Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
02-08-2011, 06:32 PM   #16
Inactive Account




Join Date: Mar 2009
Location: Australia
Posts: 207
It does not seem to me that th OP's question has been adequately answered.

I too, often wonder why short focal length lenses have such narrow maximum apertures when "normal" lenses (which have longer focal lengths can go as low as f:1.1 (and wider)?

The f-number is derived from f= focal length/diameter of the aperture.

So, one would imagine, the shorter the FL, the easier it would be to produce a smaller "f", since reducing the FL would result in a lower "f" number for any given aperture diameter.

Put another way, logically, it would appear that wider angle lenses should have smaller front elements than primes, which have smaller front elements than long focus lenses with the same "fastest aperture".

What have we overlooked?

02-08-2011, 06:52 PM   #17
Pentaxian
v5planet's Avatar

Join Date: Jan 2010
Location: Seattle
Photos: Albums
Posts: 1,904
QuoteOriginally posted by Banjo Quote
It does not seem to me that th OP's question has been adequately answered.

I too, often wonder why short focal length lenses have such narrow maximum apertures when "normal" lenses (which have longer focal lengths can go as low as f:1.1 (and wider)?

The f-number is derived from f= focal length/diameter of the aperture.

So, one would imagine, the shorter the FL, the easier it would be to produce a smaller "f", since reducing the FL would result in a lower "f" number for any given aperture diameter.

Put another way, logically, it would appear that wider angle lenses should have smaller front elements than primes, which have smaller front elements than long focus lenses with the same "fastest aperture".

What have we overlooked?
The speed of a lens is a ratio expression, that is, the focal length of the lens divided by the aperture of the lens. But I think the confusion here is that the aperture does not refer to the diameter of the front element, it refers to the diameter of the opening left by the aperture blades, which are embedded deeper in the lens, and may be substantially narrower than the big honking pieces of glass in front of them.

All those big pieces of glass that appear in faster, wide angle lenses are - and someone correct me if I'm wrong - functioning primarily to help correct for the bad chromatic aberration a wide angle lens is otherwise prone to.
02-08-2011, 07:00 PM   #18
Pentaxian
aleonx3's Avatar

Join Date: Jul 2007
Location: Brampton, Ontario
Photos: Gallery | Albums
Posts: 3,873
Don't forget the wider focal length you use, the more DOF you will want to have, not less.
02-09-2011, 08:11 AM   #19
Site Supporter
Stone G.'s Avatar

Join Date: Jun 2010
Location: North Zealand, Denmark
Photos: Gallery | Albums
Posts: 1,508
QuoteOriginally posted by Banjo Quote
It does not seem to me that th OP's question has been adequately answered.

.......

The f-number is derived from f= focal length/diameter of the aperture.

So, one would imagine, the shorter the FL, the easier it would be to produce a smaller "f", since reducing the FL would result in a lower "f" number for any given aperture diameter.

.......

What have we overlooked?
Your reasoning is basically correct. What you "overlook" may be the design requirements for short focal length lenses - that by nature must be adapted to a camera house with fixed dimensions:

A, say, 15mm lens will produce an image of an object far away at a distance just 15mm away from the OPTICAL center of that lens. Now, take a ruler and measure the distance from the PHYSICAL center of that lens to your camera's sensor and you will find that this distance is significantly larger than 15mm.

So, obviously, lens designers have to add a lot of lens elements to bring the optical center outside the lens - to "carry the optical center further away", if you will, and INTO the camera house itself

And here's why a good wide angle lens is often a heavy beast with lots of glass inside.

(At the other end, a 400mm telephoto lens is rarely 400mm long these days because designers add a lens group that places the optical center outside the lens and further AWAY from the camera. So, a high-quality fast, normal lens in the 35-55mm range just happens to be a much easier and cheaper design than either quality tele or do. wide angle lenses).

02-09-2011, 11:28 AM   #20
Pentaxian
bdery's Avatar

Join Date: Mar 2008
Location: Quebec city, Canada
Photos: Gallery
Posts: 5,658
QuoteOriginally posted by v5planet Quote
The speed of a lens is a ratio expression, that is, the focal length of the lens divided by the aperture of the lens. But I think the confusion here is that the aperture does not refer to the diameter of the front element, it refers to the diameter of the opening left by the aperture blades, which are embedded deeper in the lens, and may be substantially narrower than the big honking pieces of glass in front of them.

