Geoff Bowker's f stop corrections spreadsheet.

brit · February 26, 2010

Hi I am about to get back into LF photography and once again my dire maths is a huge problem in taking my meter reading & exposure factor to actual exposure.

However here <a href="http://www.largeformatphotography.info/bellows-factor.html">http://www.largeformatphotography.info/bellows-factor.html</a> near the bottom of the page is a spreadsheet that may help me. Maybe all I need to is print out a sizeable number of pages of the spreadsheet and maybe I can just quickly read off what my exposure time should be given a shutter reading on the lightmeter and a correction factor (from my favourite image over object plus 1 squared formula).

My problem is that although the instructions to the spreadsheet are to "just feed in the focal length" I am totally bewildered when I'm looking at the thing on my screen. I know my focal length (150mm) but what on earth to do with this spreadsheet to get my quick view printouts? Can anyone help out?

bob_salomon · February 26, 2010

Why not just get two 6 inch rulers (or metric equivelents)? Put one in the scene and measure its size on the groundglass with the other. If one inch on the ruler in the scene is one inch on the gg you are at 1:1 open 2 stops. If one inch in the scene is 1/2 inch on the gg you are 1/2 lifesize open one stop, if 1/4" on the gg open 1/2 stop, etc. If one inch in the scene is 2 inches on the gg you are at 2:1 (twice life size) add 4 stops.

You really do not need a computer or a calculator to do this.

joe_c5 · February 26, 2010

For that spreadsheet, if your focal length is 150mm, just read off the "lens 2" column, which is already 150.

If you had some focal length other than the choices already there, then in the "Enter Lens Focal Length" row just replace one of the numbers with your focal length.

It sounds as if calculating exposure corrections using the magnification may sometimes be more accurate than using the bellows extension, though.

brit · February 27, 2010

Bob I avoid that because I will be doing life size and greater than life size shots of delicate plants/flowers. Too much scope for moving my object (out of a very small dof ) in trying to rig up a ruler to be suspended in place while I view in on the GGS. Holding the ruler by hand and taking a reading there and then avoids this.

I can work out basic stops like in your example but what when the factor is something like 25 and I want to apply this to time then that would take time to figure out. I mean look at the formulae in the spreadsheet...I think you do need a calculator for that outside of the 'happy accidents' of 4 etc.

One more thing is that this will be in a greenhouse with available light, and the faster I can get from metering to shooting the better due to variability in light intensity. Hence having a printed table I can read off quickly such as 'factor 25 to 1/250' will be very very useful.

Joe I'm still not getting this...there isn't anything in lens 2 column other than 0 and #NAME?. Every cell I click on seems to have something in it already.

bob_salomon · February 27, 2010

Brian,

You do realize that the indutry specification for an accurate shutter speed is ±30%, don't you.

There is quite a bit of leeway when determing the proper shutter speeds, especially at longer times as you indicate that you will use.

You don't have to put a ruler in the scene, all you need do is measure the size of a flower or leaf and compare that top a ruler om the gg.

joe_c5 · February 27, 2010

Hmm, you know what they say about assumptions. I had assumed that the spreadsheet wasn't working correctly because I opened it with OpenOffice instead of Excel.

I ended up changing cell B3 from

=IF(A3<B2;0;(;LOG10((A3/B2)*(A3/B2))/LOG10(2);0.25))

to

=IF($A3<B$2;0;MROUND(LOG10(($A3/B$2)*($A3/B$2))/LOG10(2);0.25))

by adding MROUND, deleting the extra semicolon after MROUND, and adding the $s. Using that to fill the rest of the table looks like it gives the correct numbers in quarter stops.

brit · February 28, 2010

Bob as I see it you are saying the same as my original post "(from my favourite image over object plus 1 squared formula)"? That isn't my problem however its the maths to get the actual exposure. You might not need a calculator for IF(A3<B2;0;(;LOG10((A3/B2)*(A3/B2))/LOG10(2);0.25)) but I do.

Joe I originally opened this in OpenOffice since I don't have Excel. Can I have a go at doing the changes (I know very little about spreadsheets) you suggest in OpenOffice or has it got to be Excel?

joe_c5 · February 28, 2010

In OpenOffice you would need to change the formula like I did. The spreadsheet may work fine in Excel after all. You need to unprotect the spreadsheet, too, in Tools > Protect Document > Sheet.

