Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
03-21-2013, 04:21 PM - 1 Like   #16
Veteran Member
tromboads's Avatar

Join Date: Apr 2012
Location: Sydney
Photos: Gallery
Posts: 1,193
wait wait wait wait wait wait.

as pointed out by another, this hood is simply changing the f stop, ala stopping down the lens.

Of course its going to be a shed load clearer. 50/28 is f1.78.

I call Shenanigans.

All this magic can be yours too just be moving the aperture ring...

03-21-2013, 04:29 PM   #17
Veteran Member




Join Date: Feb 2009
Location: Alameda, CA
Posts: 3,206
QuoteOriginally posted by tromboads Quote
as pointed out by another,
Another? Another what? Who?

QuoteOriginally posted by tromboads Quote
this hood is simply changing the f stop, ala stopping down the lens.
????

I'm confused. Care to elaborate?
03-21-2013, 04:41 PM - 1 Like   #18
Veteran Member
tromboads's Avatar

Join Date: Apr 2012
Location: Sydney
Photos: Gallery
Posts: 1,193
over here.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/219144-pentax...ml#post2321807

It makes the sense. f stop if focal length / entrance pupil

for a f1.4 lens

f1.4 = 50mm / 35.7

so wide open the 50mm f1.4 has a hole 35.7mm wide :P When we change that with that step down ring, you are effectively stopping down the lens... plugging the numbers in..

f = 50 / 28
f= 1.78



No surprise it's sharper, my f1.4 stopped down a click is much sharper :P

Wombat2go said it first tho :P
03-21-2013, 04:56 PM   #19
Veteran Member




Join Date: Feb 2009
Location: Alameda, CA
Posts: 3,206
I don't have the 50mm F/1.4 any more (mine is F/1.7) so I can't say much about it.

But my test for the size of stepdown rings is very simple: with the lens attached to the camera, I set the camera to Av mode, meter off a uniformly lit surface (a white wall), then try step-down rings of gradually smaller opening until there is a drop in shutter speed, then I back up one size.

I don't think "this hood is simply changing the f stop, ala stopping down the lens."

03-21-2013, 05:04 PM   #20
Veteran Member
tromboads's Avatar

Join Date: Apr 2012
Location: Sydney
Photos: Gallery
Posts: 1,193
you are using a f1.7 with the 28mm step down ring??

well lets do the math again. with f stop = focal length / entrance diameter we can sort it pretty simply.

f1.7 = 50mm/ X
X = 29.4mm

Ie the entrance pupil wide open is 29.4mm

I'm not surprised the sensor can't determine the mm difference in entrance pupil.. With that 28mm hood your working f stop is 1.78 so bees dick all difference.

The other examples in this thread showed it being used with a f1.4 lens and the dramatic clarity difference it produced. Are you finding a dramatic clarity difference with your f1.7 lens?
03-21-2013, 09:51 PM   #21
Veteran Member




Join Date: Feb 2009
Location: Alameda, CA
Posts: 3,206
QuoteOriginally posted by tromboads Quote
you are using a f1.7 with the 28mm step down ring??
Yes. See photo on page 1 of this thread.

QuoteOriginally posted by tromboads Quote
Are you finding a dramatic clarity difference with your f1.7 lens?
No. I have not done detailed side-to-side comparison. But that doesn't mean there is no difference.

I'm sure the step-down ring helps; at least it prevents strayed fingers and the likes from touching the front element of the lens. The step-down ring does not take up any space. Unless I need to use filter, I don't even notice the step-down ring is there.

I just did the exposure test again: with the camera in Av mode, metering off a uniformly-lit scene, the shutter speeds were the same, with or without the step-down ring. If the step-down ring acted as the aperture, my camera didn't notice. That's good enough for me.
03-21-2013, 10:31 PM   #22
Site Supporter
Site Supporter
crewl1's Avatar

Join Date: Jul 2010
Location: Los Angeles, CA
Photos: Gallery | Albums
Posts: 9,805
QuoteOriginally posted by kh1234567890 Quote
I got my step-down ring from here. Turned up in 6 days. Not bad for 2 ($3).

As for the cap - 28mm clip-on caps do exist, but a better option may be a plastic push-on 51mm cap. Unfortunately these seem to be as rare as hen's teeth
Good idea, I just tested this 51mm push on cap idea and it works.
They can be purchased from littledrawe in his marketplace ad here https://www.pentaxforums.com/forums/photographic-equipment-sale/204788-sale-p...er-filter.html

03-21-2013, 11:53 PM   #23
Veteran Member
tromboads's Avatar

Join Date: Apr 2012
Location: Sydney
Photos: Gallery
Posts: 1,193
QuoteOriginally posted by SOldBear Quote
I just did the exposure test again: with the camera in Av mode, metering off a uniformly-lit scene, the shutter speeds were the same, with or without the step-down ring. If the step-down ring acted as the aperture, my camera didn't notice. That's good enough for me.
..... There isn't going to a difference in exposure, up above we worked out the the difference in placing that ring over the front wouldn't be enough to change, ie, the entrance pupil was at 28mm only marginally smaller then the normal pupil of your f1.7 50mm wide open.

This is all directed to those who did as much with a f1.4 lens and got a fantastically sharper image. All they managed to do with using this hood, is step it down to f1.8, thus some "free" clarity.
03-22-2013, 01:08 AM   #24
Veteran Member




Join Date: Oct 2011
Posts: 818
QuoteOriginally posted by tromboads Quote
..... There isn't going to a difference in exposure, up above we worked out the the difference in placing that ring over the front wouldn't be enough to change, ie, the entrance pupil was at 28mm only marginally smaller then the normal pupil of your f1.7 50mm wide open.

This is all directed to those who did as much with a f1.4 lens and got a fantastically sharper image. All they managed to do with using this hood, is step it down to f1.8, thus some "free" clarity.
I actually agree with you...

Look at the shutter speed on kh1234567890 EXIF data.

First pic, no hood: 1/1600s
Second pic, with stepdown ring as hood: 1/1000s!

No wonder....
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!
hood, metal, metal hood, tripod
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
For Sale - Sold: Revuenon 55mm f1.2 with Metal Hood cali92rs Sold Items 2 12-08-2012 11:04 AM
For Sale - Sold: Vintage Metal Square Lens Hood 49mm konradsiekierski Sold Items 6 11-18-2012 09:56 AM
For Sale - Sold: METAL LENS HOOD takumar 3.5 28mm PentaxMom Felua Sold Items 2 08-12-2011 04:15 PM
does hood shape matter? gokenin Pentax Camera and Field Accessories 8 11-02-2008 06:24 PM



All times are GMT -7. The time now is 12:05 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