, 2005 5:30:53 PM

Archived from groups: rec.photo.digital (More info?)

Yeah, I tend to agree. But I do have a mild 'astral protography' bent,
and if I could spend $70 more to get some insanely large moon shots
(who cares about corner/edge problems on a moon shot), That alone might
be worth the price of admission.

But yes, I think starting with the 300 would be wise regardless. ;) 
Anonymous
July 19, 2005 5:37:21 PM

Archived from groups: rec.photo.digital (More info?)

"2X telephoto...900mm effective" You are quite confused, it sounds.
In order for 2X to provide 900mm effective, you have to start with
450mm!!! Put it in front of 80mm, and you end up with something
similar to a 160mm lens.

Digital Optics is an example of a way to take a non-interchangeable
lens and extend its focal length. It mounts on FRONT of the existing
lens. Since you have an SLR, you don't want that! You want to use a
'teleconvertor', and most are 2X in effect like the 2X front-mounted
unit but generally better in quality of result.

--wilt
Related resources
Anonymous
July 19, 2005 5:45:23 PM

Archived from groups: rec.photo.digital (More info?)

I will admit to being confused, as I'm only starting to get my head
around the vernacular. the 900 effective comes from:

Native lens length = 300
Teleconverter = 2x => 600mm
x 1.6 to 'translate' from 35mm to digicam equivalent => ~900mm.

I'm not intentionally talking out of my arse or being obtuse, I'm just
at the start of a bit of a learning curve.
Anonymous
July 19, 2005 5:51:42 PM

Archived from groups: rec.photo.digital (More info?)

the 1.6 multiplier comes from (I nicked this from another post)
...."reduced sensor size results in a 50mm lens
having the same field of view as an 80mm lens"
Anonymous
July 19, 2005 7:08:25 PM

Archived from groups: rec.photo.digital (More info?)

Really?! Can you email me offline and let me know what you'd want for
it? Perhaps also a shipping estimate to the Vancouver area?
Anonymous
July 19, 2005 7:31:14 PM

Archived from groups: rec.photo.digital (More info?)

The one I was looking at is front-mounted.
Anonymous
July 19, 2005 7:36:57 PM

Archived from groups: rec.photo.digital (More info?)

<<Where'd you get that 80mm, though? He's asking about putting the 2x
converter on a 75-300mm lens. Given the Canon's 1.6x focal length
factor,
that works out to 960mm effective equivalent.>>

The original post did not say '960mm equivalent field of view'...it
simply said 2X on 300mm, and that is 600mm according to how I was
taught multiplication.

--Wilt
Anonymous
July 19, 2005 8:00:31 PM

Archived from groups: rec.photo.digital (More info?)

"BD" <bobby_dread@hotmail.com> wrote in message
news:1121798518.376446.71680@o13g2000cwo.googlegroups.com...
> Hi, all.
>
> I'm looking at options for my new Rebel 300d.
>
> I am considering Canon's 75-300 IS lens. I expect it would be a good
> choice.
>
> I see an inexpensive 2x telephoto out there, from a company called
> Digital Optics. this would give me 900mm effective (yikes!).

"Yikes!" is right, but what on earth would you use a 900mm (equiv.) lens
for? Do you have any idea of the problems you'd have using such a lens, even
in the (unlikely) case that such a converter could produce acceptable image
quality?


>
> Does anyone know about this crew's lenses? It's certainly a low price
> point ($60 for the 2x), so I am suspicious of the quality.

And rightly so.


> On the other
> hand, it's cheap enough that it might just be worth getting for kicks.

Yes, or you could just put the $60 in a shredder and save yourself the
bother and wasted time.


>
> But some precedent on Digital Optics' quality would be useful.
>
> Any wisdom out there?

No experience with that <ahem> brand myself, but I would be leery of any
such attachment that's essentially a no-name product. First, would it even
fit that 75-300mm lens? Most of those kinds of converters are threaded for
37mm or 52mm attachment. I'd be surprised if any 75-300 has a filter size as
small as 52mm.

Here are some comments from others on the "Digital Optics" (aka "Crystal
Optics") brand attachments:
http://www.stevesforums.com/forums/view_topic.php?id=50...

I strongly suggest you get the 75-300 first and *then* decide if you really
need or want more focal length. In any case it seems unlikely to me that any
front-end converter would be suitable for such a lens.

N.
Anonymous
July 19, 2005 9:36:11 PM

Archived from groups: rec.photo.digital (More info?)