But more importantly, after reading your original post more carefully, this spreadsheet may not be useful to you if you already have correction factor and need to use it to calculate shutter speed.

The math for that is just multiplication, e.g., 1/250 shutter speed and 25 correction factor = (1/250) * 25 = 1/10 corrected shutter speed.

So are you looking for something like this?

<tbody>

<tr>

<td width="86" align="RIGHT">Correction Factor</td>

</tr>

<tr>

<td height="50" align="LEFT">Shutter Speed (1/time)</td>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

</tbody>

</table>

The formula in OpenOffice for cell C3 is

=IF($A3/C$1>=1;CONCATENATE("1/ ";TEXT($A3/C$1;"0"));CONCATENATE(TEXT(1/($A3/C$1);"0.0");"s"))

and that was used to fill the table.

If you didn't need the formatting (the "1/ " or "s"), the formula would be much simpler: =$A3/C$1

The shutter speeds could even be rounded to nice round numbers using a lookup table, but since the full chart would be very large, instead I recommend just converting correction factor to stops, along the lines of

<tbody>

<tr>

<td width="86" height="37" align="LEFT">Correction Factor</td>

<td width="86" align="LEFT">Stops</td>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

</tbody>

</table>

where the formula for the Stops column is =MROUND(LOG(A2;2);0.5)

The math there is just log base 2, rounded to half stops.

If you really prefer the first chart, let me know and I can describe how to do the lookup table.

rob_hale1 · March 1, 2010

Hi,

If you want to do the calcs where ever, when ever then a PDA ala an Ipaq with Excel Mobile on it will handle these calcs. Mine is the rw6828 and only takes minutes to up load from a P.C./laptop.

But, But, But I find the following the following almost bomb proof for B&W and C41, E6 is something else.

Measure the distance from lens board to film at infinity focus for the lens you will be using. This is DA for this lens only.

Set up shot.

Measure the distance from lens board to film. Deduct DA from this. For every ¼ of DA left over from the deduction add ½ a f stop.

This is basically the same as Bob's method just measured a different way. Again But But note what Bob has pointed out about shutters accuracy.

Regards

Rob.

rob_hale1 · March 2, 2010

Hi Brian,

I wrote this for the other LF forum.

As a new comer to LF, with no light meter, I solved this problem by the following :-

My elderly Linhof monorail has a Graflok back so I can remove the ground glass assembly and use the slide clamps ( to hold roll backs etc ).

I made a cardboard plate from 6mm (1/4 inch) corrugated cardboard. This is 121mm (4 13/16 inches ) by 152 mm ( 6 inches). This fits in the recess that a roll back fits and can be held by the slide clamps.

I marked out the centre of this plate ( by corner to corner diagonals) and scribed a circle.

The circle is the same size as the lens mounting of an old Minolta xg1 I have. Any SLR camera with a self-lighting light meter read out will do the job. In my case I had dead lens so I removed the lens mounting ring, mounted the ring on the SLR body so I have a boss to fit snugly into the into the circle which I cut out carefully using a sharp razor blade.

How to use :-

I set up my shot. Clamp every thing down. Remove the ground glass back. Mount my card. Set the SLR ASA to the film speed ( later you will learn to read your negatives and may well find you get better results by being 1/3 or ½ a speed slower on your ASA setting than the film speed). Turn the SLR on.

Here is the trick bit. Holding the SLR to the card I refocus thro the SLR's view finder. Reason – The distance from the lens to the metering cell in the SLR needs to be allowed for due the mirror and prism, i.e. you will move the back stand towards the lens. Once focused, I have a light meter reading. There is a choice I can read just the time with the lens stopped or if the light levels are low I can open up the lens to the largest opening – read the time in the SLR and adjust the time to match my required stop down i.e. one time segment slower per f stop adjustment ( smaller opening ). Remove SLR, card and replace ground glass assembly and refocus. ( If the travel on rail is marked you can note the point you started from and can go back to that point then refocus)

I now have a wiz bang light meter but still carry my bit of card and old xg1 and will often use it to set a known datum point.

