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05-09-2010, 12:49 PM   #1
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27 year old Cosinon prime vs Sigma zoom

Invested in a K200D last year as my first DSLR partly because I have a 27 year old Cosina 200mm F4 PK mount prime lens, and wanted to be able to use it with my new camera. Having tried it with the obvious manual focus and aperture settings, was thinking about getting a zoom lens where everything would be automatic.

I'm aware that a 200mm lens designed for 35mm will actually give about the equivalent of 300mm on my camera due to sensor size. So does it therefore follow that a 70-300mm zoom will give me the same or better magnification?

I was looking at the Sigma 70-300mm APO DG, although I note that the magnification is quoted as 1:2 whereas the Cosina lens I have is stamped 1:4 - does this mean therefore that the magnification on the Cosina lens is greater?

05-09-2010, 01:49 PM   #2
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Actually the Sigma lens should give you both longer reach and greater magnification. But probably not the same image quality as a prime - even an old prime like this is probably sharper than the budget zoom from Sigma.
05-09-2010, 02:02 PM   #3
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A 200mm lens is a 200mm lens. The picture that it renders can change because of the camera that it it mounted on. Full Frame, 35mm film, will show a different ( wider ) image than a crop sensor, but is still a 200mm lens. A 300mm lens will show more magnification than a 200mm lens.
05-09-2010, 04:22 PM   #4
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QuoteOriginally posted by cozzyleicester Quote
Invested in a K200D last year as my first DSLR partly because I have a 27 year old Cosina 200mm F4 PK mount prime lens, and wanted to be able to use it with my new camera. Having tried it with the obvious manual focus and aperture settings, was thinking about getting a zoom lens where everything would be automatic.

I'm aware that a 200mm lens designed for 35mm will actually give about the equivalent of 300mm on my camera due to sensor size. So does it therefore follow that a 70-300mm zoom will give me the same or better magnification?

I was looking at the Sigma 70-300mm APO DG, although I note that the magnification is quoted as 1:2 whereas the Cosina lens I have is stamped 1:4 - does this mean therefore that the magnification on the Cosina lens is greater?
The "equivalent" focal length is compard to what you and I would call 35mm film cameras. The digital APS-C sensor is a smaller (cropped) version of that 24x36mm film frame-size. That is where the term "cropped" camera comes from. Many may argue the use of that word in that context, because some people never used the old stuff and therefore will have no basis of reference.

Regardless, a 70-300mm is still 70-300mm, and 300 is always longer than 200. That Sigma gives results that look like a 105-450mm lens on your film camera because of the smaller sensor (essentially, a smaller portion of the image circle is used and magnified, therefore it appears you are closer).

I think you are confusing some numbers on the magnification part, though. The maximum f-stop on your 200mm is f4 (also written 1:4). The f-stop on the Sigma is variable from f4-5.6. but the maximum magnification is 1:2 (which is pretty close-up).

Also, I had that Sigma. It is OK, but the image quality improves a lot when you increase the contrast, especially at the long end. Alas, that lens suffered a loose aperture blade (unknown to me until I wanted to sell it), and once I got the significantly better 55-300 Pentax, I never used it again.

05-10-2010, 05:34 AM   #5
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Many thanks everyone for your feedback - I think I will perservere with the prime lens a bit longer before looking at the zoom
05-10-2010, 07:13 AM   #6
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QuoteOriginally posted by cozzyleicester Quote
I was looking at the Sigma 70-300mm APO DG,
I have this lens. It works well for me but don't go much over 200mm or it gets a bit soft.

It's ideal for getting wild flower shots and other stuff at your feet while walking and you don't want to carry a tripod and use a true macro lens with all that hassle.

It kind of big and clumsy.

BTW a simple practical way of looking at focal length on a Pentax DSLR is to think in terms of magnification thus:

For every 100mm of FL you get 3x magnification.

So for instance the Sigma at 70mm gives 2.1X mag and at 200 6x mag and at 300mm 9x mag. FL determines magnification and it doesn't matter who or when it was made.

Taken with this lens at 190mm, 1/400 sec, ISO 800, F4.5 -

Last edited by wildman; 05-15-2010 at 06:59 AM.
05-11-2010, 05:24 AM   #7
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Great photo wildman

Thought it would be useful to explain that I mostly use the Cosina 200mm prime for aviation photography, where you are necessarily a long distance from the subject. I assumed that a 300mm zoom would be ideal for the purpose, is that the case or is it optimised for the close up work you describe?

rgds
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05-11-2010, 09:18 AM   #8
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QuoteOriginally posted by cozzyleicester Quote
I assumed that a 300mm zoom would be ideal for the purpose, is that the case or is it optimised for the close up work you describe?

rgds
Haydn
It works fine as a versatile all-around general purpose lens. It a very good value for the money and works perfectly on my K20.

I just went out on the porch and took these quick and dirty. No special PP. No more than about 1/4 mile distance. Focused on chimney in center of full frame.

1. 35mm - close to naked eye distance with FA 35mm for reference.
2. Sigma at 200mm
3 Sigma at 300mm

Perhaps this will help.


Last edited by wildman; 05-15-2010 at 07:00 AM.
05-11-2010, 10:20 AM   #9
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QuoteQuote:

I assumed that a 300mm zoom would be ideal for the purpose, is that the case or is it optimised for the close up work you describe?

rgds
Haydn

Zooms that have a "macro" capability are not optimized for the close-up shots. True macro lenses range about $450-1,000 and are corrected for a flat field, among other things.
05-11-2010, 06:21 PM   #10
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QuoteOriginally posted by SpecialK Quote
Zooms that have a "macro" capability are not optimized for the close-up shots. True macro lenses range about $450-1,000 and are corrected for a flat field, among other things.
Yep.

I think a lot of people confuse macro with close up. They think they want a macro lens when really all they want is to take close ups.

I have the Sigma 105mm macro and more often then not all I really need is something like the zoom we have been talking about. At 200mm I can stand upright and hand hold and just get a few nice flower pics at a decent distance without the full macro hassle. My FA 35 can get pretty close also but I have to stoop way down to get close enough and it's just more convenient to use the Sigma zoom at 200mm.

Whatever the Sigma's merits it's certainly worth 200 bucks.

Shots like this:

Last edited by wildman; 05-15-2010 at 06:59 AM.
05-12-2010, 05:15 AM   #11
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Once again, many thanks all for the really useful feedback!
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