Recirprocity and Bellows factors question

[ImSoNegative]

Its whatever the lens is wide open. Works everytime

[Lenny Eiger]

Bellows extension is easier than all this. It's based on the inverse square law (light decreases to the inverse square of the distance.)

f you have a 6 inch lens (150 mm) and you are focused at infinity, then the lens will be 6 inches from the ground glass. When you focus close, the lens needs to extend. If you get out to 12 inches, which is double the distance, you need to add two stops. (Twice the distance, square the 2 and put it underneath a 1 - that's an inverse square...)

So, at double the distance you have 1/4 of the light, or two stops. If you are 9 inches out, then its 1 1/2 stops to add (not exact, but close enough). This is too simple. My hand is 9 inches stretched out, I don't even carry a ruler...

Lenny

[Tin Can]

Bellows extension is easier than all this. It's based on the inverse square law (light decreases to the inverse square of the distance.)

f you have a 6 inch lens (150 mm) and you are focused at infinity, then the lens will be 6 inches from the ground glass. When you focus close, the lens needs to extend. If you get out to 12 inches, which is double the distance, you need to add two stops. (Twice the distance, square the 2 and put it underneath a 1 - that's an inverse square...)

So, at double the distance you have 1/4 of the light, or two stops. If you are 9 inches out, then its 1 1/2 stops to add (not exact, but close enough). This is too simple. My hand is 9 inches stretched out, I don't even carry a ruler...

Lenny

Great explanation and I think the simplest yet.

[basiltahan]

Great explanation and I think the simplest yet.

This can't be right.

"6 inches to 12 inches is double the distance so you add two stops"
"6 inches to 9 inches is 1.5 times the distance so you add 1.5 stops"

Both sound right.

But by that logic

"6 inches to 6 inches is one times the distance, so you add one stop" ???!!!

At what distance would you add one stop?

6 x square root of 2 (1.414, i.e. double the light) = 8.48 inches

So, 6 inches to 8.48 inches is 1.414 times the distance but you add 1 stop, not 1.4 stops.

At what distance would you add a half a stop?

6 x square root of 1.414 (1.19, 1.4 times more light) = 7.14 inches

So, 6 inches to 7.14 inches is 1.2 times the distance, but you add 1/2 stop, not 1.2 stops.

What is interesting to me is that at only one additional inch of extension with a 150mm lens, an extra half stop of light is required.

[StoneNYC]

All these inches gets me confused, it's helped me if I speak in metric since most modern lenses are in millimeters.

That makes things a little more clear I've found so maybe that will help others.

150mm lens, at 150mm extension is no stops added, 150mm at 300mm extension is double, so 1 stop added.

It's pretty simple, and MUCH easier to use the same system in measuring bellows as you use for your lens.

If you have an old lens with inches it could be easier to do the math in inches.

I don't know what it is but that decimal in my way gets me confused also, so I would also probably struggle with a lens that was 15,0cm or something silly like some oddball kodak and German lenses from the turn of the century.

Keeping it all the same helps simplify things, and not including decimal fractions also helps keep things clear for some that struggle with this concept.

So for anyone who does, try it only in millimeters and see if it makes more sense for you.

[Tracy Storer]

Hey Stone,
If extension is double the infinity focus, you are losing TWO stops of light. You will also be functioning at 1:1 magnification, and 300mm lens to subject. 225mm extension will need 1 stop comp for a 150mm lens.
Reference an earlier post from Lenny who cited the "inverse Square Rule" which will say, two times the extension means 1/4 the light at the film, or 2 stops.
Hope this makes sense to everybody, as it is absolutely correct as spelled out.
T

All these inches gets me confused, it's helped me if I speak in metric since most modern lenses are in millimeters.

That makes things a little more clear I've found so maybe that will help others.

150mm lens, at 150mm extension is no stops added, 150mm at 300mm extension is double, so 1 stop added.

It's pretty simple, and MUCH easier to use the same system in measuring bellows as you use for your lens.

If you have an old lens with inches it could be easier to do the math in inches.

I don't know what it is but that decimal in my way gets me confused also, so I would also probably struggle with a lens that was 15,0cm or something silly like some oddball kodak and German lenses from the turn of the century.

Keeping it all the same helps simplify things, and not including decimal fractions also helps keep things clear for some that struggle with this concept.

So for anyone who does, try it only in millimeters and see if it makes more sense for you.

[Doremus Scudder]

This can't be right.

"6 inches to 12 inches is double the distance so you add two stops"
"6 inches to 9 inches is 1.5 times the distance so you add 1.5 stops"

Both sound right.

But by that logic

"6 inches to 6 inches is one times the distance, so you add one stop" ???!!!

At what distance would you add one stop?

6 x square root of 2 (1.414, i.e. double the light) = 8.48 inches

So, 6 inches to 8.48 inches is 1.414 times the distance but you add 1 stop, not 1.4 stops.

At what distance would you add a half a stop?

6 x square root of 1.414 (1.19, 1.4 times more light) = 7.14 inches

So, 6 inches to 7.14 inches is 1.2 times the distance, but you add 1/2 stop, not 1.2 stops.

What is interesting to me is that at only one additional inch of extension with a 150mm lens, an extra half stop of light is required.

