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# Thread: Sekor vs Takumar vs Flektogon

1. Originally Posted by JW PHOTO
Nope! Draw out a 6x6.45 rectangle and then draw out a square 6x6. Then make an "X" by going from inside corner to inside corner. Then, with a compass or string, use the point where these lines intersect and draw a circle around each with the circle just touching the corners. You'll see for yourself the image circle for the 6x6 is larger than the 6x6.45. Not by much, but it is bigger. JW
Or stated simply, circumscribe a circle about each frame. Anyway yes, if I've done it right, I find that the 6x4.5 frame requires a circle of about 7.5 cm diameter, while the 6x6 requires one of almost 8.5 cm. (Measured exactly to the frame corners, but of course the coverage circles go further.) Anyway thanks for this, JW, because I previously had the same wrong thinking as Bob ("Wouldn't the image circle required for these formats be identical?").

Now that I think further on it, I guess I had this wrong too: I always thought the Mamiya RB/RZ needed a lens that could cover a notional 7x7 frame, since the 7cm dimension can be horizontal or vertical. Upon doing the drawing, one finds that you actually don't need to cover a notional 7x7 frame (needing about 9.9 cm diameter) but rather a "+" pattern of overlapping portrait and landscape 6x7 frames, which requires no greater coverage than a fixed portrait or landscape 6x7, so "only" a 9.2cm-diameter circle (still big).

--Dave

2. Originally Posted by Argenticien
Or stated simply, circumscribe a circle about each frame. Anyway yes, if I've done it right, I find that the 6x4.5 frame requires a circle of about 7.5 cm diameter, while the 6x6 requires one of almost 8.5 cm. (Measured exactly to the frame corners, but of course the coverage circles go further.) Anyway thanks for this, JW, because I previously had the same wrong thinking as Bob ("Wouldn't the image circle required for these formats be identical?").

Now that I think further on it, I guess I had this wrong too: I always thought the Mamiya RB/RZ needed a lens that could cover a notional 7x7 frame, since the 7cm dimension can be horizontal or vertical. Upon doing the drawing, one finds that you actually don't need to cover a notional 7x7 frame (needing about 9.9 cm diameter) but rather a "+" pattern of overlapping portrait and landscape 6x7 frames, which requires no greater coverage than a fixed portrait or landscape 6x7, so "only" a 9.2cm-diameter circle (still big).

--Dave
Except the RB needs to be able to cover 6x8 horizontally, because there is a 6x8 motorized back for it .

For 6x4.5, complicating things further, some of the Mamiya lenses have internal baffles that are essentially rectangular. As a result, the lenses cover more in one orientation than they would in the other, if there were some way of rotating the backs.

3. Originally Posted by Argenticien
Or stated simply, circumscribe a circle about each frame. Anyway yes, if I've done it right, I find that the 6x4.5 frame requires a circle of about 7.5 cm diameter, while the 6x6 requires one of almost 8.5 cm. (Measured exactly to the frame corners, but of course the coverage circles go further.) Anyway thanks for this, JW, because I previously had the same wrong thinking as Bob ("Wouldn't the image circle required for these formats be identical?").

Now that I think further on it, I guess I had this wrong too: I always thought the Mamiya RB/RZ needed a lens that could cover a notional 7x7 frame, since the 7cm dimension can be horizontal or vertical. Upon doing the drawing, one finds that you actually don't need to cover a notional 7x7 frame (needing about 9.9 cm diameter) but rather a "+" pattern of overlapping portrait and landscape 6x7 frames, which requires no greater coverage than a fixed portrait or landscape 6x7, so "only" a 9.2cm-diameter circle (still big).

--Dave
Don't feel to terrible bad about thinking the way you thought. I think that way about a lot of things and sometimes, years later, find out I was full of beans. When I was in high school my counselor wanted me to take all "college prep" courses and I got stuck in all the science classes like biology, chemistry, physics along with algebra, geometry etc.. I hated it and was so jealous of the kids in shop class that had followed the "vocational arts" plan and were running lathes and pounding steel in shop class. Needless to say, I learned very little, hated school and still like pounding steel, welding etc.. The image circle thing becomes obvious when you do it, but being a smarty had nothing to do with it from this end that's for sure. JW

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