Image circle definition

[Dario]

Hi folks

Recently I fitted my Nikkor-W 240mm lens to my 8x10 and raised the lens to see how far I could go before the image became cropped at the corners. The ground glass was in vertical format. To my surprise, the maximum allowable lift seemed to be about 40mm.

I say "surprised" because I'd calculated the maximum lift to be about 7mm, assuming an image circle of 336mm diameter. I don't think I made a mistake in the calculation, but possibly.

That led me to wonder what defines the image circle. Is it the circle of visible image (thus defined by the physical construction of the lens) or is it the circle beyond which image sharpness becomes unacceptable?

Can anyone shed any light, please?

Dario

[Leigh]

Hi Dario,

There are many ways an image circle can be defined.

The smallest is the area in which there is no image degradation at all.

The largest is the area beyond which there is no light at all.

I don't know if any manufacturer ever defined the criterion that was used for its lenses.

- Leigh

[Pfsor]

Dario, there is a difference between an "image circle" definition and a "circle of good definition" definition. Google is your friend.

[Alan9940]

You don't mention if the camera was focused at infinity, but the closer the focus the larger the image circle at any given aperture; in addition to the good advice already mentioned.

[Leigh]

Dario, there is a difference between an "image circle" definition and a "circle of good definition" definition. Google is your friend.

The major difference is that...

Image Circle appears on lens datasheets.

Circle of Good Definition does NOT appear on lens datasheets.

Google does not understand any question, and can vector you way off in the weeds.

- Leigh

[xkaes]

This has probably been discussed ad nauseum on this board, so "here we go".

4x5 film is not 4x5 inches. There are undoubtedly minor differences from one company to another for various reasons, such as the metric vs imperial measurements. This is important when someone is interested in buying a lens or dealing with "image circle" or "circle of good definition".

First, the 4x5 film I measured is 3 7/8 x 4 7/8 inches or 100 x 125mm. That produces a hypotenuse of 158-160mm. But, that is irrelevant since the entire piece of film is not usually exposed.

The ground glass is where the image falls. One my cameras, it is 95 x 120mm with a hypotenuse of 153-154mm. But, that is also irrelevant since the part of the film to be exposed can be slightly large or smaller.

I have two types of film holders: Lisco Regal II and Mido (type 1, second version). The area that can be exposed is smaller than the film size, and different from the ground glass size. Specifically, the Lisco holders are 92 x 120mm, hypotenuse 151mm, while the Mido (which I use the most) is 94 x 123mm, hypothenuse 155mm. One is smaller than the ground glass, and one is larger.

So for my gear, I need lenses that will cover 155mm -- based on the hypotenuse of my largest film holder. Most manufacturers test their gear at f22 where the "image circle" and the "circle of good definition" (however you want to define that) should be about the same. I also need to keep in mind that if I open the f-stop wider than that there will be changes to the "image circle", the "circle of good definition", and the light fall-off.

When I take a picture, I usually try to include a LITTLE bit more in the composition because, you can always cut things out on the easel, but you can't add things in!

[MAubrey]

The IC is established by Nikon based on what they view as appropriate image quality. They have a standard expectation for IQ and they set the image circle accordingly. But the circle of illumination covers 11x14+ and the results are still quite reasonable for contact printing.

[Pfsor]

I don't know if any manufacturer ever defined the criterion that was used for its lenses.
- Leigh

Image Circle appears on lens datasheets.

Circle of Good Definition does NOT appear on lens datasheets.
- Leigh

[Pfsor]

Google does not understand any question, and can vector you way off in the weeds.
- Leigh

Neither does a book - yet clever people use them to get their answers. Ever noticed that?

[Dario]

Thanks folks for your considered thoughts. MAubrey's comment seems to tally with my findings. I'll take a shot or two right to the edge of the visible image and see what the quality is like.