"wilt" <wiltw@aol.com> wrote in message
news:1121812617.933936.301340@o13g2000cwo.googlegroups.com...

> The original post did not say '960mm equivalent field of view'
> ..it
> simply said 2X on 300mm,

Nah, it said:
"I'm looking at options for my new Rebel 300d.
I am considering Canon's 75-300 IS lens. I expect
it would be a good choice. I see an inexpensive
2x telephoto out there......"

Causing you to respond to a subsequent reply with:
"In order for 2X to provide 900mm effective, you have to start with
450mm!!!"

"effective"...that's your word. So it was you who responded with an
erroneous and inaccurate comment to the "effective" range of the setup in
question. And the math is quite clear that for an a Rebel-D with 75-300 at
the long end and a 2x diopter out front the "effective" reach is 900mm+. Not
600mm. And a 450mm lens on the same setup will give an "effective" 1440mm
not 900mm.

All you did was make a mistake......so why not just say "Oops...my bad, I
missed the sensor size multiplier."?

..
Anonymous
July 19, 2005 9:38:13 PM

Archived from groups: rec.photo.digital (More info?)

"wilt" <wiltw@aol.com> wrote in message
news:1121805441.190331.54990@o13g2000cwo.googlegroups.com...
> "2X telephoto...900mm effective" You are quite confused, it sounds.
> In order for 2X to provide 900mm effective, you have to start with
> 450mm!!! Put it in front of 80mm, and you end up with something
> similar to a 160mm lens.

Where'd you get that 80mm, though? He's asking about putting the 2x
converter on a 75-300mm lens. Given the Canon's 1.6x focal length factor,
that works out to 960mm effective equivalent. Close enough, since these
numbers are approximations anyway. (The 75-300 is probably a bit less than
an actual 300mm at the long end, the 1.6x factor is just an approximation,
and the 2x conversion probably isn't exact either.


>
> Digital Optics is an example of a way to take a non-interchangeable
> lens and extend its focal length. It mounts on FRONT of the existing
> lens. Since you have an SLR, you don't want that! You want to use a
> 'teleconvertor', and most are 2X in effect like the 2X front-mounted
> unit but generally better in quality of result.

Right, but the problem there is that the rear-mounted 2x tele extender will
cut his maximum aperture by two stops, at which point his SLR will no longer
autofocus. I.e., if his 75-300 is f/5.6 at the long end it will be f/11 with
a 2x extender, too small an aperture for the SLR to autofocus (perhaps even
too small for it to be readily manually focused).

That is in fact the one advantage to a front-mounted converter: it won't
change his maximum aperture, provided it's designed so as to be suitable for
that particular lens. (Which in this case it almost certainly isn't,
however.)

N.
Anonymous
July 19, 2005 10:03:49 PM

Archived from groups: rec.photo.digital (More info?)

"BD" <bobby_dread@hotmail.com> wrote in message
news:1121805923.443417.126080@z14g2000cwz.googlegroups.com...
>I will admit to being confused, as I'm only starting to get my head
> around the vernacular. the 900 effective comes from:
>
> Native lens length = 300
> Teleconverter = 2x => 600mm
> x 1.6 to 'translate' from 35mm to digicam equivalent => ~900mm.

Yes, close enough. It works out to 960mm of course, but none of these
numbers are likely to be exactly as advertised anyway.


>
> I'm not intentionally talking out of my arse or being obtuse, I'm just
> at the start of a bit of a learning curve.

You're doing okay. But I think once you deal with an 480mm equivalent or
thereabouts, you probably will find that more than enough of a handful.

I can understand your being interested in photographing the moon. The
problem there is that the moon's been photographed 18,745,904,320,417 times
already, with several million of those being done with equipment you cannot
possibly begin to compete with. I would just buy a book with some good moon
photographs, of which there must be many. ;-)

Even a 960mm (equivalent) lens is not going to fill the frame with the moon
very well. The moon's about 2,000 miles in diameter and what, 238,000 miles
away? So with a 960mm lens on a 35 the moon image would be 2 / 238 * 960 =
8.07 mm diameter, or about one-third the height of the frame. If you take a
long-lens shot and magnify the moon with software you'll probably do about
as well as you'd do with a cheap 2x converter (if you could even get one on
that lens in the first place).

N.
Anonymous
July 19, 2005 11:31:59 PM

Archived from groups: rec.photo.digital (More info?)

On 19 Jul 2005 13:30:53 -0700, BD wrote:

> But I do have a mild 'astral protography' bent,
> and if I could spend $70 more to get some insanely large moon shots
> (who cares about corner/edge problems on a moon shot), That alone might
> be worth the price of admission.

