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[RichS]

After playing with lenses on my 8x10, I hit a question of curiosity.

I've heard many times that closing down the arperture will offer a greater sized image circle. This allows lenses that wouldn't normally cover 4x5 (or 8x10) to actually cover.

But... When I tried this on the 8x10 where I could actually see the image circle, I could see absolutely no increased coverage! The edges of the image cirlce became better defined and sharper when stopped down through f-32 to 45 or so, but no greater coverage. On lenses with a brighter center, the illumination balanced out better, but no greater coverage... I tried this on quite a few lenses with the same results.

Am I not nderstanding what people are talking about when they speak of this? Do I just have some odd lenses that don't work that way? Do only certain designed lenses do this?

Anybody know???

Thanks,

Rich...

[Nick]

I'll try. You could be seeing any of a few things.

Modern lens are limited with some sort of blocking method that keeps coverage to below a certain point. Basically it stops you from using the lens when the quality wouldn't be good enough. Someone will explain this one better-)

The point you made about the edges getting sharper to me means you are getting better coverage. You've got two things. Circle of ilumanation. The circle of light that the lens puts out. Then you've got the circle of good resolution. This is what coverage really is. The problem is how do you define "good"? If you're contacting printing then 8lpmm might be good enough. If you've enlarging then you'll need more. So coverage is sort of marketing term. Defined differently by different people at different times. When the edges went from not sharp enough to good enough you just saw your coverage increase.

[RichS]

Thanks. I know there's an obvious difference between the circle of illumination and the actual useable coverage of the lens. But from other discussions, I got the impression that the circle of illumination got larger when stopped down? Maybe things weren't explained better because those folks understood it better from the beggining? If only the usable coverage increases and the circle of illumination remains roughly the same, that I could easily understand, and see on the GG... Believe me though, some of those discussions made it very "obvious" that the circle of illumination grew as the shutter was stopped down and that just isn't what I'm seeing...

[clnfrd]

The opening of the aperture doesn't change the area of illumination of a lens. However, the smaller the aperture opening, the greater the depth-of-field(or Focus)...so, as a result,, the usable area may be increased on a lens that has poor focus around the edges with the aperture wide open.