Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Pentax Lenses

Reply
 
Thread Tools Search this Thread
Old Feb 3, 2009, 3:22 PM   #11
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,544
Default

jelpee wrote:
Quote:
I think I just blew a circuit in my brain:?!
The 50mm lens has a maximum pupil diameter of about 35.7mm. The 30mm lens has amaximum pupil diameter of about 21.4mm, or roughly 60% that of the 50mm lens.

The 30mm lens has an angle of view of about 50°. The 50mm lens has an angle of view of about 30°, or roughly 60% that of the 30mm lens.

So even though the 50mm lens has a larger maximum pupil diameter than the 30mm lens, it has a proportionally smaller angle of view, sothey both let in the same amount of light.

Keep in mind that this is a gross oversimplification of the actual math, but this iswhy the aperture is a function of the pupil diameter and the focal length.
TCav is online now   Reply With Quote
Old Feb 3, 2009, 4:18 PM   #12
Senior Member
 
bilybianca's Avatar
 
Join Date: Nov 2005
Location: Hassleholm, Sweden
Posts: 3,435
Default

And the "focal lenght" isn't the actual lenght of a lens (the A*200 mm and A*300 mm are nearly equal) but a theoretical length based on, I believe, a one element one group lens. All the lenses we use have this lenght "manipulated" with various lenses and groups. The DA 14 mm is much longer than the FA 50mm, but it's focal length is not.

If you want to get really deep into this can of worms, google nominal focal length, equivalent focal length and calibrated focal length.

Getting back to the original question, why not consider the FA 35 mm f 2.0 as a compromise. It doesn't open up to 1.4, but it's fully usable at 2.0. The Sigma 30 f 1.4 needs to be stopped down further, and the FA 50 mm 1.4 isn't really good either unless you stop it down to 2.0. Consider the 1.4 an emergency stop.:-)

Kjell
bilybianca is offline   Reply With Quote
Old Feb 3, 2009, 4:21 PM   #13
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,544
Default

bilybianca wrote:
Quote:
...an emergency stop.:-)
:-):lol::-):lol::-):lol:
TCav is online now   Reply With Quote
Old Feb 3, 2009, 4:35 PM   #14
Senior Member
 
snostorm's Avatar
 
Join Date: Jan 2004
Location: Chicago Suburb, IL, USA
Posts: 2,770
Default

Hi Dave,

I'm getting here a bit late, but something else that might be relevant is DOF. The 30 will have more DOF at the same aperture than the 50. This may be either good or bad, depending on where you're coming from.

If you want to isolate your subject with thinner DOF, then the 50 might do this better. If you want to guarantee sharpness in your subject, then the 30 should give you better tolerance to slight focus misses which could have the effect of giving you sharper shots despite the fact that the lens might not be quite as sharp inherently.

There are others that might be able to give you real-life experience with this, but unfortunately I don't shoot this FL range enough to be able to tell from first-hand experience.

This might also factor in in AF speed in very low light. . . again I don't have first hand experience, so . . .

Scott
snostorm is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 3:52 AM.