Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
02-06-2014, 07:02 PM   #151
Pentaxian




Join Date: Jul 2009
Location: Seattle
Posts: 7,611
QuoteOriginally posted by Cannikin Quote
Because for all intents and purposes...
Great, tell that to your engineering professor when he marks your answer wrong when you're asked to calculate a reaction force with am answer in kilograms.

02-06-2014, 07:04 PM   #152
Pentaxian




Join Date: Mar 2010
Location: SW Washington
Posts: 831
QuoteOriginally posted by tuco Quote
Great, tell that to your engineering professor when marks your answer wrong when you're asked to calculate a reaction force with am answer in kilograms.
Did you even read the first sentence?

Maybe your engineering professor introduced you to this formula: http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation, you know the one that explicitly defines a directly proportional relationship of mass to gravitational force, a.k.a. weight?

Last edited by Cannikin; 02-06-2014 at 07:15 PM.
02-06-2014, 07:06 PM   #153
Veteran Member
Wired's Avatar

Join Date: Aug 2009
Location: Edmonton, AB
Photos: Gallery | Albums
Posts: 2,526
A D800 with a Tokina 16-28mm f2.8 ATX Pro is 2 kilos. That's also a pretty solid landscape kit. I bet the 645 kit is heavier, but still the FF option isn't exactly light weight.
02-06-2014, 07:23 PM   #154
Pentaxian




Join Date: Jul 2009
Location: Seattle
Posts: 7,611
QuoteOriginally posted by Cannikin Quote
Did you even read the first sentence?
Yep. And what fudge factor do you use to make the units of kilogram pop out of this equation when you calculate a force with a 1 kilogram mass?

F = (mass) x (9.81 m/s^2)

02-06-2014, 07:29 PM   #155
Pentaxian
Pål Jensen's Avatar

Join Date: Dec 2007
Location: Norway
Photos: Albums
Posts: 3,317
QuoteOriginally posted by Wired Quote
A D800 with a Tokina 16-28mm f2.8 ATX Pro is 2 kilos. That's also a pretty solid landscape kit. I bet the 645 kit is heavier, but still the FF option isn't exactly light weight.
That is very true but a Pentax 645D with the 33-55 weigths about the same. That is partly what makes it a compelling alternative....
And the 75/2.8 weights only 215g!
02-06-2014, 07:54 PM   #156
Pentaxian




Join Date: Mar 2010
Location: SW Washington
Posts: 831
QuoteOriginally posted by tuco Quote
Yep. And what fudge factor do you use to make the units of kilogram pop out of this equation when you calculate a force with a 1 kilogram mass?

F = (mass) x (9.81 m/s^2)
Are you hoping to impress someone with your knowledge of elementary physics? Or do you realize that your own reasoning of giving weight as "F = m(9.81m/s^2)" is itself a mathematical shortcut born out of an observation of a real world relationship, and not a proper definition, just like the relationship between mass and weight?

How about this: An object at rest on the Earth's surface has no acceleration relative to it (ignoring the Earth's rotation). F = ma = m(0) = 0? Whoops.

Your "9.81m/s^2" figure comes from an observation that objects of negligible mass compared to the Earth will appear to accelerate towards the Earth without the application of another external force (i.e. freefall without air resistance), from the frame of reference of the Earth, at approximately 9.8 m/s^2 near the surface. This is not the "proper" way to determine force of gravity, which is given by Newton's law of universal gravitation (unless you want to get into Einstein's General Relativity). Once you have that figure, you can work out the acceleration from the definition of the unit "newton": a = f/m, giving you your shortcut of F = m(g).

Now you can accept that there are mathematical relationships between units of measurement for which a distinction is not meaningingful a difference in everyday non-engineering applications, or you can go around quoting everything (and converting everything) in "newtons", "pound-force", or whatever, and see whether it makes any meaningful difference in everyday life, or if it is just adding another unnecessary calculation to an otherwise relatively consistent and straightforward relationship.

