Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
09-15-2008, 01:19 AM   #16
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by FHPhotographer Quote
This is a bit snarky isn't it? I suspect a lot of us read the "serious and knowledgable answers," but we may not understand the answers, or we may not care, or we may not agree. But I wouldn't assume the posts go unread or unconsidered.

In simple terms, I may get it, but maybe I just don't agree with it. Besides, fact is funny stuff, a bit slippery at the very best...and even at the very best, fact is not truth.

FHPhotographer
Don't know what "snarky" means, but your reply is testimonial to my impression... See here:

QuoteOriginally posted by FHPhotographer Quote
Well, I won't pretend I understood most of your post, however, I do know that this lens does change in "discrete steps". That's what the exif data say.

Now I'm not coming onto your job site and trying to knock the shovel out of the hands of you, Ben and the other forum folk who work in this way, so don't take this as a criticism, but all I see is what the camera sees and all I know is what the camera data tell me. And the camera sees that at 33mm it's f/3.2 and at 34mm it's f/3.5.
13 (!) posts above your fact-finding, I already answered this observation of yours:

The variable aperture changes according to the focal length. Often cameras show an increase of the aperture number at a certain focal length, but that is more due to the camera electronics which will only show discrete steps in the aperture values ladder (like 1/3 f-stops).

There is a difference between not understanding a may be too technical explanation (yes, I am prone to that) and repeating the same stuff again and again, without referring to all the other answers in the thread.

AND this is, I want to really clarify that, not aimed at you in person. It is a behaviour we witness often in many forum threads and I must say, I don't find that helpful. I have seen threads, where the same answers to the same repeated question was posted six or seven times, but the thread opener simply ignored all the answers. So, what I prefer is: If an explanation is too technical, just ask for clarification or a more simple approach, instead of being repetitive, because this leads to nothing at all.

Ben

09-15-2008, 05:26 AM   #17
Veteran Member




Join Date: Aug 2007
Location: England
Posts: 309
QuoteQuote:
The open diameter is an engineering expression and should be substituted by "entrance pupil" in optics speak.
Sure, the optics book I have calls this "effective aperture" it is just a different name for the same thing.

QuoteQuote:
In simple lens constructions, this is more or less the exact dimension of the front lens. In more complicated lens constructions, the front lens is usualy much bigger, than the max. aperture suggests.
Yes, this is the point I was trying to make. For primes you can make that assumption, but zooms are much more complicated.

QuoteQuote:
If you compensate, you can for example simply open the iris diameter according to the focal length. This is more expensive, not only because of the more complex iris control mechanism, but also, because you need to make your lenses simply bigger, than in a variable aperture lens.
Again, I agree.

However, let me put this to you, to see if you can point out if I have misunderstood something.

I have a Pentax-A 1:4 70-210. If I put this at an aperture other than A (say f/8) and look through it from the camera end, when I move the zoom ring from 70 to 210 the diaphragm does not change. So there is no diaphragm adjustment for the change in focal length. This lens is constant aperture, right (the aperture ring says f/4)? Yet the diaphragm does not change so the compensation that occurs is purely optical.

This is not an expensive lens. So, did I get a real bargain here, or is there something wrong in what I have described? (By the way, I really have had good results from this lens and I tend to think that it is better than the cost would suggest.)

Richard
09-15-2008, 06:40 AM   #18
Forum Member
brownargus's Avatar

Join Date: Sep 2008
Location: Rotherham, South Yorkshire, UK
Posts: 55
Varifocal means?

QuoteOriginally posted by richard64 Quote
I have a Pentax-A 1:4 70-210. If I put this at an aperture other than A (say f/8) and look through it from the camera end, when I move the zoom ring from 70 to 210 the diaphragm does not change. So there is no diaphragm adjustment for the change in focal length. This lens is constant aperture, right (the aperture ring says f/4)? Yet the diaphragm does not change so the compensation that occurs is purely optical.

Richard
If you set a fixed aperture on a zoom lens as in the example quoted, the lens aperture will remain at f8, but if you put it on a camera with aperture priority and zoom the lens, the shutter speed will change. This is what aperture priority means. The only way you can fix the aperture and shutter while zooming and take photographs is to use the TAv (aperture and shutter priority) setting which I believe is exclusive to Pentax K10D or K20D cameras. In this case the ISO setting will change to accommodate the change in light needed at the sensor.

Varifocal is not I believe, exactly what it says; I believe it means variable focus. With a true zoom lens, as one zooms, the lens stays in focus. With a varifocal lens it does not and as one zooms one has to make adjustments to the focusing. This makes the lens somewhat cheaper for a given optical quality and makes it easier to retain a near constant aperture over the zooming range. The one I had was a Vivitar Series 1 28-105 which was a varifocal (manual) lens but had a fairly constant aperture between 2.8 and 3.5 (if I remember correctly) over the zoom range. However, if one zoomed between the extremes, one had to make focusing changes as well which could make it more difficult to use with some subjects.