This system will compensate for bellows extension and with the old xg1 I get an all over grey card reading as if I were using an incident meter. It all so reduces the number of numbers – f-stops – times etc you need to think about while you are learning. If you take notes, which I highly recommend, you will very quickly find out which speeds on which shutters are too fast or too slow. I made up small charts for each lens with 1/3 or ½ stop adjustments listed for the various speeds of my elderly shutters( these go in the back of my note book ).

Have fun and enjoy your new past time.

Regards

Rob

brit · March 3, 2010

Wow guys. I hope my absence the last 2 nights is not miscontrude that I am not interested...I just didn't get the chance to get online.

And I'm deeply greatful for your replies.

JoeC yes it was something like your first table I wanted to do altough I would have complicated it but using closer integers (basis more accurate exposures...because another idea is to get the lab to do contacts and sell the individual contacts thus all exposures on the contact sheet need to be 'consistent') and trying to decimalise the shutter speeds (because I can't do fractions either...actually I cannot even add up too well!). But I can't figure a way of 'de-decimalising' so I'd have to work backwards from known speeds, decimalised and look for closest matches...*but with your formula above I wouldn't need to do that*. Phew!

I realise the chart would be large but I was thinking of having derivations from one shutter speed per page eg 1/60 th of a second with say 30 correction factors applied to it. On another page 1/15 of a second with the same 30 correction factors.

Whoopee I can apply stops easily to f-stops. I say this because shutter error hadn't occured to me and I may need to keep the same shuter speed and alter the f-stop instead if contact sheets come back too variable. But I like playing with dof hence wanting to fit the speed around the f-stop rather than the other way around.

Thanks to you too Rob. I guess it would depend on the a) absolute light level hitting the GGS being enough and b) trying to be on axis for consistency c) probably using a grey card, but have you ever tested if you could dump your card setup and do a reflected meter reading (with your slr) straight off the GGS?

joe_c5 · March 3, 2010

What sort of control do you have over the shutter speed? Half stops? Third stops?

Assuming half stops, how about something like this?

<tbody>

<tr>

<td width="60" align="LEFT">Shutter Speed (1/time)</td>

</tr>

<tr>

<td height="34" align="RIGHT">Correction Factor</td>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

</tbody>

</table>

As the chart gets prettier, the formula gets uglier, so cell C3 is now:

=IF(C$1/$A3>1;CONCATENATE("1/ ";TEXT(LOOKUP(MROUND(LOG(C$1/$A3;2);0.5);Sheet2.$A$1:Sheet2.$A$28;Sheet2.$B$1:Sheet2.$B$28);"general"));CONCATENATE(TEXT(1/(C$1/$A3);"0.0");"s"))

and the following lookup table is on sheet 2 to keep it out of the way:

<tbody>

<tr>

<td width="83" height="50" align="LEFT">rounded log of shutter speed</td>

<td width="61" align="LEFT">rounded shutter speed</td>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

</tbody>

</table>

sergio_ortega7 · March 3, 2010

This is the device Bob suggested at the start of this thread:

http://www.calumetphoto.com/item/CC9201/

I've used one for years. It's very cheap, simple, accurate and the single most useful exposure calculator for doing close-up work with LF cameras anyone has ever devised. Spare yourself the headache using that Bowker spreadsheet.

brit · March 4, 2010

Humm...my shutter has whole click stops. My longest timed speed is 1 sec. I've aligned the timing arrow to half way between 1 second and half a second and the shutter sounds like its giving me the half a second. If that is the case I'm only on whole stops. Shooting low speed films with a good deal of enlargement may get me into B and T. I do have ND's too. If I still need to tweak say 1/3 of a stop I can do that with my aperture.

That formula is now getting crazy. Joe have you actually put these values into the spreadsheet? I was thinking that maybe you could upload it to somewhere like <a href="http://www.megaupload.com">www.megaupload.com</a> because I won't have much of a clue of what I'm doing with sheets and things. If you've done it and put in values that I can try to see what is what, and where things go, then all your help won't be wasted as I bumble around the spreadsheet too embarassed to say I can't actually work it. If you upload it to megaupload you will be given a link. Just post the link here and I will be able to get a copy. Why not even rename it in your own honour now its been tweaked to a different end? It could even be incorporated into that link to largeformat.info. I'm sure lots of people will find it a Godsend.

