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View Full Version : Wiki normal lens entry needs correction

Leonard Evens
3-May-2007, 08:19
At

en.wikipedia.org/wiki/Normal_lens

the values given for the diagonals of 4 x 5 film and 8 x 10 film are wrong. They give the diagonals of the full size film assuming it is equal to the specified dimensions in inches. It ignores the fact that the actual film area is less. Thus, the actual film area for 4 x 5 is about 95.5 x 120 mm with a diagonal of just about 153 mm. That makes the choice of 150 mm as the normal focal length seem more reasonable. Similar remarks must apply to 8 x 10, but I don't do 8 x 10 photography, so I have no direct experience. (The also seem to think the normal focal length for 8 x 10 is 355 mm, which surprises me. I thought it was 300 mm.) I know the dimensions of the actual film area in that case appear somewhere on the web, but I haven't been able to find them. I could guess, but perhaps someone who actually knows could edit the Wikipedia article for both 4 x 5 and 8 x 10.

Vaughn
3-May-2007, 11:30
At

en.wikipedia.org/wiki/Normal_lens

(The also seem to think the normal focal length for 8 x 10 is 355 mm, which surprises me. I thought it was 300 mm.) I know the dimensions of the actual film area in that case appear somewhere on the web, but I haven't been able to find them. I could guess, but perhaps someone who actually knows could edit the Wikipedia article for both 4 x 5 and 8 x 10.

Why be normal? Besides that, I would consider "normal" to be a range of focal lengths-- not an absolute number for each format.

I just measured an 8x10 neg...315mm of image area, 325mm total length, of the diagonal. I would consider 300mm to be on the wide side of "normal and 360mm to be on the long side of "Normal". I shoot a lot with my 300mm on 8x10...it seems pretty normal to me.

A 250mm lens of an 8x10 would then be on the long side of wide and a 420mm, on the wide side of long.

If there were to be any correction needed for Wiki, it would be to express wide, normal and long as ranges -- not single focal length -- for each format.

Vaughn

Bob Gentile
3-May-2007, 11:41
Just another illustration of why I've never been able to consider Wikipedia as any kind of "authority."

Ole Tjugen
3-May-2007, 13:30
The reason for the 360mm's existence as a common focal length is that it's the "normal focal length" (approximately equal to the diagonal of the image area) of the now mostly obsolete 24x30cm format.

It's only "mostly obsolete" because I have one, and every once in a while I find film for it. :D

Leonard Evens
3-May-2007, 14:10
Let me clarify the matter.

The issue is what the actual dimensions of an 8 x 10 image are. The film holder takes up some space, so the dimensions are not the external dimensions of a sheet of film. I've measured my 4 x 5 film, and the actual exposed area is just about 95.5 x 120 mm. Most people agree that it is pretty close to that.

If an 8 x 10 frame has a 325 mm diameter and it has 4:5 aspect ratio, then its dimensions would be just about 203 x 254 mm or close to 8 x 10 inches. According to Ilford, those are the external dimensions of what it sells as 8 x 10 film. I suppose some people use borderless frames of some sort or vacuum backs to hold the film in place. But I assume most people use film holders which don't expose the borders. My guess is that the actual exposed area has dimensions about 196 x 245 mm, but it would help if someone who uses 8 x 10 film would check and let us know.

The diagonal of the frame is usually taken to be the definition of the "normal" focal length for a frame (except for 35 mm). I didn't want to argue about whether or not that is a reasonable choice or whether or not the term is misleading.

Michael Graves
3-May-2007, 14:12
Don't change it in Wiki! One correct entry would mess up their perfect record!

Ole Tjugen
3-May-2007, 14:17
... If an 8 x 10 frame has a 325 mm diameter and it has 4:5 aspect ratio, then its dimensions would be just about 203 x 254 mm or close to 8 x 10 inches. According to Ilford, those are the external dimensions of what it sells as 8 x 10 film. ...

No, that's the metric equivalent of 8x10 inches. The actual nominal film size is smaller! That was done to accomodate film adapter sheaths in older 8x10" plate cameras - the plates were actually the full 8x10 inches.

And this historical "fudge" makes a standard 13x18cm film measure 127x178mm, or almost exactly 5x7 inches, while 5x7" film is smaller at 124x174mm...

3-May-2007, 14:22
The Wikipedia article is way off. But there's no standard specification for these sheet film formats - holders have varied over the years. Schneider assumes 96x120 and 153.7mm for the actual image area of 4x5, and 194x245 and 312.5mm for 8x10, which seem reasonable to me as approximations to modern holders.

FWIW, the Wikipedia article messes up some of the medium format entries too. I don't know of any modern 645 or 6x7 cameras/backs that deliver the dimensions they specify.

Alan Davenport
3-May-2007, 16:18
Why be normal? Besides that, I would consider "normal" to be a range of focal lengths-- not an absolute number for each format.

Fortunately, in practice that is true, but in fact "normal" could be considered an absolute number for each format. For sake of discussions such as this one, I maintain that it (normal defined by the diagonal) should be rigidly adhered to; it makes comparisons between lenses used on different formats simple.

Problems arise when newbies misunderstand the terminology, and believe whichever focal length they prefer to use most of the time can be considered their "normal" lens. Nothing could be further from correct; that isn't what "normal lens" means.

