Fast lenses and Olympus

[Jonas]

I didn't write it, I just quoted it. The article explicitly says "This allows the lens to be focused to different distances without changing the size of the image."

I admit that this is hard for me to conceptualize -- but for a lens to be "focused at different distances," it seems clear that the objects focused upon must also be at different distances. And if this can be done "without changing the size of the image," then it must be saying that objects at different distances will be imaged at the same size.

Yes, it sounds crazy -- but no crazier than object-space telecentricity, (...)

Hi Ranger 9,

I'm sorry I made it look as you wrote all of the part I quoted. My comment was aimed at the last sentence which I understand you wrote.

Well, I think Audii-Dudii describes it correctly and perhaps better than I put it in my reply. Maybe my last sentence should have been put this way:

As a result there would be no change in appearant focal length, or magnification, when focusing.

That doesn't mean a person standing 4 meters away is depicted the same size on the sensor as a person standing 8 meters away. It is common to see a lens get increased magnification when focusing close (the image circle grows) and a slight "zoom-out" effect when focusing at infinity. This wouldn't happen with a true image-sized telecentricity lens.

Again just to my understanding. Maybe this is a topic best discussed IRL over a beer.

Cheers, /Jonas

 

[jonoslack]

Again just to my understanding. Maybe this is a topic best discussed IRL over a beer.

Cheers, /Jonas

Perhaps, although this place seems to be the nearest to 'over a pint of beer' that you can get on the internet.

 

[Riley]

lets brush up a tad on what telecentric means
construct a rectangle across the diagonal of the sensor, and up through the exit pupil of the lens. The exit pupil contains the rectangle at its widest aperture. A truly telecentric lens would have a parallel side rectangle.

Now for 'near telecentric'
The allowable angle is 6.+ degrees, requiring an exit pupil some 80+mm out from the sensor, now you have a trapezoid shape from the sensor diagonal through the widest aperture. In this way wider angle lenses are most affected in wide open configuration, zooms at there widest angle setting. Tele lenses by there nature are more telecentric. Stopping down as with anything cures these ills, but impacts on lens speed.

Why is it so?
well they calculated that what they felt was acceptable in arrival angle to the sensor, this at a time when microlenses could cope with 12 degrees,

....thats not to say that such a microlens would entirely free you from soft edges or pixel vignetting, but it is around the determinant used by and large by APSC, in some case with exit pupils of modern digital lenses for APSC are further out (I know Nikon use this principle and it is visible in the EXIF of newer lenses), older lenses are closer to the film regime of 50+mm, which scribes a wide head trapezoid.

getting back to an aperture limitation, if the sides of the trapezoid can differ out to 6.+ degrees, this limits the max width of the wide open aperture. Another way to combat this is to add distance to the exit pupil, which somewhat rectifies the situation, but quite gently.

Truly fast lenses have exits that exceed the diagonal of the sensor, my 57/1.2 Konica is around 35mm, so it is greatly affected by the operation of the aperture which contains the arrival angle of the sides of the trapezoid. Most useful fast lenses on 4/3rds have narrower exit lenses on the rear, the Konica 40/1.8 for instance is 22mm which is near perfect, and its image quality wide open displays that.

None of this applies to mFT, which is an entirely different configuration.

I hope I explained that to everyones satisfaction, there are other things going on to do with lens design, I'll dig up some advice from Joe W and come back sometime when I clear my desk...

 

[jonoslack]

lets brush up a tad on what telecentric means
construct a rectangle across the diagonal of the sensor, and up through the exit pupil of the lens. The exit pupil contains the rectangle at its widest aperture. A truly telecentric lens would have a parallel side rectangle.

Now for 'near telecentric'
The allowable angle is 6.+ degrees, requiring an exit pupil some 80+mm out from the sensor, now you have a trapezoid shape from the sensor diagonal through the widest aperture. In this way wider angle lenses are most affected in wide open configuration, zooms at there widest angle setting. Tele lenses by there nature are more telecentric. Stopping down as with anything cures these ills, but impacts on lens speed.

