Topic: All apertures are not created equally

[Mojave]

Or am I mistaken?

4.7 on my Pentax ME is tiny compared to the 4.7 on the Wollensack LF lens but for that, Im assuming the aps are so different because the size of the negatives are so different. Is that a correct assumption?

But what about 100 year old LF lenses? I got one and its lowest ap setting is 4, though the dial goes past that, but open all the way, the size of the opening on the old Kodak is much smaller than the 4.7 ap on the Wollensack.

I hate to ask such a stupid question, but why are the aperture sizes so different in these lenses?

[will_8]

My understanding is that the aperture size depends on the focal length of the lens. For example if you have a 40mm lens and the aperture is 10mm wide then it is f4, as it is 1/4 of the size. I am guessing your lenses are of different focal lengths so that is probably why.

I'm not 100% on this though so if I'm wrong someone please correct me.

[sapata]

My understanding is that the aperture size depends on the focal length of the lens. For example if you have a 40mm lens and the aperture is 10mm wide then it is f4, as it is 1/4 of the size. I am guessing your lenses are of different focal lengths so that is probably why.

I'm not 100% on this though so if I'm wrong someone please correct me.

That's seem to be correct will, I found the aperture of my Petzval style lens by doing exactly the same.


Focal lens divided by the diameter = F number

[Mojave]

That make sense and I think I've read that before. So then how to you know what the focal length is for your lens if it isnt marked somewhere on it? I guess this all boils down to figuring out just exactly what the wide open ap setting is for the 100 year old Kodak lens I have. Its blowing out my shots and Im sure its because its bigger than 4.

[astrobeck]

It's all about ratios and about how far light is traveling through the lens to the film plane to achieve the same luminance.

[astrobeck]

is there any markings on the lens besides the f/stops?

[Francois]

Like Mauricio said, the f/stop varies according to the focal length and the physical diameter of the lens. Since the "normal" focal length is the same as the diagonal size of the film, a sheet of 4x5's normal lens is a 150mm (in reality 162mm) while a single frame of 35mm is 45mm (in reality 43mm). On a Minox, it's 17mm if I remember correctly. This will explain why f/4 on a lens can be different from f/4 on another. It also explains why f/64 is available on large format 8x10 lenses and not for 35mm film. (well... there is also diffraction in the last equation but that's a whole other story)

[sapata]

That make sense and I think I've read that before. So then how to you know what the focal length is for your lens if it isnt marked somewhere on it?

Have a look at this video...  
How to Figure Friggin Focal Length and f Stop Thingies.

[original_ann]

sapata pointed to the exact same video I was going to suggest.  I love that guy!

[Heather]

Also, f-stops are a measurement of light but f/5.6 on 35mm looks different to f/5.6 on a LF ... There's something uber technical about it but you get different depths of field/focus on each. In LF you would have a more shallow depth of field than in 35mm. Someone else can feel free to fill people in on the sciencey optical stuff; I'm just pointing out what I found between my cameras

[Francois]

Well observed Heather.
The depth of field is related to the circle of confusion...
I know this really confuses people to no end

Thing is, the circle of confusion can be loosely described as the largest out of focus spot that will still be seen as a point by the human eye. Since a large format negative gets less enlargement than a frame of 35mm, it's circle of confusion is always bigger. Surprisingly, large format lenses can be of lesser quality than those of a 35mm camera for the same image quality!

As for depth of field, it's related to this and a bunch of other stuff.
Reading the wiki link I put up is sure to give you both a headache and a "Oh! that's how it works" moment

[greenstphotography]

Most everything has been well covered here.

I'll only add one bit.  f-stop is not actually refering to a measurment of light, it is refering to aperture.  It is a very common error.

When refering to an amount of light it is a stop.  A stop is defined as either double or half of an amount of light.  A stop less would refer to half as much light and a stop more twcie as much light.

I think where things got muddled was when most cameras were manual flm cameras.  Most had apertures that were only in full stop increments (not like today's cameras that have half stop and third stop).  Since the aperture and f-stop are used interchangably, back when cameras only had apertures in incremnts of full stops (half or double) it all got mixed up since if you felt you needed less light you could only select apertures (f-stops) in full stop increments.  So with those old cameras a stop less would also mean an f-stop less.

And if it wasn't clear in the above.  While images produced at the same aperture (f-stop) will have visually different properties when looking at different format cameras.  The amount of light hitting the film is always the same for the same aperture. 

[Pete_R]

The guy in the video makes the common mistake that the rear nodal point is where the aperture is at. Could be in some lenses but certainly isn't always the case. The rear nodal point is often outside the lens. In short focal length retrofocus lenses the rear nodal point is usually behind the lens whereas with a telephoto design it's usually in front of the lens. It's also impossible to 'measure' the aperture size while looking through the lens as the lens elements will magnify (positively or negatively) the aperture.

Have a look at this (http://www.bobatkins.com/photography/technical/measuring_focal_length.html) which is a more practical way of measuring the focal length.

[Mojave]

Thank you all so much! I watched the video, which I enjoyed, and I think I get all this to a certain degree. I am going to do some measuring to see if the markings on the lens are correct.

And as far as I can tell, the only markings on the lens are the T B 25 50 100 at the top and the fstops at the bottom.