Basiltahan is exactly right here, and also points up one of the problems with "easy ways" to figure bellows extension. Sure, it's easy to say, "if I extend the lens to double the focal length, I add two stops." However, how many of us really spend a lot of time shooting at 1:1? It's all those distances in between infinity and 1:1 that are the problem. Where is the "add 1 stop" point for a 150mm lens? We know from the example above, 8.48 inches, but could you have done that quickly in your head? And for all your other lenses? Do we really think, "1.414 x my focal length = a 1-stop correction"? Can you figure in your head 1.414 x the focal lengths of all the lenses you own? Okay then, what about the in-between values? Where are the half-stop correction points? Those for 1/3-stop?

I, and I'm sure many here, want to be more accurate when exposing than 1 stop. That's why I've made a small table with all the bellows extension factors for all the lenses I own out to the length of the longest bellows I own. Then all I have to do when in doubt is consult the scale on the rail or bed of my camera (or whip out my little cloth measuring tape, which I carry in my bag) and find the right (very precisely right) correction on the table. Do the numbers once, and just pack a measure and a table with you. Easy-peasy.

Doremus

[StoneNYC]

Hey Stone,
If extension is double the infinity focus, you are losing TWO stops of light. You will also be functioning at 1:1 magnification, and 300mm lens to subject. 225mm extension will need 1 stop comp for a 150mm lens.
Reference an earlier post from Lenny who cited the "inverse Square Rule" which will say, two times the extension means 1/4 the light at the film, or 2 stops.
Hope this makes sense to everybody, as it is absolutely correct as spelled out.
T

Wait, so double extension IS two stops?

So 150mm lens, with 300mm extension is 2 stops less light? So 1/125 exposure would be 1/30th?

And 150mm lens with 600mm bellows would be 1/6th?

Did I get that right?

That's natural light, with strobe the calculation would be...

So 150mm lens, with 300mm extension is 2 stops less light? So f/64 exposure would be f/32?

And 150mm lens with 600mm bellows would be f/11?

Did I understand that right?

Basiltahan is exactly right here, and also points up one of the problems with "easy ways" to figure bellows extension. Sure, it's easy to say, "if I extend the lens to double the focal length, I add two stops." However, how many of us really spend a lot of time shooting at 1:1? It's all those distances in between infinity and 1:1 that are the problem. Where is the "add 1 stop" point for a 150mm lens? We know from the example above, 8.48 inches, but could you have done that quickly in your head? And for all your other lenses? Do we really think, "1.414 x my focal length = a 1-stop correction"? Can you figure in your head 1.414 x the focal lengths of all the lenses you own? Okay then, what about the in-between values? Where are the half-stop correction points? Those for 1/3-stop?

I, and I'm sure many here, want to be more accurate when exposing than 1 stop. That's why I've made a small table with all the bellows extension factors for all the lenses I own out to the length of the longest bellows I own. Then all I have to do when in doubt is consult the scale on the rail or bed of my camera (or whip out my little cloth measuring tape, which I carry in my bag) and find the right (very precisely right) correction on the table. Do the numbers once, and just pack a measure and a table with you. Easy-peasy.

Doremus

I was just shooting at 1:1, 1:2.5 and 1:4 the other day, so it's reasonable for some of us to shoot that way.

However I'm still confused again can you explain that in mm please?

I admit that I use an app (Reciprocity Timer App) because there's too much room for my dumb brain error to come into play. But it would be good to understand it so I don't need the app.

[Joe Smigiel]

f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128 apertures are all 1-stop apart and by extension (sic):

1", 1.4", 2", 2.8", 4", 5.6", 8", 11", 16", 22", 32", 45", 64", 90", 128" extension are all 1-stop apart;

1cm, 1.4cm, 2cm, 2.8cm, 4cm, 5.6cm, 8cm, 11cm, 16cm, 22cm, 32cm, 45cm, 64cm, 90cm, 128cm extension are all 1-stop apart;

10mm, 14mm, 20mm, 28mm, 40mm, 56mm, 80mm, 112mm, 160mm, 224mm, 320mm, 450mm, 640mm, 900mm, 1280mm extension are all 1-stop apart;

1 palm length, 1.4 palm length, 2 palm lengths, 2.8 palm lengths, 4 palm lengths, 5.6 palm lengths ...

1 l.y., 1.4 l.y., 2 l.y., 2.8 l.y, 4 l.y., 5.6 l.y., ...

or here's an extension factor table for various focal lengths that you can print and carry:

[fishbulb]

Thank you Joe. People are making it way too complicated. Just convert the measurements (mm, inches, fingers, palm widths, whatever) to apertures and look at the aperture scale on your lens for reference.

* 150mm lens at 300mm bellows length. f/15 to f/30. two stops.

* 300mm lens at 400mm bellows length. f/30 to f/40, or f/3 to f/4. a bit less than a full stop.

* 90mm lens at 900mm bellows length. f/9 to f/90, or f/0.9 to f/9. about six and a half stops.

I have literally picked up a twig off the ground, focused on the horizon, said, OK infinity is one twig length (break twig off at bellows length). Then focus close, and count the twig lengths. If my bellows are now one and a half a twigs long, then I count from f/1 to f/1.5, or a little over 1 stop. It's not rocket science.