what ƒ-stop = "reality" ??

[triplefinger]

so I was thinking the other day. I understand that the 50mm lens is considered "normal" (yes, I know some think not) and I wondered if there is a "normal" ƒ-stop.

or if it will vary based on how far away something is?

I know depth of field is a result of optics, but am curious if there is an answer to this question.

thanks, Mike

 

[Gabriel M.A.]

If someone's short-sighted, "reality" represented by a certain f-stop may be different from somebody else who is far-sighted if the subject is close-focused, or if it's far away.

I think this is one of those questions such as "what is the shutter speed that represents reality", where a kid jumping, photographed in mid-air may not represent "reality", for nobody is frozen in mid-air.

I guess what I'm trying to say is: like beauty, it's all on the eye of the beholder. Not one answer.

 

[kully]

Funny, I was thinking the same thing two weeks ago. 🙂

I don't think you can compare, as you can't really see the bokeh in the images in your head. As you as you start looking at the bokeh your eyes focus and it comes into focus.

I tried anyhoo with a pen held up at eye level and an SLR with 50mm. I'd say the closest was f/16, but that's only because the lens only stepped down to that and I couldn't test any smaller.

 

[Finder]

Yes and no. The average eye has a focal length of 17mm and an entrance pupil in the 7mm to 1.5mm range. So you could calculate depth of field. But, the eye focuses continuously so it appears we have more DOF than the optics would suggest.

 

[richard_l]

You don't know what the effect of the f-stop will be on the image without taking into consideration the focus setting. DOF depends on both focus and aperture.

Richard

 

[back alley]

f8 and be there...

 

[bobomoon]

What about with my 800asa eyes?

 

[triplefinger]

i'm so glad i started this thread...

 

[keithwms]

As already pointed out, our eyes focus actively, and the lens deforms as they do so. So the f/# (which is nothing more than the focal length divided by the pupil diameter) is not a constant.

Digression Of course, this is why some of us find focusing TTL to be a pain, it isn't natural feeling when a lens/aperture system imposes a limited DOF on our eyes. Rangefinders are more natural feeling than SLRs, right? 😉

Further Digression If you want to work out the maximum DOF for our eyes, you could take into account the pupil diameter to work out the diffraction limit, get the circle of confusion, and compute the DOF from there. As I recall the rod/cone system is quite interesting in that these photoreceptors have roughly the same size as the diameter of a diffraction limited spot on the retina.

 

[Todd.Hanz]

f5.6


the end.

 

[ray_g]

f5.6


the end.

Or wider, depending on what you smoked. 😉

 

[ray_g]

Seriously, it depends on quite a few things. Under normal circumstances: the amount of light, and the distance of the object you are focusing on.

For instance, when driving at night, your pupils are more dilated. That is why you sense "glare" when a bright headlight suddenly appears.

The opposite is true when you squint to read a freeway sigh far away. The aperture decreases, the depth of field increases, and you can see better. Of course, many other things come into play, but those are simple examples.

 

[spysmart]

Walter Zapp's Minox sees reality

Walter Zapp's Minox sees reality

The eye is a Minox http://www.minoxlab.com/PZ051897/peterd.htm

Minox enlargements give a familiar view of the world. Why should that be?

The optical system of the human eye is very, very similar to that of the Minox. When my eyes were examined recently, I asked the ophthalmologist what was the focal length, the ‘refractive strength’ of the eye. ‘On average,’ he said, ‘about 63 diopters.’ In camera terms, that is 1/63 of a meter or 15.9 mm. In bright lighting, the iris of your eye closes down to about 3 mm; in dimmer conditions the pupil enlarges to about 5 mm. Under common circumstances the human eye is an f/3.5 to f/5.6 optical system operating at a focal length only a few per cent different than the 15 mm which has been the Minox standard since 1938. The eye is a Minox; no wonder the depth of field we see and the depth of field recorded on a Minox negative are so very similar.

 

[IGMeanwell]

for 35mm ... f8

edit: Spysmart has convinced me otherwise ... maybe I should be trying my luck with Minox cameras

Here is a question.... What about digital? does the new Minox digital follow the same Minox formula?

 

[mbisc]

It really depends -- I don't think there is a "normal" definition, as in "best approximates the human eye" as in the 50mm focal lenght definition (and all of its problems).