The only good thing about using such a 2x converter for moon shots
is that even if it introduces severe corner vignetting, it wouldn't
be as much of a problem with bright round objects. :)  On the other
hand it might make focusing more difficult and inaccurate. I'd
guess that blowing up the image 2x on your computer would give you
more consistent, satisfactory results. The 2x teleconverter would
be good for one thing though. You'd more easily see the effect of
not having rock solid stability, and might decide to use a decent
tripod if you weren't already doing so. I'd think the moon would be
bright enough to allow reasonably quick shutter speeds, but the
longer focal length would require longer exposures.
Anonymous
July 20, 2005 12:25:19 AM

Archived from groups: rec.photo.digital (More info?)

I think the L lenses will be a little ways down the road. They'd be
wasted on me at this point. ;-)
Anonymous
July 20, 2005 12:53:44 AM

Archived from groups: rec.photo.digital (More info?)

BD wrote:

> The one I was looking at is front-mounted.

They're absolute gabage.. They're meant for video cameras
which have much lower resolutions and are forgiving
of imperfections in the lens. They also cause severe
vignetting when zoomed wide (75mm in your case).

These things are a giant hunk of glass that will cause
color fringing, linear distortion and lack of contrast.

There are decent ones.. Olympus made a pretty good 1.7x
lens thread mounted converter but it's quite a bit more
expensive and still not perfect.

https://emporium.olympus.com/innards/empProdDetails.asp...
Anonymous
July 20, 2005 1:32:07 AM

Archived from groups: rec.photo.digital (More info?)

In article <1121798518.376446.71680@o13g2000cwo.googlegroups.com>,
"BD" <bobby_dread@hotmail.com> wrote:

> Hi, all.
>
> I'm looking at options for my new Rebel 300d.
>
> I am considering Canon's 75-300 IS lens. I expect it would be a good
> choice.

It's great. I have one for sale if you are interested.

>
> I see an inexpensive 2x telephoto out there, from a company called
> Digital Optics. this would give me 900mm effective (yikes!).
>
> Does anyone know about this crew's lenses? It's certainly a low price
> point ($60 for the 2x), so I am suspicious of the quality. On the other
> hand, it's cheap enough that it might just be worth getting for kicks.
>
> But some precedent on Digital Optics' quality would be useful.
>
> Any wisdom out there?
>
> Thanks!
>
> DW.

--

http://home.nc.rr.com/christianbonanno/
Anonymous
July 20, 2005 1:51:37 AM

Archived from groups: rec.photo.digital (More info?)

In message <1121798518.376446.71680@o13g2000cwo.googlegroups.com>,
"BD" <bobby_dread@hotmail.com> wrote:

>Hi, all.
>
>I'm looking at options for my new Rebel 300d.
>
>I am considering Canon's 75-300 IS lens. I expect it would be a good
>choice.
>
>I see an inexpensive 2x telephoto out there, from a company called
>Digital Optics. this would give me 900mm effective (yikes!).
>
>Does anyone know about this crew's lenses? It's certainly a low price
>point ($60 for the 2x), so I am suspicious of the quality. On the other
>hand, it's cheap enough that it might just be worth getting for kicks.
>
>But some precedent on Digital Optics' quality would be useful.
>
>Any wisdom out there?

Yeah. Don't get that lens if you want to use a teleconverter. It's not
sharp enough to get any more detail out of it; it is already pushed to
its limits without a TC.

Also. you really should stop multiplying focal lengths by 1.6. That
zoom with a 2x is 600mm, not 900. It will have about as much detail as
a really sharp 200mm lens on a full-35mm-frame camera.

There's no free lunch or magic with teleconverters; they waste light,
and light isn't worth wasting unless you gain something from it.
Teleconverters are for sharp, top-end lenses, not budget consumer
lenses.

Also, the better TCs start at about $200.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
July 20, 2005 1:54:26 AM

Archived from groups: rec.photo.digital (More info?)

In message <NJSdnQvCbpJAxkDfRVn-uQ@comcast.com>,
"Nostrobino" <not@home.today> wrote:

>I strongly suggest you get the 75-300 first and *then* decide if you really
>need or want more focal length. In any case it seems unlikely to me that any
>front-end converter would be suitable for such a lens.

He'd need another lens, really. The 75-300 is only sharp at 300mm
stopped down, and it is just barely sharp. Magnifying it with a TC will
only disappoint, unless a person is confused enought to compare by
small, full-frame, downsampled images.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
July 20, 2005 1:58:37 AM

Archived from groups: rec.photo.digital (More info?)

