Questions about understanding Image Circles

[ComicDom1]

Since I am new to Large Format photography I have been reading and studying day and night. Of course one of the factors that I paid attention to while putting together my gear was the size of the Image Circle of the Lenses I was choosing. Color,Contrast, and Sharpness were also a concern.

I have been wondering on how the F stop selected affected the size of the Image Circle projected on the Film Plane. So I started looking for information on how to Calculate the Image Circle Size. Since I noticed when looking at lens specs that the manufacturers all seem to spec their lenses Image Circles at F 16 or F 22 I started to wonder what size are the Image Circles at F 4.5, 5.6, 8, 32, 45, or even F 64? Do image circles get progressively larger as you stop down and progressively smaller as you open up the lens? This might seem like a very simple question for some but also its a complicated one for someone like me who is new and might not understand yet how everything works. I certainly did not want to ruin an image because I did not stop down my lens enough to get a large enough Image Circle for coverage, and at the same time, I did not want to limit the F Stops that were usable with particular lens on my camera.

After a little searching, I found that Schneider Optics posted in one their FAQs that the Image Circle size does not really change. They claim that what happens is that the distribution of light changes as the F stop is changed on the lens. For example if I take my 75mm 4.5 Nikon SW Lens, and open it all the way up to 4.5 the brightest part of the lens is the center and the light hitting the edges of the projected Image Circle is diminished in intensity compared to the center. Of course my conclusion from this is that the wider the lens the bigger problem this would be based on the curvature of the glass and design of the lens.

So based on my long winded post, I am wondering how important it would be to know the size of your Image Circle for each F Stop of your lens?

I also understand that as you stop down your sharpness is also affected, so I am wondering how you know at what point, F stop, Image Circle Size, that the sharpness degrades for any particular lens?

All answers would be appreciated.

Thanks
Jason

 

[Leigh]

Hi Jason,

The aperture of a lens varies continuously from minimum to maximum. The detents and f-number legends are simply for operator convenience regarding exposure and depth of field calculations.

Every lens will have a point of optimum sharpness, which varies for each lens type and manufacturer. The general rule of thumb is that best performance is achieved at two stops down from full open.

Light fall-off is also a universal characteristic that varies somewhat with lens model and manufacturer. It is indeed worse with wide-angle lenses, to the point that center filters are available for some of them.

The bottom line is...
Either you obtain the information from the manufacturer of the specific lens in question, or you run some controlled tests to determine how your example works. The latter option is a worthwhile exercise in any case, since individual lenses may vary a bit from nominal.

- Leigh

 

[ComicDom1]

What individual type tests do you suggest? 3 out of 4 of my lenses are Nikon. 2 of them are Nikon SW lenses and the third a regular Nikon 210mm. The fourth lens is a Schneider 180/312 5.6/12 convertible lens.

 

[Leigh]

What individual type tests do you suggest.

Same as with any lens. Test whatever parameters are important to you.

If you're concerned about sharpness, run resolution tests at every half stop throughout the range, with high-definition targets accurately focused. Check at both center and edge.

For image circle and fall-off characteristics, shoot a uniform-reflectance subject and measure the density of the negative at different points. (An 18% gray card filling the image and close to the lens with the camera focused at infinity is optimum.) Plot the curve. Repeat the test with the lens shifted to maximum.

For distortion (barrel/pincushion) you need accurate rectangular targets like the old TV camera targets.

- Leigh

 

[Oren Grad]

To expand a bit:

There are at least five factors that affect the size of the *useful* image circle:

* Optical vignetting - the degree of light falloff inherent in the optical design

This varies across different lens designs - for example, wide angle designs generally have much more optical vignetting than normal ones. Typically improved by stopping down. How quickly and how much it improves depends on the optical design. Some manufacturers provide center filters, which can partially correct for severe optical vignetting in some wide angle designs.

* Mechanical vignetting - blockage of light by the lens mount

Again, varies with the lens - depends on the design of the optical cells, and what size shutter the cells are fitted to. With some lenses, this is the constraint that matters; with others, the image is so degraded by other factors before you hit mechanical vignetting that the mechanical blockage doesn't matter.

* Falloff in resolution and contrast due to optical aberrations

Typically improved by stopping down. How quickly and how much it improves depends on the optical design.

* Degree of enlargement intended for the resulting picture, and how closely and critically the prints will be viewed

Different users may perceive different useful image circles for a given lens, depending on their preferences and intended uses. (See the recent thread here on the 75mm Wollensak oscilloscope lens for an extreme example. Aberrations can be lovely. )

* Image circle increases proportionately with bellows extension - i.e., as you focus closer - as a consequence of the geometry of the projected image cone. At life-size magnification - 1:1 - the image circle is twice that specified for infinity focus.

____________

Further to Leigh's comments, you can't really calculate this based on lens specifications. If you must have a precise description of a lens's behavior, you'll need to test for yourself. I don't know of any manufacturer that provides image circle specifications for the full aperture range. AFAIK, only Schneider provides specifications even for two points - wide open and f/22 - for its recent lenses.

I wouldn't sweat it, though. For most lenses and for typical uses and users the manufacturer's specification for f/22 at infinity is a reasonable guide. Beyond that, you'll quickly get a good enough feel for the useful image circles of your lenses just by using them.

Good luck and have fun!

 

[ComicDom1]

I appreciate your answer. Basically my purpose in asking my question was to find a way to calculate and find the range of useful F stops for each one of my lenses. I would think this information would already be somewhere on the web but I have not found it yet. Being new to Large format, it would be very useful for me to know this information instead of having to burn film to find that range. I thought there would be a way to calculate all the information and make the task easier.

Once again thanks for your response.
Jason

 

[Leigh]

...to find a way to calculate and find the range of useful F stops for each one of my lenses.

Hi Jason,

I don't understand what you're trying to achieve.

All f/stops are "usable" for any lens. Manufacturers would go out of business if that were not the case.

Datasheets for LF lenses normally specify the largest format for which the lens is well suited. If you're really concerned about it, choose a lens designed for the next larger film size, i.e. a 5x7 lens for a 4x5 camera.

The fall-off is gradual, not abrupt, and applies uniformly over the image. If the effect is excessive in a particular image, you can compensate in the darkroom or on the computer quite easily.

The key is the fact that each image is different, and must be evaluated independently.

- Leigh

 

[SCHWARZZEIT]

When it comes to your working f-stop I recommend to use what you need in terms of DOF. Rodenstock for example specifies a working aperture range for each lens. At those f-stops the overall image quality is best. They recommend to use the larger f-number if performance at the edge of the image circle is critical.

Don't be afraid of diffraction. On most 4x5" lenses you'll only notice image degradation when stopping down much further than f/22, but even f/32 might be totally acceptable for your workflow. If you're scanning the performance of your scanner has probably a much greater impact on the quality of your final image than diffraction.

-Dominique

 

[ComicDom1]

Thanks, that is very good information to have. I am using an Epson V700 to scan in my negatives and I am finding out that the scanning techniques(settings) I use are very important as well. I am pretty much all set up with the 4x5 except for fine tuning information like I mentioned above.

Jason