focal length comparison between 35mm, 120, and 4x5. : LUSENET : Large format photography : One Thread

Ok I know that a focal length of 210 is 210 for 4x5 and about 63mm for 35mm, but what about 120 or medium format. I'm asking because ill be using 120 as well as 4x5. My real question is about exposure compensation when using 120 instead of 4x5. With the 4x5 film I know that I should employ bellows factor at any bellows draw over 210mm (as read from the scale on the rail). But, if the 210mm lens equates to say 80mm because of the difference in film size with the 120, would I compensate at any bellows draw after 80mm as read from the scale on the rail? Also I need to know what the equivalent focal legnth is for the 120 when compared to 4x5 so I can properly meter the scene.

I've checked Stroebel's book on this and did not find any information.

Any help is appreciated,

Clark King

-- Clark King (, September 30, 2001


Clark: I can't quite grasp what it is you are asking. Are you talking about changing film sizes in the 4x5, as with a roll film back? Maybe I confused myself.


-- Doug Paramore (, September 30, 2001.

There are several different image sizes for 120 film. 6x4.5, 6x6, 6x7, 6x7 among others. The actual image sizes of the above formats various somewhat from their names, for example 6x7 is approximately 56mm x 70mm. There is a fairly large difference between 6x4.5 and 6x9.

Insofar as meter compensation, I doubt there is any meaningful compensation required unless you are doing close focusing work.

-- Michael Feldman (, September 30, 2001.

A focal length of 210 is the same focal length on 35mm to 12x20 inches and beyond. The focal length of a lens does not change with the format.

I think you are looking for a way to find the 'equivalent' lens to use to get approximately the same view in different film formats.

Take your 35mm film & multiply by 3 to get a 4x5 equivalent. A 'normal' 50mm lens in 35mm is approximately a 150 in 4x5... both considered normal for the format.

If you take the other formats you are interested in and figure the math you can get an equivalent lens for each of them. This only approximate since the final film format is a bit different, but will get you in the ballpark.

-- Dan Smith (, September 30, 2001.

Thank you, for your responses!


-- Clark King (, September 30, 2001.

Frankly, I don't think it makes any sense to provide focal length "compensation" factors between two formats if the those formats don't have the same length to width ratio. (e.g. 35mm to 4x5 to 6x6.) Comparing 35mm to 4x5, there is no factor that will enable one to select a lens to help them see in a 35mm viewfinder the same image that they may have seen on a 4x5 ground glass. So, what's the point?

6x7 is close enough in ration to 4x5 that focal length compensation factors might make sense. Or 4x5 compared to 8x10. etc.

-- neil poulsen (, October 01, 2001.

Oh not again! This must have been covered about a zillion times before in the archives.

-- Pete Andrews (, October 01, 2001.

This seems to be a question about exposure compensation for bellows extension, rather than equivalent focal lengths. The questioner is confused - the exposure compensation has nothing at all to do with the size of the piece of film you place behind the lens. It is, rather, an absolute, depending just on the focal length and the extension.

This is also more than adequately covered in the archives (though sometimes incorrectly) - have a look in the 'Technique' category for 'exposure compensation' or 'bellows factor'.

Finally, metering also has nothing whatever to do with format or equivlent focal lengths.

-- Huw Evans (, October 01, 2001.

After reviewing the previous recommended archive, it does not seem to clarify the situation, especially since (as you pointed out) some of the posts are contradictory and therefore incorrect (which ones!).

I am not an expert on these matters, but it is my understanding that a 50mm macro (for a 35mm camera) extended to a distance of 200mm (with bellows attachment) requires more exposure compensation than a 200mm lens (for any format) at infinity. Am I mistaken?

-- Michael Feldman (, October 01, 2001.

Yes, a 50mm lens extended to 200mm requires exposure compensation. This has no relationship whatsoever to a 200mm lens. Now if the 200mm lens were extended to 800mm there are relationships.