In message <1121805053.966177.179030@g14g2000cwa.googlegroups.com>,
"BD" <bobby_dread@hotmail.com> wrote:

>Yeah, I tend to agree. But I do have a mild 'astral protography' bent,
>and if I could spend $70 more to get some insanely large moon shots
>(who cares about corner/edge problems on a moon shot), That alone might
>be worth the price of admission.

No, you will get horrible results with a 75-300 and a 2x TC. If it's
the kind that goes between the lens and the body, the 75-300 doesn't
have the sharpness to make it worth using; if it's the type that screws
on like a filter, it will probably be horrible, unless it is made
specifically for the lens it is mounted on. I have a screw-on 2x for my
Sony F707; it works wonderfully on the Sony, but is absolutely worthless
on my 75-300 IS on my Canons.

>But yes, I think starting with the 300 would be wise regardless. ;) 

Well, that depends on your budget. If price is not an issue, skip the
75-300, as it will probably become redundant when you decide to get a
better lens.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
July 20, 2005 2:09:25 AM

Archived from groups: rec.photo.digital (More info?)

"BD" <bobby_dread@hotmail.com> wrote in message
news:1121812274.188514.192270@g47g2000cwa.googlegroups.com...
> The one I was looking at is front-mounted.

I know. I doubt you can get one that will fit the front of a 75-300mm lens,
at least not for $60.

N.
Anonymous
July 20, 2005 2:11:03 AM

Archived from groups: rec.photo.digital (More info?)

In message <NfadnVMd5Z1f70DfRVn-hg@comcast.com>,
"Nostrobino" <not@home.today> wrote:

>"wilt" <wiltw@aol.com> wrote in message
>news:1121805441.190331.54990@o13g2000cwo.googlegroups.com...
>> "2X telephoto...900mm effective" You are quite confused, it sounds.
>> In order for 2X to provide 900mm effective, you have to start with
>> 450mm!!! Put it in front of 80mm, and you end up with something
>> similar to a 160mm lens.
>
>Where'd you get that 80mm, though? He's asking about putting the 2x
>converter on a 75-300mm lens. Given the Canon's 1.6x focal length factor,
>that works out to 960mm effective equivalent.

.... but what does that mean, in practical terms? Nothing, because a
sharp 300mm lens will capture more subject detail from the same distance
as this combo will.

--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@no.komm>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
Anonymous
July 20, 2005 2:11:04 AM

Archived from groups: rec.photo.digital (More info?)

<JPS@no.komm> wrote in message
news:3fuqd1tbf89njuem4q35bbnkfuhh8irpf2@4ax.com...
> In message <NfadnVMd5Z1f70DfRVn-hg@comcast.com>,
> "Nostrobino" <not@home.today> wrote:
>
>>"wilt" <wiltw@aol.com> wrote in message
>>news:1121805441.190331.54990@o13g2000cwo.googlegroups.com...
>>> "2X telephoto...900mm effective" You are quite confused, it sounds.
>>> In order for 2X to provide 900mm effective, you have to start with
>>> 450mm!!! Put it in front of 80mm, and you end up with something
>>> similar to a 160mm lens.
>>
>>Where'd you get that 80mm, though? He's asking about putting the 2x
>>converter on a 75-300mm lens. Given the Canon's 1.6x focal length factor,
>>that works out to 960mm effective equivalent.
>
> ... but what does that mean, in practical terms? Nothing, because a
> sharp 300mm lens will capture more subject detail from the same distance
> as this combo will.

Probably. I think it's moot anyway, since I doubt it's practical to put an
inexpensive 2x front-end converter on a 75-300mm lens.

N.
Anonymous
July 20, 2005 2:26:40 AM

Archived from groups: rec.photo.digital (More info?)

"wilt" <wiltw@aol.com> wrote in message
news:1121812617.933936.301340@o13g2000cwo.googlegroups.com...
> <<Where'd you get that 80mm, though? He's asking about putting the 2x
> converter on a 75-300mm lens. Given the Canon's 1.6x focal length
> factor,
> that works out to 960mm effective equivalent.>>
>
> The original post did not say '960mm equivalent field of view'...it
> simply said 2X on 300mm, and that is 600mm according to how I was
> taught multiplication.

When you were taught multiplication, did they teach you to just insert the
number 80 randomly into the problem? As I asked, "where'd you get that
80mm"? No one else had mentioned anything about "80mm"; it appears to be
something you just pulled out of the blue.

