Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
01-14-2021, 04:50 AM   #256
Site Supporter
Site Supporter




Join Date: Dec 2012
Photos: Albums
Posts: 1,970
QuoteOriginally posted by Jonathan Mac Quote
On a DSLR these wouldn't work as the viewfinder would be so dark it'd be unusable for auto or manual focus.
I was just looking at 15mm LTD shots for diffraction spikes, for a discussion in another thread. Many of those were shot at f/11 or smaller specifically to get starbursts and with enough ambient light autofocus worked just fine.


But I've always been confused by people who say things like "the autofocus points only are sensitive at f/2.8 or wider" or similar. For a while I didn't even have a lens that was f/2.8 or larger and autofocus worked. I probably don't understand the context.

01-14-2021, 05:34 AM   #257
Loyal Site Supporter
Loyal Site Supporter
KiloHotelphoto's Avatar

Join Date: Jan 2015
Location: Glen Mills, PA
Photos: Albums
Posts: 848
QuoteOriginally posted by ThorSanchez Quote
I was just looking at 15mm LTD shots for diffraction spikes, for a discussion in another thread. Many of those were shot at f/11 or smaller specifically to get starbursts and with enough ambient light autofocus worked just fine.
Auto focus work fine because it's a F4 lens and the body auto focuses at the max aperture before stepping down when you take the shot.
01-14-2021, 05:41 AM   #258
Pentaxian




Join Date: Dec 2011
Posts: 2,869
QuoteOriginally posted by ThorSanchez Quote
But I've always been confused by people who say things like "the autofocus points only are sensitive at f/2.8 or wider" or similar. For a while I didn't even have a lens that was f/2.8 or larger and autofocus worked. I probably don't understand the context.
Think of it as old fashioned microprisms for manual focus. AF sensors exactly like microprisms are looking through the outer edges of the mount opening / lens (left/right for horizintal line sensors; top/bottom for vertical ones) to see two differing images.

The position where they actually look through is described as "Fxxx sensitive" meaning that if you have a slower lens the sensors/microprisms will simply stare at black material and see nothing if the physical aperture opening is too slow = the light is coming in through such a small hole in the middle of the lens that the outward area is not glass.
Just as the wide open aperture figures describe a bigger and bigger opening for light this in parallel widens the area how far outward the sensors can peek through the mount opening / lens.


Current Pentax cameras have most sensors peek through F5.6 equivalent positions and 3 sensors peeking through areas so far outward that it is equivalent to a F2.8 aperture.


Since the sensors are relatively long (and parts of them still see partial images) the F5.6 ones actually tend to work somewhat (the get more and more problems) down until F9.
Attached Images
 
01-14-2021, 05:50 AM - 1 Like   #259
Pentaxian
bdery's Avatar

Join Date: Mar 2008
Location: Quebec city, Canada
Photos: Gallery
Posts: 7,963
QuoteOriginally posted by normhead Quote
They have no workable plan to replace customers such as myself.
That's what worries me. Not the quality of the products, we all understand how good they can be, but how to appeal to new customers.

In 2021, the short of it is, they need every influencer out there posting a blog post or youtube video claiming "Pentax is BACK" or something like that in bold lettering...

01-15-2021, 08:15 AM   #260
Pentaxian
Jonathan Mac's Avatar

Join Date: Apr 2009
Location: Madrid, Spain
Posts: 7,350
QuoteOriginally posted by ThorSanchez Quote
I was just looking at 15mm LTD shots for diffraction spikes, for a discussion in another thread. Many of those were shot at f/11 or smaller specifically to get starbursts and with enough ambient light autofocus worked just fine.

But I've always been confused by people who say things like "the autofocus points only are sensitive at f/2.8 or wider" or similar. For a while I didn't even have a lens that was f/2.8 or larger and autofocus worked. I probably don't understand the context.
Yes, because normal lenses focus with the aperture wide open and stop down only for the actual photo, so the viewfinder and the AF system get the benefit of whatever the maximum aperture is.