If your question is "best" f stop in terms of optical performance (and I don't want to start any flames here), the answer (as a rough rule-of-thumb) is the trade-off between sharpness/dof and defraction. Generally, something like 2 stops from maximum aperture (or 3 stops for wide angle lenses) is sort of the minimum, and in order for defraction to not get too bad, the maximum f stop is format dependent. The rule-of-thumb that I have heard is f5.6 for 35mm, f8 for medium format and f16 for 4x5.

Of course, all this can be overruled if you are trying to achieve certain effects. For example, if you need more or less dof, then select a different f stop...

Cheers,
MIke

 

[mbisc]

It really depends -- I don't think there is a "normal" definition, as in "best approximates the human eye" as in the 50mm focal lenght definition (and all of its problems).

If your question is "best" f stop in terms of optical performance (and I don't want to start any flames here), the answer (as a rough rule-of-thumb) is the trade-off between sharpness/dof and defraction. Generally, something like 2 stops from maximum aperture (or 3 stops for wide angle lenses) is sort of the minimum, and in order for defraction to not get too bad, the maximum f stop is format dependent. The rule-of-thumb that I have heard is f5.6 for 35mm, f8 for medium format and f16 for 4x5.

Of course, all this can be overruled if you are trying to achieve certain effects. For example, if you need more or less dof, then select a different f stop...

Cheers,
MIke

Well, I meant the minimum f stop -- as in, if you select f16 (instead of f5.6) for 35mm, then defraction is going to show its ugly head...

 

[telenous]

As already pointed out, our eyes focus actively, and the lens deforms as they do so. So the f/# (which is nothing more than the focal length divided by the pupil diameter) is not a constant....

... If you want to work out the maximum DOF for our eyes, you could take into account the pupil diameter to work out the diffraction limit, get the circle of confusion, and compute the DOF from there. As I recall the rod/cone system is quite interesting in that these photoreceptors have roughly the same size as the diameter of a diffraction limited spot on the retina.

As an aside to the above observations, E.Puts says that the max diameter of the human eye is around 8mm and hence the max aperture is between f2-f3 (a 'mere' CV Color Skopar 50 f2.5 for humans then). To find an animal with Noctilux-like fast eyes we have to turn to the cats. All the more reason to love cats then!

 

[jano]

As a heavily nearsighted person, I can only say bokeh without my glasses is truly exquisite. No 'cron or 'lux will match the beautiful softness and tonality, even with bokeh-killer backgrounds like trees 🙁

 

[Bryce]

Well....
I've spent some time thinking and experimenting with this. Long story short, the camera cannot represent what we actually see. Here's why I say.
As finder pointed out, the human iris has a maximum opening of about 7mm. I'd guess Finder has some background in astronomy. True?
Anyhow, under fairly normal circumstances (no drugs, daylightish lighting, etc.) the figure is in the 2-3mm range. This places our 17mm eye lens in the F/8 to F/11 range, i.e. tons of DOF.
So, why when I focus on my hand do things across the room seem so... blurry? Well, we've got two eyes, if we're lucky anyway. The distance between them is about 60 or 70 millimeters. So for the same reason that two Keck telescopes have more resolution than one, we have the equivalent of a roughly 70mm entrance pupil. With depth of field to match. That's about like a 50mm F/0.7, so no depth of field at all. Except it's only that fast in one plane...
But wait, there's more. As others have pointed out, things are focused on as we look at them (again, barring drugs). So if we take advantage of our bokeh in a fast camera lens, things look o.k as long as we're looking directly at the sharp part of the picture. When we look elsewhere, it feels awkward; shouldn't the other parts of the image come up sharp as we scan them? Well, that's what happens in live mode, rather than two dimensional Memorex.
In my (latest) final analysis, here's the best we can do with a single lens taking the picture and displaying it in two dimensions-
To portray a scene, Ansel was right- all sharp is as close as we can get. Your eye scans the scene the way it scans the real world, and the things that would be out of focus in reality will be in focus.
To portray an event or an emotion is different. Forcing the viewer's eyes to focus where the camera did can have impact.
And for real impact, the kind that makes you uncomfortable at first but later feels GOOD, use your discression.
Photography can't accurately depict reality. Accept it as art.

 

[Bryce]

OOOH- Telenous, you mentioned cat's eyes. Check this out-
I HAD to know one day what the world would look like through a slit shaped iris, so I made one to fit over an 85mm F/2 lens and took the following pictures. Lens wide open.
In focus areas look pretty ordinary, OOFS are goofy- blurry vertically but not horizontally. The second frame of the watch is focused much closer than the watch.
Just in case you were wondering.