The OP clearly knows that Canon dSLR has a 1.6 focal length factor for the
35mm equivalence which almost everyone understands and relates to. That's
where he got the ~900mm (actually 960mm if all the numbers were precisely
correct, which is unlikely) for the combination.

N.
Anonymous
July 20, 2005 3:17:04 AM

Archived from groups: rec.photo.digital (More info?)

JPS@no.komm <JPS@no.komm> wrote:
>
> Well, that depends on your budget. If price is not an issue, skip the
> 75-300, as it will probably become redundant when you decide to get a
> better lens.

.... unless you go on long (multi-day) hikes, in which case you'll want
the lightest telephoto lens you can find, and the 75-300 is it.

I'm still looking at the Sigma 80-400 to replace my 75-300 for primary
telephoto use, but I'd never try to drag that much weight into the
mountains.

--
Zed Pobre <zed@resonant.org> a.k.a. Zed Pobre <zed@debian.org>
PGP key and fingerprint available on finger; encrypted mail welcomed.
July 20, 2005 5:28:38 AM

Archived from groups: rec.photo.digital (More info?)

Hi.
I have a 75-300 IS and it suites me fine. It's not a series L lens but
have a excellent image that my money can give. ;-)

If you intend to use a 2x teleconverter, so, forget the 75-300. You are
using a Canon and I recommend that you use a canon TC too, but the
problem is: Neither the 2x TC and 1.4X TC can be mounted on the 75-300.
These TC´s accepts only some series L lens.


BD wrote:
> Hi, all.
>
> I'm looking at options for my new Rebel 300d.
>
> I am considering Canon's 75-300 IS lens. I expect it would be a good
> choice.
>
> I see an inexpensive 2x telephoto out there, from a company called
> Digital Optics. this would give me 900mm effective (yikes!).
>
> Does anyone know about this crew's lenses? It's certainly a low price
> point ($60 for the 2x), so I am suspicious of the quality. On the other
> hand, it's cheap enough that it might just be worth getting for kicks.
>
> But some precedent on Digital Optics' quality would be useful.
>
> Any wisdom out there?
>
> Thanks!
>
> DW.
>

--
gutto@iis.com.br

Carlos A. B. Coutinho
Rio de Janeiro, RJ
Brasil
Anonymous
July 20, 2005 5:35:02 AM

Archived from groups: rec.photo.digital (More info?)

"BD" <bobby_dread@hotmail.com> wrote in message
news:1121798518.376446.71680@o13g2000cwo.googlegroups.com...
> Hi, all.
>
> I'm looking at options for my new Rebel 300d.
>
> I am considering Canon's 75-300 IS lens. I expect it would be a good
> choice.
>
> I see an inexpensive 2x telephoto out there, from a company called
> Digital Optics. this would give me 900mm effective (yikes!).
>
> Does anyone know about this crew's lenses? It's certainly a low price
> point ($60 for the 2x), so I am suspicious of the quality. On the other
> hand, it's cheap enough that it might just be worth getting for kicks.
>
> But some precedent on Digital Optics' quality would be useful.
>
> Any wisdom out there?
>
> Thanks!
>
> DW.
>
BD, You mentioned astro photography in one of your replies in this thread.
In this case, I would forget about the 75-300 and save for a 300mm f/4 L IS
and a 1.4 or 2x converter. After trying the zooms, I ended up getting the L
glass. That 300mm is a great way to enter Canon's telephoto L glass series.
Here is a couple of examples with Canon's 2x converter attached:
http://home.att.net/~jriegle/moon.jpg

http://home.att.net/~jriegle/grackle1.jpg

John
Anonymous
July 20, 2005 6:48:35 AM

Archived from groups: rec.photo.digital (More info?)

In article <1121810905.678442.160390@g44g2000cwa.googlegroups.com>,
"BD" <bobby_dread@hotmail.com> wrote:

> Really?! Can you email me offline and let me know what you'd want for
> it? Perhaps also a shipping estimate to the Vancouver area?

Oh, no, sorry. I can't ship to my friends up north!

It is an excellent lens though.

--

http://home.nc.rr.com/christianbonanno/
Anonymous
July 20, 2005 4:31:26 PM

Archived from groups: rec.photo.digital (More info?)