As to compensating for film size, think of it this way. If you calculated the require exposure compensation or bellows factor for a shot using 4x5 film and then chose, in the darkroom, to crop only a section of the negative for printing, would your exposure now be wrong? If printing only the center area of the negative would it now be an underexposed negative compared to printing the entire negative area? Of course not. That is the analogy to using a smaller piece of filme (120 roll film) in your 4x5 camera.

-- Dave Schneider (, October 01, 2001.

My apologies - I guess to direct someone to the archives and then concede that some of them are wrong is a bit daft!

For the record, the technically correct way of calculating exposure compensation for a typical prime lens (not a telephoto) is as follows: Divide the total bellows extension by the focal length. Take the logarithm of this number - call it L1. Now take the logarithm of the square root of 2 - call this L2. Divide L1 by L2, and the result is the number of stops extra exposure required.

Now, there's a bit of mathematics in there that many people will not care for, so a good rule of thumb which is easier to handle is this: If L1=1 you have no required compensation; if L1=1.4, you need 1 stop; if L1=2, you need 2 stops; L1=2.8, 3 stops; and so on. You should recognise the pattern of numbers there as the standard series of f-numbers. For values of L1 in between these values a rough guess is unlikley to be too far out.

Many people have their own different rules of thumb, and this is where some of the incorrect methods creep in.

-- Huw Evans (, October 01, 2001.

Clark, to answer at least one of your questions, a 127mm on 6X7 will see about the same as the 210 0n 4X5. If you put a rollfilm back on the 4X5 and you are using the 210, bellows factor doesn't start until you go well past 210. It's still a 210, but your rollfilm will "see" approximately what a medium format with a 210 telephoto would be seeing. It is correct that you can't really compare the 3:2 ratio of 35mm with the 4:5 of 4X5, but we do it anyway. For my brain, I choose to consider the width in that ratio, not the height. since 35mm is 1.5" wide, you could fit 3 in the 5" width of 4X5 and have 1/2 inch left over. I use 3.33 to make that conversion for what it's worth, making a 165mm closer to a 50mm on 35. There's no set rule because it's technically an improper comparison. As far as metering the scene, either you're confused, or I am. If I had 3 cameras set up, a 35mm with a 63mm lens, a 120 with 127mm lens, and a 4X5 with 210mm lens, all focused at infinity, and all had the same film, ie. velvia, I would take one reading for XX ASA and set all 3 cameras the same. Hope this helps some.

-- Jim Galli (, October 01, 2001.


With regard to your query about focal length comparisons with varying formats I suggest use of the following equation in a spreadsheet such as Excel:

Angle Of View =ATAN(Image Length / 2 / Focal length) X 180 / PI() X 2

which becomes:



Column A = Focal length

Column B = Image Length

Another useful application of this equation :

Focal Length =A1/(1/(B1/C1)+1)


A1 = Subject Distance B1 = Image Width C1 = Subject width

In circumstances where space might be limited this equation will give the focal length required to fit a given subject into a particular film size. Importantly the same units of measurement must be used for each element (ie: inches, mm.) To convert a focal length in inches to mm simply multiply by 25.4.

To determine bellows factors use this equation on a seperate worksheet:

Bellows factor =(Measured Bellows Extension X Measured Bellows Extension) / (Focal length X Focal Length)


=(A1*A1)/(B1*B1) where A1 is Bellows Extension and B1 is Focal Length

A further modification of the second equation that may come in handy is:


This will give the magnification.

I have these formulae in templates on a spreadsheet in my Palm Pilot and am able to make the necessary calaculations instantly wherever I am.

Try it ... it might make light of a number of confusing issues.


-- Walter Glover (, October 01, 2001.

Thank you all for the info. I appreciate it. I'm a little new at all this and needed to clarify a few particulars. One thing about this forum is that you can learn a great deal of information, and I appreciate any and all responses.



-- Clark King (, October 01, 2001.

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