Sergio that is like the Quick Disc (see link in original post and go to bottom of page) its good but I am wanting to go to and greater than 1:1 on little delicates so am dubious about introducing things really close to the subject that might knock, or even whaft, it into a slightly different position and out of plane of sharp focus. I do notice though this goes just beyond 3 stops so covers more ground than the quickdisc. The spreadsheet held the chance I could input anything at all and get a useable result pretty much immediately. It got a bit complicated though but I think still holds the promise of being a blockbuster resource thanks now to JoeC.

joe_c5 · March 4, 2010

I do have the values in a spreadsheet, yes. I changed it to read out in full stop shutter speeds and third stop aperture adjustments. This isn't particularly based on Geoff Bowker's spreadsheet, since this one does not use bellows extension as an input.

http://www.megaupload.com/?d=IQ22NM2O

<tbody>

<tr>

<td width="60" align="LEFT">Shutter Speed (1/time)</td>

</tr>

<tr>

<td height="34" align="RIGHT">Correction Factor</td>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

<tr>

</tr>

</tbody>

</table>

brit · March 6, 2010

Wow Joe that's great news. I will DL it now to USB stick and when back on my own PC will open it in Open Office. I will report back and if it seems I know what I'm talking about then the jobs a good 'un.

rob_hale1 · March 6, 2010

Hi Brian,

Allow me to simplify, in practical terms, the Inverse square law when applied to the shutter factor for Bellows Extension.

The law states that :- shutter factor = (bellows extension) squared / (focal length) squared

This calls for the focal length of the lens, as written on the lens, to be the same as the distance between the lens and the film plane when focused at infinity. Unfortunately this is not always the case, so I will introduce “infinity focus length”. This is a “Practical” measurement of the distance between the lens board and the film plane when focused at infinity. This saves getting involved with nodal points etc when they (nodal points etc.) do not make a substantive difference.

1/ now we have, shutter factor = (bellows extension) squared divided by (infinity focus length) squared

2/ shutter factor divided by 2 = f stop adjustment (or stop adjustment) where ‘f stop adjustment’ or ‘stop adjustment’ is the extra light required.

3/ the bellows extension of any lens is proportional to its infinity focus length, thus you only need one set of shutter factors or f stops, or parts thereof, for all your lenses.

For any extension minus the infinity focus length that equals 0.25 of the infinity focus length, or increments of one quarter (eg. half, three-quarters, one etc), the following list shows the f stop/s you open the aperture by.

0.5, 1, 1.5 , 2, 2.25, 3, 3.5 or 4, 4.5, 5, 6, 7, 8

For shutter correction, you slow the shutter speed by a stop or if you have a half stop option by a combination of half stops. e.g.. a stop more light = 1/60 is slowed by a stop to 1/30.

For those of you with spread sheet minds try this.

Define a named cell, infinityfocuslength For quarter bellows extension in each of the following columns, enter into the first column :-

=(((infinityfocuslength/4)*1)+infinityfocuslength)^2/(infinityfocuslength^2)

Copy that across as many columns as you like ( 11 will give you 3 full extensions ). Having copied, go to column 2 and change *1 to *2, column 3 change *1 to *3 and so on to the end of the row.

In the named cell put in whatever focal length/infinity focus number takes your fancy and press Enter.

Try a different number.

Kinky the shutter factors in the columns do not change !!!

You can add a row to convert these shutter factors to absolute values for f stops by dividing by 2.

And a further row for practical f stop adjustment based on your own opinion.

Regards

Rob

brit · March 10, 2010

Rob I know the theory. I even did a fair amount of optics in A level physics and have a Diploma in Photography. The problem is the maths. I cannot even add up without counting on my fingers or dots on a piece of paper. People never believe how bad I say I am untill they see me trying to work something out.

I can do 1/60 to 1/30 easy as it is just a stop. What I get confused with is basic multiplication of large correction factors that are 'somewhere' between two useable settings. so x multiplied by y is between 1/30 and 1/60 but is actually 1/60 plus 2/3rd stops, so use 1/30. There is something called dyscalculia and I'd bet my life that I have it.

