Resolution of different focal lenghts of same lens

[Richard Årlin]

Take any lens that comes in different focal lenght for different formats, eg 4x5, 5x7, 8x10, would they have the same resolution regardless ? I am thinking of "compressing" the number of lenses you carry or own. A typical 8x10 wideangle, say a 158 mm Cooke wide-angle anastigmat series VIIb would it be as sharp as a 82 mm on 4x5 and could it be used as a normal standard lens replacing for example a 150 Apo Symar made especially for 4x5 without loosing quality. It would be nice to have a minimized series of lenses usable for both formats like (82, 108 ) 158, 240, 360 and maybe a 450 fuji c that could be bought selling some then unnescessary lenses.

Cheers, Richard

[Brian Vuillemenot]

You can use any lens with a big enough image circle to cover a given format for that format, as well as all smaller formats (any lens capable of covering 8X10 will cover 4X5 and 5X7). Resolution is a function of the given lens, and not the image circle. Any modern multi coated lens from the big four will have a resolution not easily destinguishable from any other modern lens under almost all real world working situations. If a lens for 8X10 has a low resolution on that format, so will it on 4X5 and 5X7, although the loss of resolution may be amplified somewhat by the smaller film size and greater enlarement factor required.

The main drawback with using 8X10 lenses for 4X5 is that they are physically much larger, many in Copal #3 shutters, and the rear element may be to big to mount on your 4X5 camera. 4X5 cameras in general are not as beefy as 8X10s, so the front standard may not be able to be kept firmly in place. A huge Copal #3 lens on a lightweight 4X5 camera will also shift the center of gravity forward, and make it suseptible to tipping over. In addition, many lenses for 4X5 will not quite cover 8X10, for example the 210 mm Apo Sironar S. I've used a large 8X10 lens for both 4X5 and 8X10 (300 mm Apo Sironar S) for quite a while, and it's worked fine, although you may be better off going with something slightly smaller (300 mm Nikkor M and Fujinon C are both in copal #1 shutters and will cover 8X10 with minimal movements).

[Dan Fromm]

Um, Richard, if you want, say to use a 158 Cooke Ser VIIb as a 4x5 normal lens instead of a 150 Apo Symmar, the thing to do is shoot the two lenses as you would normally -- same apertures, ones that you usually use -- and then ask whether both sets of negatives will give you "good enough" prints at the size you normally make. And if you want to replace an 80 shot on 4x5 with the 158 Cooke on 8x10, do the same. Your standards are the ones you have to meet, and none of us can know to what standards you work.

[paulr]

If you don't have the opportunity to test, it's worth noting that lenses for larger formats tend to perform worse than lenses for smaller formats, all else being equal.

Within the same line of lenses, you'll see performance gradually decline as the focal lengths (and image circles) increase.

[Brian Ellis]

"Within the same line of lenses, you'll see performance gradually decline as the focal lengths (and image circles) increase."

That's interesting, I don't think I've ever heard that performance declines with longer focal lengths in the same line of lenses. I'm not arguing, just curious - what's your basis for that statement?

[paulr]

Take a look at the published MTF curves from Schneider or Rodenstock. Schneider's are at schneideroptics.com; I believe Paul Buzzi has hosted Rodenstock's on his site. Look at the curves for any of their normal lens lines (like the apo symmars that Richard mentions). It's a steady downward progression with increasing focal length.

In that last post I suggested this would be true with all lens lines, but I just browsed through and it looks like it doesn't hold true (at least not as consistently) with wide angle lens designs.

[Scott Rosenberg]

isn't it pretty well established that resolution deteriorates as one moves away from the center of the lens? that said, would one therefore want to use the lens with the largest IC - maximizing the amount of image formed by the 'sweet spot'? assuming a good compendium is also employed to reduce as much non-image forming light from entering the bellows as possible. of course assuming the resolution of all lenses comprable with equivalent rates of deterioration as one moves from the center, which we also know not to be the case.

[paulr]

isn't it pretty well established that resolution deteriorates as one moves away from the center of the lens? that said, would one therefore want to use the lens with the largest IC - maximizing the amount of image formed by the 'sweet spot'?

There is some truth to this. The trouble is that in general, when you move towards a wider image circle and more even performance from corner to corner, the performance near the axis declines. At the extreme you end up with perfectly even but poor performance from corner to corner.

[Oren Grad]

>> I don't think I've ever heard that performance declines with longer focal lengths in the same line of lenses. I'm not arguing, just curious - what's your basis for that statement? <<

I've just checked this for two modern lens series. MTFs provided by Rodenstock for the Apo-Sironar-S line show performance in the center of the field at f/22 decreasing steadily but very modestly as the focal lengths increase. Remarkably, Schneider MTFs for the Apo-Symmar L line show essentially no deterioration in the center of the field at f/22 beyond the 180 focal length, though there is some deterioration at larger apertures as focal lengths increase.

Within a given focal length, lenses with bigger image circles do not necessarily deliver the same performance in the exact center of the field as those with smaller image circles. Whether it gets better or worse depends on the specific lenses, but in general among modern lenses the differences at the center of the field are not large at typical working apertures, and what happens as you depart from the center is much more important in selecting the optimal lens for a given application.

>> of course assuming the resolution of all lenses comprable with equivalent rates of deterioration as one moves from the center, which we also know not to be the case. <<

That's correct, it's not true in general.

[Brian Ellis]

So is it fair to summarize by saying that within the same line of lenses resolution deteriorates as focal length increases - sort of?