dumb question

[clarryd]

Is there a simple or easy way to determine if a particular focal length lens will cover
a particular format? I don't even know how to ask the question.
Thanks, Larry

[Gem Singer]

The easiest way is to click on LF Home Page on the blue tool bar above.

Scroll down to lens comparison charts.

Click, enter the format you need, and you'll find the answer.

[clarryd]

Thanks Gem

[mike rosenlof]

Not a dumb question. A good one. And there's no easy answer.

It all depends on the lens design, and to a smaller effect, your working aperture. A smaller aperture gives a little more coverage.

If you think of the 'normal' focal length of a format as the length of its diagonal, that focal length lens (i.e. 6 inch (150mm) on 4x5) "normally" covers that format with reasonable room for movements.

Some lenses, often with a 'W' or 'XL' in their names are designed to cover larger formats than their focal length might indicate.

Many process lenses, cover a narrower than 'normal' range. The 'narrow' aspect doesn't get displayed as loudly as Wide coverage, but it reflects the priorities of the lens designers. Apo (red dot) Artars, are one example of a process lens with a somewhat more narrow than 'standard' coverage.

In LF lenses, more elements often mean wider coverage. Not because they have to, but that's when LF lens designers usually do with more lens surfaces.

You normally need to find lens manufacturers' specs to determine what a lens covers. Most general purpose lenses are specified at infinity (or nearly so) focus. Most macro or process lenses have their circle of coverage specified at 1:1 magnification which is usually double their coverage at infinity.

[GPS]

...

If you think of the 'normal' focal length of a format as the length of its diagonal, that focal length lens (i.e. 6 inch (150mm) on 4x5) "normally" covers that format with reasonable room for movements.

...

That's like saying - if you can see the dog you can see it...
First of all, you have to know for what format the lens is made - only then can you judge if it is a "normal" focal length for that format or not. 150mm lens for SLR or 150mm lens for digital backs or 150mm lens for LF... etc.

[EdWorkman]

No
Go to cameraecccentric and peruse some old lens catalogs to get a taste.
"Image Circle" is related to focal length only in a most tenuous way, such as a 15mm lens almost certainly won't cover 4x5 [ but wait there was that........]
Then there's "circle of illumination" vs "circle of best definition"- and you gotta know what, and whose, the definition of "best" is.
The good news is that there are lots of threads here that discuss these topics and specific lenses. The most direct way to begin might be to search for information about specific lense[s]- "Will a 7 inch f6.3 Frazanar cover 7x11", or maybe "Who made Frazanars?" so you can find a catalog on line. A little searching should get you enough so that you will be able to frame good questions, and understand answers.

[Leigh]

The parameter you're looking for is the diameter of the image circle (IC).

You'll find this on the data sheet for any modern lens.
Since the IC gets larger as the lens is stopped down you'll usually find two values, one wide open (f/5.6 or ?) and one at f/22.

To cover a particular film, the IC diameter must be larger than its diagonal dimension (e.g. 163mm for 4x5).

Since large format work frequently requires lens movements, we like to have an IC larger than the minimum.

A good example of the range of values you might encounter can be found with two Nikor 150mm lenses:
The 150mm f/5.6 Nikor-W has an image circle of 210mm, which covers 4x5 adequately, while
the 150mm f/8 Nikor-SW has a huge image circle of 400mm, which easily covers 8x10.

- Leigh

[rdenney]

(e.g. 163mm for 4x5)

Less than that. 4x5 is a nominal dimension. My standard film holders provide a diagonal of 154mm. For some lenses, that's a critical difference (65/8 Super Angulon comes to mind).

For the OP, one of the challenges of lens design has been to provide a larger image circle for the same focal length. That's a big reason why tessar designs gave way to plasmat designs, and why plasmat designs were unsuitable for lenses shorter than 90 or 100mm, leading to wide-field designs like the Super Angulon and Grandagon.

Many large-format cameras have ground glass with the corners notched to allow you to look at the lens directly from the corners. If you can see the whole aperture shape through that notch, then the lens is providing coverage. If part of it is blocked off, the corner will be dark. If you can't see the aperture at all, it's outside the lens's coverage. Smaller apertures are less likely to be hidden behind the barrel of the lens, which is why coverage improves at smaller apertures.

Rick "who likes long tessars and short Super Angulons" Denney

[Leigh]

Less than that. 4x5 is a nominal dimension. My standard film holders provide a diagonal of 154mm. For some lenses, that's a critical difference (65/8 Super Angulon comes to mind).

I always use the full film diagonal as a guide for IC rating. It provides a bit of a safety margin.

I was going to put that comment in my earlier post, but that would have deprived you of the opportunity to criticize.

- Leigh

[rdenney]

I always use the full film diagonal as a guide for IC rating. It provides a bit of a safety margin.

I was going to put that comment in my earlier post, but that would have deprived you of the opportunity to criticize.

As I said, with some lenses, it matters.

Example: The published image circle of a 47/5.6 Super Angulon (not XL) is 123mm at f/22. The diagonal of 6x12 is nominally 135mm, but it is actually 125mm (56x112). One might reject that 47 for 6x12 use if they use the nominal values, but in fact that lens does work, as long as they don't need movements. Schneider already put some margin in the value. There are times when details do matter.

Rick "thank you" Denney