Camera lens 200mm maybe/maybe not

[Lester]

Hi all, I have been testing some of my camera lenses on Canon 20Da. The first is a Sigma 18-200mmDC lens. I sat down and focused it onto the TV and could fit in just over half the width of the screen. Then I fitted an old MD (minolta 35mm film type lens) Tokina 200mm, with MD-Eos adapter. I could fit in just over half the FOV of the Sigma lens at 200mm.

I realise with the size chip of the 20Da, that you magnify the FL by 1.6 when compared to standard 35mm film lenses. So I think the old Tokina 200mm lens is actually 320mm when on the 20Da. Is this correct? Also this lens is f3.5 wide open, so is it still f3.5 when on the 20Da? Or should the f3.5 be multiplied also by 1.6?

Another question; when I mount the 20Da or 40D cameras onto my telescopes, should the FL of the scopes be also magnified by 1.6?

I hope someone can understand my questions. Thanks.

[Ian Robinson]

I believe this is so.

Need to take into account the difference between the diagonal on the 35mm film (24mm x 36mm --> 43.3mm compared with the diagonal on the camera chip (22.2mm x14.8mm --> diagonal = 26.7mm) ==> 43.3/26.7 = 1.62 ==> for a lens designed for 35mm film , the effective focal length is 1.62x longer.

BUT if your MD - EF adapter is like mine, because the MD registry distance is different to the Canon EF registry distance ( http://www.photoethnography.com/ClassicCameras/index-frameset.html?Lens-adapters.html~mainFrame ), there is a corrector lens built into the adapter to move the focal plain to the correct registry distance to permit focus at infinity and you need to correct for that (I have been told the correction is multiply both the effective focal length the and the f stop being used by 1.4x.(Typically the suppliers say the corrector lens factor is 1.4x for an MD-EF adapter.)
So I believe in effect you need to use a factor of 1.6x1.4 =2.27x for the focal length on a 20D chip and 1.6x for the f.stop.

I think I have that right. ?

So f3.5 become f3.5x 1.4 = f4.9 ?
and 200mm MD on the EF become 200mmx 2.27 = 454mm Eff focal length. ?


If the 200mm Tamron had an EF bayonet and was designed for Canon EF cameras and no adapter was needed to use it, the correction is just 1.62x to get the effective focal length and I think the f number says the same.

If anyone thinks these calcs are wrong , feel free to say so.

[astro744]

True field of view (degrees) = imaging sensor size x 57.3 / focal length of telescope or lens.

If you had a 35mm camera with 36mm x 24mm sensor (film) you would have a true field of 36x57.3/200=10.3deg (long axis) and 24x57.3/200=6.9deg (short axis) for a 200mm f.l. lens. With a 2000mm focal length telescope the figures would be 1.03deg x 0.69deg.

If you use a DSLR with reduced sensor size (scale=1.6) then your sensor is not 36x24mm but 22.5x15mm. Therefore the true field will be 22.5x57.3/200=6.4deg and 15x57.3/200=4.3deg for a 200mm lens and 10x smaller for a 2000mm lens. Note this is the same as multiplying your focal length by 1.6x and using 36x24 (full size sensor) in the calculations.

The 200mm lens wide open (at 18mm mark) would be f3.5 but not at the 200mm mark typically f5.6 or greater. On a 2000mm f10 SCT the f ratio will still be f10 as the focal length and aperture and therefore focal ratio of the system has not changed, only the sensor size is in effect cropped.

Since the invention of compact digital and DSLR cameras without full 35mm film size sensors, camera manufacturers/dealers have introduced a method of comparing sensor size and effective focal lengths to that of 35mm format and whilst this is good for 35mm format it is rather meaningless when you are comparing to non 35mm format.

[Lester]

Thanks Ian and Astro, for your replies, that answer my questions very well. I will read them a number of times to understand it all.

I appreciate it.

[Lester]

I also have a MD lens 50mm f1.7 .

50mm x 2.27=113.5mm
f1.7 x 1.6 = f2.72
Could also be a very usefull astro lens.

[bojan]

Guys....
50mm FL is 50mm, ALWAYS.
Also, F-number is always the same, as it is the ratio between lens FL and lens diameter.
Both those numbers are the property of the lens ONLY.