Last edited by Cannikin; 02-06-2014 at 08:41 PM.
02-06-2014, 07:55 PM   #157
Banned




Join Date: May 2010
Location: Back to my Walkabout Creek
Photos: Gallery | Albums
Posts: 2,541
QuoteOriginally posted by mee Quote
AW as a matter of fact, provided you keep it indoors.
Yep. I need to digitise some 300 slides using new Pentax duplicator, during the next storm, on top of the lighthouse at -15C.
WR would be wonderful to have.
02-06-2014, 08:30 PM   #158
Pentaxian




Join Date: Jul 2009
Location: Seattle
Posts: 7,611
QuoteOriginally posted by Cannikin Quote
Are you hoping to impress someone with your "great knowledge" of elementary physics?
Nope. It is a sincere question. We have a fudge factor here in the US for pounds-force/pounds-mass to make the units work. Engineers over in metric countries must deal with the ambiguity all the time of what is mass for calculations vs weight/force when everything is called kilograms for both.

02-06-2014, 08:54 PM   #159
Pentaxian




Join Date: Mar 2010
Location: SW Washington
Posts: 831
QuoteOriginally posted by tuco Quote
Nope. It is a sincere question. We have a fudge factor here in the US for pounds-force/pounds-mass to make the units work. Engineers over in metric countries must deal with the ambiguity all the time of what is mass for calculations vs weight/force when everything is called kilograms for both.
If this was an honest question, then I apologize for being a bit snarky.

It all depends on application, whether it makes any real difference. Ultimately it is a matter of accepted practice in each field.

- For everyday life, there is no meaningful difference between mass and weight, as people live near the surface of the Earth and deal with objects of mass very small compared to the Earth. When people refer to weight in kilograms, they implicitly mean kilogram-force, a non-SI unit, just like in the US people rarely say "pound-force". The relationship is more or less constant, and so it is easier to conceptualize if everything is given in the same units, rather than having to do a conversion calculation (e.g. it is not really easy to visualize how much a force really is, until you compare that force to the weight of an object of certain mass that you can visualize).

- In certain engineering applications, when the primary force to counteract is gravitation, some engineers will still rely on the mass-weight relationship shortcut.

- In engineering where dynamic forces other than gravity come into play, then it really makes a difference. Often this involves machinery and other movement, such as in aerospace engineering, fluid mechanics/dynamics, etc. with many different forces in different directions. At that point it is important to work with clearly defined force units and vectors, like newtons.

Last edited by Cannikin; 02-06-2014 at 09:10 PM.
02-06-2014, 09:19 PM   #160
Pentaxian




Join Date: Jul 2009
Location: Seattle
Posts: 7,611
QuoteOriginally posted by Cannikin Quote
I
...
At that point it is important to work with clearly defined force units and vectors, like newtons.
in other words, if we all just used the SI metric system. Ironically, the old-school hold over of kilograms-force being used instead of Newtons as to not confuse the general public has created confusion. How many people grow up hearing the abbreviated kilograms instead of kilograms-focre and never end up knowing there is a difference when you have to get serious. I think John Q Public can handle it. He has no problem with units such as the Watt. And just like the time scale GMT that has been obsolete for over 40 years ever since the second has been defined atomically instead of astronomically.

Last edited by tuco; 02-06-2014 at 09:26 PM.
02-06-2014, 09:38 PM   #161
bvg
Senior Member




Join Date: Apr 2012
Location: Queensland, Australia
Photos: Albums
Posts: 272
QuoteOriginally posted by tuco Quote
Engineers over in metric countries must deal with the ambiguity all the time of what is mass for calculations vs weight/force when everything is called kilograms for both.
Yes, if they are looking for problems. To make live easier they need to use so called consistent units system. Moreover, "metric countries" have a number of consistent units systems, not only SI. But people use such strange units as minutes, hours, years, cups, tea spoons etc.

