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View Full Version : Looking to understand area of coverage vs image circle?

ryanmills
23-Mar-2016, 14:00
How does the degrees in area of coverage come into play if the image circle is dynamic based on focus distance and F-stop. Looking at a 240mm APO-Sironar N with 72 degrees vs 240mm APO-Sironar S with 75 degrees vs 250mm Fijinon 6.7 with 80 degrees. Each should see a significant improvement in movements going up but it that true at all f/stops?. I see both area of coverage and image circle are measured at f/22, do they not mean the same thing? What is that coverage degrees specifying? Does one come into play at other f/stops and focus distances. And for that matter when they measure at f/22 is that at infinity? Normally i dont get wrapped up in MTF charts and specs but I'm realizing just how little I understand and wish I knew it better.

mdarnton
23-Mar-2016, 14:07
The image circle and coverage are the same thing. Angle can mean two things, depending on the manufacturer. In modern lenses angle is a different way of stating image circle; that is, the angle that the lens covers, at infinity, will span the defined image circle. Some makers of the past (Wollensak, for instance) would define the angle that the lens would cover on the specified film size, measured diagonally on the film, which would not tell you the maximum specific image circle covered. Then they'd give you a hint, buy specifying a potentially larger film size when stopped down, IF the lens would reach that. No precise numbers though, and I suspect that's because old lenses faded out around the edges and the absolute image circle depended on how low your standards were, where many new ones are mechanically vignetted to cut off anything beyond what the manufacturer wants you to use.

F22 is usually specified because that's where a lot of people set their lenses as a minimum, and that's where a lot of LF lenses are at their best, but as you stop down coverage goes up gradually with some lenses, so f22 is a good midpoint in that sense, also.

Image circle is dynamic relative to distance, but is conventionally measured at infinity. Angle and circle are also both dynamic relatively to lens opening and conventionally specified at f22. Gotta start somewhere. As you hint that you suspect, at closer distances the specified image circle becomes irrelevant because the projected circle becomes larger the farther the lens is from the film, but if you want to do the math, the angle of coverage will stay the same and can be used to calculate the image circle if you know the extension of the lens at some specific distance.

Peter De Smidt
23-Mar-2016, 14:21
I agree with Michael, 'image circle' and 'coverage' mean the same thing. On older lenses there can be a much larger 'circle of illumination', the area that the lens would light up on a ground glass, but modern lenses often make the 'circle of illumination' come close to the 'image circle.'

While it varies a bit, the image circle tends to get larger as the aperture gets smaller. For instance, G-Clarons' coverage increases significantly from f/22 to f/45. Usually what happens is that the corners get sharper, but there might be a slight loss in the center due, probably, to diffraction.

Coverage means the size of image over which the lens will give its rated performance. The criteria might vary from manufacturer to manufacturer.

As one focuses closer than infinity, the image circle enlargers. For instance, if the size of image circle at infinity is 200mm, at 1x magnification, i.e. where image size equals subject size, the image circle will be 400mm. So a lens may be fine for a given format at usual portrait distances but not cover the format at infinity.

ryanmills
23-Mar-2016, 15:40
Thanks, I was reading old posts on the topic and at least one said they were different. I must have misunderstood. I'm assuming its not a linear plot when stopping down or opening up from the measured f/22 as well. The MTF and illumination charts are plotted across the different f stops but still at infinity focus?

A big part of trying to understand this was changing from and APO-Sironar N to S. The fall off this dramatic and the edge sharpness is just horrible well before the light dropoff with the N. I was looking at portrait examples in the ~240mm range and was really surprised to see the obviously large movements with a fujinon 250mm where I could get very little movements with the 240mm N. Shots i know were not stopped down past f/8 to f/16. Till i realized how special the coverage was on the 250mm Fujinon f/6.7. Learn something new every day I guess.

mdarnton
23-Mar-2016, 16:45
Most of the early Fujinons, the ones with inside lettering, have very large image circles for their focal lengths.

Peter De Smidt
23-Mar-2016, 16:59
The big plasmats in Copal 3 shutters, 240mm and above, were mostly likely studio lenses, due to their size and cost. The main reason, I expect, for their big apertures was ease of viewing. Back then, few of these would've been used wide open other than for viewing on the ground glass. Movements for things like fashion were probably not very extreme. There were large coverage lenses where big image circles were needed, but the bigger ones, e.g. 210mm super angulons, were very pricey, probably only justifiable for high-end architecture and similar.

Some of the less expensive smaller lenses, such as g-clarons and fujinon As, usually have significantly more coverage stopped down then the big plasmats.

ic-racer
23-Mar-2016, 17:33
How does the degrees in area of coverage come into play if the image circle is dynamic based on focus distance and F-stop.

Angle of view stays the same. As the lens is moved closer to the film, the entire image, the circle and its contents, get bigger.

mdarnton
23-Mar-2016, 18:53
Farther from the film, you mean.