All those big pieces of glass that appear in faster, wide angle lenses are - and someone correct me if I'm wrong - functioning primarily to help correct for the bad chromatic aberration a wide angle lens is otherwise prone to.
You are globally correct. But one must also mention that the aperture is in fact inverted the way we write it.

Meaning that f1,4 means 1/1,4, actually. That's quite misleading for many people.
02-09-2011, 11:28 AM   #21
Pentaxian
bdery's Avatar

Join Date: Mar 2008
Location: Quebec city, Canada
Photos: Gallery
Posts: 5,658
QuoteOriginally posted by v5planet Quote
The speed of a lens is a ratio expression, that is, the focal length of the lens divided by the aperture of the lens. But I think the confusion here is that the aperture does not refer to the diameter of the front element, it refers to the diameter of the opening left by the aperture blades, which are embedded deeper in the lens, and may be substantially narrower than the big honking pieces of glass in front of them.

All those big pieces of glass that appear in faster, wide angle lenses are - and someone correct me if I'm wrong - functioning primarily to help correct for the bad chromatic aberration a wide angle lens is otherwise prone to.
You are globally correct. But one must also mention that the aperture is in fact inverted the way we write it.

Meaning that f1,4 means 1/1,4, actually. That's quite misleading for many people.
02-09-2011, 01:46 PM   #22
Pentaxian
v5planet's Avatar

Join Date: Jan 2010
Location: Seattle
Photos: Albums
Posts: 1,904
QuoteOriginally posted by bdery Quote
You are globally correct. But one must also mention that the aperture is in fact inverted the way we write it.

Meaning that f1,4 means 1/1,4, actually. That's quite misleading for many people.
Well yes, hence ye olde way of expressing it as a ratio, ex 1:1.4.
02-09-2011, 05:36 PM   #23
Veteran Member
selar's Avatar

Join Date: Jul 2007
Location: Sydney, Australia
Photos: Gallery | Albums
Posts: 2,035
Most people think they need a fast wide-angle but most don't actually need one. Those that do need it are willing to put up with a big hunk of metal and glass, but the rest look for small and light lenses that are easy to travel with.

02-10-2011, 07:41 AM - 1 Like   #24
axl
Veteran Member




Join Date: Mar 2007
Location: Nove Zamky, Slovakia
Photos: Gallery | Albums
Posts: 7,181
QuoteOriginally posted by Abstract Quote
I have a question, I know I am on a K20D but how do you set the iso like that for example, my next step up from iso 800 is 1600 and then 3200(Also cant seem to select 6400)
You need to go custom settings menu and set Ev steps to 1/3 steps and ISO as Ev steps. Then you'll be able to set ISO in 1/3 increments...
02-10-2011, 08:19 AM   #25
Site Supporter
GeneV's Avatar

Join Date: Oct 2008
Location: Albuquerque NM
Photos: Albums
Posts: 9,762
QuoteOriginally posted by selar Quote
Most people think they need a fast wide-angle but most don't actually need one. Those that do need it are willing to put up with a big hunk of metal and glass, but the rest look for small and light lenses that are easy to travel with.
I can't say I've missed it much on APS-c. The wide angles are such short focal lengths that many of the advantages of a fast lens just don't matter as much.
02-10-2011, 10:43 AM   #26
Veteran Member
jeztastic's Avatar

Join Date: Nov 2010
Location: Canterbury
Photos: Gallery | Albums
Posts: 596
Original Poster
Thanks everyone for the replies and opinions. My questions comprehensively answered! Next - which to go for - 15mm or 21mm (assuming I win the lottery)...
02-10-2011, 11:25 AM   #27
Veteran Member
alohadave's Avatar

Join Date: Jan 2008
Location: Quincy, MA
Photos: Gallery | Albums
Posts: 2,024
QuoteOriginally posted by bdery Quote
You are globally correct. But one must also mention that the aperture is in fact inverted the way we write it.

Meaning that f1,4 means 1/1,4, actually. That's quite misleading for many people.
Only for a focal length of 1.

