Re: Best 75mm / 90mm lens?......totally kidding, but not really...read on pls
Quote:
Originally Posted by
Alan Klein
1) Why don't 35mm lenses have that falloff? Why are their designs different?
2) If it's a simple formula, why do different people say the falloff varies? There seem to be arguments on this point.
3) Why is the falloff more noticeable with chrome film as some people claim?
1. Retrofocus design (SLR, due to mirror constraint) vs. non-retro ~symmetrical design (RF / LF) PS: they do have fall-off that is corrected by stopping down, due to mechanical vignetting, just like LF lenses but there's still the inherent fall-off from traditional designs that isn't corrected.
2. As mentioned above, designs vary, and also consider that image circle may affect this (if you are closer to the edge of the IC, more fall-off).
3. Less dynamic range available - 2 stops underexposed means you are dangerously close to just having nothing usable on the film.
Re: Best 75mm / 90mm lens?......totally kidding, but not really...read on pls
Quote:
Originally Posted by
Kiwi7475
The fall off follows a cos^4, no need to waste a negative to confirm that IMO. All you need to know is the angular FOV of your lens, diagonally.
From what I find in the documentation of filters is that wideangle lenses follow cosł. The cos⁴ and cos (no power) is more for "contactcopy machines with point source light" and large format copy machines.
Re: Best 75mm / 90mm lens?......totally kidding, but not really...read on pls
Quote:
Originally Posted by
Havoc
From what I find in the documentation of filters is that wideangle lenses follow cosł. The cos⁴ and cos (no power) is more for "contactcopy machines with point source light" and large format copy machines.
Now that we get into the the weeds, let me repeat here what has already been discussed in this very forum a few times before:
“cos^4 is the basic limiting physics, cos^2 from inverse distance squared fall off, another cos factor from the incident angle of light on the film surface, and the final factor of cos from the apparent ellipticity of the aperture off axis. There are two ways that real lenses differ from this theoretical rule. First, the apex of the off-axis angle can be moved forward using a retrofocus design. Second, some superwide designs mitigate one cos factor somewhat by using the trick of tilting the enterance and exit pupils. This mostly eliminates the ellipticity of the off-axis aperture, leaving only three factors of cosine theta and an improved fall off of the off-axis illumination.
You can determine whether a particular lens uses this trick by looking at it from various angles. Stop the lens down a couple of stops and look at the front straight on -- both the outside barrel of the lens and the diaphragm will be circular. Next, keep looking at the center of the lens and slowly rotate the lens so that your eye is no longer on the optical axis. On most lenses both the outside of the barrel and the aperture diaphragm will become elliptical. On lenses using the tilting pupil principle, the diaphragm will remain almost circular as you tilt the lens.”
Re: Best 75mm / 90mm lens?......totally kidding, but not really...read on pls
Sorry I'm not sure if you mean Biogon "design" or a specific lens. I thought most of the modern short focal length lenses used this "trick". For example here is what Rodenstock said about the Grandagon-N range:
"...light fall-off at the edge has been greatly reduced thanks to an optical trick (“pupil distortion” = the entrance pupil diameter
increases when viewing at an angle)..."
Quote:
Originally Posted by
Dan Fromm
You're probably thinking of f/4.5 Biogons. One, the 75/4.5, covers 4x5. Big heavy expensive lens.
Re: Best 75mm / 90mm lens?......totally kidding, but not really...read on pls
Quote:
Originally Posted by
Michael R
Sorry I'm not sure if you mean Biogon "design" or a specific lens. I thought most of the modern short focal length lenses used this "trick". For example here is what Rodenstock said about the Grandagon-N range:
"...light fall-off at the edge has been greatly reduced thanks to an optical trick (“pupil distortion” = the entrance pupil diameter
increases when viewing at an angle)..."
I meant specific lenses. 35/4.5, 45/4.5, 53/4.5 and 75/4.5 Biogons for, respectively, 6x6, 6x7, 6x9 and 4x5.
Your R'stock quote is marketing fluff. I just looked at the "relative light fall of at ratio 0.03x" curves for Grandagon-Ns. It is worse that cos^4 at all apertures larger than f/16, marginally, as in practically speaking not at all, better at f/16.
Bob look at the charts before jumping in to tell us that R'stock is always right.
Kiwi, there's no profit in wrangling with people who think they're right.
Re: Best 75mm / 90mm lens?......totally kidding, but not really...read on pls
I don't think anyone is wrangling regarding the laws of physics. I posted the Rodenstock quote for reference/comment. It's not my quote. I didn't work there.
Re: Best 75mm / 90mm lens?......totally kidding, but not really...read on pls
Quote:
Originally Posted by
Michael R
I don't think anyone is wrangling regarding the laws of physics. I posted the Rodenstock quote for reference/comment. It's not my quote. I didn't work there.
You're not wrangling about the physics and I didn't blame the quote on you. I blamed Rodenstock's marketers.
Re: Best 75mm / 90mm lens?......totally kidding, but not really...read on pls
35mm wide angle lenses do have equivalent falloff, Alan. I personally hate that Natl Geographicky look. Falloff is more noticeable with chrome film simply because chrome film has more native contrast to begin with than negative film.
Re: Best 75mm / 90mm lens?......totally kidding, but not really...read on pls
Quote:
Originally Posted by
Drew Wiley
35mm wide angle lenses do have equivalent falloff, Alan.
No they don't, if we are talking about modern retrofocus designs. Sure, the 21mm Biogon for Contax 35mm rangefinder has the same basic fall-off characteristics of the 75mm Biogon on 4x5, but the retrofocus Nikon 14-24mm for 35mm / DSLR, when stopped down a bit, has very little fall-off comparatively.
Re: Best 75mm / 90mm lens?......totally kidding, but not really...read on pls
Retrofocus would apply mainly to SLR's. But if the lens design is similar, apples to apples, the format size won't make much difference relative to angle of view unless image circle movements are in play. But I'd hypothetically be interested in actual factory bench test results rather than general impressions, that is, if I was even in the market for something 35mm wide-angle, but I'm not.