How to calculate the needed exposure when changing formats

Qebs · May 17, 2021

Greetings everyone,

Can someone help explain how one would calculate what the increased exposure is when changing to larger formats.

I've seen some solutions through Google searches, but my math is so bad now (as compared when I was in highschool haha).

For example, how much more exposure does one need from 4x5 jumping to 8x10?

Do you need to compensate for the increased surface area as well as the bellows extension factor (if applicable)?

Thank you for reading this!
Have a great new week everyone
Best regards,
Kevin H.

Dan Fromm · May 17, 2021

Exposure required -- shutter speed and aperture -- depends on film speed and illumination. Format has nothing to do with it.

The "bellows extension factor" depends on magnification and comes into play only when shooting close up. Format has nothing to do with it.

Chan Tran · May 17, 2021

Qebs said:
Greetings everyone,

Can someone help explain how one would calculate what the increased exposure is when changing to larger formats.

I've seen some solutions through Google searches, but my math is so bad now (as compared when I was in highschool haha).

For example, how much more exposure does one need from 4x5 jumping to 8x10?

Do you need to compensate for the increased surface area as well as the bellows extension factor (if applicable)?

Thank you for reading this!
Have a great new week everyone
Best regards,
Kevin H.

May I ask did you start out with a digital camera?

Qebs · May 17, 2021

Hey Dan Fromm,

Just to confirm with an example:

Let's say you had a 500mm lens that could cover 16x20" half body shots and also cover 20x24" half body shots

I use an online calculator and it says the bellows extension factor for the 16x20" is +1.13 stops, and for the 20x24" I get +1.35 stops.

Is this the only exposure correction needed. Someone in a Facebook group said, for example, when going from 4x5" to 8x10" you need to increase 4 stops.
I think this is wrong, but I just want to confirm with more knowledgeable people here on the forums.

Many thanks!

Qebs · May 17, 2021

Chan Tran said:
May I ask did you start out with a digital camera?

Hey Chan Tran, nope, I started in the late 90's with a Canon AE-1 Program and used 4x5's and medium format later (4x5's in University)

BMbikerider · May 17, 2021

I think you have an answer looking for a question.. A 5.6 lens to cover a 4x5 neg focussed at infinity will need the same exposure with an equivalent lens focussed at infinity designed to cover 8x10. There is no magical formula just use the same meter reading as you would normally do.

I used to be a police photographer and we originally had MPP 4x5 cameras with a 135 Xenar F4.5 lens to do all our normal day to day work usually exposing at F11-F16. They upgraded to a Hasselblad with the standard 80mm Planar and using the same make/speed film as in the 4x5, but obviously 120 format, the exposure was identical. Where our meter indicated say 125 @ F11 thats what we used in both formats. The only difference was when we did close-up with either camera we used to modify the exposure slightly (No more than 1/4 - 1/2 stop) by pure experience from past tasks. In hundreds and thousands of images taken over 17 years I cannot remember either myself or my colleagues getting it wrong. It really isn't rocket science

Qebs · May 17, 2021

Awesome!

Thanks BMbikerider!
I appreciate your detailed reply!

Wow a police photographer eh? Sounds like an interested but potentially traumatizing job!
Thanks again!

BMbikerider · May 17, 2021

Qebs said:
Awesome!

Thanks BMbikerider!
I appreciate your detailed reply!

Wow a police photographer eh? Sounds like an interested but potentially traumatizing job!
Thanks again!

Not really most of it was mundane, vehicle crashes, damage, burglaries occasionally the odd death from whatever reason..

Chan Tran · May 17, 2021

Qebs said:
Hey Chan Tran, nope, I started in the late 90's with a Canon AE-1 Program and used 4x5's and medium format later (4x5's in University)

So did you use different exposures for the 35mm vs 4x5?

MattKing · May 17, 2021

Qebs said:
Someone in a Facebook group said, for example, when going from 4x5" to 8x10" you need to increase 4 stops.

That sounds like someone is trying to obtain two results in two different formats, each of which exhibits the same depth of field.
The exposure required would be the same.
But the necessary change in aperture (to obtain the same depth of field) would mandate a change in exposure time.
I'll leave it to those who use both formats to advise whether the 4 stop change in aperture is the correct one for maintaining depth of field.
And yes, bellows extension differences could factor in, if you are working at portrait distances.

Qebs · May 18, 2021

Hey,

Thanks for all the replies and help!

Chan, I think you have a good question there!
I think we just did bellows extension factor. hmm

Be well all!

wiltw · May 18, 2021

Conversion factor between any two formats:

Exposure Value(old) *1 = Exposure Value(new)

MattKing · May 18, 2021

wiltw said:
Conversion factor between any two formats:

Exposure Value(old) *1 = Exposure Value(new)

Correct to six decimals too!

wiltw · May 18, 2021

MattKing said:
Correct to six decimals too!

My abacus has 9 positions, so I can get to 8 digits to right of decimal, so I can verify that 6-digit decimal accuracy is true, in case someone has doubts.
Equation leaves out consideration of T factor, though.

gdavis · May 18, 2021

wiltw said:
Conversion factor between any two formats:

Exposure Value(old) *1 = Exposure Value(new)

Too much math; head exploded

wiltw · May 18, 2021

gdavis said:
Too much math; head exploded

I apologize. Then I should reduce the equation and simplify it, for you and others similarly confused...

1 * 1 = 1, or
A * 1 = A

and I omit what the Zone System does to the equation!

Chan Tran · May 18, 2021

wiltw said:
I apologize. Then I should reduce the equation and simplify it, for you and others similarly confused...

1 * 1 = 1, or
A * 1 = A

and I omit what the Zone System does to the equation!

You're not supposed to use math with the Zone System.

Qebs · May 18, 2021

Hahha, Thanks Matt King.

Neil Poulsen · May 20, 2021

wiltw said:
Conversion factor between any two formats:

Exposure Value(old) *1 = Exposure Value(new)

That's terrific. I love it.

Neil Poulsen · May 20, 2021

To keep the same framing AND the same depth of field in going from 4x5 to 8x10 does require higher f-stop numbers, and thereby, longer exposure times. That's because keeping the same framing requires longer (double) lenses. And to get the same depth of field in longer lenses requires higher f-stop numbers. But, that's to KEEP the same exposure, not to increase exposure.

This assumes that one is using the same film (e.g. HP5 Plus) for both formats. Imagine a dotted line that traces a 4x5 sheet of film in the center of an 8x10 sheet of film. They're going to need to same exposure.

RalphLambrecht · May 20, 2021

Qebs said:
Greetings everyone,

Can someone help explain how one would calculate what the increased exposure is when changing to larger formats.
film exposure isindependent of film format; no calculations required!
I've seen some solutions through Google searches, but my math is so bad now (as compared when I was in highschool haha).

For example, how much more exposure does one need from 4x5 jumping to 8x10?

Do you need to compensate for the increased surface area as well as the bellows extension factor (if applicable)?

Thank you for reading this!
Have a great new week everyone
Best regards,
Kevin H.

Neil Poulsen · May 20, 2021

wiltw said:
I apologize. Then I should reduce the equation and simplify it, for you and others similarly confused...

1 * 1 = 1, or
A * 1 = A

. . .

This adds absolutely nothing new to the conversation. It merely confirms the multiplicative identity.

wiltw · May 20, 2021

Neil Poulsen said:
This adds absolutely nothing new to the conversation. It merely confirms the multiplicative identity.

And that was the point; it added to the humor presented by the exploding head from 'too complex an equation' which one person contributed. That is, 'there is nothing to understand', either in the OP or in the explanation of the pseudo equation.

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