The f refers to the focal length of the lens. For a focal length of 15mm, at f/1.4, the aperture would be 10.714mm in diameter if placed at the nodal point of the lens. At f/8, it would be 1.875mm.
02-10-2011, 12:27 PM   #28
Inactive Account




Join Date: Sep 2010
Location: New York
Photos: Gallery
Posts: 388
QuoteOriginally posted by axl Quote
You need to go custom settings menu and set Ev steps to 1/3 steps and ISO as Ev steps. Then you'll be able to set ISO in 1/3 increments...
Lovely, thanks a lot! This should help me a lot at night, did not like jumping all the way to 1600, usually kept it at 800.
02-12-2011, 02:27 PM   #29
Pentaxian




Join Date: Nov 2006
Location: America's First Wilderness
Posts: 358
QuoteOriginally posted by jeztastic Quote
Thanks everyone for the replies and opinions. My questions comprehensively answered! Next - which to go for - 15mm or 21mm (assuming I win the lottery)...
Both are good, but completely different. One is a true wide angle (though not what I'd consider an ultra wide at about 24mm) and one is a wide normal. I consider 35mm in the normal range and 28mm to be a border FL straddling wide and normal. Since the 21mm is about 32mm, it's neither normal nor wide...but I love the FL since it can be used for just about anything, and I'd say it's pretty much my "normal" lens, bearing in mind I was a person that considered 35mm "normal" on a 35mm camera.

I got my 15mm at the end of last summer, and didn't use it a lot because my K-7 was in the shop 2X. Since then I've used my EX1/TL500 quite a bit along with my 645N. So the APS-C DSLRs have taken a break and so have the lenses.

However, I have had the 21mm for several years and it's my all time favorite lens. I shoot it better than 50% of the time.

F3.2 is pretty close to f/2.8 (just 1/3 of a stop), and the K10D is perfectly capable of being used above ISO 400. Shoot in RAW, learn how the camera reacts to exposure, and learn how to process. The difference between it and the K20D was marginal, and the K-7 was about the same as the K20D. So the only camera you are currently missing out on is the $1500 K-5 which is obviously a significant improvement.

The K10D is a really good camera, with one of Pentax best image file outputs, don't fool yourself into believing you cannot create wonderful images with it. Yeah, it's getting a little long in the tooth, and feature wise it pales in comparison to the K-7/K-5, but the image quality doesn't!
02-12-2011, 02:33 PM - 1 Like   #30
Pentaxian




Join Date: Nov 2006
Location: America's First Wilderness
Posts: 358
BTW, you can shoot the K10D and DA21mm combo quite often at f/5.6 and hyperfocal for work on the street or landscapes where all you care about is uniform focus or being able to shoot quickly. Take a look at a depth of field chart for the the 21mm on an APS-C camera, quite deep. No need for anything above f/8 in most cases unless you have a very close foreground...I NEVER shoot it above f/11, and rarely above F/8.


As someone noted, the advantages of the Limiteds are compact size. They tend to be usable if not close to optimal wide open. So you aren't carrying around 2 stops of glass you won't use. I sold my 20mm f/1.8 because I never (rarely, and not at all for the last 2 years) because I love the handling and size of the 21mm DA.

If you need a faster lens, the 14mm f/2.8 DA is hightly acclaimed. In the early days of that lens people actually switched to Pentax because it was the only brand with a lens of that size and speed.

The 21mm is very sharp at 3.2 while the 15mm isn't tack sharp wide open, it very good by 5.6. Considering the size, lack of distortion and flare, it really comes down to will you need to shoot it at f/4 all the time? If so then the 14mm and it's increased size is a better option.

For me, I shoot rarely wide open with a wider angled lens. I tend to often with lenses more in the normal range, but when I'm using a wide angle I usually want the DOF. So f4-5.6 is pretty much my minimum aperture

Last edited by Mountain Vision; 02-12-2011 at 02:40 PM.
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!
f4, indoors, k-mount, option, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Will you consider slower lenses for your K5? LesMizzell Pentax SLR Lens Discussion 18 11-07-2010 05:45 PM
Slower Than Molasses! Ballyclogh Pentax SLR Lens Discussion 4 01-28-2009 07:21 PM
For Sale - Sold: Three FA Limiteds and one DA super wide zoom Ed in GA Sold Items 20 07-30-2008 02:19 PM
Is the PZ-1 Really Slower than the PZ-1P? felix68 Pentax Film SLR Discussion 1 04-21-2008 06:12 PM



All times are GMT -7. The time now is 03:23 AM. | See also: NikonForums.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