John
09-17-2008, 12:53 PM   #19
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by richard64 Quote
I have a Pentax-A 1:4 70-210. If I put this at an aperture other than A (say f/8) and look through it from the camera end, when I move the zoom ring from 70 to 210 the diaphragm does not change. So there is no diaphragm adjustment for the change in focal length. This lens is constant aperture, right (the aperture ring says f/4)? Yet the diaphragm does not change so the compensation that occurs is purely optical.
Richard, your observation is spot on. If you look through the A 70-210 while zooming at a fixed aperture setting, you can easily notice, that the iris itself does not change (you can recognise that from the shapes of the aperture blades, which keep their position during zooming). Here you have a good example, that only the variation of the zoom group inside the lens, positioned in front of the aperture mechanism, varies the magnification. That in effect means, that the projected diameter of the aperture (the entrance pupil) gets bigger at the longer focal length and smaller at shorter focal lengthes. In effect the aperture number stays constant at the chosen f-number thoughout the whole zoom range.

QuoteOriginally posted by richard64 Quote
This is not an expensive lens. So, did I get a real bargain here, or is there something wrong in what I have described? (By the way, I really have had good results from this lens and I tend to think that it is better than the cost would suggest.)
The A 70-210 is a wonderful lens and it is a bargain! It is sharp, contrasty and well-made. It is very usable on digital. I can only guess it is so cheap nowadays, because people prefer AF lenses and mdoern lenses are much smaller and more lightweight. But then, modern lenses are mostly slower.

Ben

09-17-2008, 12:57 PM   #20
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by brownargus Quote
If you set a fixed aperture on a zoom lens as in the example quoted, the lens aperture will remain at f8, but if you put it on a camera with aperture priority and zoom the lens, the shutter speed will change. This is what aperture priority means. The only way you can fix the aperture and shutter while zooming and take photographs is to use the TAv (aperture and shutter priority) setting which I believe is exclusive to Pentax K10D or K20D cameras. In this case the ISO setting will change to accommodate the change in light needed at the sensor.
John, I can see what you mean. But your impression is not correct. The aperture stays fixed, espeically in the lens Richard chose as an example. And the exposure time won't change during zooming, when you use a constant aperture zoom lens. Try it, if you have such a lens at hand. I know, most modern zoom lenses, have a variable aperture, so this is not easily tried.

QuoteOriginally posted by brownargus Quote
Varifocal is not I believe, exactly what it says; I believe it means variable focus. With a true zoom lens, as one zooms, the lens stays in focus. With a varifocal lens it does not and as one zooms one has to make adjustments to the focusing. This makes the lens somewhat cheaper for a given optical quality and makes it easier to retain a near constant aperture over the zooming range. The one I had was a Vivitar Series 1 28-105 which was a varifocal (manual) lens but had a fairly constant aperture between 2.8 and 3.5 (if I remember correctly) over the zoom range. However, if one zoomed between the extremes, one had to make focusing changes as well which could make it more difficult to use with some subjects.
You are right: Varifocal is variable focus and your own observation is right on the spot.

Ben
09-22-2008, 02:59 AM   #21
Forum Member
brownargus's Avatar

Join Date: Sep 2008
Location: Rotherham, South Yorkshire, UK
Posts: 55
QuoteOriginally posted by Ben_Edict Quote
John, I can see what you mean. But your impression is not correct. The aperture stays fixed, espeically in the lens Richard chose as an example. And the exposure time won't change during zooming, when you use a constant aperture zoom lens. Try it, if you have such a lens at hand. I know, most modern zoom lenses, have a variable aperture, so this is not easily tried.
Ben
Ben, I hadn't realised that we were talking about a lens with constant aperture - my funds have only extended to zoom lenses where the aperture varies when you zoom!

John
09-22-2008, 03:54 AM   #22
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by brownargus Quote
Ben, I hadn't realised that we were talking about a lens with constant aperture - my funds have only extended to zoom lenses where the aperture varies when you zoom!
John, if you haven't got a constant aperture lens yet, the old manual focus SMC-A 70-210/4 could be a good choice for you! You will find them on ebay regularily at a price well below 100 USD (sometimes only 40-50 USD, in fact). These lenses are sharp, contrasty and all in all really worth much more money. The only thing you might miss is AF.

Ben
09-22-2008, 07:06 AM   #23
Veteran Member




Join Date: Aug 2007
Location: England
Posts: 309
Ben,

QuoteOriginally posted by Ben_Edict Quote
Here you have a good example, that only the variation of the zoom group inside the lens, positioned in front of the aperture mechanism, varies the magnification. That in effect means, that the projected diameter of the aperture (the entrance pupil) gets bigger at the longer focal length and smaller at shorter focal lengthes. In effect the aperture number stays constant at the chosen f-number thoughout the whole zoom range.
Thanks, for the detailed explanation.

QuoteQuote:
The A 70-210 is a wonderful lens and it is a bargain! It is sharp, contrasty and well-made. It is very usable on digital.
I think so too

Richard

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!
k-mount, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
can anyone explain what it means when a camera is goosed? naughty.smurf Photographic Technique 8 09-07-2010 10:16 PM
SMC Pentax-A Zoom 35-105mm Varifocal or Parafocal? writeb Pentax SLR Lens Discussion 49 05-28-2010 05:44 AM
Spring in the Netherlands means tulips! newmikey Post Your Photos! 6 04-21-2009 11:00 AM
Green Means Go: Some GIMP Silliness Mike Cash Post Your Photos! 1 12-13-2007 08:53 AM
Snow and ice means B&W to me (mostly anyway) NaClH2O Post Your Photos! 11 01-31-2007 07:06 PM



All times are GMT -7. The time now is 11:43 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