What's your favorite internet falsehood on photography?

[Alan Klein]

It means if the lens is set to f16, your "effective" aperture is now f27, so you're going to have to compensate by using more exposure time. There's really no free lunch here. You have to do a few math calculations no matter what, unless you're a mathematical savant.

So what do you set it too and how do you do the calculations?

[maltfalc]

This is way pre-internet. I recall 60+ years ago when I first got interested in photography I'd occasionally hear people assert with great authority that lenses made before World War II were not "color corrected". Have a hunch they were confounding anti-reflection coating with color correction.

David

they were talking about lenses that correct for chromatic aberration.

[Tin Can]

Agree

William Mortensen was controversial, but he was also talented and influential. I consider his book, The Model, to be one of the best sources of information for models and their photographers for the past 87 years. His books on technique show results that most photographers would never master until Photoshop, half a century later. Mortensen, like Edward Weston and other fine artists, had to struggle against the Puritan censorship that inflicted its influence even over the U. S. Postal Service. The famous feud between Mortensen and Adams had an unfortunate argument between greatly different approaches to photography. Adams excelled at presenting nature in the best possible light. (pun attempted) Mortensen went beyond that: he could create images only in his mind, and then use appropriate technique to present them as did artists for thousands of years. Eight Ansel Adams posters adorn my living room: I display nothing of Mortensen. Perhaps today I'll try to download some of his creations to correct that.

[Vaughn]

So what do you set it too and how do you do the calculations?

Keep it at f16, and use the same exposure time your meter tells you to use IF you were using f27. (Which will be a longer exposure)

One has f16 set on the lens, but because of the extended bellows, it is 'acting' like it is f27, so one uses the exposure time needed for f27.

[Alan Klein]

Keep it at f16, and use the same exposure time your meter tells you to use IF you were using f27. (Which will be a longer exposure)

One has f16 set on the lens, but because of the extended bellows, it is 'acting' like it is f27, so one uses the exposure time needed for f27.

OK That makes sense Vaughn Thanks. So f27 is about 2/3 of a stop to f/32 so changin my aperture to f11 1/3 or shutter speed 2/3 of a stop slower leaving aperture at f/16. The best combo: change to f16 1/3 and change shutter one stop slower.

[JMO]

So what do you set it too and how do you do the calculations?

Alan, It was only when I purchased a 4x5 studio camera to try my hand at table-top photography (rather than strictly landscapes as before, where my subjects were typically at "infinity" or nearly that distance) that I was confronted with the need to understand my nominal aperture (as I chose to set on my lens shutter) with my true "effective" aperture. However, I had remembered a little chart and side-bar discussion from a Schneider-Kreuznach lens brochure about exposure correction rules for those doing close-up or "macro" photography to help me understand the reciprocity relationships. A scan of that side-bar and chart is attached here as a PDF file.

Schneider Macro exposure correction rules.pdf

I suppose one should not be surprised that even such a brief technical discussion or explanation in a brochure from a fine German lens manufacturer might itself have enough math and description of the technical concepts to flummox most of us here, but the important realization I came away with is that bellows extension "reciprocity factor" corrections (adding needed exposure by adjusting shutter speed) and "effective" aperture are two sides of the same coin. So (for example) when you need to add one f-stop of exposure through shutter speed due to bellows extension, you should realize that your effective aperture is now one stop higher, and so you might want/need to adjust your aperture on the lens to avoid image softness in the film and print due to lens diffraction.

Finally, regarding these "reciprocity" considerations and how to calculate them and keep them straight, I have found it easiest to use and rely on the Reciprocity Timer app on my iPhone to make sure my exposures will be properly adjusted when doing close-up photogrpahy (in landscapes or the studio, or interiors, or wherever). There are also DOF calculator apps one can use to help you determine (mostly for close-up work) what DOF to expect, but the image softness concerns (on film and print) due to lens diffraction are, in my experience, best left to trial-and-error experience.

[Vaughn]

OK That makes sense Vaughn Thanks. So f27 is about 2/3 of a stop to f/32 so changin my aperture to f11 1/3 or shutter speed 2/3 of a stop slower leaving aperture at f/16. The best combo: change to f16 1/3 and change shutter one stop slower.

KISS...usually one does NOT change the aperture, as that has been decided on because of other factors -- depth of field, etc -- so one just adjusts the exposure time.

In most cases, when using bellows extended beyond the focal length of one's lens and using small apertures to maintain DoF, one's exposure times will be seconds -- easy to adjust exposure without changing the aperture.

[Mark Sawyer]

Pretty much what Vaughn said. To put it as simply as possible, the f/stop is the number of times the aperture diameter will fit into a focal length. Your focal length is how far your bellows is extended when focused, and increases as you focus closer.

If your aperture is two inches and you focus at infinity with a ten-inch lens, you have f/5 because two inches fits into ten inches five times. Base your exposure on f/5.

If you focus at 1:1, that ten-inch lens will take twenty inches of extension to focus that close. Your lens is now at twenty inches of focal length. The two inch aperture fits into twenty inches ten times, so at that distance and extension, it's a twenty-inch f/10 lens. Base your exposure on f/10.

[Alan Townsend]

To all, I just remembered two more of these teachings I suspect come from internet laziness.

1. To convert a lens focal length on a 4x5 to 35mm equivalent, multiply by 3. This is reapeat over and over on almost all Youtube videos. I've always used 4, because it's closer than 3 to the actual equivalent, and much easier to calculate. For example, if you are a photographer who has printed 8x10's from 35mm negatives, the factor would be...
35m vertical is 24mm. 4x5 vertical is 3 3/4 inch = 95.25mm. 95.25/24 = 3.97 which is almost exactly 4. In many enlargers, the 4x5 carrier is only 3 1/2 in.tall (in short dimession in landscape mode). When scanning, you have the full frame available. Let's calculate the diagonals. For 35mm, sqrt(24^2+36^2)=sqrt(576+1296)=43.27mm. Same for full frame 4x5 with 1/8 margins all around. sqrt(95.25^2+120.65^2)=sqrt(9072.6+14566)=153.75. 153.75/43.27=3.553 which is slightly closer to 4 than to 3. So why on earth is everyone using a 3x instead of 4x which is much closer and more practical? For example, to get same depth of field on full frame 8x10 prints from both, we convert the F8 on the 35mm X4=F32 on the 4x5.

2. Do not stop down your lens past F8 on 35mm. or F24 (using a 3x rule) or F32 (using 4x rule) on 4x5 to avoid diffraction. Last time I checked, there is always diffraction, and the airy disk is about 10 microns at F8 for green light. This would give about 100 lines/mm resolution. For 35mm 8x print or 4x5 2x print, we would get about 12.5 lines/mm = 317 lines/inch, more than 6x the required 50 lines/inch standard. The airy is always about 10 microns at F8, 20 at F16, and 40 microns, which is linear with respect to F number.

All these details make me tired.

[Vaughn]

T...
1. To convert a lens focal length on a 4x5 to 35mm equivalent, multiply by 3. ...

Divide by three?

But agreed -- excessive tiredness from worrying about deriving baseball park guesses. I print full frame (120 roll film to 11x14 sheets) and know what focal lengths are about normal for the formats I use and go from there. Start changing proportions and what's normal?