Questions about nominal vs. effective aperture for LF close-up work in a studio.

[JMO]

I am putting together a small table top studio in my basement to begin close-up photography of still life subjects (of sizes that mostly would fit inside say a microwave oven). In preparation for this I remembered reading a side bar discussion and an accompanying chart (PDF below) in one of the Schneider LF lens brochures about the difference between "nominal" and "effective" aperture when using their macro lenses for close-up work. However, I admit I don't really understand what the Schneider brochure was trying to explain, and wanted to ask for help to understand how I should prepare to adjust aperture and exposure when I begin doing this close-up photography? I understand bellows extension exposure compensations, and so if I use a macro or other lens for a close-up I will expect to adjust exposure for bellows extension, but what do I need to consider in regards to "nominal" versus "effective" aperture? My sense is that the practical reality of what the Schneider brochure is explaining is that one should not stop-down the aperture adjustment of the lens too much because you might obtain an "effective" aperture that is so high that you will run into excessive diffraction effects. Is this correct? And what practical advice can LF Forum members provide for these considerations? Thank you...

Schneider Macro exposure correction rules.pdf

[mdarnton]

I think they're just saying that when your proper exposure of f 16 is at 1:1, it becomes, "effectively", an f 32. So you need to open two stops, to a "nominal" f 8 to get an "effective" f 16. Just a different way of saying what you already know.

[Doremus Scudder]

Your bellows extension factors compensate for this, so you don't have to worry.

Depth of field is a function of magnification, so will be shallower the closer you get and the more your image is magnified. Unless you need all the math connected with magnification and DoF to figure out some aspect of setting up your scene, then you can not worry about that either; just use the gg and/or focus spread to determine optimum f-stop as you normally would.

FWIW, it's easy to find the effective aperture without all the formulas: First, find the actual size of your aperture by dividing your lens' focal length by the f-number. Let's say we have a 150mm lens at f/22: 150 divided by 22 = ~6.8mm. Now measure your actual bellows extension and divide that by the number you just got. Let's say we have 300mm of bellows extension. So, 300 divided by 6.8 = ~44. For all intents and purposes, this is f/45.

Don't use this to adjust exposure if you've already applied a bellows extension factor! Or, you can leave the bellows extension factor out and use the effective aperture. One or the other, not both! What this really tells you is that when you're using the marked f/22 with 300mm of bellows extension, you're getting into the diffraction zone. If you can, open up to optimize resolution. However, this has to be balanced against image considerations. With flat-field work, of course, you can just use the larger aperture, but with 3D subjects, often the only solution is to stop down and live with the diffraction. While it is good to know that your lens doesn't need to be stopped down so far to get to optimum performance with close-up work, in practice, the DoF constraints almost always mean you are stopping way down.

Hope this helps,

Doremus

[Mark Sawyer]

Just to confuse things further, "f/stop" is a ratio of f (focal length) to aperture (the "stop", like Waterhouse stops). Since the "f" (focal length) is the distance from the lens to the film (focal plane), when you lengthen the bellows to focus closer, you're increasing the focal length. And since the aperture stays the same, you've changed the f to stop ratio.

There are dozens of tables, apps, formulae, and other tricks to figure it out. I prefer just measuring the aperture (through the front element) and the bellows extension and divide the f by the stop to find my true f/stop.

Hence my old girlfriend's frequent admonition, "Stop f-ing around and take the picture."

[ic-racer]

That little scale in the Schneider brochure is nice. I use something similar when doing still life. I find that determining magnification with a scale like that easier than measuring the bellows extension.

[Heroique]

Applying the appropriate bellows factor takes care of the difference between nominal and effective aperture, so on a practical level, you don't really need to understand the difference between the two. ;^)

But to explain things simply as others have … the aperture ring on the lens might indicate one f-number (say f/22), but as magnification grows larger and larger (i.e., as you focus closer and closer), this indicated f-number, shown on your dial, will stay the same (always indicating f/22), but the effective f-number will grow narrower and narrower (f/32 at 1:2, f/45 at 1:1). The effective f-number is changing because your distance (lens to film) is changing.

This narrowing f-number introduces more and more diffraction – even though you haven't touched your aperture ring and the diameter of the hole isn't physically changing.

Doremus explains above what the Schneider publication doesn't: the importance of balancing diffraction issues with DOF issues. But the publication is still a nice read for more technical people.

I'll only add that after factoring in bellows compensation, which might increase your exposure time, don't forget to add any additional correction for reciprocity failure!

[JMO]

My thanks to all above, from names I'm familiar with and appreciate here! I now understand the various considerations to obtain proper aperture and exposure for my close-up work. I don't actually know how often I'll be operating at 1:1 or thereabouts, but I have a DOF calculator app on my iPhone that should help me determine my DOF, as well as some reference tables. In addition to understanding how to make exposure calculations for bellows extension, I'm also experienced in making similar corrections for film reciprocity. Best regards...

[ic-racer]

Many depth of field tables don't factor in magnification. This leads to numerous confusing threads out there. In the world of Large Format, will want to account for magnification, not only to calculate the effective aperture, but to determine the depth of field.

[Mark Sawyer]

Indeed, when once I asked a photographer whether his DoF scale was based on the f/stop indicated on the aperture scale or the actual f/stop he was working at, he looked at me funny.

[mdarnton]

If you are doing close-up work, take it as given that DOF won't be enough, no matter how small the aperture, especially with large film. Try it and you'll see, The difference is going to be between none at wide open and 3mm closed way down, if you're lucky. If DOF is important, keep the aperture as small as possible, for a start because if it's important, you probably aren't going to have enough.