Why is it so?
well they calculated that what they felt was acceptable in arrival angle to the sensor, this at a time when microlenses could cope with 12 degrees,

....thats not to say that such a microlens would entirely free you from soft edges or pixel vignetting, but it is around the determinant used by and large by APSC, in some case with exit pupils of modern digital lenses for APSC are further out (I know Nikon use this principle and it is visible in the EXIF of newer lenses), older lenses are closer to the film regime of 50+mm, which scribes a wide head trapezoid.

getting back to an aperture limitation, if the sides of the trapezoid can differ out to 6.+ degrees, this limits the max width of the wide open aperture. Another way to combat this is to add distance to the exit pupil, which somewhat rectifies the situation, but quite gently.

Truly fast lenses have exits that exceed the diagonal of the sensor, my 57/1.2 Konica is around 35mm, so it is greatly affected by the operation of the aperture which contains the arrival angle of the sides of the trapezoid. Most useful fast lenses on 4/3rds have narrower exit lenses on the rear, the Konica 40/1.8 for instance is 22mm which is near perfect, and its image quality wide open displays that.

None of this applies to mFT, which is an entirely different configuration.

I hope I explained that to everyones satisfaction, there are other things going on to do with lens design, I'll dig up some advice from Joe W and come back sometime when I clear my desk...

Thank you Riley - clear, detailed and explicit

 

[Riley]

thanks Jono

Now as it happens Joe W made a post last week answering some of these questions.

http://forums.dpreview.com/forums/read.asp?forum=1018&message=31914147

Because of the amount of light that you can cram through a given size "hole in space".

The fastest practical lens, for pretty much any SLR format (P&S, to MFT, to APS, to FF, to MF) is f1.2, and f1.4 is much more

A normal is a normal is a normal, it has a focal length about equal to the sensor diagonal (about 15% larger for fast normals on SLRs, and I won't get into the physics of that) and covers about 53 degrees. The area of that hole, and therefore, the amount of light (number of photons) is proportional to the square of the focal length. 35mm in diameter is a wonderful "hole size" for normals. It makes them weigh about 350g, and cost $400 if you make them by the 100,000, whether it's a 50mm f1.4 on FF, or a 180mm f5.6 on 4x5. (MF somehow or other managed to miss the boat, and end up with a 28mm diameter normal, 80mm f2.8. That's a long story. If MF were viable, and pursued by companies as aggressive as Nikon or Canon, we'd be seeing a lot faster and better lenses).

Pump the same number of photons into a focusing screen, and it doesn't really matter how big or small that screen is, when you enlarge it to the same part of the photographer's field of view, if it's lit by the same size hole, it's the same brightness.

Where things break down, by chance accident of physics, is formats smaller than FF. Lenses faster than f1.4 aren't easily affordable, so we don't see 30mm f1.0 normals for APS, or 25mm f0.7 normals for four thirds. We've lost the ability to maintain an affordable, relatively good performance 35mm diameter "hole in space" that we had from FF to MF to LF.


LF makes a really, really bad SLR or rangefinder. It was doomed to end up a "specialty" format.

MF has a great range of lens possibilities. F1.4 normals, wides, and short portrait teles are doable, f2.0 is not bad, and f2.8 is a piece of cake. But MF makes a lousy SLR, the mirror is huge, slow, noisy, and it kicks like a mule. It fell behind 35mm as far as lenses, metering, AF, ergonomics, etc.

FF simply got lucky. Fast lenses were doable. Small, relatively quiet and high performance SLR mechanisms were doable. The format was big enough for decent viewfinders, small enough to handle well. It ended up the "jack of all trades".

APS and four thirds aren't so lucky. Yes, you can make a wonderfully quiet and "gentle" SLR. But there's that silly f1.4 "wall" on lens design, so it ends up unable to match FF on viewfinder, DOF range, or low light ability.

thats a synopsis of the optical issues, for more detail have a look at
http://forums.dpreview.com/forums/read.asp?forum=1018&message=31899845

in particular
http://forums.dpreview.com/forums/read.asp?forum=1018&message=31916394

adding
http://forums.dpreview.com/forums/read.asp?forum=1018&message=31921836

Given EVIL and short back focus, f1.0 normals, moderate wides, and short teles are practical on four thirds, APS, and FF. They would be terribly exotic on MF, especially 6x7. On FF or APS, they're fairly affordable (check out some Voigtlander pricing), on FF, a bit less so, but still better than Canon's SLR 50mm f1.0 from the 80s, or their current 50mm and 85mm f1.2.

 

[Jonas]

Thank you Riley. And a distant Thanks! to Joe as well of course.