Coverage of short focal length lens on 8x10 at close focusing

[Scott A. Wells]

I am interested in knowing if anyone can explain the possibility of using a shor t focal length lens of say 90mm or less for closeup work on an 8x10. I'm thinkin g that a 90mm with an image circle just large enough for 4x5 coverage be capable of providing an image circle large enough to cover an 8x10 negative. I'm assumi ng that the image circle becomes larger as you move the lens from the film plane during focusing at close distances. If this is possible at what focusing distan ces would I have to move the lens from the film plane to get the required image circle for full 8x10 coverage?. What happens to the lens image quality when a sh ort focal length lens are used like this.? Basically I'm interested in short foc al length wide angle macro work with an 8x10.

[Stewart Ethier]

Yes, this should work. It is the same principle that allows the 20 x 24 Polaroid camera to make larger than lifesize portraits with modern lenses that don't come close to covering that format at infinity focus. The diameter of the image circle should be proportional to the bellows extension. Thus, for example, if a lens covers 4 x 5 at infinity, it should cover 8 x 10 at 1:1, because the bellows extension is doubled, consequently the diameter of the image circle is doubled. As for image quality, all I can say is that the 20 x 24 Polaroids made in this way that I saw recently were astonishingl

[Stewart Ethier]

.... were astonishingly sharp.

(For some reason the last few characters of my messages seem to get truncated.)

[Robert A. Zeichner]

Not only should this work for reasons previously stated, but I have it on good authority that short lenses like, perhaps the 90mm you have in mind are exceedingly sharp when used for closeups. Let us know how your results turn out!

[Sergio Ortega]

Scott,

It works very well. Just remember to allow for a substantial exposure increase due to the bellows extension and resulting light loss. Good luck, Sergio.

[David Richhart]

Scott... I have read about using shorter focal length lenses from your enlarger for this type of work. I think it was suggested that the lens be mounted backwards on the camera for better results.

Lack of a shutter would not be a problem because after figuring in the bellows factor and reciprocity of the film, the exposures would become quite long.

[Michael S. Briggs]

You are correct that the image circle becomes larger as the lens is moved away from the film. The way to think of it is that the lens projects a cone of light. The area of the film plane intersecting the cone becomes larger as the apex of the cone (the lens) moves farther from the film plane. A particularly simple case is a 1:1 reproduction ratio (life size image). The lens is twice its focal length from the film, so the coverage is also increased by a factor of two. Therefore a lens that will cover 4x5 when focused on infinity will cover 8x10 at 1:1. If it barely covers 4x5 at infinity, you will have to go all the way to 1:1 to get it to cover 8x10.

This simple geometric argument doesn't prove that the image quality will be excellent. A very asymmetric lens optimized for infinity might have rather poor image quality at 1:1. Something like a G-Claron or Apo-Ronar should do well at both. I don't know how well a wide-angle design will do. If you already have the 90 mm lens, why don't you try it and see if you like the results? If the image quality isn't good enough, then you can ask on this forum for suggestions.

[Scott A. Wells]

Thanks to all you for your assistance. My only remaining thought on the subject goes back to image degradation as Michael above pointed out. It sounds reasonable to assume that a lens optimised for work at infinity may become not so usable at 1:1 working distances. I will have to borrow a short focal length or maybe purchase one and then resell it if i cant use it. Anyhow if anyone out there has any ideas on a good candidate for a 90mm or less please respond. I dont have enough information on using the enlarging lens or maybe a process lens as these are designed for 2 demensional flat art reproduction. Thanks everyone