Oh, come on, John, you have enough of the lenses to know the answer.
Oh, come on, John, you have enough of the lenses to know the answer.
I'm Jac.Originally Posted by Dan Fromm
John is my evil twin, and an only child.
The biogons I use gain no coverage when stopped down, but I remain ignorant of variants of the so-called Biogon.
As for image circle/circle of illumination, they are two different things entirely. Some lenses do have internal baffles so that the two circles coincide, but generally most lenses will illuminate far more they will cover sharply.
I use (on 8x10) an 8 1/4" Dagor type lens which at f:32 presents an aperture that viewed from the cut corner of the GG looks like a cat's pupil. The corners need to be dodged, but they do retain contact printable detail.
Certain manufacturers have very conservative coverage figures, quite a few lenses will give contact printable detail well beyond their rated coverage.
One man's Mede is another man's Persian.
Gentlemen, thank you for the insights.
Correction to my original note, in that MTF is Modulation Transfer Function, as modulus is an engineering materials issue from my distance past.
Dan, so from the examples you provide you are saying YMMV Given the different common lens types used for ULF, which of the different designs i.e. plasmat (G-Claron), dialyte (Apo Ronar) and tessar (Nikkor M), falls off more rapid than another?
David and Oren, certainly aware of the impact of stopping down on coverage and using the more of the centre of the lens and less of the edges, but like Mark I am uncertain as to the extent, nor do I expect any means of determining, other than of course the ground glass and the print. And if we go too far down that road, then we run into diffraction impacting image quality.
Nathan, interesting regarding your comment on variation in MTF in focal length; then does it logically follow that the longer lens focal lengths have a less stringent MTF requirement? Given your requirements in the industrial photography environment, you have taken the opposite tact of what we are talking about here for ULF, in that you are essentially de-rating the image circle to address your stringent requirements for that application.
John, yes trying is the most certain way, but of course is time consuming and expensive i.e. Ilford HP5 in 8x20 cost over C$12 per sheet when I bought it last year. Hence the questions to try to understand and reduce the number of dead ends I need to travel down.
I will likely follow the common trail traveled by others, but as indicated in my thread title, I do have a curious (and analytic) mind.
FYI, for the 8x20 I do have a 355 G-Claron in a shutter, a 420 Apo Ronar in a shutter, and a 480 and a 600 Apo Ronar in barrels. The 420 Apo Ronar (IC=346mm?) will cover 8x20 at around 10 feet no problem, but need to try it for infinity, yet.
Len, if the lens doesn't have a field stop to limit coverage -- I think my little 80/6.3 WF Ektar does, that's why its claimed coverage is much less than that of other very similar 4/4 double Gauss type wide angle lenses -- MTF plummets towards the edge of the field covered. Whether that happens inside or outside of the coverage the manufacturer claims depends on the claim. Ain't no rule.Dan, so from the examples you provide you are saying YMMV Given the different common lens types used for ULF, which of the different designs i.e. plasmat (G-Claron), dialyte (Apo Ronar) and tessar (Nikkor M), falls off more rapid than another?
Apropos of "ain't no rule," for contact printing and for many situations where the negative is enlarged too discussions like this one are somewhat irrelevant. Contact printing doesn't demand as much image quality as enlargement and sharpness in the frame's corners isn't always important for the image. This last point -- who needs good image in the corners? -- has been made many times and bears repeating.
Well, come to think of it, there is a rule. People selling used lenses often claim greater coverage than the manufacturers did.
What this means for you is that you'll have to do your own testing. There's no other way. You can't trust anyone to know your requirements and standards.
If you can't afford to test, assume that the manufacturer got coverage right and and to be safe try not to set up shots with significant detail in the corners. You can't be sure that what looks sharp on the ground glass will look sharp on film or paper, you can be sure that what doesn't look sharp on the gg won't look sharp on film or paper.
This is a trick I have used with success with my 120mm Fujinon. It has an image circle of 288mm, according to the specs. However, when I focused on an object that was only about 20 feet away, the image circle adequately covered an 8x10 negative. Then, when stopped down to f22, the depth of field was sufficient that the clouds on the horizon were quite sharp.
Just an added comment...different lenses do "circle of illumination" different. Some lenses seem to drop off gradually, and while the circle of adequate sharpness is (for example) 300mm, the circle of illumination might be significantly larger. Other lenses, such as my 7.5" Ilex Paragon, stay sharp until the illumination drops off like a rock. Therefore, there is little difference between the two.
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