focal length comparison between 35mm, 120, and 4x5.

[Clark King]

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 a s 4x5. My real question is about exposure compensation when using 120 instead o f 4x5. With the 4x5 film I know that I should employ bellows factor at any bell ows draw over 210mm (as read from the scale on the rail). But, if the 210mm len s 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 compa red 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

[Doug Paramore]

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.

Regards,

[Michael Feldman]

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.

[Clark King]

Thank you, for your responses!

CK

[neil poulsen]

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.

[Pete Andrews]

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

[Huw Evans]

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.

[Michael Feldman]

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?

[Dave Schneidr]

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.

[Huw Evans]

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.