Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
06-28-2020, 08:03 PM   #31
GUB
Loyal Site Supporter
Loyal Site Supporter
GUB's Avatar

Join Date: Aug 2012
Location: Wanganui
Photos: Gallery | Albums
Posts: 5,735
QuoteOriginally posted by Alex645 Quote
I would want to know that before spending nearly $2k for an f/1.4 that was really giving me f/1.8.
Thing is if I was choosing to buy this lens for 2k in all likelihood it would be for the f1.4 of dof rather than the t1.8 of speed. ie it will still have a f1.4 dof.
If we went to t stops the dof performance would become unclear.

06-28-2020, 08:06 PM   #32
Site Supporter
Site Supporter
Alex645's Avatar

Join Date: May 2015
Location: Kaneohe, HI
Photos: Albums
Posts: 3,526
QuoteOriginally posted by swanlefitte Quote
Even the tstop is hard to quantify, at least a dxomark. Take the da 70mm f/2.4. Is it a T stop of 2.9 as it is here on the k5iis? Or is it 2.7 as on the k3? Pentax smc DA 70mm F2.4 Limited - DxOMark
��
I am not sure why DXO gets different values other than the K3 has a smaller sensor than the K5IIs and thus there is more light gathered in a smaller area? I'd have to research how they come up with their T values.

Last edited by Alex645; 06-28-2020 at 09:29 PM.
08-23-2020, 05:39 PM - 1 Like   #33
Site Supporter
Site Supporter
zkarj's Avatar

Join Date: Jan 2009
Location: Wellington
Posts: 1,287
I just stumbled across this anomaly while working out some maths to calculate depth of field tables. It's just plain weird. I plotted out all the whole (green), half (yellow), and third (white) stops from 1.4 through 11, as shown in the image below.

If we accept "the rule" as being "round to two significant figures (so no decimal from 10 up) then there are four incorrect values, and two "borrowed' values where a third and a half stop share the same number. Mostly it centres around f/4, where going one step up or down, whether thirds or halves are selected, will get you 0.5 more or less and in three out of four cases that's wrong. The 5.6 one is the only other and the only whole stop that does not match.

Where it gets even more interesting is if we look at Wikipedia's "typical" scales (which I am guessing come from the Canikon world?) which also have "errors". Namely, where Pentax rounds the half-stop 3.364 up to 3.5, the "typical" rounds it down to 3.3, neither using the more "correct" 3.4, and also when matching Pentax's "errors" in rounding 3.564 to 3.5 and 5.657 to 5.6.
Attached Images
   
08-23-2020, 08:19 PM - 1 Like   #34
Otis Memorial Pentaxian
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 42,007
QuoteOriginally posted by zkarj Quote
Where it gets even more interesting is if we look at Wikipedia's "typical" scales (which I am guessing come from the Canikon world?)
Nah...older. The confusion on this forum is at least partially traceable to the numbers displayed by the camera along with assumptions regarding half-stop values and maximum aperture values on certain Asahi/Pentax lenses. For example, the SMC Takumar 28/3.5 has an unmarked click between 3.5 and 5.6. Is this a 1/3 stop step to f/4, a half-stop step to f/4, or a 2/3 stop step to f/4.5.

While I could not find the a reference for the SMC Taks, I did find that for the SMC K-series lens brochure (LINK) where there is a table that explicitly places both f/2.5 and f/3.5 as one-third stop steps in agreement with the Wikipedia 1/3 stop increment table. (Ditto for the M-series lens brochure.)


Steve

(...ignoring that the promotional pamphlet for the Super Program places f/2.5 and f/3.5 as half-stop values...)


Last edited by stevebrot; 08-23-2020 at 08:37 PM.
08-26-2020, 04:40 PM - 1 Like   #35
Forum Member




Join Date: Jun 2020
Location: Austin
Posts: 74
QuoteOriginally posted by Alex645 Quote
So if I under this, it's really f/5.7, f/11.4 which is rounded down to f/11 and f/22.8 rounded up to f/23? At least the f/64 group is still legit.

I learned T-stops back in the late 70's and given all the advances in technology since then, I am surprised we are still using f/stops instead of T-stops.
I'd much rather it be labelled by f-stop for predictable depth of field. Having to take the shutter speed down a click is pretty much never going to ruin your image, and you can raise the ISO if need be.

I'd still love for manufacturers to publish t-stops in their specs, but unless it's at or better than the competition it doesn't really benefit them from a marketing standpoint - and often the pursuit of better image quality necessarily comes at the cost of t-stop performance.
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!
aperture, cameras, data, f/5.6, k-mount, lens, pentax, pentax lens, results, slr lens, value, values
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
To step up or not? And how far to step. Gorgarath Pentax SLR Lens Discussion 13 02-01-2020 08:23 PM
Underexposed, underdeveloped, or incorrectly scanned? Ediz7531 Film SLRs and Compact Film Cameras 28 02-14-2019 05:20 PM
Flaunt your "Miida"-labeled lenses or other photographic equipment! goatsNdonkey Pentax SLR Lens Discussion 6 11-29-2016 05:56 PM
DA* 200 and 300 are formally labeled "Not compatible with full-frame" grahame Pentax SLR Lens Discussion 22 02-22-2015 10:57 AM



All times are GMT -7. The time now is 11:02 AM. | 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