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02-12-2008, 02:10 PM   #1
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1.5 conversion newb question

Hi all,

I am sure this has been explained before, but i could not find an answer through the search, so here goes.
I understand that older lenses, when used on the digital bodies like K10D have a 1.5 conversion since the smaller APS-C sensor only captures a portion of the lens' capacity.
But, when using the new lenses, like the DA series, being designed for the new sensor, do we still need to do the conversion when applied to digital body? like the 16-45mm really what it is?

just confused since a nikon buddy brought it up on me . . .

thanks for answering a confused noob

never mind, i found the thread, but i am still confused


Last edited by pixel r0n1n; 02-12-2008 at 02:37 PM. Reason: found explanation on correct thread
02-12-2008, 02:38 PM   #2
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Well, a lens is a lens is a lens, so the 16-45mm is that focal length on whatever camera you attach it to. However, the FoV will be different on a 35mm film camera to what it is on a 1.5x cropped sensor digital camera. The FoV the 16-45mm will give you on a Pentax digital camera is about the same FoV as a 24.5-68.9mm lens would give you on a 35mm film camera. The Pentax sensors have a cropped conversion factor of 1.53.

I hope that answers your question!
02-12-2008, 06:38 PM   #3
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QuoteOriginally posted by pixel r0n1n Quote
like the 16-45mm really what it is?
Hi pixel r0n1n,

please please do not take this question personally in whatever way. You asked a great question that many have asked already before.

However, this is why I have a question on my own: May it be that elementary optics is not taught in schools anymore? Or has everybody only forgotten since?

EDIT:
Or, third possibility, may it be that people think that manufacturers print something else than the focal length onto the lens?
02-13-2008, 04:41 AM   #4
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QuoteOriginally posted by falconeye Quote
Hi pixel r0n1n,

please please do not take this question personally in whatever way. You asked a great question that many have asked already before.

However, this is why I have a question on my own: May it be that elementary optics is not taught in schools anymore? Or has everybody only forgotten since?
Well, having been out of school now for 15 years or so I can tell you, that at least here in my part the US, focal length on optics was never disscussed. Maybe a few paragraphs at best.

02-14-2008, 12:03 PM   #5
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pixel r0n1n, if you're still reading, check out this explanation; hopefully it will clear things up for you.
02-20-2008, 11:19 PM   #6
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Further Questions on Focal Length

OK, so lets say I have three lenses: DA18-55, A80-200, 50mm

The DA18-55 is the lens that came with the K100D. The A80-200 and 50mm are lenses I picked up that are old film lenses. When I use the 80-200 or the 50 and try to use the shake reduction I am questioned about the focal length of the lens. Do I calculate the focal length of the old lenses as x1.5 the focal length or enter them with the listed values on the lens?

I guess what I am understanding is that focal length is focal length no matter if it is a 30 year old 35mm film body lens on a modern digital body. Is that correct? If that is right the 1.5 measurement only relates to what I would see in comparable prints from 35mm vs the digital body. Do I have this concept correct. Sorry to ask stupid questions. I'm new to anything beyond point-and-shoot 35mm and digital cameras.
02-20-2008, 11:30 PM   #7
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QuoteOriginally posted by brobers Quote
OK, so lets say I have three lenses: DA18-55, A80-200, 50mm

The DA18-55 is the lens that came with the K100D. The A80-200 and 50mm are lenses I picked up that are old film lenses. When I use the 80-200 or the 50 and try to use the shake reduction I am questioned about the focal length of the lens. Do I calculate the focal length of the old lenses as x1.5 the focal length or enter them with the listed values on the lens?

I guess what I am understanding is that focal length is focal length no matter if it is a 30 year old 35mm film body lens on a modern digital body. Is that correct? If that is right the 1.5 measurement only relates to what I would see in comparable prints from 35mm vs the digital body. Do I have this concept correct. Sorry to ask stupid questions. I'm new to anything beyond point-and-shoot 35mm and digital cameras.
Yes, you are correct. Your 50mm lens will always be a 50mm lens, no matter where you mount it. The difference is in the size of the medium you use to project the image on (35mm film, APS-C size CCD, etc). Your field of vie (FoV) will be different depending on the size of the capturing medium, but your focal length will be the same.
02-21-2008, 12:45 AM   #8
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Thanks that clears up some misconceptions I had on the matter.

02-21-2008, 01:20 AM   #9
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QuoteOriginally posted by falconeye Quote
Or, third possibility, may it be that people think that manufacturers print something else than the focal length onto the lens?
You bet!!
On my KM prosumer, the zoom scale marking is 28, 35, 50, 100, 200mm!
You have to look at the front of the lens to find the true focal length of 7.2-50.8mm.

So I am not surprised at all that many people moving from P&S or Prosumer to DSLR are totally confused by all the focal length and crop factor conversion.
02-21-2008, 01:27 AM   #10
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QuoteOriginally posted by nosnoop Quote
So I am not surprised at all that many people moving from P&S or Prosumer to DSLR are totally confused by all the focal length and crop factor conversion.
Usually though, P&S's will say 28-70mm (or whatever it happens to be) equivalent. Any interchangeable lens I'm aware of though would be labeled in the focal length (which deals with the distance certain parts of the lens will be from the focal plane, to focus to infinity - yea, that's oversimplified - but it's the general idea).
02-21-2008, 07:11 AM   #11
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QuoteOriginally posted by falconeye Quote
Hi pixel r0n1n,

please please do not take this question personally in whatever way. You asked a great question that many have asked already before.

However, this is why I have a question on my own: May it be that elementary optics is not taught in schools anymore? Or has everybody only forgotten since?

EDIT:
Or, third possibility, may it be that people think that manufacturers print something else than the focal length onto the lens?

optics are very briefly covered in physics class, if you are lucky and your teacher likes optics and decides to devote some time to it, since no teacher in my experience has ever finished teaching a full science/physics textbook within a single semester, and even then, they teach general optics and light bending, focal length is taught but not in relationship to film

and few students in north american highschools elect to take photography vs drama/music/art classes

when i was in highschool my friend was in the photography class, her and 5 other people, i wasnt even into it back then.
02-21-2008, 07:12 AM   #12
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QuoteOriginally posted by nosnoop Quote
You bet!!
On my KM prosumer, the zoom scale marking is 28, 35, 50, 100, 200mm!
You have to look at the front of the lens to find the true focal length of 7.2-50.8mm.

So I am not surprised at all that many people moving from P&S or Prosumer to DSLR are totally confused by all the focal length and crop factor conversion.
You also have to remember that until digital P&S, the majoprity of all P&S cameras used 35mm film, so the prosumer market, or what ever, was accustomed to seeing focal lenghts of 35mm cameras also, but were not expected to be the techno geeks than SLR photographers tend to be.

So it is not unrealistic for them to quote the 35mm equivelent. Also note that there is a vast difference in the prosumer market in sensors and sizes, and therefore, to make it easier when purchasing, it is better to quote the 35mm equivelent. I don't have an issue with that. In fact, when I read some camera reviews or buyers guides, I find that quoting true focal lentghts of the lenses, and not quoting the sensor dimensions makes it really frustrating in picking a camera.
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