Nostrobino wrote:
>
> "BD" <bobby_dread@hotmail.com> wrote in message
> news:1121805923.443417.126080@z14g2000cwz.googlegroups.com...
> >I will admit to being confused, as I'm only starting to get my head
> > around the vernacular. the 900 effective comes from:
> >
> > Native lens length = 300
> > Teleconverter = 2x => 600mm
> > x 1.6 to 'translate' from 35mm to digicam equivalent => ~900mm.
>
> Yes, close enough. It works out to 960mm of course, but none of these
> numbers are likely to be exactly as advertised anyway.
>
> >
> > I'm not intentionally talking out of my arse or being obtuse, I'm just
> > at the start of a bit of a learning curve.
>
> You're doing okay. But I think once you deal with an 480mm equivalent or
> thereabouts, you probably will find that more than enough of a handful.
>
> I can understand your being interested in photographing the moon. The
> problem there is that the moon's been photographed 18,745,904,320,417 times
> already, with several million of those being done with equipment you cannot
> possibly begin to compete with. I would just buy a book with some good moon
> photographs, of which there must be many. ;-)
>
> Even a 960mm (equivalent) lens is not going to fill the frame with the moon
> very well. The moon's about 2,000 miles in diameter and what, 238,000 miles
> away? So with a 960mm lens on a 35 the moon image would be 2 / 238 * 960 =
> 8.07 mm diameter, or about one-third the height of the frame. If you take a
> long-lens shot and magnify the moon with software you'll probably do about
> as well as you'd do with a cheap 2x converter (if you could even get one on
> that lens in the first place).
>
> N.

The *actual* focal length of the combo is 600mm, not 960. 960 is the
35mm equivalent, not actual. The image size is independent of any
sensor, so it will be 2/238*600 = 5.04 mm, about a third of the 300D
sensor size.

Colin
Anonymous
July 20, 2005 4:31:27 PM

Archived from groups: rec.photo.digital (More info?)

"Colin D" <ColinD@killspam.127.0.0.1> wrote in message
news:42DD9B5E.57734A81@killspam.127.0.0.1...
>
>
> Nostrobino wrote:
>>
>> "BD" <bobby_dread@hotmail.com> wrote in message
>> news:1121805923.443417.126080@z14g2000cwz.googlegroups.com...
>> >I will admit to being confused, as I'm only starting to get my head
>> > around the vernacular. the 900 effective comes from:
>> >
>> > Native lens length = 300
>> > Teleconverter = 2x => 600mm
>> > x 1.6 to 'translate' from 35mm to digicam equivalent => ~900mm.
>>
>> Yes, close enough. It works out to 960mm of course, but none of these
>> numbers are likely to be exactly as advertised anyway.
>>
>> >
>> > I'm not intentionally talking out of my arse or being obtuse, I'm just
>> > at the start of a bit of a learning curve.
>>
>> You're doing okay. But I think once you deal with an 480mm equivalent or
>> thereabouts, you probably will find that more than enough of a handful.
>>
>> I can understand your being interested in photographing the moon. The
>> problem there is that the moon's been photographed 18,745,904,320,417
>> times
>> already, with several million of those being done with equipment you
>> cannot
>> possibly begin to compete with. I would just buy a book with some good
>> moon
>> photographs, of which there must be many. ;-)
>>
>> Even a 960mm (equivalent) lens is not going to fill the frame with the
>> moon
>> very well. The moon's about 2,000 miles in diameter and what, 238,000
>> miles
>> away? So with a 960mm lens on a 35 the moon image would be 2 / 238 * 960
>> =
>> 8.07 mm diameter, or about one-third the height of the frame. If you take
>> a
>> long-lens shot and magnify the moon with software you'll probably do
>> about
>> as well as you'd do with a cheap 2x converter (if you could even get one
>> on
>> that lens in the first place).
>>
>> N.
>
> The *actual* focal length of the combo is 600mm, not 960. 960 is the
> 35mm equivalent, not actual.

Which is exactly what I said it was:
>> Even a 960mm (equivalent) lens [etc., etc.]

I think everyone understands what "(equivalent)" means in this context.


> The image size is independent of any
> sensor, so it will be 2/238*600 = 5.04 mm, about a third of the 300D
> sensor size.

The salient point is that it *is* about a third of the frame height,
whichever way you look at it. As I said,
>> So with a 960mm lens on a 35 the moon image would be 2 / 238 * 960 =
>> 8.07 mm diameter, or about one-third the height of the frame.

Change that to the actual focal length and sensor dimensions and the result
is the same, it's still about a third of the frame height.

The OP was using the 35mm equivalencies which are familiar to most of us,
and I continued the same usage.

N.
!