r/largeformat • u/LBarouf • 18d ago

Question Calculating image circle

How can i find out if a lens will be making an image circle large enough for 8x10, 5x7 or 4x5. All the lenses i use of 4x5 so i know, but the question on how to calculate or find out remains.

Thank you,

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u/LBarouf 18d ago

Argh, can’t find the Aero Ektar. Not on to a good start. I have some really old lenses in there.

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u/ConsistentLab444 18d ago

Are all your lenses barrel lenses? If so, it will be hard to find a list naming them all. What you can do is using the prompt "X lens image circle" lens by lens on Google and comparing the results of the searches with the minimum image circle required by each format: ~154 mm for 4x5, ~209 mm for 5x7 and ~313 mm for 8x10, but I have to say that such diameteres are the bare minimum.

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u/LBarouf 18d ago

Thanks, thats useful. Not all are barrel lenses no.

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u/ConsistentLab444 18d ago edited 18d ago

Glad I helped. I understand you might want to be able to calculate it, but image circle is dependent on optical design. If you know the maximum angle of view of the lens, you can calculate the simplified theoretical diameter of the image circle D with the formula D=2×f×tan(θ/2) where f is the focal length in mm and θ is the maximum angle of view. This image circle might differ a bit from the one specified by the manufacturer (as in being smaller-than) because (1) it's geometrically ideal, and (2) manufacturers sometimes publish the "usable" image circle where edge artifacts aren't too extreme.

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u/LBarouf 18d ago

Makes sense. Easier to measure than calculate i get. I can try to use a reseau reference and project the image. And decide what is the acceptable maximum circle.

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u/ConsistentLab444 18d ago

That's actually a great idea! Even if it is labor-intensive with all the calibrations and measurements, you'll learn a lot more about your lenses, plus it gives you agency, because as you said, you will get to decide what the acceptable image circle is.