And btw, other planets have completely different values of the coefficient g!
02-06-2014, 09:46 PM   #162
Pentaxian




Join Date: Mar 2010
Location: SW Washington
Posts: 831
QuoteOriginally posted by tuco Quote
I think John Q Public can handle it. He has no problem with units such as the Watt.
I wouldn't be so sure about that. People think of units in terms of their understanding of everyday things they grew up with. Even now, most people think of "watts" as in "how many watts these appliances use" and not "1 watt = the rate of work done in applying a force of 1 newton over a distance of 1 meter in 1 second". Or how bright CFLs are by "incandescent light bulb equivalent wattage" instead of lumens.

It is better to teach people units of mass first, because that does not change for a given object no matter where you take it, unlike weight. You tell a kid to pick up a liter of water and tell him "You feel that? This is 1kg of water". Afterwards, if it makes a difference in his field, he will be educated on the difference between mass and weight. Otherwise it is good enough to say "If I put two 500g lenses in my bag, it will weigh the same as if I put a 1kg lens in my bag".

Last edited by Cannikin; 02-06-2014 at 10:50 PM.
02-06-2014, 11:04 PM   #163
Pentaxian
Zygonyx's Avatar

Join Date: Jun 2011
Location: Ile de France
Posts: 3,244
QuoteOriginally posted by lesmore49 Quote
How do you know this ? The camera is not available yet ?

I generally used my Mamiya medium format with tripod...but I also used it hand held with out difficulty. I had a small level affixed to it so I could quickly ensure things were level.
I own current 645D : I doubt you can take neat shots with this 50 Mpix sensor handheld using live view

Last edited by Zygonyx; 02-06-2014 at 11:11 PM.
02-07-2014, 01:55 AM - 1 Like   #164
Veteran Member
philbaum's Avatar

Join Date: Oct 2007
Location: Port Townsend, Washington State, USA
Posts: 3,659
QuoteOriginally posted by RonHendriks1966 Quote
Looks great, but expensive.

How about that tilting screen. That's a real photo-beginners-item I guess.
Tilting screens are useful for anyone searching for a new perspective. I own 2 tilting screen cameras: the Sony Nex 5n and Nex 6. Those tilting screens are incredibly sturdy and handy. If not, Sony would be dealing with a ton of them in their warranty repairs. Over the last 2 years, i've yet to see ANYONE mention a broken tiltable screen on Sony forums for dpreview.

Another thing you don't see, is owners of tilting screen cameras saying: gee, i wish my camera didn't have one There is a great fear out there about the "unknown" experience of tilting screen - i have no idea why.

I recently bought a K3, but it was despite the camera not having a tilting screen. Sony puts them on several if not all of their FF cameras.
02-07-2014, 02:00 AM   #165
Pentaxian




Join Date: Oct 2010
Photos: Gallery | Albums
Posts: 3,406
Tilting screen. If I had money to buy this new 645, it would not be a problem for me. you can always choose to not use it, if necessary It can be handy too.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
50mp sony cmos, 645d, ability, advantage, bit, blackmagic, camera, canon, cards, cmos, crop factor, fujitsu, heat, image, k3, lens, macro, market, max, medium, medium-format, mf, minutes, model, mp, name, pentax, pentax 645d2014, pentax news, pentax rumors, phase, products, sensor, size, slr, sony, status, texas, video
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Phase One announces IQ250 50MP CMOS medium format back with Sony sensor jogiba Non-Pentax Cameras: Canon, Nikon, etc. 8 03-04-2014 12:30 PM
Hasselblad H5D-50c: 50Mpix CMOS sensor from Sony -soon in Pentax 645D II? Mistral75 Non-Pentax Cameras: Canon, Nikon, etc. 8 01-22-2014 05:42 AM
Sony SLT-A58 [20.1 Exmor HD CMOS] (??Maybe K3 Sensor??) joe.penn Non-Pentax Cameras: Canon, Nikon, etc. 11 03-07-2013 01:04 PM
Do the new Sony CMOS sensors attract dirt? GeneV Pentax DSLR Discussion 10 08-31-2012 09:26 AM
New Sony APS-C size CMOS Image Sensor with 12.47 Effective Resolution for use in DSLR Matjazz Pentax DSLR Discussion 4 08-20-2007 02:42 PM



All times are GMT -7. The time now is 03:56 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top