These Canon lenses are f/11 with a constant aperture, so it cannot be changed either way. It's always f/11.
01-15-2021, 08:37 AM   #261
Pentaxian
normhead's Avatar

Join Date: Jun 2007
Location: Near Algonquin Park
Photos: Gallery | Albums
Posts: 37,238
Also, the smaller the aperture, the wider the DoF will be, making it more difficult for the camera to decide which of the points in acceptable focus is where you want your focal plane. The focal plane could be moved forward and backward within the wider DoF. You won't know what it's done until you see it in on a monitor or TV. Hence the preference or using Live View for manual focus. In Live View with focus assist you actually watch the DoF shift as you manually focus, as opposed to letting the camera decide where it thinks you should focus.

In images like this...


I use Live View to determine the front and back of the depth of field, because the front and back edges of the DoF is more important than selecting the exact position of the focal plane. This is something that s far as I know, AF cannot be programmed to emulate.

Using an f4 lens stopped down for landscape with everything from 10 feet to infinity in focus, you have a lot of leeway with the exact technical focal plane is. Most of the time, it's not an issue. When your actual focus is 30 feet to infinity, and your lens can do 10 feet to infinity, you really have to mess up to have part of your image out of focus, but it can be done. I've had it happen. If precision is required, AF is always a crap shoot.

Last edited by normhead; 01-15-2021 at 08:53 AM.
01-25-2021, 07:41 AM   #262
Lev
Pentaxian




Join Date: Aug 2015
Location: Tbilisi, Georgia
Posts: 677
Original Poster
QuoteOriginally posted by que es tu Quote
CROMULENT!!! Wow! I had to look it up, but I like it!! I'll have to work that into conversations!! Not only do I learn photography on PF, but also English!!

---------- Post added 01-09-21 at 03:28 PM ----------



Whoa!! Butt photos!!?? Is this safe for work??
I think that butt photos should be at the first position of category list in photograhy :d

---------- Post added 01-25-21 at 07:44 AM ----------

QuoteOriginally posted by normhead Quote
Also, the smaller the aperture, the wider the DoF will be, making it more difficult for the camera to decide which of the points in acceptable focus is where you want your focal plane. The focal plane could be moved forward and backward within the wider DoF. You won't know what it's done until you see it in on a monitor or TV. Hence the preference or using Live View for manual focus. In Live View with focus assist you actually watch the DoF shift as you manually focus, as opposed to letting the camera decide where it thinks you should focus.

In images like this...


I use Live View to determine the front and back of the depth of field, because the front and back edges of the DoF is more important than selecting the exact position of the focal plane. This is something that s far as I know, AF cannot be programmed to emulate.

Using an f4 lens stopped down for landscape with everything from 10 feet to infinity in focus, you have a lot of leeway with the exact technical focal plane is. Most of the time, it's not an issue. When your actual focus is 30 feet to infinity, and your lens can do 10 feet to infinity, you really have to mess up to have part of your image out of focus, but it can be done. I've had it happen. If precision is required, AF is always a crap shoot.
true!! I always try to use live view when I go wider than 2.8 with focus assist
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!
aps-c, beginner, body, brand, brands, butt, camera, cameras, card, criticism, dslr, files, images, k-50, k-70, kp, lens, market, mirrorless, pentax, people, phone, photography, photos, stock, time, users
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Nature Not beautiful,not pretty, but it seems interesting and appealing. Tonytee Post Your Photos! 11 05-07-2020 09:31 PM
My blog about Baby Steps into Photography : Beginners Photography anu l Pentax SLR Lens Discussion 12 01-17-2016 10:21 AM
10 Useful Digital Photography Tutorials for Beginners (& not so beginners) interested_observer General Photography 2 04-24-2014 07:50 AM
Like it or not, the K-01 is appealing! Adam Pentax K-01 65 02-07-2012 08:47 AM
Casey Anthony Appealing Conviction For Lying To Police jogiba General Talk 28 08-03-2011 07:24 AM



All times are GMT -7. The time now is 03:42 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