Please forget about that crop factor nonsense.. which is creeping into our discussions every now and then. Very confusing and misused term, it may have some relevance in everyday photography (and among people who do not have proper understanding of geometrical optics), but in astrophotography it should never be mentioned.

The size of the sensor (in combination with FL of the said objective lens) will determine the field of view for that particular camera-lens combination (and this can be calculated by using simple trigonometric relations), and it will be expressed in degrees (for frame diagonal, or frame hight, or frame width).
The sensor resolution for the same camera-lens combination , on the other hand, will be determined by pixel size and lens FL (again the same trigonometric relations will apply), and this will be expressed in arcsec/pixel or pixel/arrcsec. This of course should not be mixed with optical lens resolution (which depends on lens quality)

The excel file which does some field size calculations (and pixel resolution or scale0 could be found in the following thread:
http://www.iceinspace.com.au/forum/showthread.php?t=50828&highlight=crop+factor

[Lester]

Hi Bojan, I like to keep things simple. So I put the Sigma DC 18-200mm lens on the Canon 20Da and set it at 200mm FL and see a set FOV; then I put the old Tokina 200mm lens onto the 20Da and then the FOV has been reduced by almost 50%. So to my understanding the Tokina is magnifying the image more than the Sigma lens and this result can be equated to an increase in FL.

[Ian Robinson]

Me too, and an older MD 58mm f1.4 as well.

Did some experiental images not long after I bought the 40D with the MD 50mm f1.7 and it has nice wide fov but I did notice some Spherical Aberrations (?) on bright starts in the edges operated wide open.
Seems even more striking with the MD 58mm f1.4. Might be a consequence of the optics in the adapter ?

Been a long time since I used the old XD5 for astroimaging, so I can't remember if I saw aberrations with these lenses on negatives.
I wonder if that is correctable in Photoshop ? (Not got that yet).
Or in Paint Shop ? (Have it).

[bojan]

This only means that the Sigma states "equivalent" FL.
"Equivalent" FL is NOT actual FL of the lens.. It is the focal dlength of the lens that would have the same field of view as the lens mounted on so called "full frame" (24x36mm, Leica format) sensor.

Tokina (being older lens, right?) states ACTUAL FL.
"Simple" does not mean "correct".. but it often means misleading, like in your example.

However, you are not to blame for some manufacturers trying to "simplify" things.. All they managed to do is to introduce and to increase confusion.

Try this "simple" way on, say, Toucam CCD sensor.. which is 3x4mm or something like that, and you will see what I mean..
You can calculate correct results ONLY if you are using actual focal lengths.. not that "equivalent" nonsense.

There is no "magnification" observed here.
What you guys saw when experimenting with lenses was the effect of using two lenses of *different* FL on the same sensor - resulting in different field of view.
What was written on the lens (as FL) is irrelevant. One was 50% shorter ant that's it.

Of course, all this above is valid if no active adapters were used (adapters with lenses in them).
If you used such an adapter, then there was a magnification factor introduced (together with distortions, usually visible in corners of the frame).
But, then, the combination of adapter (with lens) + Lens has different FL.. original FL corrected by factor introduced by active adapter.

I would advise anyone to avoid adapters with lenses used for astro-photography. It is far better to modify the lens (even if it is not reversible) than to distort your images in non-predictable way.

[Lester]

I am beginning to agree that the MD-Eos adapter will not give good results for wide field star images. To reach infinity focus the lens is screwed hard around and to me it doesn't quite go far enough to get a crisp image.

Thanks for your input chaps. All the best.

[bojan]

For -> EOS modification, have a look at what I have done..
It may be helpful to you too.
http://www.iceinspace.com.au/forum/showthread.php?t=39293&highlight=canon+50mm+lens
http://www.iceinspace.com.au/forum/showthread.php?t=51866&highlight=lens+modification

This requires some machining to be done on adapter.. And some adjustment of focus stops on the lens itself (easy and reversible.. they are accessible from the front or after removal of rubber grip ring).

I have no experience with MD lenses, but I am sure it can be done.

[Lester]

Thanks Bojan, I will keep this in mind for the future, if I decide to go further with adapting my lenses.