confusion over focal length of lenses

grebeman · #1 November 13th, 2010, 01:22 AM

Why is there so much fuss made about this aspect of m4/3 and this constant comparison to 35mm. Go back to basics. Take the diagonal of the negative, or sensor in this digital age and divide it by the focal length of the lens. The resulting number gives you a quotient (just a fancy way of saying a number) that gives you a measure of the angle of view of the lens. This is independent of the format of the camera.
So a m4/3 sensor has a diagonal of just under 22mm, a 35mm negative has a diagonal of 43mm, a 21/4" square negative has a diagonal of 79mm, a 6x9 negative has a diagonal of 105mm, a 4x5" negative has a diagonal of 155mm. The standard lenses for these formats were always regarded as 50mm for 35mm, 80mm for 21/4" square, 105mm for 6x9 and 150mm for 4x5".
When you divide the diagonal of the negative by the standard focal length for each format the result is approximately 1 for each of these. If you use a lens of shorter focal length the number obtained is greater than 1, if a longer focal length lens is used, the number is less than 1.
Therefore on m4/3 the standard lens should be about 22mm focal length, lets call it 20mm and for simplicity and easy calculation lets call the sensor diagonal 20mm
Fit a 50mm focal length lens and do the simple mathmatics and the result is 20/50 = 0.4, therefore on m4/3 a 50mm lens is a long focal length lens, on 35mm the result is 43/50 = 0.86, on 21/4" the result is 79/50 = 1.58. Exactly the same focal length lens, different results on each format, different perspective obtained.

diagonal/focal length vs angle of view
0.35 = 20 deg
0.54 = 30 deg
0.73 = 40 deg
0.93 = 50 deg
1.15 = 60deg
1.40 = 70 deg
1.68 = 80 deg
2.00 = 90 deg
2.38 = 100 deg
2.86 = 110 deg

Barrie

Luckypenguin · #2 November 13th, 2010, 03:08 AM

I personally have no problems with the traditional means of relating the field-of-view (FOV) or angle-of-view of a lens back to an equivalent focal length lens on a 35mm film frame. It has effectively become a base unit relative to the most widely used photographic format of the last century. Just as base units of mass, force, etc relate back to a measurement which may no longer mean anything to us, the same will most likely happen with lens classification as 35mm film becomes a distant memory.

BillN · #3 November 13th, 2010, 03:50 AM

Barrie

Thanks for the posting - there are always quite a few postings all over the internet on this subject

I wouldn't get too concerned the debate will go on forever

Cheers

grebeman · #4 November 13th, 2010, 05:23 AM

Yes I guess you're right there Bill, I should learn to relax

Barrie

Ray Sachs · #5 November 13th, 2010, 06:21 AM

Quote:

Originally Posted by grebeman

Yes I guess you're right there Bill, I should learn to relax

Barrie

If that system works for you, good enough. But I've been using 35mm equivalencies for so long, its the way my limited brain is wired. I know what 24mm or 35 or 50 or 200 means. I have no idea what 60 degrees means. I mean I KNOW what 60 degrees means, but I just don't think about it that way. I could learn, but I already had to learn to convert non-35mm format gear to 35mm equivalents and I'm over 50 now, so I don't want to learn anything else!

Or, at least, unless there's a really compelling reason to do so and I don't see one here.

Don't worry, there are similar arguments in the cycling world over "gear inches" versus other methods of measuring gearing (some of which take the size of the tire and the length of the crank into consideration, and some ofwhich don't). And there are good arguments for moving on from gear inches, but I know what 27 gear inches means and what 50 and 108 gear inches means and feel like and I just don't see a compelling reason to rewire my increasingly difficult-to-rewire head to some other system.

I'm sure there are similar arguments blazing in every technical pursuit in the world. Its a long and proud tradition!

-Ray

Rudi · #6 November 13th, 2010, 06:42 AM

I'll have to agree with Ray here, Barrie. Your classification just seems too complicated to me (I know it's not, that's why I typed "seems"). Since I am so used to the 35mm system, it must by now be a part of my DNA, so a 35mm-equivalent FOV makes perfect sense to me without even having to think about it. Considering how many people use that convention, I'd hazard a guess that I am not alone!

grebeman · #7 November 13th, 2010, 07:13 AM

I don't think in terms of angle of view, all you need is the ratio of the sensor diagonal to the focal length of the lens, then you know if your lens is wide angle or telephoto, and how much by how much that number deviates from 1.00. Perhaps my ideas are coloured by being more of a medium format convert, and thus 35mm sizes mean little to me.

Barrie

grebeman · #8 November 13th, 2010, 07:41 AM

To anyone who is interested my thoughts are based on fundamental photographic theory, the table of diagonal length to angle of view is taken from "The Manual of Photography", my edition being the 1975 reprint, this was formerly "The Ilford Manual of Photography" originally published in 1890.
The ratio of diagonal length (of the sensor) to focal length of the lens is format independent.

Barrie

NetizenSmith · #9 November 13th, 2010, 09:20 AM

I understand the 35mm film frame and therefore what to expect from particular focal lengths on that system. It's only natural that I will double the focal length in my head on a 4/3 mount to understand how much I'm going to get in the shot.

Brianetta · #10 November 13th, 2010, 09:35 AM

I think we should stop comparing everything to 35mm and accept that this is a micro four thirds forum.

Personally, I'm unfamiliar with 35mm fields of view for given focal lengths. What really gets on my wick is when people start throwing in 35mm focal lengths (occasionally with the unqualified adjective "equivalent") without explaining why.

My previous camera has a zoom lens that goes from 6.1 to 61mm, and that's recorded in its EXIF data. If I were to start converting all my focal lengths to whatever gave the same field on that, others would feel much as I do when they start doing so with 35mm.

Grebeman: Rather than going with the diagonal, I'd rather just go with the horizontal. Then I can estimate the calculated field angle against the floor plane.