The "normal" lens is based on the focal length that provides a similar angle of view as a viewer sees when viewing an 8x10 inch print at the standard distance of 10 inches. It turns out that focal length correlates closely with the diagonal measurement of the film. Thus a 300mm lens, which dramatically compresses distance on a 35mm photo, doesn't make anything look at all unusual when used with your 8x10 -- it is a normal lens for 8x10.

Vaughn
3-May-2007, 16:34
The Wikipedia article is way off...<snip>...Schneider assumes 96x120 and 153.7mm for the actual image area of 4x5, and 194x245 and 312.5mm for 8x10, which seem reasonable to me as approximations to modern holders.

As I stated above, an 8x10 negative of mine from an unspecified brand/age of film holder, measured 315mm diangonally across its image area. That's plus or minus a couple mm's.

When a discussion generalizing about what a "normal" lens is for a format gets around to talking about half millimeters of film dimensions, then it is time to put the slide rules down, set aside the calipers, close the books of specifications, and take some photos or make some prints! ;) Or have a nice dark beer! :p Or on my budget, and by what I already have open, a glass of Two-Buck Chuck Merlot!

Vaughn

Jason Greenberg Motamedi
3-May-2007, 16:41
The entry also claims that "longer focal length lenses are called telephoto lenses", which we also (should) know to be false.

Vaughn
3-May-2007, 17:01
I understand your point, Alan. Perhaps "normal" is not the right word at all. It is a generalistic term...even when one considers it, the "normal" temperature of a human body (live) is a range and not precisly 98.6F.

Calumet had a great chart in their catalogs giving any focal length lens its 35mm equivilent for each format. Using the diagonal of the film image area as a good standard. But then again, only for giving ballpark estimations of equivilent focal lengths across format sizes. Unless one is doing the math and needs the accuracy, being within 5mm or so is surely close enough.

For those moving between formats, those mm's won't make any difference compared to interesting problem of working with the proportions of a new format. How close to "normal" one's lenses are for 35mm or 6x6 becomes almost immaterial compared to working with the change in proportion from the rectangle to the square.

Vaughn

Leonard Evens
4-May-2007, 05:41
Vaughn,

My apoloogies! I didn't understand from your wording what the 315 designated. From that and Oren't s information, I conclude that 194 x 245 mm is close enough to be taken as the typical image area for 8 x 10.

I don't have a 6 x 4.5 format camera, but my mesaurements for 6 x 6, 6 x 7, and 6 x 9 are not too far off from what the Wiki page gives. My 6 x 7 Horseman roll film holders which I got in the 70s for my Horseman 980 have openings about 56 x 70 mm, which is not too far from Wiki's 56 x 68 mm.

Brian C. Miller
4-May-2007, 07:20
Hasn't anybody realised that those who make film and filmholders don't use Wikipedia as a reference??

Jason Greenberg Motamedi
4-May-2007, 08:12
Hasn't anybody realised that those who make film and filmholders don't use Wikipedia as a reference??

Of course not, but many people do. The number of university papers I have recieved using Wikipedia as a reference (or as a source of information to plagiarize) is amazing.

Brian C. Miller
4-May-2007, 23:33
OK, so let's redefine everything and then laugh our butts off!

Some young lady did a study of deliberately entering errors into Wikipedia and seeing how long it took for them to be corrected. Of course the popular articles were corrected sooner than the obscure articles, if the obscure articles were updated at all.

Let's see, the Roswell incident happened because someone set off a flash in some alien's face, Dagguereotypes were actually invented by Thor, and the list is endless! Kodak is a CIA plot...

RDKirk
5-May-2007, 09:24
The "normal" lens is based on the focal length that provides a similar angle of view as a viewer sees when viewing an 8x10 inch print at the standard distance of 10 inches. It turns out that focal length correlates closely with the diagonal measurement of the film. Thus a 300mm lens, which dramatically compresses distance on a 35mm photo, doesn't make anything look at all unusual when used with your 8x10 -- it is a normal lens for 8x10.

Alan, while I greatly agree with you, I'd quibble on, "provides a similar angle of view." I would say, "privides a similar perspective relationship of near and far objects in the image."

It is very important to say, "when viewing an 8x10 print at the standard viewing distance," but realizing that the viewing distance and print size are relative. It's just important that they remain standardized as one views the prints from various formats.

I tremendously agree with you that understanding "normal lens" makes comparing formats trivially easy. I currently shoot with 24x36mm, 6x7cm, and 4x5-iin. (Yes, I know none of them is precisely those measurements, but a "mile" has never been precisely a "thousand paces," either).

When I shift between lenses and formats, to achieve similar perspective and framing from one format to the next, I've never trying to calculate it mathmatically. I merely think in terms of "difference from normal." If I like the framing and perspective using a lens that is 2xNormal on my 24x36mm camera, then I mount a lens that's 2xNormal on my 6x7 camera. Simple.

false_Aesthetic
13-May-2007, 06:48
Shouldn't you all be out taking pictures?

C. D. Keth
13-May-2007, 15:20
Whenever you read a wiki article, append "ish" to every measurement in your head. Then and only then will it be a really useful resource.