OK Joe I am on my own PC now with the spreadsheet open...still trying to figure it out. Will get back to you when the penny drops.

brit · March 10, 2010

Right I think I've got it! :)

One thing though..as all these times are 1 second and less at metering stage can I combine these with times above 1 second?

Example...

Lets say I meter and get 2 and an eight of a second (I know I know I just want to test this) and my correction factor is 14.

14 x 2 seconds = 28 seconds

For the tricky fractions, from the spreadsheet (oh joy!) I get the Correction Factor 14 and 1/8 of a second shuter speed intersecting at at 1.8seconds.

So my total (ex reciprosity and long live Acros) shutter time is 28 + 1.8 = 29.8 aka 30 seconds? I think I can use it that way but just asking incase I'm wrong.

Is sheet 2 some sort of 'code' for sheet 1? of can I use it in some way? Not figured that one out yet.

joe_c5 · March 10, 2010

You can use the spreadsheet that way, yes, where the exposure time for 2 1/8 seconds with a correction factor of 14 would be 28 + 1.8 = 29.8 seconds. Perhaps I should have written the spreadsheet to handle longer exposures, but at that point it becomes just multiplication without any fractions or rounding to third stops.

The stuff on sheet 2 is used by the formula, and isn't useful by itself.

rob_hale1 · March 12, 2010

Hi Brian,

I have no problem with your discomfort with maths and numbers. The difficulty I see is how to get your extensions sorted.

I will take a punt and guess that your 150 lens is not a tele focus or something unusual.

Buy yourself a small steel tape measure. With a felt tip marker pen black out from zero to 150 mm. At 187 mm draw a stripe across the tape with marker pen and next to it put a small sticky label with “ .5 f “ on it. At 225 mm draw the stripe and put a small sticky label with “ 1 f “ on it. At 262 mm draw the line and label “ 1.5 f “ At 300 mm draw the line and label “ 2 f “ At 337 mm draw the line and label “ 2.5 f “ At 375 mm draw the line and label “ 3 f “ At 412 mm draw the line and label “ 3.5 or 4 f “ At 450 mm draw the line and label “ 4.5 f “

To use.

Compose your picture, dof, lighting etc. Take light reading and set the shutter to match. Now take your newly marked out tape and measure from the lens board to the film plane. If the distance is equal to or greater than any of the mark points on the tape use the number on the label next to the marked point. If the number is a whole number, slow the shutter down by that number of stops. If the number has a part number, open your aperture by that decimal.

Example: 1/ The distance is 310 mm which equals 2 f, because 310 mm is in between 300 mm and 337 mm. The shutter is set on 1/125 so you slow the shutter down 1 stop to 1/60 then a second stop 1/30.

2/ In this example I have dropped the fractional time notation. The distance is 345 which equals 2.5 f. The shutter is set on 30 and the aperture is f 16. So you slow shutter down 1 stop to 15 then the second stop to 8 now we now open the aperture by .5 f stop so you move the from 16 to 11.5. If it had been set at 16.5 you would move it to 16.

There is a very nice analogue light meter with a disk system of read out which allows one to see all this information at once. The f stops and time side by side far simpler to comprehend than digital systems. These are the Gossen Lunasix F, the Profisix sbc and all of the 9 volt battery ones.

OK give this a try and remember that your shutter is anything up to plus or minus 30% so do test with film.

P.S. in the previous post there is a typo in the list of numbers 2.25 should read 2.5.

Regards Rob.

brit · March 14, 2010

That's clever. I totally lost my post by clicking the Notify button.

Hi JoeC.

Thanks for this spreadsheet. Now I think I can get to grips with 'difficult' sets of numbers. Thanks again.

Hi Rob Hale,

I get your drift but just for now as I'm starting over again and have got to grips with the spreadsheet I think I will stick with that for now. If I ever get a wandering mind or want to double check my spreadsheet against another method then I think I will do what you suggest in your last post as this seems a straight forward way of getting results. But just for now I'll stick to one way rather than try two ways.

Sign In

Geoff Bowker's f stop corrections spreadsheet.

Recommended Posts

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Link to comment

Share on other sites

Create an account or sign in to comment

Create an account

Sign in