Is anyone good at eyeballing approximate scale? Can you give me a hand here? :)

[qnr]

I'm trying to determine the zoom ratio of my spotting scope. Math and I aren't on speaking terms, and I haven't gotten around to doing real measurements.

Here's the story. I bought a discontinued spotting scope used. It's a 15x-60x 60mm scope ... the documentation I downloaded says that at 15x, when used as a telephoto lens, it is equivalent to a 1000mm lens (the eyepiece gets adjusted depending upon its use as a spotting scope or telephoto lens, which explains the difference (50mm is considered 'normal' for 35mm film cameras, times 15 would seem to be 750mm)).

OK - I've got that, however, when I use it as a lens, using the eyepiece and a T2-mount, even at its lowest zoom of 15x, it seems to be way more than 15x/1000mm (and, the image in a spotting scope alongside, set to 20x, is much smaller). I attribute this to the T-mount, which acts a bit like macro extension tubes do, enlarging the image/focal length, but I could be wrong on this.

So... does anyone think they can eyeball what the actual zoom ratio and/or focal length might be? The first image is at 50mm (equiv, actually 25mm with my 4/3 sensor) and the second image is at the lowest zoom on the spotting scope. The images are links, if you wish to view them full size.

scene at 50mm (equiv)

scene at lowest zoom on spotting scope

The scope in question (a Celestron 52200), in case anyone was struck by idle curiosity on the matter:



Edit: thanks for looking

[JeffR]

1. Eyeballing it got me nowhere

but I opened a copy of the 50mm scene in Photoshop and put a grid over it and then looked at the tele shot. If I'm right, it's about a 20x magnification, which would be 1000mm compared to the nominal 50mm one.

[qnr]

2. ok, thank you - so it is about 1000mm at 15x then. I generally stay in the 15x to 30x range,

but do sometimes go to 60x ... that gives me a place to start.

Interesting though. When I shoot it with my 300mm (600mm eqiuv), the whole water tower only takes up about a 6th of the screen, but like I said, math and I don't get along, so maybe that's the way it should look.

[JeffR]

3. I'm pretty math challenged myself.

And still can't get my head around how two different brands of, say, 100mm lenses can have different fields of view on the same camera body.

[qnr]

7. Yeah :) -- well, I'm going to put some other shot up downthread (in a bit)

to show where my confusion stems from....

[ManiacJoe]

4. The math in the manual seems to assume a full-frame 35mm camera.

If your digital camera is not a full-frame SLR, you will probably need to add the "crop factor" into the math.

[qnr]

6. Thank you - yeah, my conversions are easy with a 4/3 sensor, a straight 2x.

which is why I have the first at 50mm equiv (it's actually taken at 25mm).

[Schema Thing]

9. so it's a 2000mm equiv on your 4/3 camera, right?

[qnr]

10. right - assuming 1000mm -- but I'm thinking the T-mount increases it a bit -- just

judging from another spotting scope that was sitting adjacent to mine, which had a smaller image at a higher zoom. On the other hand, I might just be overthinking things and confusing myself :)

[Schema Thing]

11. hmmm, my limited experience with having to shim lens mounts

tells me the T-mount probably gets your lens the same distance from the sensor as your normal mount. If it's not dead on, the lens won't focus at infinity. Of course, maybe it doesn't? Even so, I wouldn't think it would affect the magnification. (I'm no expert on optics math though)

But I'd also guess that their "1000mm" equiv spec is just an approximation. So if it's really, say an 1100mm equiv, that makes it a 2200mm on your camera.

[qnr]

12. Yeah, I'm sure you're right. It will focus at various distances. If I use a different

t-mount that has a short extension built in, the magnification is less, and the image is brighter, but there is serious vignetting. If I put the tube I'm currently using on to the other t-mount that has the extension, the magnification increases, with a drastic loss of light - having done some experiments like that is what made me believe that it might be the mount increasing magnification. However, I'm beginning to feel that yeah, it is above the stated 1000mm, but only by a small amount -- maybe 1100mm(2200mm equiv) or 1200mm(2400mm equiv).

[Schema Thing]

13. I guess one way to figure out fairly accurately would be if

you could measure an object in the distance, and then shoot it with your scope and your 800mm, then print it out and compare the size differences mathematically.

[qnr]

14. arrrrgh math!

/me runs away and hides :)

[Schema Thing]

15. lol, I know, but that math should be relatively simple.

if the larger image was, for instance, 300% larger than the smaller image, then your lens would be a 2400mm lens (300% of 800mm)

[qnr]

5. self-delete - repiled to wrong post n/t

[qnr]

8. 2 shots, legacy lenses, 400mm and 400mm and 2x teleconverter

Which is 800mm and 1600mm with my sensor.

The 1600mm is handheld on a windy day, so it's blurry, but useful for scale (it's also crooked, but not straightened so as not to change the scale).

This is why hearing that it's at 1000mm is confusing to me, the images from the scope seem significantly larger.

Actually though, I think I'm fixating on the "1000mm" part. After all, 1000mm would look like 2000mm in my photos. However, looking at the 800mm (1600mm equiv) one, I'd say it's a bit higher than 1000mm.. maybe 1100 or 1200mm.

Scene at 800mm equiv
(400mm telephoto with 2x ratio for sensor)

Scene at 1600mm equiv
(400mm telephoto, 2x telephoto teleconverter, with 2x ratio for sensor)

[WannaJumpMyScooter]

16. see, those two make perfect sense to me

the guy in the riser is twice as tall at 1600 than at 800, just like you would expect.

[qnr]

17. same here - I think I'm getting a handle on the overall deal though. n/t

[alfredo]

18. Would a Maksutov-Cassegrain be appropriate for

photography?

This one come with a T adapter threads.


Maybe a standard spotting scope would have clearer optics because of the simplicity?
Not sure if this one does.

[qnr]

19. They're both appropriate. I've seen excellent results from both.

It really depends upon what you want to do with it. The mirror lens is nice and bright, and sharp, though generally you're stuck with a single aperture. The spotting scopes give you great range, and in addition, you can use them by eye, something you can't do with a lens. Being on a fixed income, I just went with the spotting scope, since it only cost me about $50.00. I have other lenses, the scope was just to play with.

[alfredo]

22. I am expecting to make enough working the polls to

buy a decent spotting scope. I'm into astronomy and need a small tele because I am only allowed to carry thirty lbs. I have a 10" dob I can't use because it is too heavy for me to carry. The Mak looks like it would be good for both.

[qnr]

24. Yeah, they're very popular - I'd say that's a good route. n/t

[alfredo]

27. Both my wife and I are birders. It will get used.

[qnr]

20. BTW, according to Celestron's site, which I did a lot of looking around on after buying

the Celestron scope, any of their spotting scopes will accept a T-mount, for use with cameras.

[alfredo]

21. My main camera is a Canon G10, a point and shoot. Would

they have a mount like the T mount.


These Two by Alpen look good.

[qnr]

23. The P&S and bridge cameras can't use a T-mount. A T-mount requires that you

remove the lens and use the scope as an actual lens. However, in lots of respects, the P&S and bridge cameras are great matches for scopes. There are about 80 different adapters that I can think of offhand - heck, you can even make your own pretty easily, for years people just held their cameras up to the eyepiece, or sometimes used the cap to a milk bottle as a kind of connector.

[alfredo]

25. I will be getting either a DSLR or a 4/3 in the not too

distant future.

[qnr]

26. Lots of lightweight options coming along from various manufacturers, so you

should have a nice selection to choose from.

Edit: Silly apostrophe

[alfredo]

28. The GF1 looks very nice.